NAME¶
traceroute - print the route packets trace to network host
SYNOPSIS¶
traceroute [
-46dFITUnreAV] [
-f first_ttl] [
-g
gate,...]
[
-i device] [
-m max_ttl] [
-p port] [
-s
src_addr]
[
-q nqueries] [
-N squeries] [
-t tos]
[
-l flow_label] [
-w waittime] [
-z sendwait]
[
-UL] [
-D]
[
-P proto] [
--sport=port] [
-M method] [
-O
mod_options]
[
--mtu] [
--back]
host [
packet_len]
traceroute6 [
options]
tcptraceroute [
options]
lft [
options]
DESCRIPTION¶
traceroute tracks the route packets taken from an IP network on their way
to a given host. It utilizes the IP protocol's time to live (TTL) field and
attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the
path to the host.
traceroute6 is equivalent to
traceroute -6
tcptraceroute is equivalent to
traceroute -T
lft , the Layer Four Traceroute, performs a TCP traceroute, like
traceroute -T , but attempts to provide compatibility with the
original such implementation, also called "lft".
The only required parameter is the name or IP address of the destination
host . The optional
packet_len`gth is the total size of
the probing packet (default 60 bytes for IPv4 and 80 for IPv6). The specified
size can be ignored in some situations or increased up to a minimal value.
This program attempts to trace the route an IP packet would follow to some
internet host by launching probe packets with a small ttl (time to live) then
listening for an ICMP "time exceeded" reply from a gateway. We start
our probes with a ttl of one and increase by one until we get an ICMP
"port unreachable" (or TCP reset), which means we got to the
"host", or hit a max (which defaults to 30 hops). Three probes (by
default) are sent at each ttl setting and a line is printed showing the ttl,
address of the gateway and round trip time of each probe. The address can be
followed by additional information when requested. If the probe answers come
from different gateways, the address of each responding system will be
printed. If there is no response within a 5.0 seconds (default), an
"*" (asterisk) is printed for that probe.
After the trip time, some additional annotation can be printed:
!H,
!N, or
!P (host, network or protocol unreachable),
!S
(source route failed),
!F (fragmentation needed),
!X
(communication administratively prohibited),
!V (host precedence
violation),
!C (precedence cutoff in effect), or
!<num>
(ICMP unreachable code <num>). If almost all the probes result in some
kind of unreachable, traceroute will give up and exit.
We don't want the destination host to process the UDP probe packets, so the
destination port is set to an unlikely value (you can change it with the
-p flag). There is no such a problem for ICMP or TCP tracerouting (for
TCP we use half-open technique, which prevents our probes to be seen by
applications on the destination host).
In the modern network environment the traditional traceroute methods can not be
always applicable, because of widespread use of firewalls. Such firewalls
filter the "unlikely" UDP ports, or even ICMP echoes. To solve this,
some additional tracerouting methods are implemented (including tcp), see
LIST OF AVAILABLE METHODS below. Such methods try to use particular
protocol and source/destination port, in order to bypass firewalls (to be seen
by firewalls just as a start of allowed type of a network session).
OPTIONS¶
- --help
- Print help info and exit.
- -4, -6
- Explicitly force IPv4 or IPv6 tracerouting. By default, the program will
try to resolve the name given, and choose the appropriate protocol
automatically. If resolving a host name returns both IPv4 and IPv6
addresses, traceroute will use IPv4.
- -I, --icmp
- Use ICMP ECHO for probes
- -T, --tcp
- Use TCP SYN for probes
- -d, --debug
- Enable socket level debugging (when the Linux kernel supports it)
- -F, --dont-fragment
- Do not fragment probe packets. (For IPv4 it also sets DF bit, which tells
intermediate routers not to fragment remotely as well).
Varying the size of the probing packet by the packet_len command line
parameter, you can manually obtain information about the MTU of individual
network hops. The --mtu option (see below) tries to do this
automatically.
Note, that non-fragmented features (like -F or --mtu) work
properly since the Linux kernel 2.6.22 only. Before that version, IPv6 was
always fragmented, IPv4 could use the once the discovered final mtu only
(from the route cache), which can be less than the actual mtu of a
device.
- -f first_ttl, --first=first_ttl
- Specifies with what TTL to start. Defaults to 1.
- -g gateway, --gateway=gateway
- Tells traceroute to add an IP source routing option to the outgoing packet
that tells the network to route the packet through the specified
gateway (most routers have disabled source routing for security
reasons). In general, several gateway's is allowed (comma
separated). For IPv6, the form of
num,addr,addr... is allowed,
where num is a route header type (default is type 2). Note the
type 0 route header is now deprecated (rfc5095).
- -i interface, --interface=interface
- Specifies the interface through which traceroute should send
packets. By default, the interface is selected according to the routing
table.
