|NC(1)||General Commands Manual||NC(1)|
nc — arbitrary TCP
and UDP connections and listens
utility is used for just about anything under the sun involving TCP, UDP, or
UNIX-domain sockets. It can open TCP connections,
send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning,
and deal with both IPv4 and IPv6. Unlike telnet(1),
nc scripts nicely, and separates error messages onto
standard error instead of sending them to standard output, as
telnet(1) does with some.
Common uses include:
- simple TCP proxies
- shell-script based HTTP clients and servers
- network daemon testing
- a SOCKS or HTTP ProxyCommand for ssh(1)
- and much, much more
The options are as follows:
- Use IPv4 addresses only.
- Use IPv6 addresses only.
- Allow broadcast.
- Send CRLF as line-ending. Each line feed (LF) character from the input data is translated into CR+LF before being written to the socket. Line feed characters that are already preceded with a carriage return (CR) are not translated. Received data is not affected.
- Enable debugging on the socket.
- Do not attempt to read from stdin.
- Pass the first connected socket using sendmsg(2) to
stdout and exit. This is useful in conjunction with
ncperform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config(5)
ProxyUseFdpassoption). Cannot be used with
- Print out the
nchelp text and exit.
- Specify the size of the TCP receive buffer.
- Sleep for interval seconds between lines of text sent and received. Also causes a delay time between connections to multiple ports.
- When a connection is completed, listen for another one. Requires
-l. When used together with the
-uoption, the server socket is not connected and it can receive UDP datagrams from multiple hosts.
- Listen for an incoming connection rather than initiating a connection to a
remote host. The destination and
port to listen on can be specified either as
non-optional arguments, or with options
-prespectively. Cannot be used together with
-z. Additionally, any timeouts specified with the
-woption are ignored.
- Set the TTL / hop limit of outgoing packets.
- Ask the kernel to drop incoming packets whose TTL / hop limit is under minttl.
- shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.
- Do not perform domain name resolution. If a name cannot be resolved without DNS, an error will be reported.
- Specify the size of the TCP send buffer.
- Specifies a username to present to a proxy server that requires authentication. If no username is specified then authentication will not be attempted. Proxy authentication is only supported for HTTP CONNECT proxies at present.
- Specify the source port
ncshould use, subject to privilege restrictions and availability.
- after EOF on stdin, wait the specified number of
seconds and then quit. If
seconds is negative, wait forever (default).
Specifying a non-negative seconds implies
- Choose source and/or destination ports randomly instead of sequentially within a range or in the order that the system assigns them.
- Enable the RFC 2385 TCP MD5 signature option.
- Set the source address to send packets from, which is useful on machines
with multiple interfaces. For UNIX-domain datagram
sockets, specifies the local temporary socket file to create and use so
that datagrams can be received. Cannot be used together with
- Change the IPv4 TOS/IPv6 traffic class value.
keyword may be one of
reliability, or one of the DiffServ Code Points:
cs7; or a number in either hex or decimal.
- Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
This makes it possible to use
ncto script telnet sessions.
- Use UNIX-domain sockets. Cannot be used together
- Use UDP instead of TCP. Cannot be used together with
-x. For UNIX-domain sockets, use a datagram socket instead of a stream socket. If a UNIX-domain socket is used, a temporary receiving socket is created in /tmp unless the
-sflag is given.
- Set the routing table to be used.
- Produce more verbose output.
- Terminate after receiving recvlimit packets from the network.
- Connections which cannot be established or are idle timeout after
timeout seconds. The
-wflag has no effect on the
ncwill listen forever for a connection, with or without the
-wflag. The default is no timeout.
- Use proxy_protocol when talking to the proxy server.
Supported protocols are
5(SOCKS v.5) and
connect(HTTPS proxy). If the protocol is not specified, SOCKS version 5 is used.
- Connect to destination using a proxy at
proxy_address and port. If
port is not specified, the well-known port for the
proxy protocol is used (1080 for SOCKS, 3128 for HTTPS). An IPv6 address
can be specified unambiguously by enclosing
proxy_address in square brackets. A proxy cannot be
used with any of the options
- DCCP mode.
- Only scan for listening daemons, without sending any data to them. Cannot
be used together with
destination can be a numerical IP address or
a symbolic hostname (unless the
-n option is given).
In general, a destination must be specified, unless the
-l option is given (in which case the local host is
used). For UNIX-domain sockets, a destination is
required and is the socket path to connect to (or listen on if the
-l option is given).
port can be specified as a numeric port
number or as a service name. Port ranges may be specified as numeric port
numbers of the form nn-mm. In
general, a destination port must be specified, unless the
-U option is given.
