table of contents
- NAME
- SYNOPSIS
- DESCRIPTION
- OPTIONS SUMMARY
- TARGET SPECIFICATION
- HOST DISCOVERY
- PORT SCANNING BASICS
- PORT SCANNING TECHNIQUES
- PORT SPECIFICATION AND SCAN ORDER
- SERVICE AND VERSION DETECTION
- OS DETECTION
- NMAP SCRIPTING ENGINE (NSE)
- TIMING AND PERFORMANCE
- FIREWALL/IDS EVASION AND SPOOFING
- OUTPUT
- MISCELLANEOUS OPTIONS
- RUNTIME INTERACTION
- EXAMPLES
- NMAP BOOK
- BUGS
- AUTHOR
- LEGAL NOTICES
- NOTES
NMAP(1) | Nmap Reference Guide | NMAP(1) |
NAME¶
nmap - Network exploration tool and security / port scannerSYNOPSIS¶
nmap [Scan Type...] [Options]
{target specification}
DESCRIPTION¶
Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime. The output from Nmap is a list of scanned targets, with supplemental information on each depending on the options used. Key among that information is the “interesting ports table”.. That table lists the port number and protocol, service name, and state. The state is either open, filtered, closed, or unfiltered. Open. means that an application on the target machine is listening for connections/packets on that port. Filtered. means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed. Closed. ports have no application listening on them, though they could open up at any time. Ports are classified as unfiltered. when they are responsive to Nmap's probes, but Nmap cannot determine whether they are open or closed. Nmap reports the state combinations open|filtered. and closed|filtered. when it cannot determine which of the two states describe a port. The port table may also include software version details when version detection has been requested. When an IP protocol scan is requested ( -sO), Nmap provides information on supported IP protocols rather than listening ports. In addition to the interesting ports table, Nmap can provide further information on targets, including reverse DNS names, operating system guesses, device types, and MAC addresses. A typical Nmap scan is shown in Example 1. The only Nmap arguments used in this example are -A, to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution; and then the two target hostnames. Example 1. A representative Nmap scan# nmap -A -T4 scanme.nmap.org Nmap scan report for scanme.nmap.org (74.207.244.221) Host is up (0.029s latency). rDNS record for 74.207.244.221: li86-221.members.linode.com Not shown: 995 closed ports PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0) | ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA) |_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA) 80/tcp open http Apache httpd 2.2.14 ((Ubuntu)) |_http-title: Go ahead and ScanMe! 646/tcp filtered ldp 1720/tcp filtered H.323/Q.931 9929/tcp open nping-echo Nping echo Device type: general purpose Running: Linux 2.6.X OS CPE: cpe:/o:linux:linux_kernel:2.6.39 OS details: Linux 2.6.39 Network Distance: 11 hops Service Info: OS: Linux; CPE: cpe:/o:linux:kernel TRACEROUTE (using port 53/tcp) HOP RTT ADDRESS [Cut first 10 hops for brevity] 11 17.65 ms li86-221.members.linode.com (74.207.244.221) Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
OPTIONS SUMMARY¶
This options summary is printed when Nmap is run with no arguments, and the latest version is always available at https://svn.nmap.org/nmap/docs/nmap.usage.txt. It helps people remember the most common options, but is no substitute for the in-depth documentation in the rest of this manual. Some obscure options aren't even included here.Nmap 6.47 ( http://nmap.org ) Usage: nmap [Scan Type(s)] [Options] {target specification} TARGET SPECIFICATION: Can pass hostnames, IP addresses, networks, etc. Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254 -iL <inputfilename>: Input from list of hosts/networks -iR <num hosts>: Choose random targets --exclude <host1[,host2][,host3],...>: Exclude hosts/networks --excludefile <exclude_file>: Exclude list from file HOST DISCOVERY: -sL: List Scan - simply list targets to scan -sn: Ping Scan - disable port scan -Pn: Treat all hosts as online -- skip host discovery -PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes -PO[protocol list]: IP Protocol Ping -n/-R: Never do DNS resolution/Always resolve [default: sometimes] --dns-servers <serv1[,serv2],...>: Specify custom DNS servers --system-dns: Use OS's DNS resolver --traceroute: Trace hop path to each host SCAN TECHNIQUES: -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans -sU: UDP Scan -sN/sF/sX: TCP Null, FIN, and Xmas scans --scanflags <flags>: Customize TCP scan flags -sI <zombie host[:probeport]>: Idle scan -sY/sZ: SCTP INIT/COOKIE-ECHO scans -sO: IP protocol scan -b <FTP relay host>: FTP bounce scan PORT SPECIFICATION AND SCAN ORDER: -p <port ranges>: Only scan specified ports Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9 -F: Fast mode - Scan fewer ports than the default scan -r: Scan ports consecutively - don't randomize --top-ports <number>: Scan <number> most common ports --port-ratio <ratio>: Scan ports more common than <ratio> SERVICE/VERSION DETECTION: -sV: Probe open ports to determine service/version info --version-intensity <level>: Set from 0 (light) to 9 (try all probes) --version-light: Limit to most likely probes (intensity 2) --version-all: Try every single probe (intensity 9) --version-trace: Show detailed version scan activity (for debugging) SCRIPT SCAN: -sC: equivalent to --script=default --script=<Lua scripts>: <Lua scripts> is a comma separated list of directories, script-files or script-categories --script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts --script-args-file=filename: provide NSE script args in a file --script-trace: Show all data sent and received --script-updatedb: Update the script database. --script-help=<Lua scripts>: Show help about scripts. <Lua scripts> is a comma-separated list of script-files or script-categories. OS DETECTION: -O: Enable OS detection --osscan-limit: Limit OS detection to promising targets --osscan-guess: Guess OS more aggressively TIMING AND PERFORMANCE: Options which take <time> are in seconds, or append 'ms' (milliseconds), 's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m). -T<0-5>: Set timing template (higher is faster) --min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes --min-parallelism/max-parallelism <numprobes>: Probe parallelization --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies probe round trip time. --max-retries <tries>: Caps number of port scan probe retransmissions. --host-timeout <time>: Give up on target after this long --scan-delay/--max-scan-delay <time>: Adjust delay between probes --min-rate <number>: Send packets no slower than <number> per second --max-rate <number>: Send packets no faster than <number> per second FIREWALL/IDS EVASION AND SPOOFING: -f; --mtu <val>: fragment packets (optionally w/given MTU) -D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys -S <IP_Address>: Spoof source address -e <iface>: Use specified interface -g/--source-port <portnum>: Use given port number --proxies <url1,[url2],...>: Relay connections through HTTP/SOCKS4 proxies --data-length <num>: Append random data to sent packets --ip-options <options>: Send packets with specified ip options --ttl <val>: Set IP time-to-live field --spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address --badsum: Send packets with a bogus TCP/UDP/SCTP checksum OUTPUT: -oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3, and Grepable format, respectively, to the given filename. -oA <basename>: Output in the three major formats at once -v: Increase verbosity level (use -vv or more for greater effect) -d: Increase debugging level (use -dd or more for greater effect) --reason: Display the reason a port is in a particular state --open: Only show open (or possibly open) ports --packet-trace: Show all packets sent and received --iflist: Print host interfaces and routes (for debugging) --log-errors: Log errors/warnings to the normal-format output file --append-output: Append to rather than clobber specified output files --resume <filename>: Resume an aborted scan --stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML --webxml: Reference stylesheet from Nmap.Org for more portable XML --no-stylesheet: Prevent associating of XSL stylesheet w/XML output MISC: -6: Enable IPv6 scanning -A: Enable OS detection, version detection, script scanning, and traceroute --datadir <dirname>: Specify custom Nmap data file location --send-eth/--send-ip: Send using raw ethernet frames or IP packets --privileged: Assume that the user is fully privileged --unprivileged: Assume the user lacks raw socket privileges -V: Print version number -h: Print this help summary page. EXAMPLES: nmap -v -A scanme.nmap.org nmap -v -sn 192.168.0.0/16 10.0.0.0/8 nmap -v -iR 10000 -Pn -p 80 SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
TARGET SPECIFICATION¶
Everything on the Nmap command-line that isn't an option (or option argument) is treated as a target host specification. The simplest case is to specify a target IP address or hostname for scanning. Sometimes you wish to scan a whole network of adjacent hosts. For this, Nmap supports CIDR-style. addressing. You can append / numbits to an IPv4 address or hostname and Nmap will scan every IP address for which the first numbits are the same as for the reference IP or hostname given. For example, 192.168.10.0/24 would scan the 256 hosts between 192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and 192.168.10.255 (binary: 11000000 10101000 00001010 11111111), inclusive. 192.168.10.40/24 would scan exactly the same targets. Given that the host scanme.nmap.org. is at the IP address 64.13.134.52, the specification scanme.nmap.org/16 would scan the 65,536 IP addresses between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0, which targets the whole Internet. The largest value is /32, which scans just the named host or IP address because all address bits are fixed. CIDR notation is short but not always flexible enough. For example, you might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or .255 because they may be used as subnet network and broadcast addresses. Nmap supports this through octet range addressing. Rather than specify a normal IP address, you can specify a comma-separated list of numbers or ranges for each octet. For example, 192.168.0-255.1-254 will skip all addresses in the range that end in .0 or .255, and 192.168.3-5,7.1 will scan the four addresses 192.168.3.1, 192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may be omitted; the default values are 0 on the left and 255 on the right. Using - by itself is the same as 0-255, but remember to use 0- in the first octet so the target specification doesn't look like a command-line option. Ranges need not be limited to the final octets: the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for all IP addresses ending in 13.37. This sort of broad sampling can be useful for Internet surveys and research. IPv6 addresses can only be specified by their fully qualified IPv6 address or hostname. CIDR and octet ranges aren't yet supported for IPv6. IPv6 addresses with non-global scope need to have a zone ID suffix. On Unix systems, this is a percent sign followed by an interface name; a complete address might be fe80::a8bb:ccff:fedd:eeff%eth0. On Windows, use an interface index number in place of an interface name: fe80::a8bb:ccff:fedd:eeff%1. You can see a list of interface indexes by running the command netsh.exe interface ipv6 show interface. Nmap accepts multiple host specifications on the command line, and they don't need to be the same type. The command nmap scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.- does what you would expect. While targets are usually specified on the command lines, the following options are also available to control target selection: -iL inputfilename (Input from list) .Reads target specifications from inputfilename.
Passing a huge list of hosts is often awkward on the command line, yet it is a
common desire. For example, your DHCP server might export a list of 10,000
current leases that you wish to scan. Or maybe you want to scan all IP
addresses except for those to locate hosts using unauthorized static IP
addresses. Simply generate the list of hosts to scan and pass that filename to
Nmap as an argument to the -iL option. Entries can be in any of the
formats accepted by Nmap on the command line (IP address, hostname, CIDR,
IPv6, or octet ranges). Each entry must be separated by one or more spaces,
tabs, or newlines. You can specify a hyphen (-) as the filename if you want
Nmap to read hosts from standard input rather than an actual file.
The input file may contain comments that start with # and extend to the end of
the line.
-iR num hosts (Choose random targets) .
For Internet-wide surveys and other research, you may
want to choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain private,
multicast, or unallocated address ranges are automatically skipped. The
argument 0 can be specified for a never-ending scan. Keep in mind that some
network administrators bristle at unauthorized scans of their networks and may
complain. Use this option at your own risk! If you find yourself really bored
one rainy afternoon, try the command nmap -Pn -sS -p 80 -iR 0 --open.
to locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude
hosts/networks) .
Specifies a comma-separated list of targets to be
excluded from the scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can include
hostnames, CIDR netblocks, octet ranges, etc. This can be useful when the
network you wish to scan includes untouchable mission-critical servers,
systems that are known to react adversely to port scans, or subnets
administered by other people.
--excludefile exclude_file (Exclude list from file) .
This offers the same functionality as the
--exclude option, except that the excluded targets are provided in a
newline-, space-, or tab-delimited exclude_file rather than on the
command line.
The exclude file may contain comments that start with # and extend to the end of
the line.
HOST DISCOVERY¶
One of the very first steps in any network reconnaissance mission is to reduce a (sometimes huge) set of IP ranges into a list of active or interesting hosts. Scanning every port of every single IP address is slow and usually unnecessary. Of course what makes a host interesting depends greatly on the scan purposes. Network administrators may only be interested in hosts running a certain service, while security auditors may care about every single device with an IP address. An administrator may be comfortable using just an ICMP ping to locate hosts on his internal network, while an external penetration tester may use a diverse set of dozens of probes in an attempt to evade firewall restrictions. Because host discovery needs are so diverse, Nmap offers a wide variety of options for customizing the techniques used. Host discovery is sometimes called ping scan, but it goes well beyond the simple ICMP echo request packets associated with the ubiquitous ping tool. Users can skip the ping step entirely with a list scan ( -sL) or by disabling ping (-Pn), or engage the network with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of these probes is to solicit responses which demonstrate that an IP address is actually active (is being used by a host or network device). On many networks, only a small percentage of IP addresses are active at any given time. This is particularly common with private address space such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it used by companies with less than a thousand machines. Host discovery can find those machines in a sparsely allocated sea of IP addresses. If no host discovery options are given, Nmap sends an ICMP echo request, a TCP SYN packet to port 443, a TCP ACK packet to port 80, and an ICMP timestamp request. (For IPv6, the ICMP timestamp request is omitted because it is not part of ICMPv6.) These defaults are equivalent to the -PE -PS443 -PA80 -PP options. The exceptions to this are the ARP (for IPv4) and Neighbor Discovery. (for IPv6) scans which are used for any targets on a local ethernet network. For unprivileged Unix shell users, the default probes are a SYN packet to ports 80 and 443 using the connect system call.. This host discovery is often sufficient when scanning local networks, but a more comprehensive set of discovery probes is recommended for security auditing. The -P* options (which select ping types) can be combined. You can increase your odds of penetrating strict firewalls by sending many probe types using different TCP ports/flags and ICMP codes. Also note that ARP/Neighbor Discovery ( -PR). is done by default against targets on a local ethernet network even if you specify other -P* options, because it is almost always faster and more effective. By default, Nmap does host discovery and then performs a port scan against each host it determines is online. This is true even if you specify non-default host discovery types such as UDP probes ( -PU). Read about the -sn option to learn how to perform only host discovery, or use -Pn to skip host discovery and port scan all target hosts. The following options control host discovery: -sL (List Scan) .The list scan is a degenerate form of host discovery that
simply lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does reverse-DNS
resolution on the hosts to learn their names. It is often surprising how much
useful information simple hostnames give out. For example, fw.chi is the name
of one company's Chicago firewall. Nmap also reports the total number of IP
addresses at the end. The list scan is a good sanity check to ensure that you
have proper IP addresses for your targets. If the hosts sport domain names you
do not recognize, it is worth investigating further to prevent scanning the
wrong company's network.