- -m max_ttl, --max-hops=max_ttl
- Specifies the maximum number of hops (max time-to-live value)
traceroute will probe. The default is 30.
- -N squeries, --sim-queries=squeries
- Specifies the number of probe packets sent out simultaneously. Sending
several probes concurrently can speed up traceroute considerably.
The default value is 16.
Note that some routers and hosts can use ICMP rate throttling. In such a
situation specifying too large number can lead to loss of some
responses.
- -n
- Do not try to map IP addresses to host names when displaying them.
- -p port, --port=port
- For UDP tracing, specifies the destination port base traceroute
will use (the destination port number will be incremented by each probe).
For ICMP tracing, specifies the initial ICMP sequence value (incremented by
each probe too).
For TCP and others specifies just the (constant) destination port to
connect. When using the tcptraceroute wrapper, -p specifies the source
port.
- -t tos, --tos=tos
- For IPv4, set the Type of Service (TOS) and Precedence value. Useful
values are 16 (low delay) and 8 (high throughput). Note that in order to
use some TOS precedence values, you have to be super user.
For IPv6, set the Traffic Control value.
- -l flow_label, --flowlabel=flow_label
- Use specified flow_label for IPv6 packets.
- -w waittime, --wait=waittime
- Set the time (in seconds) to wait for a response to a probe (default 5.0
sec).
- -q nqueries, --queries=nqueries
- Sets the number of probe packets per hop. The default is 3.
- -r
- Bypass the normal routing tables and send directly to a host on an
attached network. If the host is not on a directly-attached network, an
error is returned. This option can be used to ping a local host through an
interface that has no route through it.
- -s source_addr, --source=source_addr
- Chooses an alternative source address. Note that you must select the
address of one of the interfaces. By default, the address of the outgoing
interface is used.
- -z sendwait, --sendwait=sendwait
- Minimal time interval between probes (default 0). If the value is more
than 10, then it specifies a number in milliseconds, else it is a number
of seconds (float point values allowed too). Useful when some routers use
rate-limit for ICMP messages.
- -e, --extensions
- Show ICMP extensions (rfc4884). The general form is
CLASS/TYPE : followed by a hexadecimal
dump. The MPLS (rfc4950) is shown parsed, in a form:
MPLS:L=label,E=exp_use,S=stack_bottom,T=TTL
(more objects separated by / ).
- -A, --as-path-lookups
- Perform AS path lookups in routing registries and print results directly
after the corresponding addresses.
- -V, --version
- Print the version and exit.
There is a couple of additional options, intended for an advanced usage (another
trace methods etc.):
- --sport=port
- Chooses the source port to use. Implies -N 1. Normally
source ports (if applicable) are chosen by the system.
- --fwmark=mark
- Set the firewall mark for outgoing packets (since the Linux kernel
2.6.25).
- -M method, --module=name
- Use specified method for traceroute operations. Default traditional udp
method has name default, icmp (-I) and tcp
(-T) have names icmp and tcp
respectively.
Method-specific options can be passed by -O . Most methods
have their simple shortcuts, (-I means -M icmp,
etc).
- -O option, --options=options
- Specifies some method-specific option. Several options are separated by
comma (or use several -O on cmdline). Each method may have its own
specific options, or many not have them at all. To print information about
available options, use -O help.
- -U, --udp
- Use UDP to particular destination port for tracerouting (instead of
increasing the port per each probe). Default port is 53 (dns).
- -UL
- Use UDPLITE for tracerouting (default port is 53).
- -D, --dccp
- Use DCCP Requests for probes.
- -P protocol, --protocol=protocol
- Use raw packet of specified protocol for tracerouting. Default protocol is
253 (rfc3692).
- --mtu
- Discover MTU along the path being traced. Implies
-F -N 1. New mtu is printed once in a form of
F=NUM at the first probe of a hop which requires such
mtu to be reached. (Actually, the correspond "frag
needed" icmp message normally is sent by the previous hop).
Note, that some routers might cache once the seen information on a
fragmentation. Thus you can receive the final mtu from a closer hop. Try
to specify an unusual tos by -t , this can help for one
attempt (then it can be cached there as well).
See -F option for more info.
- --back
- Print the number of backward hops when it seems different with the forward
direction. This number is guessed in assumption that remote hops send
reply packets with initial ttl set to either 64, or 128 or 255 (which
seems a common practice). It is printed as a negate value in a form of
'-NUM' .
LIST OF AVAILABLE METHODS¶
In general, a particular traceroute method may have to be chosen by
-M name, but most of the methods have their simple cmdline
switches (you can see them after the method name, if present).
default¶
The traditional, ancient method of tracerouting. Used by default.
