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traceroute - print the route packets take to network host
traceroute [ -aDFPIdlMnrvx ] [ -f first_ttl ]
[ -g gateway ] [ -i iface ] [ -m max_ttl ]
[ -p port ] [ -q nqueries ] [ -s src_addr ]
[ -t tos ] [ -w waittime ] [ -z pausemsecs
] ] [ -A as_server ]
host [ packetlen ]
The Internet is a large and complex aggregation of network hardware,
connected together by gateways. Tracking the route one's packets fol-
low (or finding the miscreant gateway that's discarding your packets)
can be difficult. Traceroute uses 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 40 bytes, but this may be
increased by specifying a packet length (in bytes) after the destina-
tion host name.
Other options are:
-a Turn on AS# lookups for each hop encountered.
-A Turn on AS# lookups and use the given server instead of the
-d Turn on socket-level debugging.
-D Dump the packet data to standard error before transmitting it.
-f Set the initial time-to-live used in the first outgoing probe
-F Set the "don't fragment" bit.
-g Specify a loose source route gateway (8 maximum).
-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.)
-I Use ICMP ECHO instead of UDP datagrams.
-l Display the ttl value of the returned packet. This is useful
for checking for asymmetric routing.
-m Set the max time-to-live (max number of hops) used in outgoing
probe packets. The default value is taken from the
net.inet.ip.ttl sysctl(3) variable.
-M If found, show the MPLS Label and the Experimental (EXP) bit for
-n Print hop addresses numerically rather than symbolically and
numerically (saves a nameserver address-to-name lookup for each
gateway found on the path).
-p Set the base UDP port number used in probes (default is 33434).
Traceroute hopes 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 something is listening on a port in the default
range, this option can be used to pick an unused port range.
-P Set the "don't fragment" bit, and use the next hop mtu each time
we get the "need fragmentation" error, thus probing the path
-q Set the number of probe packets sent for each hop. By default,
traceroute sends three probe packets.
-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(8)).
-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. 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.)
-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).
-v Verbose output. Received ICMP packets other than TIME_EXCEEDED
and UNREACHABLEs are listed.
-w Set the time (in seconds) to wait for a response to a probe
(default 5 sec.).
-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 calculated). Note that
checksums are usually required for the last hop when using ICMP
ECHO probes (-I). So they are always calculated when using
-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).
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 nis.nsf.net.
traceroute to nis.nsf.net (184.108.40.206), 30 hops max, 38 byte packet
1 helios.ee.lbl.gov (220.127.116.11) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (18.104.22.168) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (126.96.36.199) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (188.8.131.52) 39 ms 39 ms 39 ms
6 184.108.40.206 (220.127.116.11) 40 ms 59 ms 59 ms
7 18.104.22.168 (22.214.171.124) 59 ms 59 ms 59 ms
8 126.96.36.199 (188.8.131.52) 99 ms 99 ms 80 ms
9 184.108.40.206 (220.127.116.11) 139 ms 239 ms 319 ms
10 18.104.22.168 (22.214.171.124) 220 ms 199 ms 199 ms
11 nic.merit.edu (126.96.36.199) 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 - lilac-dmc.Berkeley.EDU - 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 allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (188.8.131.52), 30 hops max
1 helios.ee.lbl.gov (184.108.40.206) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (220.127.116.11) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (18.104.22.168) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (22.214.171.124) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (126.96.36.199) 20 ms 39 ms 39 ms
6 188.8.131.52 (184.108.40.206) 59 ms 119 ms 39 ms
7 220.127.116.11 (18.104.22.168) 59 ms 59 ms 39 ms
8 22.214.171.124 (126.96.36.199) 80 ms 79 ms 99 ms
9 188.8.131.52 (184.108.40.206) 139 ms 139 ms 159 ms
10 220.127.116.11 (18.104.22.168) 199 ms 180 ms 300 ms
11 22.214.171.124 (126.96.36.199) 300 ms 239 ms 239 ms
12 * * *
13 188.8.131.52 (184.108.40.206) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (220.127.116.11) 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.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 destina-
1 helios.ee.lbl.gov (18.104.22.168) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (126.96.36.199) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (188.8.131.52) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (184.108.40.206) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (220.127.116.11) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (18.104.22.168) 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 (HP-UX) soft-
ware, 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 (host, net-
work or protocol unreachable), !S (source route failed), !F-<pmtu>
(fragmentation needed - the RFC1191 Path MTU Discovery value is dis-
played), !X (communication administratively prohibited), !V (host
precedence violation), !C (precedence cutoff in effect), or !<N> (ICMP
unreachable code <num>). These are defined by RFC1812 (which super-
sedes RFC1716). If almost all the probes result in some kind of
unreachable, traceroute will give up and exit.
traceroute -g 10.3.0.5 22.214.171.124
will show the path from the Cambridge Mailbridge to PSC, while
traceroute -g 126.96.36.199 -g 10.3.0.5 188.8.131.52
will show the path from the Cambridge Mailbridge to Merit, using PSC to
reach the Mailbridge.
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.
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.
The current version is available via anonymous ftp:
Please send bug reports to email@example.com.
The AS number capability reports information that may sometimes be
inaccurate due to discrepancies between the contents of the routing
database server and the current state of the Internet.
4.3 Berkeley Distribution 21 September 2000 TRACEROUTE(8)