Since the idea is to simply print a list of target hosts, options for higher
level functionality such as port scanning, OS detection, or ping scanning
cannot be combined with this. If you wish to disable ping scanning while still
performing such higher level functionality, read up on the -Pn (skip
ping) option.
-sn (No port scan) .
This option tells Nmap not to do a port scan after host
discovery, and only print out the available hosts that responded to the scan.
This is often known as a “ping scan”, but you can also request
that traceroute and NSE host scripts be run. This is by default one step more
intrusive than the list scan, and can often be used for the same purposes. It
allows light reconnaissance of a target network without attracting much
attention. Knowing how many hosts are up is more valuable to attackers than
the list provided by list scan of every single IP and host name.
Systems administrators often find this option valuable as well. It can easily be
used to count available machines on a network or monitor server availability.
This is often called a ping sweep, and is more reliable than pinging the
broadcast address because many hosts do not reply to broadcast queries.
The default host discovery done with -sn consists of an ICMP echo
request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP timestamp
request by default. When executed by an unprivileged user, only SYN packets
are sent (using a connect call) to ports 80 and 443 on the target. When
a privileged user tries to scan targets on a local ethernet network, ARP
requests are used unless --send-ip was specified. The -sn option
can be combined with any of the discovery probe types (the -P* options,
excluding -Pn) for greater flexibility. If any of those probe type and
port number options are used, the default probes are overridden. When strict
firewalls are in place between the source host running Nmap and the target
network, using those advanced techniques is recommended. Otherwise hosts could
be missed when the firewall drops probes or their responses.
In previous releases of Nmap, -sn was known as -sP..
-Pn (No ping) .
This option skips the Nmap discovery stage altogether.
Normally, Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as port scans,
version detection, or OS detection against hosts that are found to be up.
Disabling host discovery with -Pn causes Nmap to attempt the requested
scanning functions against every target IP address specified. So if a
class B target address space (/16) is specified on the command line, all
65,536 IP addresses are scanned. Proper host discovery is skipped as with the
list scan, but instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target IP is active. To
skip ping scan and port scan, while still allowing NSE to run, use the
two options -Pn -sn together.
For machines on a local ethernet network, ARP scanning will still be performed
(unless --disable-arp-ping or --send-ip is specified) because
Nmap needs MAC addresses to further scan target hosts. In previous versions of
Nmap, -Pn was -P0. and -PN..
-PS port list (TCP SYN Ping) .
This option sends an empty TCP packet with the SYN flag
set. The default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC. in nmap.h).. Alternate ports can
be specified as a parameter. The syntax is the same as for the -p
except that port type specifiers like T: are not allowed. Examples are
-PS22 and -PS22-25,80,113,1050,35000. Note that there can be no
space between -PS and the port list. If multiple probes are specified
they will be sent in parallel.
The SYN flag suggests to the remote system that you are attempting to establish
a connection. Normally the destination port will be closed, and a RST (reset)
packet sent back. If the port happens to be open, the target will take the
second step of a TCP three-way-handshake. by responding with a SYN/ACK TCP
packet. The machine running Nmap then tears down the nascent connection by
responding with a RST rather than sending an ACK packet which would complete
the three-way-handshake and establish a full connection. The RST packet is
sent by the kernel of the machine running Nmap in response to the unexpected
SYN/ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the RST or SYN/ACK
response discussed previously tell Nmap that the host is available and
responsive.
On Unix boxes, only the privileged user root. is generally able to send and
receive raw TCP packets.. For unprivileged users, a workaround is
automatically employed. whereby the connect system call is initiated
against each target port. This has the effect of sending a SYN packet to the
target host, in an attempt to establish a connection. If connect
returns with a quick success or an ECONNREFUSED failure, the underlying TCP
stack must have received a SYN/ACK or RST and the host is marked available. If
the connection attempt is left hanging until a timeout is reached, the host is
marked as down.
-PA port list (TCP ACK Ping) .
The TCP ACK ping is quite similar to the just-discussed
SYN ping. The difference, as you could likely guess, is that the TCP ACK flag
is set instead of the SYN flag. Such an ACK packet purports to be
acknowledging data over an established TCP connection, but no such connection
exists. So remote hosts should always respond with a RST packet, disclosing
their existence in the process.
The -PA option uses the same default port as the SYN probe (80) and can
also take a list of destination ports in the same format. If an unprivileged
user tries this, the connect workaround discussed previously is used.
This workaround is imperfect because connect is actually sending a SYN
packet rather than an ACK.
The reason for offering both SYN and ACK ping probes is to maximize the chances
of bypassing firewalls. Many administrators configure routers and other simple
firewalls to block incoming SYN packets except for those destined for public
services like the company web site or mail server. This prevents other
incoming connections to the organization, while allowing users to make
unobstructed outgoing connections to the Internet. This non-stateful approach
takes up few resources on the firewall/router and is widely supported by
hardware and software filters. The Linux Netfilter/iptables. firewall software
offers the --syn convenience option to implement this stateless
approach. When stateless firewall rules such as this are in place, SYN ping
probes ( -PS) are likely to be blocked when sent to closed target
ports. In such cases, the ACK probe shines as it cuts right through these
rules.
Another common type of firewall uses stateful rules that drop unexpected
packets. This feature was initially found mostly on high-end firewalls, though
it has become much more common over the years. The Linux Netfilter/iptables
system supports this through the --state option, which categorizes
packets based on connection state. A SYN probe is more likely to work against
such a system, as unexpected ACK packets are generally recognized as bogus and
dropped. A solution to this quandary is to send both SYN and ACK probes by
specifying -PS and -PA.
-PU port list (UDP Ping) .
Another host discovery option is the UDP ping, which
sends a UDP packet to the given ports. For most ports, the packet will be
empty, though for a few a protocol-specific payload will be sent that is more
likely to get a response.. The payload database is described at
http://nmap.org/book/nmap-payloads.html.
The --data-length. option can be used to send a fixed-length random
payload to every port or (if you specify a value of 0) to disable payloads.
You can also disable payloads by specifying --data-length 0.
The port list takes the same format as with the previously discussed -PS
and -PA options. If no ports are specified, the default is 40125.. This
default can be configured at compile-time by changing
DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly uncommon port is used
by default because sending to open ports is often undesirable for this
particular scan type.
Upon hitting a closed port on the target machine, the UDP probe should elicit an
ICMP port unreachable packet in return. This signifies to Nmap that the
machine is up and available. Many other types of ICMP errors, such as
host/network unreachables or TTL exceeded are indicative of a down or
unreachable host. A lack of response is also interpreted this way. If an open
port is reached, most services simply ignore the empty packet and fail to
return any response. This is why the default probe port is 40125, which is
highly unlikely to be in use. A few services, such as the Character Generator
(chargen) protocol, will respond to an empty UDP packet, and thus disclose to
Nmap that the machine is available.
The primary advantage of this scan type is that it bypasses firewalls and
filters that only screen TCP. For example, I once owned a Linksys BEFW11S4
wireless broadband router. The external interface of this device filtered all
TCP ports by default, but UDP probes would still elicit port unreachable
messages and thus give away the device.
-PY port list (SCTP INIT Ping) .
This option sends an SCTP packet containing a minimal
INIT chunk. The default destination port is 80 (configurable at compile time
by changing DEFAULT_SCTP_PROBE_PORT_SPEC. in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the -p
except that port type specifiers like S: are not allowed. Examples are
-PY22 and -PY22,80,179,5060. Note that there can be no space
between -PY and the port list. If multiple probes are specified they
will be sent in parallel.
The INIT chunk suggests to the remote system that you are attempting to
establish an association. Normally the destination port will be closed, and an
ABORT chunk will be sent back. If the port happens to be open, the target will
take the second step of an SCTP four-way-handshake. by responding with an
INIT-ACK chunk. If the machine running Nmap has a functional SCTP stack, then
it tears down the nascent association by responding with an ABORT chunk rather
than sending a COOKIE-ECHO chunk which would be the next step in the
four-way-handshake. The ABORT packet is sent by the kernel of the machine
running Nmap in response to the unexpected INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the ABORT or
INIT-ACK response discussed previously tell Nmap that the host is available
and responsive.
On Unix boxes, only the privileged user root. is generally able to send and
receive raw SCTP packets.. Using SCTP INIT Pings is currently not possible for
unprivileged users..
-PE; -PP; -PM (ICMP Ping Types) .
In addition to the unusual TCP, UDP and SCTP host
discovery types discussed previously, Nmap can send the standard packets sent
by the ubiquitous ping program. Nmap sends an ICMP type 8 (echo request)
packet to the target IP addresses, expecting a type 0 (echo reply) in return
from available hosts.. Unfortunately for network explorers, many hosts and
firewalls now block these packets, rather than responding as required by
RFC 1122[2].. For this reason, ICMP-only scans are rarely reliable
enough against unknown targets over the Internet. But for system
administrators monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does not stop there.
The ICMP standards ( RFC 792[3]. and RFC 950[4]. “a host
SHOULD NOT implement these messages”. Timestamp and address mask
queries can be sent with the -PP and -PM options, respectively.
A timestamp reply (ICMP code 14) or address mask reply (code 18) discloses
that the host is available. These two queries can be valuable when
administrators specifically block echo request packets while forgetting that
other ICMP queries can be used for the same purpose.
-PO protocol list (IP Protocol Ping) .
One of the newer host discovery options is the IP
protocol ping, which sends IP packets with the specified protocol number set
in their IP header. The protocol list takes the same format as do port lists
in the previously discussed TCP, UDP and SCTP host discovery options. If no
protocols are specified, the default is to send multiple IP packets for ICMP
(protocol 1), IGMP (protocol 2), and IP-in-IP (protocol 4). The default
protocols can be configured at compile-time by changing
DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h. Note that for the ICMP, IGMP,
TCP (protocol 6), UDP (protocol 17) and SCTP (protocol 132), the packets are
sent with the proper protocol headers. while other protocols are sent with no
additional data beyond the IP header (unless the --data-length. option
is specified).
This host discovery method looks for either responses using the same protocol as
a probe, or ICMP protocol unreachable messages which signify that the given
protocol isn't supported on the destination host. Either type of response
signifies that the target host is alive.
-PR (ARP Ping) .
One of the most common Nmap usage scenarios is to scan an
ethernet LAN. On most LANs, especially those using private address ranges
specified by RFC 1918[5], the vast majority of IP addresses are unused
at any given time. When Nmap tries to send a raw IP packet such as an ICMP
echo request, the operating system must determine the destination hardware
(ARP) address corresponding to the target IP so that it can properly address
the ethernet frame. This is often slow and problematic, since operating
systems weren't written with the expectation that they would need to do
millions of ARP requests against unavailable hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP requests. And
if it gets a response back, Nmap doesn't even need to worry about the IP-based
ping packets since it already knows the host is up. This makes ARP scan much
faster and more reliable than IP-based scans. So it is done by default when
scanning ethernet hosts that Nmap detects are on a local ethernet network.
Even if different ping types (such as -PE or -PS) are specified,
Nmap uses ARP instead for any of the targets which are on the same LAN. If you
absolutely don't want to do an ARP scan, specify --disable-arp-ping.
For IPv6 (-6 option), -PR uses ICMPv6 Neighbor Discovery instead of ARP.
Neighbor Discovery, defined in RFC 4861, can be seen as the IPv6 equivalent of
ARP.
--disable-arp-ping (No ARP or ND Ping) .
Nmap normally does ARP or IPv6 Neighbor Discovery (ND)
discovery of locally connected ethernet hosts, even if other host discovery
options such as -Pn or -PE are used. To disable this implicit
behavior, use the --disable-arp-ping option.
The default behavior is normally faster, but this option is useful on networks
using proxy ARP, in which a router speculatively replies to all ARP requests,
making every target appear to be up according to ARP scan.
--traceroute (Trace path to host) .
Traceroutes are performed post-scan using information
from the scan results to determine the port and protocol most likely to reach
the target. It works with all scan types except connect scans ( -sT)
and idle scans ( -sI). All traces use Nmap's dynamic timing model and
are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live) in an attempt
to elicit ICMP Time Exceeded messages from intermediate hops between the
scanner and the target host. Standard traceroute implementations start with a
TTL of 1 and increment the TTL until the destination host is reached. Nmap's
traceroute starts with a high TTL and then decrements the TTL until it reaches
zero. Doing it backwards lets Nmap employ clever caching algorithms to speed
up traces over multiple hosts. On average Nmap sends 5–10 fewer packets
per host, depending on network conditions. If a single subnet is being scanned
(i.e. 192.168.0.0/24) Nmap may only have to send two packets to most
hosts.
-n (No DNS resolution) .
Tells Nmap to never do reverse DNS resolution on
the active IP addresses it finds. Since DNS can be slow even with Nmap's
built-in parallel stub resolver, this option can slash scanning times.
-R (DNS resolution for all targets) .
Tells Nmap to always do reverse DNS resolution on
the target IP addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver) .
By default, Nmap resolves IP addresses by sending queries
directly to the name servers configured on your host and then listening for
responses. Many requests (often dozens) are performed in parallel to improve
performance. Specify this option to use your system resolver instead (one IP
at a time via the getnameinfo call). This is slower and rarely useful
unless you find a bug in the Nmap parallel resolver (please let us know if you
do). The system resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]]
(Servers to use for reverse DNS queries) .
By default, Nmap determines your DNS servers (for rDNS
resolution) from your resolv.conf file (Unix) or the Registry (Win32).
Alternatively, you may use this option to specify alternate servers. This
option is not honored if you are using --system-dns or an IPv6 scan.
Using multiple DNS servers is often faster, especially if you choose
authoritative servers for your target IP space. This option can also improve
stealth, as your requests can be bounced off just about any recursive DNS
server on the Internet.
This option also comes in handy when scanning private networks. Sometimes only a
few name servers provide proper rDNS information, and you may not even know
where they are. You can scan the network for port 53 (perhaps with version
detection), then try Nmap list scans ( -sL) specifying each name server
one at a time with --dns-servers until you find one which works.
