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 ]

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 gateway along the path to some host.


Other options are:




Set the "don’t fragment" bit.


Enable socket level debugging.


Specify a loose source route gateway (8 maximum).


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.)


Use ICMP ECHO instead of UDP datagrams.










Use the following IP address (which usually is given as an IP number, not a hostname) 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.)


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 meaningful − see the IP spec for definitions. Useful values are probably ‘-t 16’ (low delay) and ‘-t 8’ (high throughput).






Toggle checksums. Normally, this prevents traceroute from calculating checksums. 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).

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 probe.

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
traceroute to (, 30 hops max, 38 byte packet
 1 (  19 ms  19 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
 3  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
 6 (  40 ms  59 ms  59 ms
 7 (  59 ms  59 ms  59 ms
 8 (  99 ms  99 ms  80 ms
 9 (  139 ms  239 ms  319 ms
10 (  220 ms  199 ms  199 ms
11 (  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 − − 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
traceroute to (, 30 hops max
 1 (  0 ms  0 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (  19 ms  19 ms  19 ms
 3  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  19 ms
 6 (  59 ms  119 ms  39 ms
 7 (  59 ms  59 ms  39 ms
 8 (  80 ms  79 ms  99 ms
 9 (  139 ms  139 ms  159 ms
10 (  199 ms  180 ms  300 ms
11 (  300 ms  239 ms  239 ms
12  * * *
13 (  259 ms  499 ms  279 ms
14  * * *
15  * * *
16  * * *
17  * * *
18  ALLSPICE.LCS.MIT.EDU (  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 with 12.


 1 (  0 ms  0 ms  0 ms
 2  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  39 ms
 3  lilac-dmc.Berkeley.EDU (  19 ms  39 ms  19 ms
 6  csgw.Berkeley.EDU (  39 ms  59 ms  39 ms
 7  * * *
 8  * * *
 9  * * *
10  * * *
11  * * *
12  * * *
13  rip.Berkeley.EDU (  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 problem 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 (got a host, network or protocol unreachable, respectively), !S or !F (source route failed or fragmentation needed − neither of these should ever occur and the associated gateway is busted if you see one), !X (communication administratively prohibited), or !<N> (ICMP unreachable code N). If almost all the probes result in some kind of unreachable, traceroute will give up and exit.






The current version is available via anonymous ftp:


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