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 40 bytes, but this may be
increased by specifying a packet length (in bytes) after the destination
Other options are:
- Turn on AS# lookups for each hop encountered.
- Turn on AS# lookups and use the given server instead of the default.
- Firewall evasion mode. Use fixed destination ports for UDP, UDP-Lite, TCP
and SCTP probes. The destination port does NOT increment with each packet
- Set the initial time-to-live used in the first outgoing probe packet.
- Set the "don't fragment" bit.
- Enable socket level debugging.
- When an ICMP response to our probe datagram is received, print the
differences between the transmitted packet and the packet quoted by the
ICMP response. A key showing the location of fields within the transmitted
packet is printed, followed by the original packet in hex, followed by the
quoted packet in hex. Bytes that are unchanged in the quoted packet are
shown as underscores. Note, the IP checksum and the TTL of the quoted
packet are not expected to match. By default, only one probe per hop is
sent with this option.
- Specify a loose source route gateway (8 maximum).
- 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
-s flag for another way to do this.)
- Use ICMP ECHO instead of UDP datagrams. (A synonym for "-P
- Set the initial time-to-live value used in outgoing probe packets. The
default is 1, i.e., start with the first hop.
- Set the max time-to-live (max number of hops) used in outgoing probe
packets. The default is the value of the
(the same default used for TCP connections).
- Print hop addresses numerically rather than symbolically and numerically
(saves a nameserver address-to-name lookup for each gateway found on the
- Send packets of specified IP protocol. The currently supported protocols
are: UDP, UDP-Lite, TCP, SCTP, GRE and ICMP. Other protocols may also be
specified (either by name or by number), though
traceroute does not implement any special
knowledge of their packet formats. This option is useful for determining
which router along a path may be blocking packets based on IP protocol
number. But see BUGS below.
- Protocol specific. For UDP, UDP-Lite, TCP and SCTP, sets the base
port number used in probes (default is 33434).
Traceroute hopes that nothing is listening on UDP ports (or UDP-Lite ports
if used by
traceroute and supported by the peer)
port + 1 to port + (max_ttl - first_ttl +
1) * nprobes 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.
- Set the number of probes per hop (default is 3, unless
-D is specified, when it is 1).
- 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
- 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.)
- Print a summary of how many probes were not answered for each hop.
- 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).
- Verbose output. Received ICMP packets other than
UNREACHABLEs are listed.
- Set the time (in seconds) to wait for a response to a probe (default 5
- 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
causes them to be calculated). Note that checksums are usually required
for the last hop when using ICMP ECHO probes (
So they are always calculated when using ICMP.
- 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
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 the amount of hops
specified by the net.inet.ip.ttl
and can be changed with the
-m flag). Three probes
-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
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
A sample use and output might be:
% traceroute nis.nsf.net.
traceroute to nis.nsf.net (220.127.116.11), 64 hops max, 38 byte packet
1 helios.ee.lbl.gov (18.104.22.168) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (188.8.131.52) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (184.108.40.206) 39 ms 39 ms 39 ms
6 220.127.116.11 (18.104.22.168) 40 ms 59 ms 59 ms
7 22.214.171.124 (126.96.36.199) 59 ms 59 ms 59 ms
8 188.8.131.52 (184.108.40.206) 99 ms 99 ms 80 ms
9 220.127.116.11 (18.104.22.168) 139 ms 239 ms 319 ms
10 22.214.171.124 (126.96.36.199) 220 ms 199 ms 199 ms
11 nic.merit.edu (188.8.131.52) 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
A more interesting example is:
% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (184.108.40.206), 64 hops max
1 helios.ee.lbl.gov (220.127.116.11) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (18.104.22.168) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (22.214.171.124) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (126.96.36.199) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (188.8.131.52) 20 ms 39 ms 39 ms
6 184.108.40.206 (220.127.116.11) 59 ms 119 ms 39 ms
7 18.104.22.168 (22.214.171.124) 59 ms 59 ms 39 ms
8 126.96.36.199 (188.8.131.52) 80 ms 79 ms 99 ms
9 184.108.40.206 (220.127.116.11) 139 ms 139 ms 159 ms
10 18.104.22.168 (22.214.171.124) 199 ms 180 ms 300 ms
11 126.96.36.199 (188.8.131.52) 300 ms 239 ms 239 ms
12 * * *
13 184.108.40.206 (220.127.116.11) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (18.104.22.168) 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
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 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
1 helios.ee.lbl.gov (22.214.171.124) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (188.8.131.52) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (184.108.40.206) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (18.104.22.168) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (22.214.171.124) 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) software, expect to
see this problem frequently and/or take care picking the target host of your
Other possible annotations after the time are:
- Host unreachable.
- Network unreachable.
- Protocol unreachable.
- Source route failed.
- Fragmentation needed. The RFC1191 Path MTU Discovery value is
- Destination network unknown.
- Destination host unknown.
- Source host is isolated.
- Communication with destination network administratively prohibited.
- Communication with destination host administratively prohibited.
- For this ToS the destination network is unreachable.
- For this ToS the destination host is unreachable.
- Communication administratively prohibited.
- Host precedence violation.
- Precedence cutoff in effect.
- ICMP unreachable code <num>.
These are defined by RFC1812 (which supersedes RFC1716). If almost
all the probes result in some kind of unreachable,
traceroute will give up and exit.
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