traceroute(8) — Unsupported
Name
traceroute − print the route that packets take to the network host
Syntax
/usr/etc/traceroute [ −m max_ttl ] [ −n ] [ −p port ] [ −q nqueries ] [ −r ] [ −s src_addr ] [ −t tos ] [ −w ] [ −w waittime ] host [ packetsize ]
Description
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 is discarding your packets) can be difficult. The traceroute command uses the IP protocol ‘time to live’ field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to a particular host.
The only mandatory parameter is the destination host name or IP number. The default probe datagram length is 38 bytes, but this may be increased by specifying a packet size (in bytes) after the destination host name.
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 and can be changed with the −m flag). Three probes (which can be changed with the −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 3 second timeout interval (which can be changed with the −w flag), a "*" is printed for that probe.
We do not want the destination host to process the UDP probe packets so the destination port is set to an unlikely value. The destination port value can be changed with the −p flag, if necessary.
Options
Additional traceroute options are:
−m Sets the max time-to-live (max number of hops) used in outgoing probe packets. The default is 30 hops which is the same default used for TCP connections.
−n Prints hop addresses numerically rather than symbolically and numerically. This saves a nameserver address-to-name lookup for each gateway found on the path.
−p Sets the base UDP port number used in probes (default is 33434). The traceroute command presumes 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 another process islistening on a port in the default range, this option can be used to pick an unused port range.
−r Bypasses the normal routing tables and sends 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 (for example, after the interface was dropped by routed(.).
−s Uses the following IP address (which must be given as an IP number, not a hostname) as the source address in outgoing probe packets. On hosts 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 on which the probe packet is sent. If the IP address is not one of this machine’s interface addresses, an error is returned and nothing is sent.
−t Sets 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 option is not relevant to you because services like telnet and ftp do not let you control the type-of-service .) Not all values of type-of-service are legal or meaningful − 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 are listed.
−w Sets the time (in seconds) to wait for a response to a probe. The default is 3 seconds.
Examples
The following command traces the route a packet takes from localhost to the host nis.nsf.net:
localhost> 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 and 3 are identical. This is due to a bug in the 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) does not supply address-to-name translations for its NSSes.
The following is another example of output from the traceroute command. Packets from localhost to the host allspice.lcs.mit.edu are being traced:
localhost> 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
Note that the gateways 12, 14, 15, 16 and 17 hops away either do not send ICMP "time exceeded" messages or send them with a ttl too small to reach localhost. Gateways 14 through 17 are running the MIT C Gateway code that does not send "time exceeded" messages. It is unclear what is happening with gateway 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.
When this bug appears on the destination system it behaves as follows:
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 !
Note that there are 12 "gateways" (13 is the final destination) and the last half of them are "missing". What is happening is that the host rip (a Sun-3 running Sun OS3.5) is using the ttl from our arriving datagram as the ttl in its ICMP reply. The reply will time out on the return path (with no notice sent to anyone since ICMP’s are not sent for ICMP’s) until we probe with a ttl that is at least twice the path length. This means that the host rip is really only 7 hops away.
A reply that returns with a ttl of 1 is a clue this problem exists. The traceroute prints a "!" after the time if the ttl is less than or equal to 1. Since many systems continue to run obsolete or non-standard software, expect to see this problem frequently.
Other possible annotations after the time are !H, !N, !P (got a host, network or protocol unreachable, respectively), !S or !F (source route failed or fragmentation needed − if either of these occurs, the associated gateway is broken). If almost all the probes result in some kind of unreachable, the traceroute command 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, you should not use traceroute during normal operations or from automated scripts.