PORT SCANNING BASICS¶
While Nmap has grown in functionality over the years, it began as an efficient port scanner, and that remains its core function. The simple command nmap target scans 1,000 TCP ports on the host target. While many port scanners have traditionally lumped all ports into the open or closed states, Nmap is much more granular. It divides ports into six states: open, closed, filtered, unfiltered, open|filtered, or closed|filtered. These states are not intrinsic properties of the port itself, but describe how Nmap sees them. For example, an Nmap scan from the same network as the target may show port 135/tcp as open, while a scan at the same time with the same options from across the Internet might show that port as filtered. The six port states recognized by NmapAn application is actively accepting TCP connections, UDP
datagrams or SCTP associations on this port. Finding these is often the
primary goal of port scanning. Security-minded people know that each open port
is an avenue for attack. Attackers and pen-testers want to exploit the open
ports, while administrators try to close or protect them with firewalls
without thwarting legitimate users. Open ports are also interesting for
non-security scans because they show services available for use on the
network.
A closed port is accessible (it receives and responds to
Nmap probe packets), but there is no application listening on it. They can be
helpful in showing that a host is up on an IP address (host discovery, or ping
scanning), and as part of OS detection. Because closed ports are reachable, it
may be worth scanning later in case some open up. Administrators may want to
consider blocking such ports with a firewall. Then they would appear in the
filtered state, discussed next.
Nmap cannot determine whether the port is open because
packet filtering prevents its probes from reaching the port. The filtering
could be from a dedicated firewall device, router rules, or host-based
firewall software. These ports frustrate attackers because they provide so
little information. Sometimes they respond with ICMP error messages such as
type 3 code 13 (destination unreachable: communication administratively
prohibited), but filters that simply drop probes without responding are far
more common. This forces Nmap to retry several times just in case the probe
was dropped due to network congestion rather than filtering. This slows down
the scan dramatically.
The unfiltered state means that a port is accessible, but
Nmap is unable to determine whether it is open or closed. Only the ACK scan,
which is used to map firewall rulesets, classifies ports into this state.
Scanning unfiltered ports with other scan types such as Window scan, SYN scan,
or FIN scan, may help resolve whether the port is open.
Nmap places ports in this state when it is unable to
determine whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could also mean that a
packet filter dropped the probe or any response it elicited. So Nmap does not
know for sure whether the port is open or being filtered. The UDP, IP
protocol, FIN, NULL, and Xmas scans classify ports this way.
This state is used when Nmap is unable to determine
whether a port is closed or filtered. It is only used for the IP ID idle
scan.
PORT SCANNING TECHNIQUES¶
As a novice performing automotive repair, I can struggle for hours trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to the task at hand. When I fail miserably and tow my jalopy to a real mechanic, he invariably fishes around in a huge tool chest until pulling out the perfect gizmo which makes the job seem effortless. The art of port scanning is similar. Experts understand the dozens of scan techniques and choose the appropriate one (or combination) for a given task. Inexperienced users and script kiddies,. on the other hand, try to solve every problem with the default SYN scan. Since Nmap is free, the only barrier to port scanning mastery is knowledge. That certainly beats the automotive world, where it may take great skill to determine that you need a strut spring compressor, then you still have to pay thousands of dollars for it. Most of the scan types are only available to privileged users.. This is because they send and receive raw packets,. which requires root access on Unix systems. Using an administrator account on Windows is recommended, though Nmap sometimes works for unprivileged users on that platform when WinPcap has already been loaded into the OS. Requiring root privileges was a serious limitation when Nmap was released in 1997, as many users only had access to shared shell accounts. Now, the world is different. Computers are cheaper, far more people have always-on direct Internet access, and desktop Unix systems (including Linux and Mac OS X) are prevalent. A Windows version of Nmap is now available, allowing it to run on even more desktops. For all these reasons, users have less need to run Nmap from limited shared shell accounts. This is fortunate, as the privileged options make Nmap far more powerful and flexible. While Nmap attempts to produce accurate results, keep in mind that all of its insights are based on packets returned by the target machines (or firewalls in front of them). Such hosts may be untrustworthy and send responses intended to confuse or mislead Nmap. Much more common are non-RFC-compliant hosts that do not respond as they should to Nmap probes. FIN, NULL, and Xmas scans are particularly susceptible to this problem. Such issues are specific to certain scan types and so are discussed in the individual scan type entries. This section documents the dozen or so port scan techniques supported by Nmap. Only one method may be used at a time, except that UDP scan ( -sU) and any one of the SCTP scan types ( -sY, -sZ) may be combined with any one of the TCP scan types. As a memory aid, port scan type options are of the form -sC, where C is a prominent character in the scan name, usually the first. The one exception to this is the deprecated FTP bounce scan ( -b). By default, Nmap performs a SYN Scan, though it substitutes a connect scan if the user does not have proper privileges to send raw packets (requires root access on Unix). Of the scans listed in this section, unprivileged users can only execute connect and FTP bounce scans. -sS (TCP SYN scan) .SYN scan is the default and most popular scan option for
good reasons. It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by restrictive firewalls. It is also
relatively unobtrusive and stealthy since it never completes TCP connections.
SYN scan works against any compliant TCP stack rather than depending on
idiosyncrasies of specific platforms as Nmap's FIN/NULL/Xmas, Maimon and idle
scans do. It also allows clear, reliable differentiation between the open,
closed, and filtered states.
This technique is often referred to as half-open scanning, because you don't
open a full TCP connection. You send a SYN packet, as if you are going to open
a real connection and then wait for a response. A SYN/ACK indicates the port
is listening (open), while a RST (reset) is indicative of a non-listener. If
no response is received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable error (type
3, code 1, 2, 3, 9, 10, or 13) is received. The port is also considered open
if a SYN packet (without the ACK flag) is received in response. This can be
due to an extremely rare TCP feature known as a simultaneous open or split
handshake connection (see
http://nmap.org/misc/split-handshake.pdf).
-sT (TCP connect scan) .
TCP connect scan is the default TCP scan type when SYN
scan is not an option. This is the case when a user does not have raw packet
privileges. Instead of writing raw packets as most other scan types do, Nmap
asks the underlying operating system to establish a connection with the target
machine and port by issuing the connect system call. This is the same
high-level system call that web browsers, P2P clients, and most other
network-enabled applications use to establish a connection. It is part of a
programming interface known as the Berkeley Sockets API. Rather than read raw
packet responses off the wire, Nmap uses this API to obtain status information
on each connection attempt.
When SYN scan is available, it is usually a better choice. Nmap has less control
over the high level connect call than with raw packets, making it less
efficient. The system call completes connections to open target ports rather
than performing the half-open reset that SYN scan does. Not only does this
take longer and require more packets to obtain the same information, but
target machines are more likely to log the connection. A decent IDS will catch
either, but most machines have no such alarm system. Many services on your
average Unix system will add a note to syslog, and sometimes a cryptic error
message, when Nmap connects and then closes the connection without sending
data. Truly pathetic services crash when this happens, though that is
uncommon. An administrator who sees a bunch of connection attempts in her logs
from a single system should know that she has been connect scanned.
-sU (UDP scans) .
While most popular services on the Internet run over the
TCP protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most common.
Because UDP scanning is generally slower and more difficult than TCP, some
security auditors ignore these ports. This is a mistake, as exploitable UDP
services are quite common and attackers certainly don't ignore the whole
protocol. Fortunately, Nmap can help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined with a TCP
scan type such as SYN scan ( -sS) to check both protocols during the
same run.
UDP scan works by sending a UDP packet to every targeted port. For some common
ports such as 53 and 161, a protocol-specific payload is sent, but for most
ports the packet is empty.. The --data-length option can be used to
send a fixed-length random payload to every port or (if you specify a value of
0) to disable payloads. If an ICMP port unreachable error (type 3, code 3) is
returned, the port is closed. Other ICMP unreachable errors (type 3, codes 1,
2, 9, 10, or 13) mark the port as filtered. Occasionally, a service will
respond with a UDP packet, proving that it is open. If no response is received
after retransmissions, the port is classified as open|filtered. This means
that the port could be open, or perhaps packet filters are blocking the
communication. Version detection ( -sV) can be used to help
differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and filtered ports
rarely send any response, leaving Nmap to time out and then conduct
retransmissions just in case the probe or response were lost. Closed ports are
often an even bigger problem. They usually send back an ICMP port unreachable
error. But unlike the RST packets sent by closed TCP ports in response to a
SYN or connect scan, many hosts rate limit. ICMP port unreachable messages by
default. Linux and Solaris are particularly strict about this. For example,
the Linux 2.4.20 kernel limits destination unreachable messages to one per
second (in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid flooding the
network with useless packets that the target machine will drop. Unfortunately,
a Linux-style limit of one packet per second makes a 65,536-port scan take
more than 18 hours. Ideas for speeding your UDP scans up include scanning more
hosts in parallel, doing a quick scan of just the popular ports first,
scanning from behind the firewall, and using --host-timeout to skip
slow hosts.
-sY (SCTP INIT scan) .
SCTP[7] is a relatively new alternative to the TCP and UDP protocols,
combining most characteristics of TCP and UDP, and also adding new features
like multi-homing and multi-streaming. It is mostly being used for SS7/SIGTRAN
related services but has the potential to be used for other applications as
well. SCTP INIT scan is the SCTP equivalent of a TCP SYN scan. It can be
performed quickly, scanning thousands of ports per second on a fast network
not hampered by restrictive firewalls. Like SYN scan, INIT scan is relatively
unobtrusive and stealthy, since it never completes SCTP associations. It also
allows clear, reliable differentiation between the open, closed, and filtered
states.
This technique is often referred to as half-open scanning, because you don't
open a full SCTP association. You send an INIT chunk, as if you are going to
open a real association and then wait for a response. An INIT-ACK chunk
indicates the port is listening (open), while an ABORT chunk is indicative of
a non-listener. If no response is received after several retransmissions, the
port is marked as filtered. The port is also marked filtered if an ICMP
unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .
These three scan types (even more are possible with the
--scanflags option described in the next section) exploit a subtle
loophole in the TCP RFC[8] to differentiate between open and closed
ports. Page 65 of RFC 793 says that “if the [destination] port state is
CLOSED .... an incoming segment not containing a RST causes a RST to be sent
in response.” Then the next page discusses packets sent to open ports
without the SYN, RST, or ACK bits set, stating that: “you are unlikely
to get here, but if you do, drop the segment, and return.”
When scanning systems compliant with this RFC text, any packet not containing
SYN, RST, or ACK bits will result in a returned RST if the port is closed and
no response at all if the port is open. As long as none of those three bits
are included, any combination of the other three (FIN, PSH, and URG) are OK.
Nmap exploits this with three scan types:
Null scan ( -sN)
-sA (TCP ACK scan) .
Does not set any bits (TCP flag header is 0)
FIN scan ( -sF)
Sets just the TCP FIN bit.
Xmas scan ( -sX)
Sets the FIN, PSH, and URG flags, lighting the packet up
like a Christmas tree.
These three scan types are exactly the same in behavior except for the TCP flags
set in probe packets. If a RST packet is received, the port is considered
closed, while no response means it is open|filtered. The port is marked
filtered if an ICMP unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is
received.
The key advantage to these scan types is that they can sneak through certain
non-stateful firewalls and packet filtering routers. Another advantage is that
these scan types are a little more stealthy than even a SYN scan. Don't count
on this though—most modern IDS products can be configured to detect
them. The big downside is that not all systems follow RFC 793 to the letter. A
number of systems send RST responses to the probes regardless of whether the
port is open or not. This causes all of the ports to be labeled closed. Major
operating systems that do this are Microsoft Windows, many Cisco devices,
BSDI, and IBM OS/400. This scan does work against most Unix-based systems
though. Another downside of these scans is that they can't distinguish open
ports from certain filtered ones, leaving you with the response
open|filtered.This scan is different than the others discussed so far
in that it never determines open (or even open|filtered) ports. It is used to
map out firewall rulesets, determining whether they are stateful or not and
which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you use
--scanflags). When scanning unfiltered systems, open and closed ports
will both return a RST packet. Nmap then labels them as unfiltered, meaning
that they are reachable by the ACK packet, but whether they are open or closed
is undetermined. Ports that don't respond, or send certain ICMP error messages
back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan) .
Window scan is exactly the same as ACK scan except that
it exploits an implementation detail of certain systems to differentiate open
ports from closed ones, rather than always printing unfiltered when a RST is
returned. It does this by examining the TCP Window field of the RST packets
returned. On some systems, open ports use a positive window size (even for RST
packets) while closed ones have a zero window. So instead of always listing a
port as unfiltered when it receives a RST back, Window scan lists the port as
open or closed if the TCP Window value in that reset is positive or zero,
respectively.
This scan relies on an implementation detail of a minority of systems out on the
Internet, so you can't always trust it. Systems that don't support it will
usually return all ports closed. Of course, it is possible that the machine
really has no open ports. If most scanned ports are closed but a few common
port numbers (such as 22, 25, 53) are filtered, the system is most likely
susceptible. Occasionally, systems will even show the exact opposite behavior.
If your scan shows 1,000 open ports and three closed or filtered ports, then
those three may very well be the truly open ones.
-sM (TCP Maimon scan) .
The Maimon scan is named after its discoverer, Uriel
Maimon.. He described the technique in Phrack Magazine issue #49 (November
1996).. Nmap, which included this technique, was released two issues later.
This technique is exactly the same as NULL, FIN, and Xmas scans, except that
the probe is FIN/ACK. According to RFC 793[8] (TCP), a RST packet
should be generated in response to such a probe whether the port is open or
closed. However, Uriel noticed that many BSD-derived systems simply drop the
packet if the port is open.
--scanflags (Custom TCP scan) .
Truly advanced Nmap users need not limit themselves to
the canned scan types offered. The --scanflags option allows you to
design your own scan by specifying arbitrary TCP flags.. Let your creative
juices flow, while evading intrusion detection systems. whose vendors simply
paged through the Nmap man page adding specific rules!
The --scanflags argument can be a numerical flag value such as 9 (PSH and
FIN), but using symbolic names is easier. Just mash together any combination
of URG, ACK, PSH, RST, SYN, and FIN. For example, --scanflags
URGACKPSHRSTSYNFIN sets everything, though it's not very useful for
scanning. The order these are specified in is irrelevant.
In addition to specifying the desired flags, you can specify a TCP scan type
(such as -sA or -sF). That base type tells Nmap how to interpret
responses. For example, a SYN scan considers no-response to indicate a
filtered port, while a FIN scan treats the same as open|filtered. Nmap will
behave the same way it does for the base scan type, except that it will use
the TCP flags you specify instead. If you don't specify a base type, SYN scan
is used.
-sZ (SCTP COOKIE ECHO scan) .
