traceroute - print route IP packets follow going to a remote host

traceroute [ options ] host [ size ]

Traceroute attempts to trace the route an IP packet follows to some internet host. It finds out intermediate hops by launching probe packets with a small time-to-live (TTL) value, and then listens for an ICMP reply of time exceeded from an intermediate router. Traceroute starts probing with a TTL of one, and increments by one until an ICMP port unreachable reply is received. This means the probe either got through to host, or hit the maximum TTL.

host is the only mandatory argument, and specifies the target system, either as an IP address, or as a host name. Parameter size determines the size of the probe packets in bytes.

-a

Abort after 10 consecutive hops without an answer.

-d

Turn on socket level debugging. This option is only available to the super-user (root).

-m max_ttl

Set the maximum time-to-live (TTL) value that will be used for probing. Hosts that are farther than max_ttl hops away will not be traced (default 30).

-n

Report only IP addresses, but no hostnames.

-p port

Start probing at an alternate UDP port (default 33434). Traceroute by default sends out UDP packets with increasing port numbers starting at port, and listens for ICMP errors returned from remote hosts. This scheme only works if there are no UDP servers listening on the probed hosts in the range from port to port + max_ttl.

-q n

Send out n queries for each TTL (each intermediate host) (default 3).

-r

Set Dont Route option, advising routers to drop the packets. In other words, only probe within the local subnet.

-s addr

Set source address of outgoing packets to addr, given either as numeric IP address, or as hostname.

-t tos

Set the type-of-service field in the outgoing IP packets (default 0). tos is valid in the range of 0 to 255.

-u

Use microsecond timestamps.

-v

Turn on verbose output.

-w wait

Set timeout for replies to wait seconds (default 5 sec). If no ICMP reply is received within wait after a packet has been sent out, the probe is considered as failed.

-A

Report the Autonomous System Number (ASN) at each hop. Roughly speaking, the ASN tells which administration a router is subject to. See RFC 1930 for all the details, and section ENVIRONMENT below on how to fine tune the lookup.

-I proto

Send out probe packets using IP protocol proto, given either as name or numerical value (default UDP). Some features like parallel probing are only available when using UDP.

-M

Determine the maximum transfer unit (MTU) along the path. See RFC 1191 for details.

-O

At each hop, perform a DNS lookup and report the owner as listed in the SOA record.

-P

Send out multiple probes in parallel. The default behaviour probes each hop in turn, starting from the nearest. Parallel mode is faster, but less reliable. Many routers rate limit ICMP packets from a single host, so dropouts are much more likely in parallel mode, and need not indicate a networking problem.

-Q

Report detailed statistics on the round trip times at each hop (minimum / average +- standard deviation / maximum). The values are given in milli seconds.

-S min_ttl

Set the time-to-live (TTL) value in the first packet sent out to min_ttl (default 1). This option determines the first (nearest) host that will show up in the trace.

-T t

End each line with t instead of a newline. This comes in handy, for example, when including traceroute's output in an HTML page.

-U

Move on to probing the next hop as soon as the first successful probe arrives.

-$

Send out nothing but a single ping with a very large time-to-live.

Usually the round trip time is printed for each probe at each hop. Special symbols denote when something went wrong:

*

No reply received within wait seconds.

!

Reply arrived with a time-to-live value of one or lower.

!H

Received a reply telling that the destination host is unreachable.

!N

Received a reply telling that the destination network is unreachable.

!P

Received a reply telling that the desired protocol is unavailable.

!S

Received a reply telling that source routing failed. Should never occur--unless the probed gateway is screwed.

!F

Received a reply telling that fragmentation is needed. Should never occur--unless the probed gateway is screwed.

(This section is taken almost verbatim from the documentation in the traceroute sourcecode.)

[yak 71]% traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 30 hops max, 56 byte packet
 1  helios.ee.lbl.gov (128.3.112.1)  19 ms  19 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  39 ms  19 ms
 3  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  39 ms  19 ms
 4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  39 ms  40 ms  39 ms
 5  ccn-nerif22.Berkeley.EDU (128.32.168.22)  39 ms  39 ms  39 ms
 6  128.32.197.4 (128.32.197.4)  40 ms  59 ms  59 ms
 7  131.119.2.5 (131.119.2.5)  59 ms  59 ms  59 ms
 8  129.140.70.13 (129.140.70.13)  99 ms  99 ms  80 ms
 9  129.140.71.6 (129.140.71.6)  139 ms  239 ms  319 ms
10  129.140.81.7 (129.140.81.7)  220 ms  199 ms  199 ms
11  nic.merit.edu (35.1.1.48)  239 ms  239 ms  239 ms

Note that lines 2 & 3 are the same. This is due to a buggy kernel on the 2nd hop system -- lbl-csam.arpa -- that forwards packets with a zero TTL.

