SYNOPSIS
traceroute parameters
The traceroute command displays the route that packets take to a
network host.
1 – FLAGS
-A Looks 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 probes all addresses if this option is
set. Normally only the first address as returned by
the resolver is attempted.
-c Specifies a delay (in seconds) to pause between
stoptime probe packets. This may be necessary if the final
destination is a router that does not accept
undeliverable packets in bursts.
-f Disables 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 and the indicator !F is printed. If a
gate-way returns the value of the proper MTU size
to be used, traceroute decreases the packet size
automatically to this new value. If the proper MTU
size is not returned, traceroute chooses a shorter
packet size.
-g Enables the IP LSRR (Loose Source Record Route)
gateway option. This is useful for asking how somebody else,
at the specified gateway, reaches a particular target.
-h server Specifies the name or IP address of the whois server
that is contacted for the AS-number lookup, if the -A
option is given.
-i Sets the starting time-to-live value to initial_ttl,
initial_ to override the default value of 1. Effectively this
ttl skips processing for those intermediate hosts that are
less than initial_ttl hops away.
-k Keeps the connection to the whois server permanently
open. This makes lookups considerably quicker, because
connection setup for each individual lookup is not
necessary. However, all whois servers do not support
this feature.
-l Prints the value of the ttl field in each received
packet (this can be used to help detect asymmetric
routing).
-m max_ Sets the max time-to-live (max number of hops) used in
ttl outgoing probe packets. The default is 30 hops, which
is the same default used for TCP connections.
-N Displays the network name for each hop. If a BIND
resolver cannot be reached, network names are
retrieved just from the /etc/networks file.
-n Prints hop IP addresses numerically rather than both
symbolically and numerically. This 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 Sets the base UDP port number used in probes (default
is 33434). The traceroute command presumes that
nothing is listening on UDP ports base to base+nhops-1
at the destination host (so an ICMP "port unreachable"
message is returned to terminate the route tracing).
If another process is listening on a port in the
default range, use this option to pick an unused port
range.
-Q Stops probing this hop after maxquit consecutive
maxquit timeouts are detected. The default value is 5. Useful
in combination with -S if you have specified a big
nqueries probe count.
-q Sets the number of probes launched at each ttl setting
nqueries (default is 3).
-r Bypasses the normal routing tables and sends 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(8) or gated(8)).
-S Prints a per-hop minimum/average/maximum rtt (round-
trip time) 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 the -Q option.
-s Uses the following IP address (which must be given as
source_ an IP number, not a hostname) as the source address
addr in outgoing probe packets. On hosts with more than
one IP address, use this option to force the source
address to be something other than the IP address of
the interface on which the probe packet is sent. If
the IP address is not one of this machine's interface
addresses, an error is returned and nothing is sent.
-t tos Sets 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. Use this option
to determine if different types-of-service result in
different paths. Not all values of TOS are legal or
meaningful. See the IP specification for definitions.
Useful values are probably -t 16 (low delay) and -t 8
(high throughput).
-v Produces verbose output. Lists any received ICMP
packets other than "time exceeded" and "unreachable".
-w Sets the time (in seconds) to wait for a response to a
waittime probe. The default is 3 seconds.
2 – DESCRIPTION
The Internet is a large and complex aggregation of network
hardware, connected together by gateways. The traceroute command
tracks the route packets follow from gateway to gateway. The
command uses the IP protocol `time to live' field and attempts to
elicit an ICMP "time exceeded" response from each gateway along
the path to a particular host.
The only mandatory parameter is the destination host name or IP
number. The default probe datagram length is 38 bytes, but you
can increase this by specifying a packet size (in bytes) after
the destination host name. This is useful when the -f option is
given for MTU discovery along the route. You should start with
the maximum packet size for your own network interface (if the
given value is even bigger, traceroute attempts to select a more
appropriate value). If no packet size is given when using the -f
option, traceroute determines the initial MTU automatically.
To track the route of an IP packet, traceroute launches UDP probe
packets with a small ttl (time to live) and then listens for an
ICMP "time exceeded" reply from a gateway. Probes start with
a ttl of one and increase by one until either an ICMP "port
unreachable" is returned (indicating that the packet reached
the host) or the maximum number of hops is exceeded (the default
is 30 hops and can be changed with the -m option). At each ttl
setting, traceroute launches three probes (you can change the
number with the -q option) and prints a line showing the ttl,
address of the gateway, and round trip time of each probe. If the
probe answers come from different gateways, traceroute prints the
address of each responding system. If there is no response within
a 3 second timeout interval (which you can change with the -w
option), an asterisk (*) is printed for that probe.
To prevent the destination host from processing the UDP probe
packets, the destination port is set to an unlikely value. You
can change the destination port value with the -p option, if
necessary.
3 – Special Annotations
Other possible annotations after the time are:
!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.
This should not occur under normal circumstances and the
associated gateway might be broken 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 in 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 Some 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.
If all the probes result in an unreachable status,
traceroute stops sending probes and exits.
4 – TTL Indication
(ttl=n!)
This indicates that the ttl value in the ICMP "time exceeded"
packet that we received was unexpected. We expected some initial
value, for example, the number of routers between our system and
another system. 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 4.
There are several common initial values for ICMP ttls: 255, 60,
59, 30 and 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. Tru64 UNIX uses an
initial ttl of 64. The traceroute command checks against all of
these, making it hard to detect some small routing asymmetries.
If you want to see the ttl values in all the packets, use the -l
option.
NOTE
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, do not use traceroute during normal
operations or from automated scripts.
5 – Examples
1. The following command traces the route a packet takes from
localhost to the host nis.nsf.net:
localhost> 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
Note that lines 2 and 3 are identical. This is due to a bug in the
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). The
NSFNet (129.140) does not supply address-to-name translations for
its NSSes. Therefore, you cannot be certain of the path the packets
take cross-country.
2. The following is another example of output from the
traceroute com mand. Packets from localhost to the host
allspice.lcs.mit.edu are being traced:
localhost> 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 and 17 hops away either
do not send ICMP "time exceeded" messages or send them with
a ttl too small to reach localhost. Further investigation is
required to determine the cause. For example, by contacting
the system administrators for gateways 14 through 17, you
could discover that these gateways are running the MIT C
Gateway code that does not send "time exceeded" messages.
The silent gateway 12 in the 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.
When this bug appears on the destination system it behaves as
follows:
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 !
Note that there are 12 gateways (13 is the final destination
and the last half of them are missing. What is happening is
that the host rip (a Sun-3 running Sun OS3.5) is using the ttl
from our arriving datagram as the ttl in its ICMP reply. The
reply will time out on the return path (with no notice sent to
anyone since ICMPs are not sent for ICMPs) until we probe with
a ttl that is at least twice the path length. This means that
the host rip is really only 7 hops away. A reply that returns
with a ttl of 1 is a clue this problem exists. The traceroute
command prints a ! after the time if the ttl is less than or
equal to 1. Since many systems continue to run obsolete or
non-standard software, expect to see this problem frequently.