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It
takes advantage of the fact that SCTP implementations should silently drop
packets containing COOKIE ECHO chunks on open ports, but send an ABORT if the
port is closed. The advantage of this scan type is that it is not as obvious a
port scan than an INIT scan. Also, there may be non-stateful firewall rulesets
blocking INIT chunks, but not COOKIE ECHO chunks. Don't be fooled into
thinking that this will make a port scan invisible; a good IDS will be able to
detect SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO scans
cannot differentiate between open and filtered ports, leaving you with the
state open|filtered in both cases.
-sI zombie host[:probeport] (idle scan) .
This advanced scan method allows for a truly blind TCP
port scan of the target (meaning no packets are sent to the target from your
real IP address). Instead, a unique side-channel attack exploits predictable
IP fragmentation ID sequence generation on the zombie host to glean
information about the open ports on the target. IDS systems will display the
scan as coming from the zombie machine you specify (which must be up and meet
certain criteria). This fascinating scan type is too complex to fully describe
in this reference guide, so I wrote and posted an informal paper with full
details at http://nmap.org/book/idlescan.html.
Besides being extraordinarily stealthy (due to its blind nature), this scan type
permits mapping out IP-based trust relationships between machines. The port
listing shows open ports from the perspective of the zombie host. So
you can try scanning a target using various zombies that you think might be
trusted. (via router/packet filter rules).
You can add a colon followed by a port number to the zombie host if you wish to
probe a particular port on the zombie for IP ID changes. Otherwise Nmap will
use the port it uses by default for TCP pings (80).
-sO (IP protocol scan) .
IP protocol scan allows you to determine which IP
protocols (TCP, ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol numbers rather
than TCP or UDP port numbers. Yet it still uses the -p option to select
scanned protocol numbers, reports its results within the normal port table
format, and even uses the same underlying scan engine as the true port
scanning methods. So it is close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates the power of
open-source software. While the fundamental idea is pretty simple, I had not
thought to add it nor received any requests for such functionality. Then in
the summer of 2000, Gerhard Rieger. conceived the idea, wrote an excellent
patch implementing it, and sent it to the announce mailing list. (then called
nmap-hackers).. I incorporated that patch into the Nmap tree and released a
new version the next day. Few pieces of commercial software have users
enthusiastic enough to design and contribute their own improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of iterating
through the port number field of a UDP packet, it sends IP packet headers and
iterates through the eight-bit IP protocol field. The headers are usually
empty, containing no data and not even the proper header for the claimed
protocol. The exceptions are TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol
header for those is included since some systems won't send them otherwise and
because Nmap already has functions to create them. Instead of watching for
ICMP port unreachable messages, protocol scan is on the lookout for ICMP
protocol unreachable messages. If Nmap receives any response in any
protocol from the target host, Nmap marks that protocol as open. An ICMP
protocol unreachable error (type 3, code 2) causes the protocol to be marked
as closed Other ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13)
cause the protocol to be marked filtered (though they prove that ICMP is open
at the same time). If no response is received after retransmissions, the
protocol is marked open|filtered
-b FTP relay host (FTP bounce scan) .
An interesting feature of the FTP protocol (RFC
959[9]) is support for so-called proxy FTP connections. This allows a user
to connect to one FTP server, then ask that files be sent to a third-party
server. Such a feature is ripe for abuse on many levels, so most servers have
ceased supporting it. One of the abuses this feature allows is causing the FTP
server to port scan other hosts. Simply ask the FTP server to send a file to
each interesting port of a target host in turn. The error message will
describe whether the port is open or not. This is a good way to bypass
firewalls because organizational FTP servers are often placed where they have
more access to other internal hosts than any old Internet host would. Nmap
supports FTP bounce scan with the -b option. It takes an argument of
the form username:password@server:port.
Server is the name or IP address of a vulnerable FTP server. As with a
normal URL, you may omit username:password, in which case
anonymous login credentials (user: anonymous password:-wwwuser@) are used. The
port number (and preceding colon) may be omitted as well, in which case the
default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released, but has
largely been fixed. Vulnerable servers are still around, so it is worth trying
when all else fails. If bypassing a firewall is your goal, scan the target
network for port 21 (or even for any FTP services if you scan all ports with
version detection) and use the ftp-bounce. NSE script. Nmap will tell you
whether the host is vulnerable or not. If you are just trying to cover your
tracks, you don't need to (and, in fact, shouldn't) limit yourself to hosts on
the target network. Before you go scanning random Internet addresses for
vulnerable FTP servers, consider that sysadmins may not appreciate you abusing
their servers in this way.
PORT SPECIFICATION AND SCAN ORDER¶
In addition to all of the scan methods discussed previously, Nmap offers options for specifying which ports are scanned and whether the scan order is randomized or sequential. By default, Nmap scans the most common 1,000 ports for each protocol. -p port ranges (Only scan specified ports) .This option specifies which ports you want to scan and
overrides the default. Individual port numbers are OK, as are ranges separated
by a hyphen (e.g. 1-1023). The beginning and/or end values of a range may be
omitted, causing Nmap to use 1 and 65535, respectively. So you can specify
-p- to scan ports from 1 through 65535. Scanning port zero. is allowed
if you specify it explicitly. For IP protocol scanning ( -sO), this
option specifies the protocol numbers you wish to scan for (0–255).
When scanning a combination of protocols (e.g. TCP and UDP), you can specify a
particular protocol by preceding the port numbers by T: for TCP, U: for UDP,
S: for SCTP, or P: for IP Protocol. The qualifier lasts until you specify
another qualifier. For example, the argument -p
U:53,111,137,T:21-25,80,139,8080 would scan UDP ports 53, 111,and 137, as
well as the listed TCP ports. Note that to scan both UDP and TCP, you have to
specify -sU and at least one TCP scan type (such as -sS,
-sF, or -sT). If no protocol qualifier is given, the port
numbers are added to all protocol lists. Ports can also be specified by name
according to what the port is referred to in the nmap-services. You can even
use the wildcards * and ? with the names. For example, to scan FTP and all
ports whose names begin with “http”, use -p ftp,http*. Be
careful about shell expansions and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate ports inside
that range that appear in nmap-services. For example, the following will scan
all ports in nmap-services equal to or below 1024: -p [-1024]. Be
careful with shell expansions and quote the argument to -p if
unsure.
-F (Fast (limited port) scan) .
Specifies that you wish to scan fewer ports than the
default. Normally Nmap scans the most common 1,000 ports for each scanned
protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in order to know
which ports are the most common. If port frequency information isn't
available, perhaps because of the use of a custom nmap-services file, Nmap
scans all named ports plus ports 1-1024. In that case, -F means to scan
only ports that are named in the services file.
-r (Don't randomize ports) .
By default, Nmap randomizes the scanned port order
(except that certain commonly accessible ports are moved near the beginning
for efficiency reasons). This randomization is normally desirable, but you can
specify -r for sequential (sorted from lowest to highest) port scanning
instead.
--port-ratio ratio<decimal number between 0 and
1>
Scans all ports in nmap-services file with a ratio
greater than the one given. ratio must be between 0.0 and 1.1.
--top-ports n
Scans the n highest-ratio ports found in
nmap-services file. n must be 1 or greater.
SERVICE AND VERSION DETECTION¶
Point Nmap at a remote machine and it might tell you that ports 25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services. database of about 2,200 well-known services,. Nmap would report that those ports probably correspond to a mail server (SMTP), web server (HTTP), and name server (DNS) respectively. This lookup is usually accurate—the vast majority of daemons listening on TCP port 25 are, in fact, mail servers. However, you should not bet your security on this! People can and do run services on strange ports.. Even if Nmap is right, and the hypothetical server above is running SMTP, HTTP, and DNS servers, that is not a lot of information. When doing vulnerability assessments (or even simple network inventories) of your companies or clients, you really want to know which mail and DNS servers and versions are running. Having an accurate version number helps dramatically in determining which exploits a server is vulnerable to. Version detection helps you obtain this information. After TCP and/or UDP ports are discovered using one of the other scan methods, version detection interrogates those ports to determine more about what is actually running. The nmap-service-probes. database contains probes for querying various services and match expressions to recognize and parse responses. Nmap tries to determine the service protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC BIND, Apache httpd, Solaris telnetd), the version number, hostname, device type (e.g. printer, router), the OS family (e.g. Windows, Linux). When possible, Nmap also gets the Common Platform Enumeration (CPE). representation of this information. Sometimes miscellaneous details like whether an X server is open to connections, the SSH protocol version, or the KaZaA user name, are available. Of course, most services don't provide all of this information. If Nmap was compiled with OpenSSL support, it will connect to SSL servers to deduce the service listening behind that encryption layer.. Some UDP ports are left in the open|filtered state after a UDP port scan is unable to determine whether the port is open or filtered. Version detection will try to elicit a response from these ports (just as it does with open ports), and change the state to open if it succeeds. open|filtered TCP ports are treated the same way. Note that the Nmap -A option enables version detection among other things. A paper documenting the workings, usage, and customization of version detection is available at http://nmap.org/book/vscan.html. When RPC services are discovered, the Nmap RPC grinder. is automatically used to determine the RPC program and version numbers. It takes all the TCP/UDP ports detected as RPC and floods them with SunRPC program NULL commands in an attempt to determine whether they are RPC ports, and if so, what program and version number they serve up. Thus you can effectively obtain the same info as rpcinfo -p even if the target's portmapper is behind a firewall (or protected by TCP wrappers). Decoys do not currently work with RPC scan.. When Nmap receives responses from a service but cannot match them to its database, it prints out a special fingerprint and a URL for you to submit if to if you know for sure what is running on the port. Please take a couple minutes to make the submission so that your find can benefit everyone. Thanks to these submissions, Nmap has about 6,500 pattern matches for more than 650 protocols such as SMTP, FTP, HTTP, etc.. Version detection is enabled and controlled with the following options: -sV (Version detection) .Enables version detection, as discussed above.
Alternatively, you can use -A, which enables version detection among
other things.
-sR. is an alias for -sV. Prior to March 2011, it was used to
active the RPC grinder separately from version detection, but now these
options are always combined.
--allports (Don't exclude any ports from version detection) .
By default, Nmap version detection skips TCP port 9100
because some printers simply print anything sent to that port, leading to
dozens of pages of HTTP GET requests, binary SSL session requests, etc. This
behavior can be changed by modifying or removing the Exclude directive in
nmap-service-probes, or you can specify --allports to scan all ports
regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity)
.
When performing a version scan (-sV), Nmap sends a
series of probes, each of which is assigned a rarity value between one and
nine. The lower-numbered probes are effective against a wide variety of common
services, while the higher-numbered ones are rarely useful. The intensity
level specifies which probes should be applied. The higher the number, the
more likely it is the service will be correctly identified. However, high
intensity scans take longer. The intensity must be between 0 and 9.. The
default is 7.. When a probe is registered to the target port via the
nmap-service-probes ports directive, that probe is tried regardless of
intensity level. This ensures that the DNS probes will always be attempted
against any open port 53, the SSL probe will be done against 443, etc.
--version-light (Enable light mode) .
This is a convenience alias for --version-intensity
2. This light mode makes version scanning much faster, but it is slightly
less likely to identify services.
--version-all (Try every single probe) .
An alias for --version-intensity 9, ensuring that
every single probe is attempted against each port.
--version-trace (Trace version scan activity) .
This causes Nmap to print out extensive debugging info
about what version scanning is doing. It is a subset of what you get with
--packet-trace.
OS DETECTION¶
One of Nmap's best-known features is remote OS detection using TCP/IP stack fingerprinting. Nmap sends a series of TCP and UDP packets to the remote host and examines practically every bit in the responses. After performing dozens of tests such as TCP ISN sampling, TCP options support and ordering, IP ID sampling, and the initial window size check, Nmap compares the results to its nmap-os-db. database of more than 2,600 known OS fingerprints and prints out the OS details if there is a match. Each fingerprint includes a freeform textual description of the OS, and a classification which provides the vendor name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type (general purpose, router, switch, game console, etc). Most fingerprints also have a Common Platform Enumeration (CPE). representation, like cpe:/o:linux:linux_kernel:2.6. If Nmap is unable to guess the OS of a machine, and conditions are good (e.g. at least one open port and one closed port were found), Nmap will provide a URL you can use to submit the fingerprint if you know (for sure) the OS running on the machine. By doing this you contribute to the pool of operating systems known to Nmap and thus it will be more accurate for everyone. OS detection enables some other tests which make use of information that is gathered during the process anyway. One of these is TCP Sequence Predictability Classification. This measures approximately how hard it is to establish a forged TCP connection against the remote host. It is useful for exploiting source-IP based trust relationships (rlogin, firewall filters, etc) or for hiding the source of an attack. This sort of spoofing is rarely performed any more, but many machines are still vulnerable to it. The actual difficulty number is based on statistical sampling and may fluctuate. It is generally better to use the English classification such as “worthy challenge” or “trivial joke”. This is only reported in normal output in verbose ( -v) mode. When verbose mode is enabled along with -O, IP ID sequence generation is also reported. Most machines are in the “incremental” class, which means that they increment the ID field in the IP header for each packet they send. This makes them vulnerable to several advanced information gathering and spoofing attacks. Another bit of extra information enabled by OS detection is a guess at a target's uptime. This uses the TCP timestamp option ( RFC 1323[10]) to guess when a machine was last rebooted. The guess can be inaccurate due to the timestamp counter not being initialized to zero or the counter overflowing and wrapping around, so it is printed only in verbose mode. A paper documenting the workings, usage, and customization of OS detection is available at http://nmap.org/book/osdetect.html. OS detection is enabled and controlled with the following options: -O (Enable OS detection) .Enables OS detection, as discussed above. Alternatively,
you can use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets) .
OS detection is far more effective if at least one open
and one closed TCP port are found. Set this option and Nmap will not even try
OS detection against hosts that do not meet this criteria. This can save
substantial time, particularly on -Pn scans against many hosts. It only
matters when OS detection is requested with -O or -A.
--osscan-guess; --fuzzy (Guess OS detection results) .
When Nmap is unable to detect a perfect OS match, it
sometimes offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these (equivalent) options
make Nmap guess more aggressively. Nmap will still tell you when an imperfect
match is printed and display its confidence level (percentage) for each
guess.
--max-os-tries (Set the maximum number of OS detection tries against a
target) .