A more interesting example is:

[yak 72]% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.26.0.115), 30 hops max
 1  helios.ee.lbl.gov (128.3.112.1)  0 ms  0 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (128.32.216.1)  19 ms  19 ms  19 ms
 3  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  19 ms  19 ms
 4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  19 ms  39 ms  39 ms
 5  ccn-nerif22.Berkeley.EDU (128.32.168.22)  20 ms  39 ms  39 ms
 6  128.32.197.4 (128.32.197.4)  59 ms  119 ms  39 ms
 7  131.119.2.5 (131.119.2.5)  59 ms  59 ms  39 ms
 8  129.140.70.13 (129.140.70.13)  80 ms  79 ms  99 ms
 9  129.140.71.6 (129.140.71.6)  139 ms  139 ms  159 ms
10  129.140.81.7 (129.140.81.7)  199 ms  180 ms  300 ms
11  129.140.72.17 (129.140.72.17)  300 ms  239 ms  239 ms
12  * * *
13  128.121.54.72 (128.121.54.72)  259 ms  499 ms  279 ms
14  * * *
15  * * *
16  * * *
17  * * *
18  ALLSPICE.LCS.MIT.EDU (18.26.0.115)  339 ms  279 ms  279 ms

(I start to see why I'm having so much trouble with mail to MIT.) Note that the gateways 12, 14, 15, 16 & 17 hops away either don't send ICMP "time exceeded" messages or send them with a TTL too small to reach us. 14 - 17 are running the MIT C Gateway code that doesn't send "time exceeded"s. God only knows what's going on with 12.

The silent gateway 12 in the above may be the result of a bug in the 4.[23]BSD network code (and its derivatives): 4.x (x <= 3) sends an unreachable message using whatever TTL remains in the original datagram. Since, for gateways, the remaining TTL is zero, the icmp "time exceeded" is guaranteed to not make it back to us. The behavior of this bug is slightly more interesting when it appears on the destination system:

 1  helios.ee.lbl.gov (128.3.112.1)  0 ms  0 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  19 ms  39 ms
 3  lilac-dmc.Berkeley.EDU (128.32.216.1)  19 ms  39 ms  19 ms
 4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  39 ms  40 ms  19 ms
 5  ccn-nerif35.Berkeley.EDU (128.32.168.35)  39 ms  39 ms  39 ms
 6  csgw.Berkeley.EDU (128.32.133.254)  39 ms  59 ms  39 ms
 7  * * *
 8  * * *
 9  * * *
10  * * *
11  * * *
12  * * *
13  rip.Berkeley.EDU (128.32.131.22)  59 ms !  39 ms !  39 ms !

Notice that there are 12 "gateways" (13 is the final destination) and exactly the last half of them are "missing". What's really happening is that rip (a Sun-3 running Sun OS3.5) is using the TTL from our arriving datagram as the TTL in its icmp reply. So, the reply will time out on the return path until we probe with a TTL that's at least twice the path length. I.e., rip is really only 7 hops away.

The lookup process of Autonomous System Numbers (ASN, see option -A above) can be configured via several environment variables. By default, traceroute issues a whois query on the Routing Assets Database (RADB) at whois.ra.net, which should be sufficient in most cases. Chances are that you don't want to change anything here, unless you know very well what you are doing.

The contents of the following environment variables are limited to 100 characters at most. Any trailing characters are silently ignored. If unset, compiled-in defaults are used.

RA_SERVER

Server to issue a RADB whois query on, given either as hostname, or dotted-quad IP address. Defaults to whois.ra.net.

RA_SERVICE

TCP port to connect to on the whois server, given either as name or port number. Defaults to whois.

The following variables determine how traceroute attempts to extract an ASN from the whois reply.

DATA_DELIMITER

Each line containing an ASN starts with this tag. Defaults to origin:.

The RADB may contain more than one entry for a given IP address. To find out the correct entry, traceroute has to look up the subnet that is the most specific to this IP.

ROUTE_DELIMITER

Each line containing a subnet entry starts with this tag. Defaults to route:.

PREFIX_DELIMITER

The network IP and the prefix are separated by this tag. Defaults to /.

This is not the standard version of traceroute (as included in the netkit package), but an alternative implementation maintained by Ehud Gavron. It is based on the Van Jacobson/BSD traceroute, and includes additional features including AS lookup, TOS support, microsecond timestamps, path MTU discovery, and parallel probing. It is known as trACESroute or traceroute-nanog.

mtr(8), netstat(8), pchar(8), ping(8)

Please send any bugs to Ehud Gavron <gavron@wetwork.net> and/or report them to the Debian Bug Tracking System at http://bugs.debian.org/traceroute-nanog.

TrACESroute is maintained by Ehud Gavron <ehud.gavron@login.com>.

The first man page was written by Brian Russo for use with Debian/GNU, and was later rewritten by Daniel Kobras <kobras@debian.org> and Martin A. Godisch <martin@godisch.de>. Some parts are taken from the documentation in the source code. Still, this man page may be used by others.