It is quite simple to build a very basic client/server model using
nc. On one console, start
listening on a specific port for a connection. For example:
$ nc -l 1234
nc is now listening on port 1234 for a
connection. On a second console (or a second machine), connect to the
machine and port being listened on:
$ nc -N 127.0.0.1 1234
There should now be a connection between the ports. Anything typed
at the second console will be concatenated to the first, and vice-versa.
After the connection has been set up,
nc does not
really care which side is being used as a ‘server’ and which
side is being used as a ‘client’. The connection may be
terminated using an
EOF (‘^D’), as the
-N flag was given.
There is no
-e option in this netcat, but you still can execute
a command after connection being established by redirecting file
descriptors. Be cautious here because opening a port and let anyone
connected execute arbitrary command on your site is DANGEROUS. If you really
need to do this, here is an example:
On ‘server’ side:
$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
On ‘client’ side:
$ nc host.example.com 1234
$ (shell prompt from host.example.com)
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.
When connection is terminated,
nc quits as
-k if you want it keep listening, but if
the command quits this option won't restart it or keep
nc running. Also don't forget to remove the file
descriptor once you don't need it anymore:
$ rm -f /tmp/f
The example in the previous section can be expanded to build a basic data transfer model. Any information input into one end of the connection will be output to the other end, and input and output can be easily captured in order to emulate file transfer.
Start by using
nc to listen on a specific
port, with output captured into a file:
$ nc -l 1234 > filename.out
Using a second machine, connect to the listening
nc process, feeding it the file which is to be
$ nc -N host.example.com 1234 < filename.in
After the file has been transferred, the connection will close automatically.
TALKING TO SERVERS¶
It is sometimes useful to talk to servers “by hand” rather than through a user interface. It can aid in troubleshooting, when it might be necessary to verify what data a server is sending in response to commands issued by the client. For example, to retrieve the home page of a web site:
$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
Note that this also displays the headers sent by the web server. They can be filtered, using a tool such as sed(1), if necessary.
More complicated examples can be built up when the user knows the format of requests required by the server. As another example, an email may be submitted to an SMTP server using:
$ nc [-C] localhost 25 << EOF HELO host.example.com MAIL FROM:<firstname.lastname@example.org> RCPT TO:<email@example.com> DATA Body of email. . QUIT EOF
It may be useful to know which ports are open and running services
on a target machine. The
-z flag can be used to tell
nc to report open ports, rather than initiate a
connection. Usually it's useful to turn on verbose output to stderr by use
this option in conjunction with
$ nc -zv host.example.com 20-30 Connection to host.example.com 22 port [tcp/ssh] succeeded! Connection to host.example.com 25 port [tcp/smtp] succeeded!
The port range was specified to limit the search to ports 20 - 30,
and is scanned by increasing order (unless the
flag is set).
You can also specify a list of ports to scan, for example:
$ nc -zv host.example.com http 20 22-23 nc: connect to host.example.com 80 (tcp) failed: Connection refused nc: connect to host.example.com 20 (tcp) failed: Connection refused Connection to host.example.com port [tcp/ssh] succeeded! nc: connect to host.example.com 23 (tcp) failed: Connection refused
The ports are scanned by the order you given (unless the
-r flag is set).
Alternatively, it might be useful to know which server software is
running, and which versions. This information is often contained within the
greeting banners. In order to retrieve these, it is necessary to first make
a connection, and then break the connection when the banner has been
retrieved. This can be accomplished by specifying a small timeout with the
-w flag, or perhaps by issuing a
QUIT" command to the server:
$ echo "QUIT" | nc host.example.com 20-30 SSH-1.99-OpenSSH_3.6.1p2 Protocol mismatch. 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 host.example.com 42
Open a UDP connection to port 53 of host.example.com:
$ nc -u host.example.com 53
Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection:
$ nc -s 10.1.2.3 host.example.com 42
Create and listen on a UNIX-domain stream socket:
$ nc -lU /var/tmp/dsocket
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
The same example again, this time enabling proxy authentication with username “ruser” if the proxy requires it:
$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
UDP port scans using the
of flags will always report success irrespective of the target machine's
state. However, in conjunction with a traffic sniffer either on the target
machine or an intermediary device, the
combination could be useful for communications diagnostics. Note that the
amount of UDP traffic generated may be limited either due to hardware
resources and/or configuration settings.
|March 31, 2021||Debian|