When Nmap performs OS detection against a target and
fails to find a perfect match, it usually repeats the attempt. By default,
Nmap tries five times if conditions are favorable for OS fingerprint
submission, and twice when conditions aren't so good. Specifying a lower
--max-os-tries value (such as 1) speeds Nmap up, though you miss out on
retries which could potentially identify the OS. Alternatively, a high value
may be set to allow even more retries when conditions are favorable. This is
rarely done, except to generate better fingerprints for submission and
integration into the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE)¶
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and flexible features. It allows users to write (and share) simple scripts (using the Lua programming language[11], Tasks we had in mind when creating the system include network discovery, more sophisticated version detection, vulnerability detection. NSE can even be used for vulnerability exploitation. To reflect those different uses and to simplify the choice of which scripts to run, each script contains a field associating it with one or more categories. Currently defined categories are auth, broadcast, default. discovery, dos, exploit, external, fuzzer, intrusive, malware, safe, version, and vuln. These are all described at http://nmap.org/book/nse-usage.html#nse-categories. Scripts are not run in a sandbox and thus could accidentally or maliciously damage your system or invade your privacy. Never run scripts from third parties unless you trust the authors or have carefully audited the scripts yourself. The Nmap Scripting Engine is described in detail at http://nmap.org/book/nse.html and is controlled by the following options: -sC .Performs a script scan using the default set of scripts.
It is equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a target
network without permission.
--script
filename|category|
directory |expression[,...] .
Runs a script scan using the comma-separated list of
filenames, script categories, and directories. Each element in the list may
also be a Boolean expression describing a more complex set of scripts. Each
element is interpreted first as an expression, then as a category, and finally
as a file or directory name.
There are two special features for advanced users only. One is to prefix script
names and expressions with + to force them to run even if they normally
wouldn't (e.g. the relevant service wasn't detected on the target port). The
other is that the argument all may be used to specify every script in Nmap's
database. Be cautious with this because NSE contains dangerous scripts such as
exploits, brute force authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute. Absolute names are used
directly. Relative paths are looked for in the scripts of each of the
following places until found:
--script-args
n1=v1,n2
={n3=v3},n4={v4,v5}
.
--datadir
$NMAPDIR.
~/.nmap (not searched on Windows).
HOME\AppData\Roaming\nmap (only on Windows).
the directory containing the nmap executable
the directory containing the nmap executable, followed by
../share/nmap
NMAPDATADIR.
the current directory.
When a directory name is given, Nmap loads every file in the directory whose
name ends with .nse. All other files are ignored and directories are not
searched recursively. When a filename is given, it does not have to have the
.nse extension; it will be added automatically if necessary. Nmap scripts are
stored in a scripts subdirectory of the Nmap data directory by default (see
http://nmap.org/book/data-files.html).
For efficiency, scripts are indexed in a database stored in scripts/script.db,.
which lists the category or categories in which each script belongs. When
referring to scripts from script.db by name, you can use a shell-style
‘*’ wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as
http-auth and http-open-proxy. The argument to --script had to be in
quotes to protect the wildcard from the shell.
More complicated script selection can be done using the and, or, and not
operators to build Boolean expressions. The operators have the same
precedence[12] as in Lua: not is the highest, followed by and and then
or. You can alter precedence by using parentheses. Because expressions contain
space characters it is necessary to quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive
category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script
"default,safe". It loads all scripts that are in the default
category or the safe category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default
and safe categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive
categories, except for those whose names start with http-.
Lets you provide arguments to NSE scripts. Arguments are
a comma-separated list of name=value pairs. Names and values may be strings
not containing whitespace or the characters ‘{’,
‘}’, ‘=’, or ‘,’. To include one of
these characters in a string, enclose the string in single or double quotes.
Within a quoted string, ‘\’ escapes a quote. A backslash is only
used to escape quotation marks in this special case; in all other cases a
backslash is interpreted literally. Values may also be tables enclosed in {},
just as in Lua. A table may contain simple string values or more name-value
pairs, including nested tables. Many scripts qualify their arguments with the
script name, as in xmpp-info.server_name. You may use that full qualified
version to affect just the specified script, or you may pass the unqualified
version (server_name in this case) to affect all scripts using that argument
name. A script will first check for its fully qualified argument name (the
name specified in its documentation) before it accepts an unqualified argument
name. A complex example of script arguments is --script-args
'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'.
The online NSE Documentation Portal at http://nmap.org/nsedoc/ lists
the arguments that each script accepts.
--script-args-file filename .
Lets you load arguments to NSE scripts from a file. Any
arguments on the command line supersede ones in the file. The file can be an
absolute path, or a path relative to Nmap's usual search path (NMAPDIR, etc.)
Arguments can be comma-separated or newline-separated, but otherwise follow
the same rules as for --script-args, without requiring special quoting
and escaping, since they are not parsed by the shell.
--script-help
filename|category|
directory
|expression|all[,...] .
Shows help about scripts. For each script matching the
given specification, Nmap prints the script name, its categories, and its
description. The specifications are the same as those accepted by
--script; so for example if you want help about the ftp-anon script,
you would run nmap --script-help ftp-anon. In addition to getting help
for individual scripts, you can use this as a preview of what scripts will be
run for a specification, for example with nmap --script-help
default.
--script-trace .
This option does what --packet-trace does, just
one ISO layer higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed information
includes the communication protocol, the source, the target and the
transmitted data. If more than 5% of all transmitted data is not printable,
then the trace output is in a hex dump format. Specifying
--packet-trace enables script tracing too.
--script-updatedb .
This option updates the script database found in
scripts/script.db which is used by Nmap to determine the available default
scripts and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts directory or if you
have changed the categories of any script. This option is generally used by
itself: nmap --script-updatedb.
TIMING AND PERFORMANCE¶
One of my highest Nmap development priorities has always been performance. A default scan ( nmap hostname) of a host on my local network takes a fifth of a second. That is barely enough time to blink, but adds up when you are scanning hundreds or thousands of hosts. Moreover, certain scan options such as UDP scanning and version detection can increase scan times substantially. So can certain firewall configurations, particularly response rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate these scans, the user has ultimate control over how Nmap runs. Expert users carefully craft Nmap commands to obtain only the information they care about while meeting their time constraints. Techniques for improving scan times include omitting non-critical tests, and upgrading to the latest version of Nmap (performance enhancements are made frequently). Optimizing timing parameters can also make a substantial difference. Those options are listed below. Some options accept a time parameter. This is specified in seconds by default, though you can append ‘ms’, ‘s’, ‘m’, or ‘h’ to the value to specify milliseconds, seconds, minutes, or hours. So the --host-timeout arguments 900000ms, 900, 900s, and 15m all do the same thing. --min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel scan group sizes) .Nmap has the ability to port scan or version scan
multiple hosts in parallel. Nmap does this by dividing the target IP space
into groups and then scanning one group at a time. In general, larger groups
are more efficient. The downside is that host results can't be provided until
the whole group is finished. So if Nmap started out with a group size of 50,
the user would not receive any reports (except for the updates offered in
verbose mode) until the first 50 hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It starts out
with a group size as low as five so the first results come quickly and then
increases the groupsize to as high as 1024. The exact default numbers depend
on the options given. For efficiency reasons, Nmap uses larger group sizes for
UDP or few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap will
never exceed that size. Specify a minimum size with --min-hostgroup and
Nmap will try to keep group sizes above that level. Nmap may have to use
smaller groups than you specify if there are not enough target hosts left on a
given interface to fulfill the specified minimum. Both may be set to keep the
group size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery phase of a scan.
This includes plain ping scans ( -sn). Host discovery always works in
large groups of hosts to improve speed and accuracy.
The primary use of these options is to specify a large minimum group size so
that the full scan runs more quickly. A common choice is 256 to scan a network
in Class C sized chunks. For a scan with many ports, exceeding that number is
unlikely to help much. For scans of just a few port numbers, host group sizes
of 2048 or more may be helpful.
--min-parallelism numprobes; --max-parallelism
numprobes (Adjust probe parallelization) .
These options control the total number of probes that may
be outstanding for a host group. They are used for port scanning and host
discovery. By default, Nmap calculates an ever-changing ideal parallelism
based on network performance. If packets are being dropped, Nmap slows down
and allows fewer outstanding probes. The ideal probe number slowly rises as
the network proves itself worthy. These options place minimum or maximum
bounds on that variable. By default, the ideal parallelism can drop to one if
the network proves unreliable and rise to several hundred in perfect
conditions.
The most common usage is to set --min-parallelism to a number higher than
one to speed up scans of poorly performing hosts or networks. This is a risky
option to play with, as setting it too high may affect accuracy. Setting this
also reduces Nmap's ability to control parallelism dynamically based on
network conditions. A value of 10 might be reasonable, though I only adjust
this value as a last resort.
The --max-parallelism option is sometimes set to one to prevent Nmap from
sending more than one probe at a time to hosts. The --scan-delay
option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout
time, --initial-rtt-timeout time
(Adjust probe timeouts) .
Nmap maintains a running timeout value for determining
how long it will wait for a probe response before giving up or retransmitting
the probe. This is calculated based on the response times of previous probes.
If the network latency shows itself to be significant and variable, this timeout
can grow to several seconds. It also starts at a conservative (high) level and
may stay that way for a while when Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout
than the defaults can cut scan times significantly. This is particularly true
for pingless ( -Pn) scans, and those against heavily filtered networks.
Don't get too aggressive though. The scan can end up taking longer if you
specify such a low value that many probes are timing out and retransmitting
while the response is in transit.
If all the hosts are on a local network, 100 milliseconds ( --max-rtt-timeout
100ms) is a reasonable aggressive value. If routing is involved, ping a
host on the network first with the ICMP ping utility, or with a custom packet
crafter such as Nping. that is more likely to get through a firewall. Look at
the maximum round trip time out of ten packets or so. You might want to double
that for the --initial-rtt-timeout and triple or quadruple it for the
--max-rtt-timeout. I generally do not set the maximum RTT below
100 ms, no matter what the ping times are. Nor do I exceed
1000 ms.
--min-rtt-timeout is a rarely used option that could be useful when a
network is so unreliable that even Nmap's default is too aggressive. Since
Nmap only reduces the timeout down to the minimum when the network seems to be
reliable, this need is unusual and should be reported as a bug to the nmap-dev
mailing list..
--max-retries numtries (Specify the maximum number of port
scan probe retransmissions) .
When Nmap receives no response to a port scan probe, it
could mean the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has rate
limiting enabled that temporarily blocked the response. So Nmap tries again by
retransmitting the initial probe. If Nmap detects poor network reliability, it
may try many more times before giving up on a port. While this benefits
accuracy, it also lengthen scan times. When performance is critical, scans may
be sped up by limiting the number of retransmissions allowed. You can even
specify --max-retries 0 to prevent any retransmissions, though that is
only recommended for situations such as informal surveys where occasional
missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten retransmissions. If a
network seems reliable and the target hosts aren't rate limiting, Nmap usually
only does one retransmission. So most target scans aren't even affected by
dropping --max-retries to a low value such as three. Such values can
substantially speed scans of slow (rate limited) hosts. You usually lose some
information when Nmap gives up on ports early, though that may be preferable
to letting the --host-timeout expire and losing all information about
the target.
--host-timeout time (Give up on slow target hosts) .
Some hosts simply take a long time to scan. This
may be due to poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few percent of
the scanned hosts can eat up a majority of the scan time. Sometimes it is best
to cut your losses and skip those hosts initially. Specify
--host-timeout with the maximum amount of time you are willing to wait.
For example, specify 30m to ensure that Nmap doesn't waste more than half an
hour on a single host. Note that Nmap may be scanning other hosts at the same
time during that half an hour, so it isn't a complete loss. A host that times
out is skipped. No port table, OS detection, or version detection results are
printed for that host.
--scan-delay time; --max-scan-delay
time (Adjust delay between probes) .
This option causes Nmap to wait at least the given amount
of time between each probe it sends to a given host. This is particularly
useful in the case of rate limiting.. Solaris machines (among many others)
will usually respond to UDP scan probe packets with only one ICMP message per
second. Any more than that sent by Nmap will be wasteful. A
--scan-delay of 1s will keep Nmap at that slow rate. Nmap tries to
detect rate limiting and adjust the scan delay accordingly, but it doesn't
hurt to specify it explicitly if you already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate limiting, the scan
slows down dramatically. The --max-scan-delay option specifies the
largest delay that Nmap will allow. A low --max-scan-delay can speed up
Nmap, but it is risky. Setting this value too low can lead to wasteful packet
retransmissions and possible missed ports when the target implements strict
rate limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS)..
--min-rate number; --max-rate number
(Directly control the scanning rate) .
Nmap's dynamic timing does a good job of finding an
appropriate speed at which to scan. Sometimes, however, you may happen to know
an appropriate scanning rate for a network, or you may have to guarantee that
a scan will be finished by a certain time. Or perhaps you must keep Nmap from
scanning too quickly. The --min-rate and --max-rate options are
designed for these situations.
When the --min-rate option is given Nmap will do its best to send packets
as fast as or faster than the given rate. The argument is a positive real
number representing a packet rate in packets per second. For example,
specifying --min-rate 300 means that Nmap will try to keep the sending
rate at or above 300 packets per second. Specifying a minimum rate does not
keep Nmap from going faster if conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given maximum. Use
--max-rate 100, for example, to limit sending to 100 packets per second
on a fast network. Use --max-rate 0.1 for a slow scan of one packet
every ten seconds. Use --min-rate and --max-rate together to
keep the rate inside a certain range.
These two options are global, affecting an entire scan, not individual hosts.
They only affect port scans and host discovery scans. Other features like OS
detection implement their own timing.
There are two conditions when the actual scanning rate may fall below the
requested minimum. The first is if the minimum is faster than the fastest rate
at which Nmap can send, which is dependent on hardware. In this case Nmap will
simply send packets as fast as possible, but be aware that such high rates are
likely to cause a loss of accuracy. The second case is when Nmap has nothing
to send, for example at the end of a scan when the last probes have been sent
and Nmap is waiting for them to time out or be responded to. It's normal to
see the scanning rate drop at the end of a scan or in between hostgroups. The
sending rate may temporarily exceed the maximum to make up for unpredictable
delays, but on average the rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning faster than a
network can support may lead to a loss of accuracy. In some cases, using a
faster rate can make a scan take longer than it would with a slower
rate. This is because Nmap's
adaptive retransmission algorithms will detect the network congestion caused by
an excessive scanning rate and increase the number of retransmissions in order
to improve accuracy. So even though packets are sent at a higher rate, more
packets are sent overall. Cap the number of retransmissions with the
--max-retries option if you need to set an upper limit on total scan
time.
--defeat-rst-ratelimit .
Many hosts have long used rate limiting. to reduce the
number of ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset) packets they
generate. This can slow Nmap down dramatically as it adjusts its timing to
reflect those rate limits. You can tell Nmap to ignore those rate limits (for
port scans such as SYN scan which don't treat non-responsive ports as
open) by specifying --defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear non-responsive
because Nmap didn't wait long enough for a rate-limited RST response. With a
SYN scan, the non-response results in the port being labeled filtered rather
than the closed state we see when RST packets are received. This option is
useful when you only care about open ports, and distinguishing between closed
and filtered ports isn't worth the extra time.
