traceroute(8) traceroute(8)
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NAME
traceroute - print the route packets take to network host
SYNOPSIS
traceroute [-A] [-a] [-c stoptime] [-f] [-g gateway] [-h server]
[-i initial_ttl] [-k] [-l] [-m maxhops] [-N] [-n] [-p port]
[-Q maxquit] [-q nqueries] [-r] [-S] [-s source_addr] [-t tos] [-v]
[-w waittime] host [packetsize]
DESCRIPTION
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. This becomes especially interesting when the -f option is
given for MTU discovery along the route. One should start with the
maximum size for your own network interface (if the given value is
even bigger, traceroute will try to select a more appropriate value).
Other options are:
OPTIONS
-A Look up the AS-number (Autonomous System) for each hop's network
address at the whois server specified by the -h option.
-a If the destination host has multiple addresses, traceroute will
probe all addresses if this option is set. Normally only the
first address as returned by the resolver is attempted.
-c stoptime
Specify a delay (in seconds) to pause between probe packets.
This may be necessary if the final destination is a Cisco router
which does not accept undeliverable packets in bursts.
-f Disable IP fragmentation. If the given packetsize is too big to
be handled unfragmented by a machine along the route, a
``fragmentation needed'' status is returned. The indicator !F is
printed. Some routers will return the value of the proper MTU
size to be used. traceroute will trim down the packet size
automatically to this new value. If the proper MTU size is not
returned, a shorter packet size will be chosen according to the
suggestions in RFC 1191. (Note that this option has no effect
on those platforms which do not allow raw ip socket
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manipulation).
-g gateway
Enable the IP LSRR (Loose Source Record Route) option. This is
useful for asking how somebody else, at the specified gateway,
reaches a particular target.
-h server
Specify the name or IP address of the whois server that will be
contacted for the AS-number lookup, if the -A option is given.
-i initial_ttl
Set the starting time-to-live value to initial_ttl, to override
the default value 1. Effectively this skips processing for those
intermediate hosts which are less than initial_ttl hops away.
-k Keep the connection to the whois server permanently open. This
will speedup lookups considerably, since otherwise a connection
needs to be set up for each individual lookup. Unfortunately,
not all whois servers support this feature.
-l Print the value of the ttl field in each received packet (this
can be used to help detect asymmetric routing).
-m maxhops
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).
-N Display the network name for each hop. This will be useful only
if you have linked traceroute with the BIND 4.9.3 resolver
library, and the network names have been stored in DNS according
to RFC 1101. If this is not the case, network names are
retrieved just from the /etc/networks file.
-n Print hop addresses numerically rather than both symbolically
and numerically (saves a nameserver address-to-name lookup for
each gateway found on the path). It also prevents a reverse
lookup for numeric dotted quad addresses given on the command
line (destination host, or -g gateway addresses).
-p port
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.
-Q maxquit
Stop probing this hop after maxquit consecutive timeouts are
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detected. The default value is 5. Useful in combination with -S
if you have specified a big nqueries probe count.
-q nqueries
Set the desired number of probe queries. The default is 3.
-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)).
-S Print a per-hop min/avg/max rtt statistics summary. This
suppresses the per-probe rtt and ttl reporting. For better
statistics you need to increase the default nqueries probe
count. See also -Q.
-s source_addr
Use the following IP address (which must be given as an IP
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 tos
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 "unreachable" are listed.
-w waittime
Set the time (in seconds) to wait for a response to a probe
(default 3 seconds).
DEFAULT OPTIONS
Default options and parameters can be preset in an environment
variable TRACEROUTE_DEFAULTS using the same syntax as on the command
line. They will be evaluated before the command line arguments.
CUSTOMIZING AS-number LOOKUPS
The built-in defaults for AS-number lookups can be overruled by
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defining three environment variables.
AS_SERVER_HOST
Defines the name of the whois server to be queried. The command
line option -h always takes precedence. If not set, and if no
server is given on the command line, lookups will not be
performed.
AS_SERVER_PORT
Can be used to overrule the default whois port 43. There is no
command line option to define an alternative whois port.
AS_SERVER_OPTIONS
Gives special options to be sent to the whois server for each
lookup. By default, no special options are defined.
INTERNALS
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 (change with -i flag)
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 and
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 3
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).
EXAMPLES
A sample use and output might be:
[yak 71]% 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
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
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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.
A more interesting example is:
[yak 72]% 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
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
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 (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 * * *
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9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (128.32.131.22) 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
SPECIAL ANNOTATIONS
!H Host is unreachable.
!N Network is unreachable.
!P Protocol is unreachable.
!F Fragmentation needed. This indicator may show up if the -f
command line option is being used, and the associated gateway
requires further fragmentation. In case the desired new MTU size
is known, it is indicated.
!S Source route failed. Should not occur under normal
circumstances and the associated gateway is busted if you see
one.
!T Host or network is unreachable for the given tos.
!U Destination is unreachable. This indicator is printed for some
of the new unreachable subcodes as defined per RFC 1812.
!A Some routers fail to generate an ICMP "port unreachable"
message, but send an ICMP "time exceeded" message instead, if
they are the target host. The indicator is printed if this is
detected.
!G Other routers erroneously generate ICMP "port unreachable"
instead of "time exceeded", if they are specified as loose
source route gateway hosts. The indicator is printed if this is
detected.
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If all the probes result in some kind of unreachable, traceroute will
give up and exit.
TTL INDICATION
(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. (4.3 tahoe BSD 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.
OTHER EXAMPLES
traceroute -g 10.3.0.5 128.182.0.0
will show the path from the Cambridge Mailbridge to PSC while
traceroute -g 192.5.146.4 -g 10.3.0.5 35.0.0.0
shows how the Cambridge Mailbridge reaches Merit, by using PSC to
reach the Mailbridge.
WARNING
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.
AUTHOR
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.
Rewritten by Eric Wassenaar, Nikhef-H, <e07@nikhef.nl>
SEE ALSO
netstat(1), ping(8)
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