Join GitHub today
GitHub is home to over 20 million developers working together to host and review code, manage projects, and build software together.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
|Failed to load latest commit information.|
traceroute host - trace the route ip packets follow going to "host". Attempt to trace the route an ip packet would follow to some internet host. We find out intermediate hops by launching 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 net.inet.ip.ttl 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. Probe packets are UDP format. We don't want the destination host to process them 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 might be: [yak 71]% traceroute nis.nsf.net. traceroute to nis.nsf.net (220.127.116.11), 64 hops max, 56 byte packet 1 helios.ee.lbl.gov (18.104.22.168) 19 ms 19 ms 0 ms 2 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 39 ms 19 ms 3 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 19 ms 4 ccngw-ner-cc.Berkeley.EDU (188.8.131.52) 39 ms 40 ms 39 ms 5 ccn-nerif22.Berkeley.EDU (184.108.40.206) 39 ms 39 ms 39 ms 6 220.127.116.11 (18.104.22.168) 40 ms 59 ms 59 ms 7 22.214.171.124 (126.96.36.199) 59 ms 59 ms 59 ms 8 188.8.131.52 (184.108.40.206) 99 ms 99 ms 80 ms 9 220.127.116.11 (18.104.22.168) 139 ms 239 ms 319 ms 10 22.214.171.124 (126.96.36.199) 220 ms 199 ms 199 ms 11 nic.merit.edu (188.8.131.52) 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 (184.108.40.206), 64 hops max 1 helios.ee.lbl.gov (220.127.116.11) 0 ms 0 ms 0 ms 2 lilac-dmc.Berkeley.EDU (18.104.22.168) 19 ms 19 ms 19 ms 3 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 19 ms 19 ms 4 ccngw-ner-cc.Berkeley.EDU (126.96.36.199) 19 ms 39 ms 39 ms 5 ccn-nerif22.Berkeley.EDU (188.8.131.52) 20 ms 39 ms 39 ms 6 184.108.40.206 (220.127.116.11) 59 ms 119 ms 39 ms 7 18.104.22.168 (22.214.171.124) 59 ms 59 ms 39 ms 8 126.96.36.199 (188.8.131.52) 80 ms 79 ms 99 ms 9 184.108.40.206 (220.127.116.11) 139 ms 139 ms 159 ms 10 18.104.22.168 (22.214.171.124) 199 ms 180 ms 300 ms 11 126.96.36.199 (188.8.131.52) 300 ms 239 ms 239 ms 12 * * * 13 184.108.40.206 (220.127.116.11) 259 ms 499 ms 279 ms 14 * * * 15 * * * 16 * * * 17 * * * 18 ALLSPICE.LCS.MIT.EDU (18.104.22.168) 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.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 (22.214.171.124) 0 ms 0 ms 0 ms 2 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 19 ms 39 ms 3 lilac-dmc.Berkeley.EDU (188.8.131.52) 19 ms 39 ms 19 ms 4 ccngw-ner-cc.Berkeley.EDU (184.108.40.206) 39 ms 40 ms 19 ms 5 ccn-nerif35.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 39 ms 6 csgw.Berkeley.EDU (18.104.22.168) 39 ms 59 ms 39 ms 7 * * * 8 * * * 9 * * * 10 * * * 11 * * * 12 * * * 13 rip.Berkeley.EDU (22.214.171.124) 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, !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). If almost all the probes result in some kind of unreachable, traceroute will give up and exit. Notes ----- This program must be run by root or be setuid. (I suggest that you *don't* make it setuid -- casual use could result in a lot of unnecessary traffic on our poor, congested nets.) This program requires a kernel mod that does not appear in any system available from Berkeley: A raw ip socket using proto IPPROTO_RAW must interpret the data sent as an ip datagram (as opposed to data to be wrapped in a ip datagram). See the README file that came with the source to this program for a description of the mods I made to /sys/netinet/raw_ip.c. Your mileage may vary. But, again, ANY 4.x (x < 4) BSD KERNEL WILL HAVE TO BE MODIFIED TO RUN THIS PROGRAM. The udp port usage may appear bizarre (well, ok, it is bizarre). The problem is that an icmp message only contains 8 bytes of data from the original datagram. 8 bytes is the size of a udp header so, if we want to associate replies with the original datagram, the necessary information must be encoded into the udp header (the ip id could be used but there's no way to interlock with the kernel's assignment of ip id's and, anyway, it would have taken a lot more kernel hacking to allow this code to set the ip id). So, to allow two or more users to use traceroute simultaneously, we use this task's pid as the source port (the high bit is set to move the port number out of the "likely" range). To keep track of which probe is being replied to (so times and/or hop counts don't get confused by a reply that was delayed in transit), we increment the destination port number before each probe. Don't use this as a coding example. I was trying to find a routing problem and this code sort-of popped out after 48 hours without sleep. I was amazed it ever compiled, much less ran. I stole the idea for this program from Steve Deering. Since the first release, I've learned that had I attended the right IETF working group meetings, I also could have stolen it from Guy Almes or Matt Mathis. I don't know (or care) who came up with the idea first. I envy the originators' perspicacity and I'm glad they didn't keep the idea a secret. Tim Seaver, Ken Adelman and C. Philip Wood provided bug fixes and/or enhancements to the original distribution. I've hacked up a round-trip-route version of this that works by sending a loose-source-routed udp datagram through the destination back to yourself. Unfortunately, SO many gateways botch source routing, the thing is almost worthless. Maybe one day... -- Van Jacobson (email@example.com) Tue Dec 20 03:50:13 PST 1988