--nsock-engine epoll|kqueue|poll|select .
Enforce use of a given nsock IO multiplexing engine. Only
the select(2)-based fallback engine is guaranteed to be available on your
system. Engines are named after the name of the IO management facility they
leverage. Engines currently implemented are epoll, kqueue, poll, and select,
but not all will be present on any platform. Use nmap -V to see which
engines are supported.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template) .
While the fine-grained timing controls discussed in the
previous section are powerful and effective, some people find them confusing.
Moreover, choosing the appropriate values can sometimes take more time than
the scan you are trying to optimize. So Nmap offers a simpler approach, with
six timing templates. You can specify them with the -T option and their
number (0–5) or their name. The template names are
paranoid ( 0), sneaky (1),
polite ( 2), normal (3),
aggressive ( 4), and insane (5). The
first two are for IDS evasion. Polite mode slows down the scan to use less
bandwidth and target machine resources. Normal mode is the default and so
-T3 does nothing. Aggressive mode speeds scans up by making the
assumption that you are on a reasonably fast and reliable network. Finally
insane mode. assumes that you are on an extraordinarily fast network or are
willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish to be, while
leaving Nmap to pick the exact timing values. The templates also make some
minor speed adjustments for which fine-grained control options do not
currently exist. For example, -T4. prohibits the dynamic scan delay
from exceeding 10 ms for TCP ports and -T5 caps that value at
5 ms. Templates can be used in combination with fine-grained controls,
and the fine-grained controls will you specify will take precedence over the
timing template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that option even when
you add fine-grained controls so that you benefit from those extra minor
optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would recommend
always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because they
think it is less likely to crash hosts or because they consider themselves to
be polite in general. They often don't realize just how slow -T polite.
really is. Their scan may take ten times longer than a default scan. Machine
crashes and bandwidth problems are rare with the default timing options (
-T3) and so I normally recommend that for cautious scanners. Omitting
version detection is far more effective than playing with timing values at
reducing these problems.
While -T0. and -T1. may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of machines or ports.
For such a long scan, you may prefer to set the exact timing values you need
rather than rely on the canned -T0 and -T1 values.
The main effects of T0 are serializing the scan so only one port is
scanned at a time, and waiting five minutes between sending each probe.
T1 and T2 are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes. T3 is Nmap's default behavior,
which includes parallelization.. -T4 does the equivalent of
--max-rtt-timeout 1250ms --initial-rtt-timeout 500ms --max-retries 6
and sets the maximum TCP scan delay to 10 milliseconds. T5 does the
equivalent of --max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m as well as
setting the maximum TCP scan delay to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING¶
Many Internet pioneers envisioned a global open network with a universal IP address space allowing virtual connections between any two nodes. This allows hosts to act as true peers, serving and retrieving information from each other. People could access all of their home systems from work, changing the climate control settings or unlocking the doors for early guests. This vision of universal connectivity has been stifled by address space shortages and security concerns. In the early 1990s, organizations began deploying firewalls for the express purpose of reducing connectivity. Huge networks were cordoned off from the unfiltered Internet by application proxies, network address translation, and packet filters. The unrestricted flow of information gave way to tight regulation of approved communication channels and the content that passes over them. Network obstructions such as firewalls can make mapping a network exceedingly difficult. It will not get any easier, as stifling casual reconnaissance is often a key goal of implementing the devices. Nevertheless, Nmap offers many features to help understand these complex networks, and to verify that filters are working as intended. It even supports mechanisms for bypassing poorly implemented defenses. One of the best methods of understanding your network security posture is to try to defeat it. Place yourself in the mind-set of an attacker, and deploy techniques from this section against your networks. Launch an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel through one of your own proxies. In addition to restricting network activity, companies are increasingly monitoring traffic with intrusion detection systems (IDS). All of the major IDSs ship with rules designed to detect Nmap scans because scans are sometimes a precursor to attacks. Many of these products have recently morphed into intrusion prevention systems (IPS). that actively block traffic deemed malicious. Unfortunately for network administrators and IDS vendors, reliably detecting bad intentions by analyzing packet data is a tough problem. Attackers with patience, skill, and the help of certain Nmap options can usually pass by IDSs undetected. Meanwhile, administrators must cope with large numbers of false positive results where innocent activity is misdiagnosed and alerted on or blocked. Occasionally people suggest that Nmap should not offer features for evading firewall rules or sneaking past IDSs. They argue that these features are just as likely to be misused by attackers as used by administrators to enhance security. The problem with this logic is that these methods would still be used by attackers, who would just find other tools or patch the functionality into Nmap. Meanwhile, administrators would find it that much harder to do their jobs. Deploying only modern, patched FTP servers is a far more powerful defense than trying to prevent the distribution of tools implementing the FTP bounce attack. There is no magic bullet (or Nmap option) for detecting and subverting firewalls and IDS systems. It takes skill and experience. A tutorial is beyond the scope of this reference guide, which only lists the relevant options and describes what they do. -f (fragment packets); --mtu (using the specified MTU) .The -f option causes the requested scan (including
ping scans) to use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters, intrusion
detection systems, and other annoyances to detect what you are doing. Be
careful with this! Some programs have trouble handling these tiny packets. The
old-school sniffer named Sniffit segmentation faulted immediately upon
receiving the first fragment. Specify this option once, and Nmap splits the
packets into eight bytes or less after the IP header. So a 20-byte TCP header
would be split into three packets. Two with eight bytes of the TCP header, and
one with the final four. Of course each fragment also has an IP header.
Specify -f again to use 16 bytes per fragment (reducing the number of
fragments).. Or you can specify your own offset size with the --mtu
option. Don't also specify -f if you use --mtu. The offset must
be a multiple of eight. While fragmented packets won't get by packet filters
and firewalls that queue all IP fragments, such as the
CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some networks can't
afford the performance hit this causes and thus leave it disabled. Others
can't enable this because fragments may take different routes into their
networks. Some source systems defragment outgoing packets in the kernel. Linux
with the iptables. connection tracking module is one such example. Do a scan
while a sniffer such as Wireshark. is running to ensure that sent packets are
fragmented. If your host OS is causing problems, try the --send-eth.
option to bypass the IP layer and send raw ethernet frames.
Fragmentation is only supported for Nmap's raw packet features, which includes
TCP and UDP port scans (except connect scan and FTP bounce scan) and OS
detection. Features such as version detection and the Nmap Scripting Engine
generally don't support fragmentation because they rely on your host's TCP
stack to communicate with target services.
-D decoy1[,decoy2][,ME][,...]
(Cloak a scan with decoys) .
Causes a decoy scan to be performed, which makes it
appear to the remote host that the host(s) you specify as decoys are scanning
the target network too. Thus their IDS might report 5–10 port scans
from unique IP addresses, but they won't know which IP was scanning them and
which were innocent decoys. While this can be defeated through router path
tracing, response-dropping, and other active mechanisms, it is generally an
effective technique for hiding your IP address.
Separate each decoy host with commas, and you can optionally use ME. as one of
the decoys to represent the position for your real IP address. If you put ME
in the sixth position or later, some common port scan detectors (such as Solar
Designer's. excellent Scanlogd). are unlikely to show your IP address at all.
If you don't use ME, Nmap will put you in a random position. You can also use
RND. to generate a random, non-reserved IP address, or RND: number to
generate number addresses.
Note that the hosts you use as decoys should be up or you might accidentally SYN
flood your targets. Also it will be pretty easy to determine which host is
scanning if only one is actually up on the network. You might want to use IP
addresses instead of names (so the decoy networks don't see you in their
nameserver logs).
Decoys are used both in the initial ping scan (using ICMP, SYN, ACK, or
whatever) and during the actual port scanning phase. Decoys are also used
during remote OS detection ( -O). Decoys do not work with version
detection or TCP connect scan. When a scan delay is in effect, the delay is
enforced between each batch of spoofed probes, not between each individual
probe. Because decoys are sent as a batch all at once, they may temporarily
violate congestion control limits.
It is worth noting that using too many decoys may slow your scan and potentially
even make it less accurate. Also, some ISPs will filter out your spoofed
packets, but many do not restrict spoofed IP packets at all.
-S IP_Address (Spoof source address) .
In some circumstances, Nmap may not be able to determine
your source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish to send
packets through.
Another possible use of this flag is to spoof the scan to make the targets think
that someone else is scanning them. Imagine a company being repeatedly
port scanned by a competitor! The -e option and -Pn are
generally required for this sort of usage. Note that you usually won't receive
reply packets back (they will be addressed to the IP you are spoofing), so
Nmap won't produce useful reports.
-e interface (Use specified interface) .
Tells Nmap what interface to send and receive packets on.
Nmap should be able to detect this automatically, but it will tell you if it
cannot.
--source-port portnumber; -g
portnumber (Spoof source port number) .
One surprisingly common misconfiguration is to trust
traffic based only on the source port number. It is easy to understand how
this comes about. An administrator will set up a shiny new firewall, only to
be flooded with complaints from ungrateful users whose applications stopped
working. In particular, DNS may be broken because the UDP DNS replies from
external servers can no longer enter the network. FTP is another common
example. In active FTP transfers, the remote server tries to establish a
connection back to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of application-level
proxies or protocol-parsing firewall modules. Unfortunately there are also
easier, insecure solutions. Noting that DNS replies come from port 53 and
active FTP from port 20, many administrators have fallen into the trap of
simply allowing incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other cases,
administrators consider this a short-term stop-gap measure until they can
implement a more secure solution. Then they forget the security upgrade.
Overworked network administrators are not the only ones to fall into this trap.
Numerous products have shipped with these insecure rules. Even Microsoft has
been guilty. The IPsec filters that shipped with Windows 2000 and Windows XP
contain an implicit rule that allows all TCP or UDP traffic from port 88
(Kerberos). In another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the source port 53
(DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent)
to exploit these weaknesses. Simply provide a port number and Nmap will send
packets from that port where possible. Most scanning operations that use raw
sockets, including SYN and UDP scans, support the option completely. The
option notably doesn't have an effect for any operations that use normal
operating system sockets, including DNS requests, TCP connect scan,.
version detection, and script scanning. Setting the source port also doesn't
work for OS detection, because Nmap must use different port numbers for
certain OS detection tests to work properly.
--data-length number (Append random data to sent packets) .
Normally Nmap sends minimalist packets containing only a
header. So its TCP packets are generally 40 bytes and ICMP echo requests are
just 28. Some UDP ports. and IP protocols. get a custom payload by default.
This option tells Nmap to append the given number of random bytes to most of
the packets it sends, and not to use any protocol-specific payloads. (Use
--data-length 0 for no random or protocol-specific payloads.. OS
detection ( -O) packets are not affected. because accuracy there
requires probe consistency, but most pinging and portscan packets support
this. It slows things down a little, but can make a scan slightly less
conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ;
--ip-options hex string (Send packets with specified ip
options) .
The IP protocol[13] offers several options which
may be placed in packet headers. Unlike the ubiquitous TCP options, IP options
are rarely seen due to practicality and security concerns. In fact, many
Internet routers block the most dangerous options such as source routing. Yet
options can still be useful in some cases for determining and manipulating the
network route to target machines. For example, you may be able to use the
record route option to determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being dropped by a
certain firewall, you may be able to specify a different route with the strict
or loose source routing options.
The most powerful way to specify IP options is to simply pass in values as the
argument to --ip-options. Precede each hex number with \x then the two
digits. You may repeat certain characters by following them with an asterisk
and then the number of times you wish them to repeat. For example,
\x01\x07\x04\x00*36\x01 is a hex string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options. Simply pass the
letter R, T, or U to request record-route,. record-timestamp,. or both options
together, respectively. Loose or strict source routing. may be specified with
an L or S followed by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received, specify
--packet-trace. For more information and examples of using IP options
with Nmap, see http://seclists.org/nmap-dev/2006/q3/52.
--ttl value (Set IP time-to-live field) .
Sets the IPv4 time-to-live field in sent packets to the
given value.
--randomize-hosts (Randomize target host order) .
Tells Nmap to shuffle each group of up to 16384 hosts
before it scans them. This can make the scans less obvious to various network
monitoring systems, especially when you combine it with slow timing options.
If you want to randomize over larger group sizes, increase
PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution is to
generate the target IP list with a list scan ( -sL -n -oN
filename), randomize it with a Perl script, then provide the
whole list to Nmap with -iL..
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC
address) .
Asks Nmap to use the given MAC address for all of the raw
ethernet frames it sends. This option implies --send-eth. to ensure
that Nmap actually sends ethernet-level packets. The MAC given can take
several formats. If it is simply the number 0, Nmap chooses a completely
random MAC address for the session. If the given string is an even number of
hex digits (with the pairs optionally separated by a colon), Nmap will use
those as the MAC. If fewer than 12 hex digits are provided, Nmap fills in the
remainder of the six bytes with random values. If the argument isn't a zero or
hex string, Nmap looks through nmap-mac-prefixes to find a vendor name
containing the given string (it is case insensitive). If a match is found,
Nmap uses the vendor's OUI (three-byte prefix). and fills out the remaining
three bytes randomly. Valid --spoof-mac argument examples are Apple, 0,
01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco. This option only affects
raw packet scans such as SYN scan or OS detection, not connection-oriented
features such as version detection or the Nmap Scripting Engine.
--proxies Comma-separated list of proxy URLs (Relay TCP
connections through a chain of proxies) .
Asks Nmap to establish TCP connections with a final
target through supplied chain of one or more HTTP or SOCKS4
--max-parallelism may help because some proxies refuse to handle as
many concurrent connections as Nmap opens by default.
This option takes a list of proxies as argument, expressed as URLs in the format
proto://host:port. Use commas to separate node URLs in a chain. No
authentication is supported yet. Valid protocols are HTTP and SOCKS4.
Warning: this feature is still under development and has limitations. It is
implemented within the nsock library and thus has no effect on the ping, port
scanning and OS discovery phases of a scan. Only NSE and version scan benefit
from this option so far—other features may disclose your true address.
SSL connections are not yet supported, nor is proxy-side DNS resolution
(hostnames are always resolved by Nmap).
--badsum (Send packets with bogus TCP/UDP checksums) .
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for
packets sent to target hosts. Since virtually all host IP stacks properly drop
these packets, any responses received are likely coming from a firewall or IDS
that didn't bother to verify the checksum. For more details on this technique,
see http://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums) .
