TRACEROUTE(1M) TRACEROUTE(1M)
NAME
traceroute - print the route packets take to a network host
SYNOPSIS
/usr/etc/traceroute [ -g addr ] [ -l ] [ -m max_ttl ]
[ -M min_ttl ] [ -n ] [ -p port ] [ -q nqueries ]
[ -r ] [ -s src_addr ] [ -t tos ] [ -w waittime ]
host [ datalen ]
DESCRIPTION
The Internet is a large and complex aggregation of network hardware,
connected by gateways. Tracking the route your packets follow (or
finding the miscreant gateway that's discarding your packets) can be
difficult. traceroute utilizes the IP protocol ``time-to-live'' (TTL)
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 address.
The default probe datagram length is 40 bytes, but this may be increased
by specifying the additional length (in bytes) after the destination host
name.
The options are:
-g Enable the IP LSRR (Loose Source Record Route) option in addition to
the TTL tests. This is useful for asking how somebody else, at
addr, (either an IP address or a hostname) reaches a particular
target.
-l Print the value of the TTL field in each received packet (this can
be used to help detect asymmetric routing).
-m Set the maximum time-to-live (maximum number of hops) used in
outgoing probe packets. The default is 30 hops or the minimum TTL
plus 1, whichever is larger.
-M Set the minimum time-to-live used in outgoing probe packets. The
default is 1 hop.
-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.
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-q Set the number of probe packets to send. The default is 3 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 (for
example, after the interface was dropped by routed(1M)).
-s Use the following IP address (which must be given as a 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 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.
-t Set the type-of-service (TOS) 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. Not all values of TOS are
legal or meaningful: see the IP RFC 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
PORT_UNREACHABLEs are listed.
-w Set the time (in seconds) to wait for a response to a probe (default
is 3 seconds).
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 then,
listening for an ICMP TIME_EXCEEDED reply from a gateway. The probes
begin with a TTL of one and increase by one until an ICMP
PORT_UNREACHABLE message is received, which means we got to ``host'' or
hit the maximum (which defaults to 30 hops but 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
gateways, the address of each responding system will be printed. If
there is no response within a 3-second timeout interval (changed with the
-w flag), a ``*'' is printed for that probe.
So that the destination host will not process the UDP probe packets, the
destination port is set to an unlikely value. If someone on the
destination is using that value, it can be changed with the -p flag.
A sample use and output might be:
% 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
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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
Notice that lines 2 and 3 are the same because of a buggy kernel on the
second hop system - lbl-csam.arpa - that forwards packets with a zero TTL
(a bug in the distributed version of 4.3BSD). 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:
% 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
Notice that the gateways 12, 14, 15, 16 and 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_EXCEEDEDs.
The silent gateway 12 in the above example 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:
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% traceroute rip.berkeley.edu
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 !
Notice of the 12 ``gateways'' (13 is the final destination), exactly the
half of them are ``missing''. In this example, rip, a Sun-3 running Sun
OS3.5, is using the TTL from the arriving datagram as the TTL in its ICMP
reply. The reply will then time out on the return path, with no notice
sent to anyone since ICMP packets aren't sent for ICMP packets, until we
probe with a TTL that's at least twice the path length - that is, 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 some vendors ship obsolete or nonstandard software,
expect to see this problem frequently and/or take care selecting 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 broken if you see one). If almost all the
probes result in some kind of unreachable, traceroute will give up and
exit.
(ttl=n!) indicates that the TTL value in the ICMP TIME_EXCEEDED packet
that we received was "unexpected". What we expect is that the value will
be (some initial value - the number of routers between us). In other
words, if the path from hop 5 to us is the same as the path from us to
hop 5, we expect to receive a TTL value of (some initial value - 4).
Unfortunately, there are several common "initial value"s for ICMP TTLs.
The most common are 255, 60, 59, 30, 29. (IRIX, 4.3BSD-tahoe and cisco
routers use 255, Proteon routers use either 59 or 29 depending on
software release, several other implementations use 60 and 30.)
Traceroute checks against all of these, making it hard to detect some
"off by one" routing asymmetries. If you want to see all the TTL values
in all the packets, use the -l option.
For example,
% traceroute -g 10.3.0.5 128.182.0.0
will show the path from the Cambridge Mailbridge to PSC while
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% traceroute -g 192.5.146.4 -g 10.3.0.5 35.0.0.0
shows how the Cambridge Mailbrige reaches Merit, by 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. It
is unwise to use traceroute during normal operations or from automated
scripts due to the load it could impose on the network.
AUTHORS
Van Jacobson, Steve Deering, C. Philip Wood, Tim Seaver, and Ken Adelman.
SEE ALSO
netstat(1), ping(1M)
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