TRACEROUTE(8) |
traceroute - print the route packets take to network host |
traceroute [ -m max_ttl ] [ -n ] [ -p port ] [ -q nqueries ] [ -r ] [ -s src_addr ] |
[ -t tos ] [ -w waittime ] host [ packetsize ] |
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. 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. Other options are: -m max_ttl -n Print hop addresses numerically rather than symbolically and numerically (saves a nameserver ad -p port -q nqueries -r Bypass the normal routing tables and send directly to a host on an attached network. If the host is -s src_addr -t tos -v Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs |
4.3 Berkeley Distribution June 6, 1993 1
TRACEROUTE(8) |
are listed. |
-w Set the time (in seconds) to wait for a response to a probe (default 3 sec.). |
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 gate way. 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 (changed 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 gate ways, the address of each responding system will be printed. If there is no response within a 3 sec. time out 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 un likely 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. Note that lines 2 & 3 are the same. This is due to a buggy kernel on the 2nd hop system - lblcsam.arpa - that forwards packets with a zero ttl (a bug in the distributed version of 4.3 BSD). Note that you have to guess what path the packets are taking crosscountry since the NSFNet (129.140) doesn't supply ad dresstoname translations for its NSSes. A more interesting example is: [yak 72]% traceroute allspice.lcs.mit.edu. |
4.3 Berkeley Distribution |
TRACEROUTE(8) |
18 ALLSPICE.LCS.MIT.EDU (18.26.0.115) 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. 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 data gram. 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 sys tem: 1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms Notice that there are 12 "gateways" (13 is the final destination) and exactly the last half of them are "miss ing". What's really happening is that rip (a Sun3 running Sun OS3.5) is using the ttl from our arriving data gram 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 nonstandard ( 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, !P (got a host, network or proto col 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). 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 pri marily 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 scripts. |
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. |
netstat(1), ping(8) |
The traceroute command is currently in beta test. |
4.3 Berkeley Distribution |