Asks Nmap to use the deprecated Adler32 algorithm for
calculating the SCTP checksum. If --adler32 is not given, CRC-32C
(Castagnoli) is used. RFC 2960[14] originally defined Adler32 as
checksum algorithm for SCTP; RFC 4960[7] later redefined the SCTP
checksums to use CRC-32C. Current SCTP implementations should be using
CRC-32C, but in order to elicit responses from old, legacy SCTP
implementations, it may be preferable to use Adler32.
OUTPUT¶
Any security tool is only as useful as the output it generates. Complex tests and algorithms are of little value if they aren't presented in an organized and comprehensible fashion. Given the number of ways Nmap is used by people and other software, no single format can please everyone. So Nmap offers several formats, including the interactive mode for humans to read directly and XML for easy parsing by software. In addition to offering different output formats, Nmap provides options for controlling the verbosity of output as well as debugging messages. Output types may be sent to standard output or to named files, which Nmap can append to or clobber. Output files may also be used to resume aborted scans. Nmap makes output available in five different formats. The default is called interactive output,. and it is sent to standard output (stdout).. There is also normal output,. which is similar to interactive except that it displays less runtime information and warnings since it is expected to be analyzed after the scan completes rather than interactively. XML output. is one of the most important output types, as it can be converted to HTML, easily parsed by programs such as Nmap graphical user interfaces, or imported into databases. The two remaining output types are the simple grepable output. which includes most information for a target host on a single line, and sCRiPt KiDDi3 0utPUt. for users who consider themselves |<-r4d. While interactive output is the default and has no associated command-line options, the other four format options use the same syntax. They take one argument, which is the filename that results should be stored in. Multiple formats may be specified, but each format may only be specified once. For example, you may wish to save normal output for your own review while saving XML of the same scan for programmatic analysis. You might do this with the options -oX myscan.xml -oN myscan.nmap. While this chapter uses the simple names like myscan.xml for brevity, more descriptive names are generally recommended. The names chosen are a matter of personal preference, though I use long ones that incorporate the scan date and a word or two describing the scan, placed in a directory named after the company I'm scanning. While these options save results to files, Nmap still prints interactive output to stdout as usual. For example, the command nmap -oX myscan.xml target prints XML to myscan.xml and fills standard output with the same interactive results it would have printed if -oX wasn't specified at all. You can change this by passing a hyphen character as the argument to one of the format types. This causes Nmap to deactivate interactive output, and instead print results in the format you specified to the standard output stream. So the command nmap -oX - target will send only XML output to stdout.. Serious errors may still be printed to the normal error stream, stderr.. Unlike some Nmap arguments, the space between the logfile option flag (such as -oX) and the filename or hyphen is mandatory. If you omit the flags and give arguments such as -oG- or -oXscan.xml, a backwards compatibility feature of Nmap will cause the creation of normal format output files named G- and Xscan.xml respectively. All of these arguments support strftime-like. conversions in the filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D is the same as %m%d%y. A % followed by any other character just yields that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml' will use an XML file with a name in the form of scan-144840-121307.xml. Nmap also offers options to control scan verbosity and to append to output files rather than clobbering them. All of these options are described below. Nmap Output Formats -oN filespec (normal output) .Requests that normal output be directed to the given
filename. As discussed above, this differs slightly from interactive
output.
-oX filespec (XML output) .
Requests that XML output be directed to the given
filename. Nmap includes a document type definition (DTD) which allows XML
parsers to validate Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML output. The DTD
defines the legal elements of the format, and often enumerates the attributes
and values they can take on. The latest version is always available from
https://svn.nmap.org/nmap/docs/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free XML parsers
are available for all major computer languages, including C/C++, Perl, Python,
and Java. People have even written bindings for most of these languages to
handle Nmap output and execution specifically. Examples are
Nmap::Scanner[15]. and Nmap::Parser[16]. in Perl CPAN. In almost
all cases that a non-trivial application interfaces with Nmap, XML is the
preferred format.
The XML output references an XSL stylesheet which can be used to format the
results as HTML. The easiest way to use this is simply to load the XML output
in a web browser such as Firefox or IE. By default, this will only work on the
machine you ran Nmap on (or a similarly configured one) due to the hard-coded
nmap.xsl filesystem path. Use the --webxml or --stylesheet
options to create portable XML files that render as HTML on any web-connected
machine.
-oS filespec (ScRipT KIdd|3 oUTpuT) .
Script kiddie output is like interactive output, except
that it is post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and spelling. Humor
impaired people should note that this option is making fun of the script
kiddies before flaming me for supposedly “helping them”.
-oG filespec (grepable output) .
This output format is covered last because it is
deprecated. The XML output format is far more powerful, and is nearly as
convenient for experienced users. XML is a standard for which dozens of
excellent parsers are available, while grepable output is my own simple hack.
XML is extensible to support new Nmap features as they are released, while I
often must omit those features from grepable output for lack of a place to put
them.
Nevertheless, grepable output is still quite popular. It is a simple format that
lists each host on one line and can be trivially searched and parsed with
standard Unix tools such as grep, awk, cut, sed, diff, and Perl. Even I
usually use it for one-off tests done at the command line. Finding all the
hosts with the SSH port open or that are running Solaris takes only a simple
grep to identify the hosts, piped to an awk or cut command to print the
desired fields.
Grepable output consists of comments (lines starting with a pound (#)). and
target lines. A target line includes a combination of six labeled fields,
separated by tabs and followed with a colon. The fields are Host, Ports,
Protocols, Ignored State, OS, Seq Index, IP ID, and Status.
The most important of these fields is generally Ports, which gives details on
each interesting port. It is a comma separated list of port entries. Each port
entry represents one interesting port, and takes the form of seven slash (/)
separated subfields. Those subfields are: Port number, State, Protocol, Owner,
Service, SunRPC info, and Version info.
As with XML output, this man page does not allow for documenting the entire
format. A more detailed look at the Nmap grepable output format is available
from http://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats) .
As a convenience, you may specify -oA
basename to store scan results in normal, XML, and grepable
formats at once. They are stored in basename.nmap, basename.xml,
and basename.gnmap, respectively. As with most programs, you can prefix
the filenames with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level) .
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as they are found
and completion time estimates are provided when Nmap thinks a scan will take
more than a few minutes. Use it twice or more for even greater verbosity:
-vv, or give a verbosity level directly, for example -v3..
Most changes only affect interactive output, and some also affect normal and
script kiddie output. The other output types are meant to be processed by
machines, so Nmap can give substantial detail by default in those formats
without fatiguing a human user. However, there are a few changes in other
modes where output size can be reduced substantially by omitting some detail.
For example, a comment line in the grepable output that provides a list of all
ports scanned is only printed in verbose mode because it can be quite
long.
-d (Increase debugging level) .
When even verbose mode doesn't provide sufficient data
for you, debugging is available to flood you with much more! As with the
verbosity option ( -v), debugging is enabled with a command-line flag
(-d) and the debug level can be increased by specifying it multiple
times,. as in -dd, or by setting a level directly. For example,
-d9 sets level nine. That is the highest effective level and will
produce thousands of lines unless you run a very simple scan with very few
ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if you are simply
confused as to what Nmap is doing and why. As this feature is mostly intended
for developers, debug lines aren't always self-explanatory. You may get
something like: Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987
==> srtt: 14987 rttvar: 14987 to: 100000. If you don't understand a line,
your only recourses are to ignore it, look it up in the source code, or
request help from the development list (nmap-dev).. Some lines are self
explanatory, but the messages become more obscure as the debug level is
increased.
--reason (Host and port state reasons) .
Shows the reason each port is set to a specific state and
the reason each host is up or down. This option displays the type of the
packet that determined a port or hosts state. For example, A RST packet from a
closed port or an echo reply from an alive host. The information Nmap can
provide is determined by the type of scan or ping. The SYN scan and SYN ping (
-sS and -PS) are very detailed, but the TCP connect scan
(-sT) is limited by the implementation of the connect system
call. This feature is automatically enabled by the debug option ( -d).
and the results are stored in XML log files even if this option is not
specified.
--stats-every time (Print periodic timing stats) .
Periodically prints a timing status message after each
interval of time. The time is a specification of the kind described in
the section called “TIMING AND PERFORMANCE”; so for example, use
--stats-every 10s to get a status update every 10 seconds. Updates are
printed to interactive output (the screen) and XML output.
--packet-trace (Trace packets and data sent and received) .
Causes Nmap to print a summary of every packet sent or
received. This is often used for debugging, but is also a valuable way for new
users to understand exactly what Nmap is doing under the covers. To avoid
printing thousands of lines, you may want to specify a limited number of ports
to scan, such as -p20-30. If you only care about the goings on of the
version detection subsystem, use --version-trace instead. If you only
care about script tracing, specify --script-trace. With
--packet-trace, you get all of the above.
--open (Show only open (or possibly open) ports) .
Sometimes you only care about ports you can actually
connect to (open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is normally done
after the scan using tools such as grep, awk, and Perl, but this feature was
added due to overwhelming requests. Specify --open to only see hosts
with at least one open, open|filtered, or unfiltered port, and only see ports
in those states. These three states are treated just as they normally are,
which means that open|filtered and unfiltered may be condensed into counts if
there are an overwhelming number of them.
--iflist (List interfaces and routes) .
Prints the interface list and system routes as detected
by Nmap. This is useful for debugging routing problems or device
mischaracterization (such as Nmap treating a PPP connection as
ethernet).
Miscellaneous output options
--append-output (Append to rather than clobber output files) .
When you specify a filename to an output format flag such
as -oX or -oN, that file is overwritten by default. If you
prefer to keep the existing content of the file and append the new results,
specify the --append-output option. All output filenames specified in
that Nmap execution will then be appended to rather than clobbered. This
doesn't work well for XML ( -oX) scan data as the resultant file
generally won't parse properly until you fix it up by hand.
--resume filename (Resume aborted scan) .
Some extensive Nmap runs take a very long time—on
the order of days. Such scans don't always run to completion. Restrictions may
prevent Nmap from being run during working hours, the network could go down,
the machine Nmap is running on might suffer a planned or unplanned reboot, or
Nmap itself could crash. The administrator running Nmap could cancel it for
any other reason as well, by pressing ctrl-C. Restarting the whole scan from
the beginning may be undesirable. Fortunately, if normal ( -oN) or
grepable ( -oG) logs were kept, the user can ask Nmap to resume
scanning with the target it was working on when execution ceased. Simply
specify the --resume option and pass the normal/grepable output file as
its argument. No other arguments are permitted, as Nmap parses the output file
to use the same ones specified previously. Simply call Nmap as nmap
--resume logfilename. Nmap will append new results to the
data files specified in the previous execution. Resumption does not support
the XML output format because combining the two runs into one valid XML file
would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform
XML output) .
Nmap ships with an XSL. stylesheet. named nmap.xsl. for
viewing or translating XML output to HTML.. The XML output includes an
xml-stylesheet directive which points to nmap.xml where it was initially
installed by Nmap. Run the XML file through an XSLT processor such as
xsltproc[17]. to produce an HTML file. Directly opening the XML file in
a browser no longer works well because modern browsers limit the locations a
stylesheet may be loaded from. If you wish to use a different stylesheet,
specify it as the argument to --stylesheet. You must pass the full
pathname or URL. One common invocation is --stylesheet
http://nmap.org/svn/docs/nmap.xsl. This tells an XSLT processor to load
the latest version of the stylesheet from Nmap.Org. The --webxml option
does the same thing with less typing and memorization. Loading the XSL from
Nmap.Org makes it easier to view results on a machine that doesn't have Nmap
(and thus nmap.xsl) installed. So the URL is often more useful, but the local
filesystem location of nmap.xsl is used by default for privacy reasons.
--webxml (Load stylesheet from Nmap.Org) .
This is a convenience option, nothing more than an alias
for --stylesheet http://nmap.org/svn/docs/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML) .
Specify this option to prevent Nmap from associating any
XSL stylesheet with its XML output. The xml-stylesheet directive is
omitted.
MISCELLANEOUS OPTIONS¶
This section describes some important (and not-so-important) options that don't really fit anywhere else. -6 (Enable IPv6 scanning) .Nmap has IPv6 support for its most popular features. Ping
scanning, port scanning, version detection, and the Nmap Scripting Engine all
support IPv6. The command syntax is the same as usual except that you also add
the -6 option. Of course, you must use IPv6 syntax if you specify an
address rather than a hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are recommended. The
output looks the same as usual, with the IPv6 address on the
“interesting ports” line being the only IPv6 giveaway.
While IPv6 hasn't exactly taken the world by storm, it gets significant use in
some (usually Asian) countries and most modern operating systems support it.
To use Nmap with IPv6, both the source and target of your scan must be
configured for IPv6. If your ISP (like most of them) does not allocate IPv6
addresses to you, free tunnel brokers are widely available and work fine with
Nmap. I use the free IPv6 tunnel broker. service at
http://www.tunnelbroker.net. Other tunnel brokers are listed at
Wikipedia[18]. 6to4 tunnels are another popular, free approach.
On Windows, raw-socket IPv6 scans are supported only on ethernet devices (not
tunnels), and only on Windows Vista. and later. Use the --unprivileged.
option in other situations.
-A (Aggressive scan options) .
This option enables additional advanced and aggressive
options. I haven't decided exactly which it stands for yet. Presently this
enables OS detection ( -O), version scanning (-sV), script
scanning ( -sC) and traceroute (--traceroute).. More features
may be added in the future. The point is to enable a comprehensive set of scan
options without people having to remember a large set of flags. However,
because script scanning with the default set is considered intrusive, you
should not use -A against target networks without permission. This
option only enables features, and not timing options (such as -T4) or
verbosity options ( -v) that you might want as well.
--datadir directoryname (Specify custom Nmap data file
location) .
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of these files has
been specified (using the --servicedb or --versiondb options),
that location is used for that file. After that, Nmap searches these files in
the directory specified with the --datadir option (if any). Any files
not found there, are searched for in the directory specified by the
NMAPDIR. environment variable. Next comes ~/.nmap. for real and
effective UIDs; or on Windows, HOME\AppData\Roaming\nmap (where
HOME is the user's home directory, like C:\Users\user). This is
followed by the location of the nmap executable and the same location with
../share/nmap appended. Then a compiled-in location such as
/usr/local/share/nmap or /usr/share/nmap.
--servicedb services file (Specify custom services file) .
Asks Nmap to use the specified services file rather than
the nmap-services data file that comes with Nmap. Using this option also
causes a fast scan ( -F) to be used. See the description for
--datadir for more information on Nmap's data files.
--versiondb service probes file (Specify custom service
probes file) .
