traceroute - print the route packets take to network host
traceroute [ -dFInrvx ] [ -f first_ttl ] [ -g gateway ]
[ -i iface ] [ -m max_ttl ] [ -p port ]
[ -q nqueries ] [ -s src_addr ] [ -t tos ]
[ -w waittime ] [ -z pausemsecs ]
host [ packetlen ]
The Internet is a large and complex aggregation of network hardware, connected together by
gateways. Tracking the route one's packets follow (or finding the miscreant gateway
that's discarding your packets) can be difficult. Traceroute utilizes the IP protocol
`time to live' field and attempts to elicit an ICMP TIME_EXCEEDED response from each gate-
way along the path to some host.
The only mandatory parameter is the destination host name or IP number. The default probe
datagram length is 40 bytes, but this may be increased by specifying a packet length (in
bytes) after the destination host name.
Other options are:
-f Set the initial time-to-live used in the first outgoing probe packet.
-F Set the "don't fragment" bit.
-d Enable socket level debugging.
-g Specify a loose source route gateway (8 maximum).
-i Specify a network interface to obtain the source IP address for outgoing probe
packets. This is normally only useful on a multi-homed host. (See the -s flag for
another way to do this.)
-I Use ICMP ECHO instead of UDP datagrams.
-m Set the max time-to-live (max number of hops) used in outgoing probe packets. The
default is 30 hops (the same default used for TCP connections).
-n Print hop addresses numerically rather than symbolically and numerically (saves a
nameserver address-to-name lookup for each gateway found on the path).
-p Set the base UDP port number used in probes (default is 33434). Traceroute hopes
that nothing is listening on UDP ports base to base + nhops - 1 at the destination
host (so an ICMP PORT_UNREACHABLE message will be returned to terminate the route
tracing). If something is listening on a port in the default range, this option
can be used to pick an unused port range.
-r Bypass the normal routing tables and send directly to a host on an attached net-
work. 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 (e.g., after the interface was dropped by routed(8C)).
-s Use the following IP address (which usually is given as an IP number, not a host-
name) as the source address in outgoing probe packets. On multi-homed hosts (those
with more than one IP address), this option can be used to force the source address
to be something other than the IP address of the interface the probe packet is sent
on. If the IP address is not one of this machine's interface addresses, an error
is returned and nothing is sent. (See the -i flag for another way to do this.)
-t Set the type-of-service in probe packets to the following value (default zero).
The value must be a decimal integer in the range 0 to 255. This option can be used
to see if different types-of-service result in different paths. (If you are not
running 4.4bsd, this may be academic since the normal network services like telnet
and ftp don't let you control the TOS). Not all values of TOS are legal or mean-
ingful - see the IP spec for definitions. Useful values are probably `-t 16' (low
delay) and `-t 8' (high throughput).
-v Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs
-w Set the time (in seconds) to wait for a response to a probe (default 5 sec.).
-x Toggle ip checksums. Normally, this prevents traceroute from calculating ip check-
sums. In some cases, the operating system can overwrite parts of the outgoing
packet but not recalculate the checksum (so in some cases the default is to not
calculate checksums and using -x causes them to be calcualted). Note that checksums
are usually required for the last hop when using ICMP ECHO probes (-I). So they
are always calculated when using ICMP.
-z Set the time (in milliseconds) to pause between probes (default 0). Some systems
such as Solaris and routers such as Ciscos rate limit icmp messages. A good value
to use with this this is 500 (e.g. 1/2 second).
This program attempts to trace the route an IP packet would follow to some internet host
by launching UDP 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" (which means we got to "host") or hit a max
(which defaults to 30 hops & can be changed with the -m flag). Three probes (change with
-q flag) 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. 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 sec. timeout interval (changed with the -w flag), a "*" is printed for that
We don't want the destination host to process the UDP probe packets so the destination
port is set to an unlikely value (if some clod on the destination is using that value, it
can be changed with the -p flag).
A sample use and output might be:
[yak 71]% traceroute nis.nsf.net.
traceroute to nis.nsf.net (18.104.22.168), 30 hops max, 38 byte packet
1 helios.ee.lbl.gov (22.214.171.124) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (184.108.40.206) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 39 ms
6 18.104.22.168 (22.214.171.124) 40 ms 59 ms 59 ms
7 126.96.36.199 (188.8.131.52) 59 ms 59 ms 59 ms
8 184.108.40.206 (220.127.116.11) 99 ms 99 ms 80 ms
9 18.104.22.168 (22.214.171.124) 139 ms 239 ms 319 ms
10 126.96.36.199 (188.8.131.52) 220 ms 199 ms 199 ms
11 nic.merit.edu (184.108.40.206) 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 bug in the distributed version of
4.3BSD). Note that you have to guess what path the packets are taking cross-country since
the NSFNet (129.140) doesn't supply address-to-name translations for its NSSes.
A more interesting example is:
[yak 72]% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (220.127.116.11), 30 hops max
1 helios.ee.lbl.gov (18.104.22.168) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (22.214.171.124) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (188.8.131.52) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (184.108.40.206) 20 ms 39 ms 39 ms
6 220.127.116.11 (18.104.22.168) 59 ms 119 ms 39 ms
7 22.214.171.124 (126.96.36.199) 59 ms 59 ms 39 ms
8 188.8.131.52 (184.108.40.206) 80 ms 79 ms 99 ms
9 220.127.116.11 (18.104.22.168) 139 ms 139 ms 159 ms
10 22.214.171.124 (126.96.36.199) 199 ms 180 ms 300 ms
11 188.8.131.52 (184.108.40.206) 300 ms 239 ms 239 ms
12 * * *
13 220.127.116.11 (18.104.22.168) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (22.214.171.124) 339 ms 279 ms 279 ms
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
The silent gateway 12 in the above may be the result of a bug in the 4.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 (126.96.36.199) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (184.108.40.206) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (220.127.116.11) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (18.104.22.168) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (22.214.171.124) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (126.96.36.199) 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 (with no notice sent to anyone since ICMP's aren't
sent for ICMP's) until we probe with a ttl that's at least twice the path length. I.e.,
rip is really only 7 hops away. A reply that returns with a ttl of 1 is a clue this prob-
lem exists. Traceroute prints a "!" after the time if the ttl is <= 1. Since vendors
ship a lot of obsolete (DEC's Ultrix, Sun 3.x) or non-standard (HPUX) software, expect to
see this problem frequently and/or take care picking the target host of your probes.
Other possible annotations after the time are !H, !N, or !P (host, network or protocol
unreachable), !S (source route failed), !F-<pmtu> (fragmentation needed - the RFC1191 Path
MTU Discovery value is displayed), !X (communication administratively prohibited), !V
(host precedence violation), !C (precedence cutoff in effect), or !<num> (ICMP unreachable
code <num>). These are defined by RFC1812 (which supersedes RFC1716). If almost all the
probes result in some kind of unreachable, traceroute will give up and exit.
This program is intended for use in network testing, measurement and management. It
should be used primarily for manual fault isolation. Because of the load it could impose
on the network, it is unwise to use traceroute during normal operations or from automated
pathchar(8), netstat(1), ping(8)
Implemented by Van Jacobson from a suggestion by Steve Deering. Debugged by a cast of
thousands with particularly cogent suggestions or fixes from C. Philip Wood, Tim Seaver
and Ken Adelman.
The current version is available via anonymous ftp:
Please send bug reports to firstname.lastname@example.org.
4.3 Berkeley Distribution 21 September 2000 TRACEROUTE(8)