TRACEROUTE(8) OpenBSD System Manager's Manual TRACEROUTE(8)NAMEtraceroute - print the route packets take to network host
SYNOPSIStraceroute [-cDdIlnrSvx] [-f first_ttl] [-g gateway_addr] [-m max_ttl]
[-P proto] [-p port] [-q nqueries] [-s src_addr] [-t tos]
[-V rtable] [-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.
The options are as follows:
-c 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 specified).
-D Dump the packet data to standard error before transmitting it.
-d Turn on socket-level debugging.
-f first_ttl
Set the first time-to-live 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 value is 1 (skip no
hosts).
-g gateway_addr
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.
-I Equivalent to -P 1. Used for compatibility with other OSes.
-l Display the ttl value of the returned packet. This is useful for
checking for asymmetric routing.
-m max_ttl
Set the max time-to-live (max number of hops) used in outgoing
probe packets. The default is the value of the system's
net.inet.ip.ttl MIB variable, which defaults to 64.
-n Print hop addresses numerically rather than symbolically and
numerically (saves a nameserver address-to-name lookup for each
gateway found on the path).
-P proto
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 ping(8)).
-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*nqueries-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 nqueries
Set the number of probes per ``ttl'' to nqueries (default is
three probes).
-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 corresponds to the
SO_DONTROUTE socket option; it 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 a routing daemon).
-S Print how many probes were not answered for each hop.
-s src_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
and the user is not the superuser, 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 a
4.3BSD-Tahoe or later system, 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 rtable
Set the routing table to be used. The default is 0.
-v Verbose output. Received ICMP packets other than TIME_EXCEEDED
and UNREACHABLEs are listed.
-w waittime
Set the time (in seconds) to wait for a response to a probe
(default 5).
-x Print the ICMP extended headers if available.
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 out 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 64 hops and can be changed with the -m flag). Three
probes (changed 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 5 second 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).
A sample use and output might be:
$ traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 64 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 & 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 4.3 BSD). 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 (18.26.0.115), 64 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 * * *
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 (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 !H, !N, !P (got a host,
network or protocol unreachable, respectively), !A, !C (access to the
network or host, respectively, is prohibited), !X (communication
administratively prohibited by filtering), !S or !F (source route failed
or fragmentation needed - neither of these should ever occur and the
associated gateway is busted if you see one), !U (destination network or
host unknown), !T (destination network or host unreachable for TOS),
!<code> (other ICMP unreachable code). If almost all the probes result
in some kind of unreachable, traceroute will give up and exit.
$ 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
will show the path from the Cambridge Mailbridge to Merit, using PSC to
reach the Mailbridge.
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.
SEE ALSOnetstat(1), ping(8)HISTORY
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. traceroute 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 re-implemented.
The traceroute command first appeared in 4.4BSD.
AUTHORS
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.
OpenBSD 4.9 July 8, 2010 OpenBSD 4.9
