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3 JohnMcPherson 1 See also the PublicTraceroutes page on this wiki.
2
1 perry 3 ----
4 !!NAME
5 traceroute - print the route packets take to network host
3 JohnMcPherson 6
1 perry 7 !!SYNOPSIS
3 JohnMcPherson 8 __traceroute__ [[ __-dFIlnrvx__ ] [[ __-f__ ''first_ttl'' ] [[ __-g__ ''gateway'' ]
9 [[ __-i__ ''iface'' ] [[ __-m__ max_ttl ] [[ __-p__ ''port'' ]
10 [[ __-q__ ''nqueries'' ] [[ __-s__ ''src_addr'' ] [[ __-t__ ''tos'' ]
11 [[ __-w__ ''waittime'' ] [[ __-z__ ''pausemsecs'' ]
12 ''host'' [[ ''packetlen'' ]
1 perry 13
14 !!DESCRIPTION
3 JohnMcPherson 15 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.
1 perry 16
3 JohnMcPherson 17 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.
1 perry 18
19 Other options are:
3 JohnMcPherson 20 ;__-f__ : Set the initial time-to-live used in the first outgoing probe packet.
21 ;__-F__ : Set the "don't fragment" bit.
22 ;__-d__ : Enable socket level debugging.
23 ;__-g__ : Specify a loose source route gateway (8 maximum).
24 ;__-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.)
25 ;__-I__ : Use ICMP ECHO instead of [UDP] datagrams.
26 ;__-l__ : Display the ttl value of the returned packet. This is useful for checking for assymetric routing.
27 ;__-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).
28 ;__-n__ : Print hop addresses numerically rather than symbolically and numerically (saves a nameserver address-to-name lookup for each gateway found on the path).
29 ;__-p__ : Set the base UDP port number used in probes (default is 33434). Traceroute hopes that nothing is listening on UDP ports ''base + 1'' to ''base + nhops'' 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.
30 ;__-r__ : Bypass the normal routing tables and send 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 (e.g., after the interface was dropped by routed(8C)).
31 ;__-s__ : 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. This option can only be used by the super-user. (See the __-i__ flag for another way to do this.)
32 ;__-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 meaningful - see the IP spec for definitions. Useful values are probably ` __-t__ ''16'' ' (low delay) and ` __-t__ ''8'' ' (high throughput).
33 ;__-v__ : Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs are listed.
34 ;__-w__ : Set the time (in seconds) to wait for a response to a probe (default 5 sec.).
35 ;__-x__ : Toggle ip checksums. Normally, this prevents traceroute from calculating ip 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__ ). So they are always calculated when using ICMP.
36 ;__-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).
1 perry 37
3 JohnMcPherson 38 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.
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3 JohnMcPherson 40 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).
1 perry 41
3 JohnMcPherson 42 A sample use and output might be:
1 perry 43
44
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3 JohnMcPherson 46 [[yak 71]% traceroute nis.nsf.net.
47 traceroute to nis.nsf.net (35.1.1.48), 30 hops max, 38 byte packet
1 perry 48
3 JohnMcPherson 49 1 helios.ee.lbl.gov (128.3.112.1) 19 ms 19 ms 0 ms
50 2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
51 3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
52 4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 39 ms
53 5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 39 ms 39 ms 39 ms
54 6 128.32.197.4 (128.32.197.4) 40 ms 59 ms 59 ms
55 7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 59 ms
56 8 129.140.70.13 (129.140.70.13) 99 ms 99 ms 80 ms
57 9 129.140.71.6 (129.140.71.6) 139 ms 239 ms 319 ms
58 10 129.140.81.7 (129.140.81.7) 220 ms 199 ms 199 ms
59 11 nic.merit.edu (35.1.1.48) 239 ms 239 ms 239 ms
1 perry 60
3 JohnMcPherson 61 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.
1 perry 62
3 JohnMcPherson 63 A more interesting example is:
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3 JohnMcPherson 66 [[yak 72]% traceroute allspice.lcs.mit.edu.
67 traceroute to allspice.lcs.mit.edu (18.26.0.115), 30 hops max
1 perry 68
3 JohnMcPherson 69 1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
70 2 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 19 ms 19 ms
71 3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 19 ms
72 4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 19 ms 39 ms 39 ms
73 5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 20 ms 39 ms 39 ms
74 6 128.32.197.4 (128.32.197.4) 59 ms 119 ms 39 ms
75 7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 39 ms
76 8 129.140.70.13 (129.140.70.13) 80 ms 79 ms 99 ms
77 9 129.140.71.6 (129.140.71.6) 139 ms 139 ms 159 ms
78 10 129.140.81.7 (129.140.81.7) 199 ms 180 ms 300 ms
79 11 129.140.72.17 (129.140.72.17) 300 ms 239 ms 239 ms
80 12 * * *
81 13 128.121.54.72 (128.121.54.72) 259 ms 499 ms 279 ms
82 14 * * *
83 15 * * *
84 16 * * *
85 17 * * *
86 18 ALLSPICE.LCS.MIT.EDU (18.26.0.115) 339 ms 279 ms 279 ms
1 perry 87
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3 JohnMcPherson 90 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 perry 91
3 JohnMcPherson 92 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. The behavior of this bug is slightly more interesting when it appears on the destination system:
1 perry 93
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3 JohnMcPherson 97 1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
98 2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 39 ms
99 3 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 39 ms 19 ms
100 4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 19 ms
101 5 ccn-nerif35.Berkeley.EDU (128.32.168.35) 39 ms 39 ms 39 ms
102 6 csgw.Berkeley.EDU (128.32.133.254) 39 ms 59 ms 39 ms
103 7 * * *
104 8 * * *
105 9 * * *
106 10 * * *
107 11 * * *
108 12 * * *
109 13 rip.Berkeley.EDU (128.32.131.22) 59 ms ! 39 ms ! 39 ms !
1 perry 110
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3 JohnMcPherson 114 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.
1 perry 115
3 JohnMcPherson 116 Other possible annotations after the time are __!H__ , __!N__ , or __!P__ (host, network or protocol unreachable), __!A__ , __!C__ (access to the network or host, respectively, is prohibited), __!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.
1 perry 117
3 JohnMcPherson 118 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 scripts.
1 perry 119
3 JohnMcPherson 120 !!DIAGNOSTICS
121 If your kernel does not support rtnetlink (routing messages), you will get a warning of the form
1 perry 122
3 JohnMcPherson 123 traceroute: Warning: findsaddr: error sending netlink message: <reason>
124 traceroute: Warning: ip checksums disabled
1 perry 125
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3 JohnMcPherson 127 This is harmless as IP checksums will be provided by the kernel, and UDP checksums (which are also disabled when rtnetlink is unavailable) are optional.
1 perry 128
129 !!SEE ALSO
3 JohnMcPherson 130 pchar(8), netstat(1), ping(8)
1 perry 131
132 !!AUTHOR
3 JohnMcPherson 133 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.
1 perry 134
3 JohnMcPherson 135 The current version is available via anonymous ftp:
1 perry 136
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3 JohnMcPherson 138 ''ftp://ftp.ee.lbl.gov/traceroute.tar.gz''
1 perry 139
140
141 !!BUGS
3 JohnMcPherson 142 Please send bug reports to traceroute@ee.lbl.gov.
1 perry 143
3 JohnMcPherson 144 4.3 Berkeley Distribution 21 September 2000 traceroute(8)
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