print the route packets take to network
The Internet is a large and complex aggregation of network hardware, connected
together by gateways. Tracking the route packets follow (or finding the
miscreant gateway that's discarding packets) can be difficult.
attempt to elicit
responses from each gateway
along the path to host
, in order to determine
their route. traceroute
is used for IPv4 networks
The options are as follows:
- Look up the AS number for each hop address. Uses the DNS
service described at
- Do not increment the destination port number in successive
UDP packets. Rather, all UDP packets will have the same destination port,
as set via the -p flag (or 33434 if none is
- Dump the packet data to standard error before transmitting
- Turn on socket-level debugging.
- Set the first TTL or hop limit used in outgoing probe
packets. The effect is that the first first_ttl - 1 hosts will be skipped
in the output of traceroute. The default is 1
(skip no hosts).
- Add gateway_addr to the
list of addresses in the IP Loose Source Record Route (LSRR) option. If no
gateways are specified, the LSRR option is omitted. This option is not
available for IPv6.
- Use ICMP or ICMP6 ECHO instead of UDP datagrams.
- Display the TTL or hop limit value of the returned packet.
This is useful for checking for asymmetric routing.
- Set the maximum TTL or hop limit. The default is the value
of the system's net.inet.ip.ttl or
variable, which defaults to 64.
- Print hop addresses numerically rather than symbolically
and numerically (saves a nameserver address-to-name lookup for each
gateway found on the path).
- Change the protocol being used from UDP to a numeric
protocol or a name as specified in
/etc/protocols. This will not work reliably
for most protocols. If set to 1 (ICMP), then ICMP Echo Request messages
will be used (same as
option is not available for IPv6.
- Set the base UDP port
number used in probes. The default is 33434.
traceroute hopes that nothing is listening on
UDP ports base to
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
- Set the number of probes per TTL to
nqueries. The default is three
- Print how many probes were not answered for each hop.
- 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 and the user is not the superuser, an
error is returned and nothing is sent.
- Set the type-of-service (TOS) in probe packets. The value
may be one of critical,
reliability, or one of the DiffServ Code
Points: ef, af11 ...
af43, cs0 ... cs7; or a number in either
hex or decimal. The default is zero. This option can be used to see if
different types-of-service result in different paths. If this option is
used, changes to the type-of-service in the returned packets are
displayed. Not all values of TOS are legal or meaningful - see the IP spec
for definitions. Useful values are probably
throughput. This option is not available for
- Set the routing table to be used.
- Verbose output. Received ICMP packets other than
UNREACHABLEs are listed.
- Set the time, in seconds, to wait for a response to a
probe. The default is 5.
- Print the ICMP extended headers if available. This option
is not available for IPv6.
- The destination host, specified as a host name or numeric
- The probe datagram length. The default is 40 bytes for IPv4
UDP and 60 bytes for ICMP, IPv6 UDP and ICMP6.
The program attempts to trace the route an IP packet would follow to a host by
launching UDP probe packets with a small TTL or hop limit, then listening for
an ICMP "time exceeded" reply from a gateway. It starts using probes
with a TTL/hop limit of one and increases by one until it gets an ICMP
"port unreachable" (which means it reached the host) or hits a
maximum limit (which defaults to 64, but can be changed using the
option). Three probes (the exact number can be
changed using the -q
option) are sent and a line
is printed showing the TTL or hop limit, 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 second timeout interval (which can be changed using the
option), a "*" is printed for that
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 using the
A sample use and output might be:
$ traceroute nis.nsf.net.
traceroute to nis.nsf.net (22.214.171.124), 64 hops max, 56 byte packet
1 helios.ee.lbl.gov (126.96.36.199) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (184.108.40.206) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (220.127.116.11) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (18.104.22.168) 39 ms 39 ms 39 ms
6 22.214.171.124 (126.96.36.199) 40 ms 59 ms 59 ms
7 188.8.131.52 (184.108.40.206) 59 ms 59 ms 59 ms
8 220.127.116.11 (18.104.22.168) 99 ms 99 ms 80 ms
9 22.214.171.124 (126.96.36.199) 139 ms 239 ms 319 ms
10 188.8.131.52 (184.108.40.206) 220 ms 199 ms 199 ms
11 nic.merit.edu (220.127.116.11) 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 - 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:
$ traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.104.22.168), 64 hops max
1 helios.ee.lbl.gov (22.214.171.124) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (126.96.36.199) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (184.108.40.206) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (220.127.116.11) 20 ms 39 ms 39 ms
6 18.104.22.168 (22.214.171.124) 59 ms 119 ms 39 ms
7 126.96.36.199 (188.8.131.52) 59 ms 59 ms 39 ms
8 184.108.40.206 (220.127.116.11) 80 ms 79 ms 99 ms
9 18.104.22.168 (22.214.171.124) 139 ms 139 ms 159 ms
10 126.96.36.199 (188.8.131.52) 199 ms 180 ms 300 ms
11 184.108.40.206 (220.127.116.11) 300 ms 239 ms 239 ms
12 * * *
13 18.104.22.168 (22.214.171.124) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (126.96.36.199) 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.
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 (188.8.131.52) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (184.108.40.206) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (220.127.116.11) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (18.104.22.168) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (22.214.171.124) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (126.96.36.199) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (188.8.131.52) 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 ICMPs aren't sent for ICMPs)
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
"!" 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) software, expect
to see this problem frequently and/or take care picking the target host of
Other possible annotations after the time are !H
(got a host,
network or protocol unreachable, respectively),
the network or host, respectively, is prohibited),
(communication administratively prohibited by
(source route failed or fragmentation needed - neither of these should ever
occur and the associated gateway is busted if you see one),
(destination network or host unknown),
(destination network or host unreachable for
(other ICMP unreachable
(TOS bit in returned packet
differs from last hop). If almost all the probes result in some kind of
will give up and exit.
$ traceroute -g 10.3.0.5
will show the path from the Cambridge Mailbridge to PSC, while
$ traceroute -g 184.108.40.206 -g 10.3.0.5
will show the path from the Cambridge Mailbridge to Merit, using PSC to reach
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
during normal operations or from
The very first traceroute
(never released) used
ICMP ECHO_REQUEST datagrams as probe packets. During the first night of
testing it was discovered that more than half the router vendors of the time
would not return an ICMP TIME_EXCEEDED for an ECHO_REQUEST.
was then changed to use UDP probe
packets. Most modern TCP/IP implementations will now generate an ICMP error
message to ICMP query messages, and the option to use ECHO_REQUEST probes was
command first appeared in
. The traceroute6
command first appeared in the WIDE Hydrangea IPv6 protocol stack kit.
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