print the route packets take to network host
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.
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 host name.
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
- 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
and supported by the peer) base + 1 to
base + nhops * 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
(low delay) and
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
-x causes them to be calculated). Note
that checksums are usually required for the last hop when using ICMP ECHO
-I). 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
and can be changed with the
probes (change with
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
flag), a "*" is printed for
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 (188.8.131.52), 64 hops max, 38 byte packet
1 helios.ee.lbl.gov (184.108.40.206) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (18.104.22.168) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (22.214.171.124) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 39 ms
6 188.8.131.52 (184.108.40.206) 40 ms 59 ms 59 ms
7 220.127.116.11 (18.104.22.168) 59 ms 59 ms 59 ms
8 22.214.171.124 (126.96.36.199) 99 ms 99 ms 80 ms
9 188.8.131.52 (184.108.40.206) 139 ms 239 ms 319 ms
10 220.127.116.11 (18.104.22.168) 220 ms 199 ms 199 ms
11 nic.merit.edu (22.214.171.124) 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 NSSes.
A more interesting example is:
% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (126.96.36.199), 64 hops max
1 helios.ee.lbl.gov (188.8.131.52) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (184.108.40.206) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (220.127.116.11) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (18.104.22.168) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (22.214.171.124) 20 ms 39 ms 39 ms
6 126.96.36.199 (188.8.131.52) 59 ms 119 ms 39 ms
7 184.108.40.206 (220.127.116.11) 59 ms 59 ms 39 ms
8 18.104.22.168 (22.214.171.124) 80 ms 79 ms 99 ms
9 126.96.36.199 (188.8.131.52) 139 ms 139 ms 159 ms
10 184.108.40.206 (220.127.116.11) 199 ms 180 ms 300 ms
11 18.104.22.168 (22.214.171.124) 300 ms 239 ms 239 ms
12 * * *
13 126.96.36.199 (188.8.131.52) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (184.108.40.206) 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.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 (220.127.116.11) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (18.104.22.168) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (22.214.171.124) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (126.96.36.199) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (188.8.131.52) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (184.108.40.206) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (220.127.116.11) 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 probes.
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,
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
during normal operations or from
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.
When using protocols other than UDP, functionality is reduced. In particular,
the last packet will often appear to be lost, because even though it reaches
the destination host, there's no way to know that because no ICMP message is
sent back. In the TCP case,
should listen for a RST from the destination host (or an intermediate router
that's filtering packets), but this is not implemented yet.
The AS number capability reports information that may sometimes be inaccurate
due to discrepancies between the contents of the routing database server and
the current state of the Internet.