Probe packets are udp datagrams with so-called "unlikely" destination
ports. The "unlikely" port of the first probe is 33434, then for
each next probe it is incremented by one. Since the ports are expected to be
unused, the destination host normally returns "icmp unreach port" as
a final response. (Nobody knows what happens when some application listens for
such ports, though).
This method is allowed for unprivileged users.
icmp -I¶
Most usual method for now, which uses icmp echo packets for probes.
If you can
ping(8) the destination host, icmp tracerouting is applicable as
well.
This method may be allowed for unprivileged users since the kernel 3.0 (IPv4
only), which supports new
dgram icmp (or
"ping") sockets. To allow such sockets, sysadmin
should provide
net/ipv4/ping_group_range sysctl range to match any
group of the user.
Options:
- raw
- Use only raw sockets (the traditional way).
This way is tried first by default (for compatibility reasons), then new
dgram icmp sockets as fallback.
- dgram
- Use only dgram icmp sockets.
tcp -T¶
Well-known modern method, intended to bypass firewalls.
Uses the constant destination port (default is 80, http).
If some filters are present in the network path, then most probably any
"unlikely" udp ports (as for
default method) or even icmp
echoes (as for
icmp) are filtered, and whole tracerouting will just
stop at such a firewall. To bypass a network filter, we have to use only
allowed protocol/port combinations. If we trace for some, say, mailserver,
then more likely
-T -p 25 can reach it, even when
-I can not.
This method uses well-known "half-open technique", which prevents
applications on the destination host from seeing our probes at all. Normally,
a tcp syn is sent. For non-listened ports we receive tcp reset, and all is
done. For active listening ports we receive tcp syn+ack, but answer by tcp
reset (instead of expected tcp ack), this way the remote tcp session is
dropped even without the application ever taking notice.
There is a couple of options for
tcp method:
- syn,ack,fin,rst,psh,urg,ece,cwr
- Sets specified tcp flags for probe packet, in any combination.
- flags=num
- Sets the flags field in the tcp header exactly to num.
- ecn
- Send syn packet with tcp flags ECE and CWR (for Explicit Congestion
Notification, rfc3168).
- sack,timestamps,window_scaling
- Use the corresponding tcp header option in the outgoing probe packet.
- sysctl
- Use current sysctl (/proc/sys/net/*) setting for the tcp
header options above and ecn. Always set by default, if nothing
else specified.
- mss=num
- Use value of num for maxseg tcp header option (when
syn).
- info
- Print tcp flags of final tcp replies when the target host is reached.
Allows to determine whether an application listens the port and other
useful things.
Default options is
syn,sysctl.
tcpconn¶
An initial implementation of tcp method, simple using
connect(2) call, which
does full tcp session opening. Not recommended for normal use, because a
destination application is always affected (and can be confused).
udp -U¶
Use udp datagram with constant destination port (default 53, dns).
Intended to bypass firewall as well.
Note, that unlike in
tcp method, the correspond application on the
destination host
always receive our probes (with random data), and most
can easily be confused by them. Most cases it will not respond to our packets
though, so we will never see the final hop in the trace. (Fortunately, it
seems that at least dns servers replies with something angry).
This method is allowed for unprivileged users.
udplite -UL¶
Use udplite datagram for probes (with constant destination port, default 53).
This method is allowed for unprivileged users.
Options:
- coverage=num
- Set udplite send coverage to num.
dccp -D¶
Use DCCP Request packets for probes (rfc4340).
This method uses the same "half-open technique" as used for TCP. The
default destination port is 33434.
Options:
- service=num
- Set DCCP service code to num (default is 1885957735).
raw -P proto¶
Send raw packet of protocol
proto.
No protocol-specific headers are used, just IP header only.
Implies
-N 1.
Options:
- protocol=proto
- Use IP protocol proto (default 253).
NOTES¶
To speed up work, normally several probes are sent simultaneously. On the other
hand, it creates a "storm of packages", especially in the reply
direction. Routers can throttle the rate of icmp responses, and some of
replies can be lost. To avoid this, decrease the number of simultaneous
probes, or even set it to 1 (like in initial traceroute implementation), i.e.
-N 1
The final (target) host can drop some of the simultaneous probes, and might even
answer only the latest ones. It can lead to extra "looks like
expired" hops near the final hop. We use a smart algorithm to auto-detect
such a situation, but if it cannot help in your case, just use
-N 1
too.
For even greater stability you can slow down the program's work by
-z
option, for example use
-z 0.5 for half-second pause between probes.
If some hops report nothing for every method, the last chance to obtain
something is to use
ping -R command (IPv4, and for nearest 8 hops
only).
SEE ALSO¶
ping(8),
ping6(8),
tcpdump(8),
netstat(8)