Asks Nmap to use the specified service probes file rather
than the nmap-service-probes data file that comes with Nmap. See the
description for --datadir for more information on Nmap's data
files.
--send-eth (Use raw ethernet sending) .
Asks Nmap to send packets at the raw ethernet (data link)
layer rather than the higher IP (network) layer. By default, Nmap chooses the
one which is generally best for the platform it is running on. Raw sockets (IP
layer). are generally most efficient for Unix machines, while ethernet frames
are required for Windows operation since Microsoft disabled raw socket
support. Nmap still uses raw IP packets on Unix despite this option when there
is no other choice (such as non-ethernet connections).
--send-ip (Send at raw IP level) .
Asks Nmap to send packets via raw IP sockets rather than
sending lower level ethernet frames. It is the complement to the
--send-eth option discussed previously.
--privileged (Assume that the user is fully privileged) .
Tells Nmap to simply assume that it is privileged enough
to perform raw socket sends, packet sniffing, and similar operations that
usually require root privileges. on Unix systems. By default Nmap quits if
such operations are requested but geteuid is not zero.
--privileged is useful with Linux kernel capabilities and similar
systems that may be configured to allow unprivileged users to perform
raw-packet scans. Be sure to provide this option flag before any flags for
options that require privileges (SYN scan, OS detection, etc.). The
NMAP_PRIVILEGED. environment variable may be set as an equivalent
alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket privileges) .
This option is the opposite of --privileged. It
tells Nmap to treat the user as lacking network raw socket and sniffing
privileges. This is useful for testing, debugging, or when the raw network
functionality of your operating system is somehow broken. The
NMAP_UNPRIVILEGED. environment variable may be set as an equivalent
alternative to --unprivileged.
--release-memory (Release memory before quitting) .
This option is only useful for memory-leak debugging. It
causes Nmap to release allocated memory just before it quits so that actual
memory leaks are easier to spot. Normally Nmap skips this as the OS does this
anyway upon process termination.
-V; --version (Print version number) .
Prints the Nmap version number and exits.
-h; --help (Print help summary page) .
Prints a short help screen with the most common command
flags. Running Nmap without any arguments does the same thing.
RUNTIME INTERACTION¶
During the execution of Nmap, all key presses are captured. This allows you to interact with the program without aborting and restarting it. Certain special keys will change options, while any other keys will print out a status message telling you about the scan. The convention is that lowercase letters increase the amount of printing, and uppercase letters decrease the printing. You may also press ‘ ?’ for help. v / VIncrease / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
EXAMPLES¶
Here are some Nmap usage examples, from the simple and routine to a little more complex and esoteric. Some actual IP addresses and domain names are used to make things more concrete. In their place you should substitute addresses/names from your own network. While I don't think port scanning other networks is or should be illegal, some network administrators don't appreciate unsolicited scanning of their networks and may complain. Getting permission first is the best approach. For testing purposes, you have permission to scan the host scanme.nmap.org.. This permission only includes scanning via Nmap and not testing exploits or denial of service attacks. To conserve bandwidth, please do not initiate more than a dozen scans against that host per day. If this free scanning target service is abused, it will be taken down and Nmap will report Failed to resolve given hostname/IP: scanme.nmap.org. These permissions also apply to the hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do not currently exist. nmap -v scanme.nmap.org This option scans all reserved TCP ports on the machine scanme.nmap.org . The -v option enables verbose mode. nmap -sS -O scanme.nmap.org/24 Launches a stealth SYN scan against each machine that is up out of the 256 IPs on the class C sized network where Scanme resides. It also tries to determine what operating system is running on each host that is up and running. This requires root privileges because of the SYN scan and OS detection. nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127 Launches host enumeration and a TCP scan at the first half of each of the 255 possible eight-bit subnets in the 198.116 class B address space. This tests whether the systems run SSH, DNS, POP3, or IMAP on their standard ports, or anything on port 4564. For any of these ports found open, version detection is used to determine what application is running. nmap -v -iR 100000 -Pn -p 80 Asks Nmap to choose 100,000 hosts at random and scan them for web servers (port 80). Host enumeration is disabled with -Pn since first sending a couple probes to determine whether a host is up is wasteful when you are only probing one port on each target host anyway. nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap 216.163.128.20/20 This scans 4096 IPs for any web servers (without pinging them) and saves the output in grepable and XML formats.NMAP BOOK¶
While this reference guide details all material Nmap options, it can't fully demonstrate how to apply those features to quickly solve real-world tasks. For that, we released Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning. Topics include subverting firewalls and intrusion detection systems, optimizing Nmap performance, and automating common networking tasks with the Nmap Scripting Engine. Hints and instructions are provided for common Nmap tasks such as taking network inventory, penetration testing, detecting rogue wireless access points, and quashing network worm outbreaks. Examples and diagrams show actual communication on the wire. More than half of the book is available free online. See http://nmap.org/book for more information.BUGS¶
Like its author, Nmap isn't perfect. But you can help make it better by sending bug reports or even writing patches. If Nmap doesn't behave the way you expect, first upgrade to the latest version available from http://nmap.org. If the problem persists, do some research to determine whether it has already been discovered and addressed. Try searching for the error message on our search page at http://insecure.org/search.html or at Google. Also try browsing the nmap-dev archives at http://seclists.org/.. Read this full manual page as well. If nothing comes of this, mail a bug report to dev@nmap.org. Please include everything you have learned about the problem, as well as what version of Nmap you are running and what operating system version it is running on. Problem reports and Nmap usage questions sent to dev@nmap.org are far more likely to be answered than those sent to Fyodor directly. If you subscribe to the nmap-dev list before posting, your message will bypass moderation and get through more quickly. Subscribe at http://nmap.org/mailman/listinfo/dev. Code patches to fix bugs are even better than bug reports. Basic instructions for creating patch files with your changes are available at https://svn.nmap.org/nmap/HACKING. Patches may be sent to nmap-dev (recommended) or to Fyodor directly.AUTHOR¶
Gordon “Fyodor” Lyon fyodor@nmap.org ( http://insecure.org) Hundreds of people have made valuable contributions to Nmap over the years. These are detailed in the CHANGELOG. file which is distributed with Nmap and also available from http://nmap.org/changelog.html.LEGAL NOTICES¶
Nmap Copyright and Licensing¶
The Nmap Security Scanner is (C) 1996–2013 Insecure.Com LLC. Nmap is also a registered trademark of Insecure.Com LLC. This program is free software; you may redistribute and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; Version 2 (“GPL”), BUT ONLY WITH ALL OF THE CLARIFICATIONS AND EXCEPTIONS DESCRIBED HEREIN. This guarantees your right to use, modify, and redistribute this software under certain conditions. If you wish to embed Nmap technology into proprietary software, we sell alternative licenses (contact sales@nmap.com). Dozens of software vendors already license Nmap technology such as host discovery, port scanning, OS detection, version detection, and the Nmap Scripting Engine. Note that the GPL places important restrictions on “derivative works”, yet it does not provide a detailed definition of that term. To avoid misunderstandings, we interpret that term as broadly as copyright law allows. For example, we consider an application to constitute a derivative work for the purpose of this license if it does any of the following with any software or content covered by this license (“Covered Software”):•Integrates source code from Covered
Software.
•Reads or includes copyrighted data files, such as
Nmap's nmap-os-db or nmap-service-probes.
•Is designed specifically to execute Covered
Software and parse the results (as opposed to typical shell or execution-menu
apps, which will execute anything you tell them to).
•Includes Covered Software in a proprietary
executable installer. The installers produced by InstallShield are an example
of this. Including Nmap with other software in compressed or archival form
does not trigger this provision, provided appropriate open source
decompression or de-archiving software is widely available for no charge. For
the purposes of this license, an installer is considered to include Covered
Software even if it actually retrieves a copy of Covered Software from another
source during runtime (such as by downloading it from the Internet).
•Links (statically or dynamically) to a library
which does any of the above.
•Executes a helper program, module, or script to
do any of the above.
This list is not exclusive, but is meant to clarify our interpretation of
derived works with some common examples. Other people may interpret the plain
GPL differently, so we consider this a special exception to the GPL that we
apply to Covered Software. Works which meet any of these conditions must
conform to all of the terms of this license, particularly including the GPL
Section 3 requirements of providing source code and allowing free
redistribution of the work as a whole.
As another special exception to the GPL terms, Insecure.Com LLC grants
permission to link the code of this program with any version of the OpenSSL
library which is distributed under a license identical to that listed in the
included docs/licenses/OpenSSL.txt file, and distribute linked combinations
including the two..
Any redistribution of Covered Software, including any derived works, must obey
and carry forward all of the terms of this license, including obeying all GPL
rules and restrictions. For example, source code of the whole work must be
provided and free redistribution must be allowed. All GPL references to
"this License", are to be treated as including the terms and
conditions of this license text as well.
Because this license imposes special exceptions to the GPL, Covered Work may not
be combined (even as part of a larger work) with plain GPL software. The
terms, conditions, and exceptions of this license must be included as well.
This license is incompatible with some other open source licenses as well. In
some cases we can relicense portions of Nmap or grant special permissions to
use it in other open source software. Please contact fyodor@nmap.org with any
such requests. Similarly, we don't incorporate incompatible open source
software into Covered Software without special permission from the copyright
holders.
If you have any questions about the licensing restrictions on using Nmap in
other works, are happy to help. As mentioned above, we also offer alternative
license to integrate Nmap into proprietary applications and appliances. These
contracts have been sold to dozens of software vendors, and generally include
a perpetual license as well as providing for priority support and updates.
They also fund the continued development of Nmap. Please email sales@nmap.com
for further information.
If you have received a written license agreement or contract for Covered
Software stating terms other than these, you may choose to use and
redistribute Covered Software under those terms instead of these.
Creative Commons License for this Nmap Guide¶
This Nmap Reference Guide is (C) 2005–2012 Insecure.Com LLC. It is hereby placed under version 3.0 of the Creative Commons Attribution License[19]. This allows you redistribute and modify the work as you desire, as long as you credit the original source. Alternatively, you may choose to treat this document as falling under the same license as Nmap itself (discussed previously).Source Code Availability and Community Contributions¶
Source is provided to this software because we believe users have a right to know exactly what a program is going to do before they run it. This also allows you to audit the software for security holes (none have been found so far). Source code also allows you to port Nmap to new platforms, fix bugs, and add new features. You are highly encouraged to send your changes to dev@nmap.org for possible incorporation into the main distribution. By sending these changes to Fyodor or one of the Insecure.Org development mailing lists, it is assumed that you are offering the Nmap Project (Insecure.Com LLC) the unlimited, non-exclusive right to reuse, modify, and relicense the code. Nmap will always be available open source,. but this is important because the inability to relicense code has caused devastating problems for other Free Software projects (such as KDE and NASM). We also occasionally relicense the code to third parties as discussed above. If you wish to specify special license conditions of your contributions, just say so when you send them.No Warranty.¶
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License v2.0 for more details at http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file included with Nmap. It should also be noted that Nmap has occasionally been known to crash poorly written applications, TCP/IP stacks, and even operating systems.. While this is extremely rare, it is important to keep in mind. Nmap should never be run against mission critical systems unless you are prepared to suffer downtime. We acknowledge here that Nmap may crash your systems or networks and we disclaim all liability for any damage or problems Nmap could cause.Inappropriate Usage¶
Because of the slight risk of crashes and because a few black hats like to use Nmap for reconnaissance prior to attacking systems, there are administrators who become upset and may complain when their system is scanned. Thus, it is often advisable to request permission before doing even a light scan of a network. Nmap should never be installed with special privileges (e.g. suid root).. That would open up a major security vulnerability as other users on the system (or attackers) could use it for privilege escalation.Third-Party Software and Funding Notices¶
This product includes software developed by the Apache Software Foundation[20]. A modified version of the Libpcap portable packet capture library[21]. is distributed along with Nmap. The Windows version of Nmap utilized the Libpcap-derived WinPcap library[22]. instead. Regular expression support is provided by the PCRE library[23],. which is open-source software, written by Philip Hazel.. Certain raw networking functions use the Libdnet[24]. networking library, which was written by Dug Song.. A modified version is distributed with Nmap. Nmap can optionally link with the OpenSSL cryptography toolkit[25]. for SSL version detection support. The Nmap Scripting Engine uses an embedded version of the Lua programming language[26].. The Liblinear linear classification library[27] is used for our IPv6 OS detection machine learning techniques[28]. All of the third-party software described in this paragraph is freely redistributable under BSD-style software licenses. Binary packages for Windows and Mac OS X include support libraries necessary to run Zenmap and Ndiff with Python and PyGTK. (Unix platforms commonly make these libraries easy to install, so they are not part of the packages.) A listing of these support libraries and their licenses is included in the LICENSES files. This software was supported in part through the Google Summer of Code[29] and the DARPA CINDER program[30] (DARPA-BAA-10-84).United States Export Control.¶
Nmap only uses encryption when compiled with the optional OpenSSL support and linked with OpenSSL. When compiled without OpenSSL support, Insecure.Com LLC believes that Nmap is not subject to U.S. Export Administration Regulations (EAR)[31] export control. As such, there is no applicable ECCN (export control classification number) and exportation does not require any special license, permit, or other governmental authorization. When compiled with OpenSSL support or distributed as source code, Insecure.Com LLC believes that Nmap falls under U.S. ECCN 5D002[32] (“Information Security Software”). We distribute Nmap under the TSU exception for publicly available encryption software defined in EAR 740.13(e)[33].NOTES¶
- 1.
- Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning
- 2.
- RFC 1122
- 3.
- RFC 792
- 4.
- RFC 950
- 5.
- RFC 1918
- 6.
- UDP
- 7.
- SCTP
- 8.
- TCP RFC
- 9.
- RFC 959
- 10.
- RFC 1323
- 11.
- Lua programming language
- 12.
- precedence
- 13.
- IP protocol
- 14.
- RFC 2960
- 15.
- Nmap::Scanner
- 16.
- Nmap::Parser
- 17.
- xsltproc
- 18.
- listed at Wikipedia
- 19.
- Creative Commons Attribution License
- 20.
- Apache Software Foundation
- 21.
- Libpcap portable packet capture library
- 22.
- WinPcap library
- 23.
- PCRE library
- 24.
- Libdnet
- 25.
- OpenSSL cryptography toolkit
- 26.
- Lua programming language
- 27.
- Liblinear linear classification library
- 28.
- IPv6 OS detection machine learning techniques
- 29.
- Google Summer of Code
- 30.
- DARPA CINDER program
- 31.
- Export Administration Regulations (EAR)
- 32.
- 5D002
- 33.
- EAR 740.13(e)
08/13/2014 | Nmap |