OpenBSD-4.6/usr.sbin/traceroute/traceroute.8
.\" $OpenBSD: traceroute.8,v 1.41 2009/06/05 06:47:12 jmc Exp $
.\" $NetBSD: traceroute.8,v 1.6 1995/10/12 03:05:50 mycroft Exp $
.\"
.\" Copyright (c) 1990, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Van Jacobson.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" notice, this list of conditions and the following disclaimer in the
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.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
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.\" @(#)traceroute.8 8.1 (Berkeley) 6/6/93
.\"
.Dd $Mdocdate: June 5 2009 $
.Dt TRACEROUTE 8
.Os
.Sh NAME
.Nm traceroute
.Nd print the route packets take to network host
.Sh SYNOPSIS
.Nm traceroute
.Bk -words
.Op Fl cDdIlnrSv
.Op Fl f Ar first_ttl
.Op Fl g Ar gateway_addr
.Op Fl m Ar max_ttl
.Op Fl P Ar proto
.Op Fl p Ar port
.Op Fl q Ar nqueries
.Op Fl s Ar src_addr
.Op Fl t Ar tos
.Op Fl V Ar rdomain
.Op Fl w Ar waittime
.Ar host
.Op Ar packetsize
.Ek
.Sh 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.
.Nm
utilizes the IP protocol `time to live' field and attempts to elicit an
.Tn ICMP
.Dv TIME_EXCEEDED
response from each gateway along the path to some
host.
.Pp
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.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl 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
.Fl p
flag (or 33434 if none is specified).
.It Fl D
Dump the packet data to standard error before transmitting it.
.It Fl d
Turn on socket-level debugging.
.It Fl f Ar 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
.Nm traceroute .
The default value is 1 (skip no hosts).
.It Fl g Ar gateway_addr
Add
.Ar 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.
.It Fl I
Equivalent to
.Fl P
1.
Used for compatibility with other OSes.
.It Fl l
Display the ttl value of the returned packet.
This is useful for checking for asymmetric routing.
.It Fl m Ar 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
.Cm net.inet.ip.ttl
MIB variable, which defaults to 64.
.It Fl n
Print hop addresses numerically rather than symbolically and numerically
(saves a nameserver address-to-name lookup for each gateway found on the
path).
.It Fl P Ar proto
Change the protocol being used from
.Tn UDP
to a numeric protocol or a name as specified in
.Pa /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
.Xr ping 8 ) .
.It Fl p Ar port
Set the base
.Tn UDP
.Ar port
number used in probes (default is 33434).
.Nm
hopes that nothing is listening on
.Tn UDP
ports
.Em base
to
.Em base+nhops*nqueries-1
at the destination host (so an
.Tn ICMP
.Dv 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.
.It Fl q Ar nqueries
Set the number of probes per ``ttl'' to
.Ar nqueries
(default is three probes).
.It Fl 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
.Dv 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).
.It Fl S
Print how many probes were not answered for each hop.
.It Fl s Ar 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.
.It Fl t Ar tos
Set the
.Em 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
.Bx 4.3 tahoe
or later system, this may be academic since the normal network
services like telnet and ftp don't let you control the
.Dv TOS ) .
Not all values of
.Dv TOS
are legal or
meaningful \- see the IP spec for definitions.
Useful values are probably
.Ql -t 16
(low delay) and
.Ql -t 8
(high throughput).
.It Fl V Ar rdomain
Set the routing domain.
The default is 0.
.It Fl v
Verbose output.
Received
.Tn ICMP
packets other than
.Dv TIME_EXCEEDED
and
.Dv UNREACHABLE Ns s
are listed.
.It Fl w Ar waittime
Set the time (in seconds) to wait for a response to a probe (default 3).
.El
.Pp
This program attempts to trace the route an IP packet would follow to some
internet host by launching
.Tn UDP
probe
packets with a small ttl (time to live) then listening for an
.Tn ICMP
"time exceeded" reply from a gateway.
We start out probes with a ttl of one and increase by one until we get an
.Tn ICMP
"port unreachable"
(which means we got to "host") or hit a max (which
defaults to 64 hops and can be changed with the
.Fl m
flag).
Three probes (changed with
.Fl 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
.Fl w
flag), a "*" is printed for that
probe.
.Pp
We don't want the destination
host to process the
.Tn 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
.Fl p
flag).
.Pp
A sample use and output might be:
.Bd -literal -offset indent
$ 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
.Ed
.Pp
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
.Tn BSD ) .
Note that you have to guess what path
the packets are taking cross-country since the
.Tn NSFNET
(129.140)
doesn't supply address-to-name translations for its
.Tn NSS Ns es .
.Pp
A more interesting example is:
.Bd -literal -offset indent
$ 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
.Ed
.Pp
Note that the gateways 12, 14, 15, 16 & 17 hops away
either don't send
.Tn ICMP
"time exceeded" messages or send them
with a ttl too small to reach us.
14 \- 17 are running the
.Tn MIT
C Gateway code that doesn't send "time exceeded"s.
God only knows what's going on with 12.
.Pp
The silent gateway 12 in the above may be the result of a bug in
the 4.[23]
.Tn 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
.Tn 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:
.Bd -literal -offset indent
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 !
.Ed
.Pp
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
.Tn ICMP
reply.
So, the reply will time out on the return path
(with no notice sent to anyone since
.Tn ICMP's
aren't sent for
.Tn 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.
.Nm
prints a "!" after the time if the ttl is <= 1.
Since vendors ship a lot of obsolete
.Pf ( Tn DEC Ns \'s
Ultrix, Sun 3.x) or
non-standard
.Pq Tn HP-UX
software, expect to see this problem
frequently and/or take care picking the target host of your
probes.
.Pp
Other possible annotations after the time are
.Sy !H ,
.Sy !N ,
.Sy !P
(got a host, network or protocol unreachable, respectively),
.Sy !A ,
.Sy !C
(access to the network or host, respectively, is prohibited),
.Sy !X
(communication administratively prohibited by filtering),
.Sy !S
or
.Sy !F
(source route failed or fragmentation needed \- neither of these should
ever occur and the associated gateway is busted if you see one),
.Sy !U
(destination network or host unknown),
.Sy !T
(destination network or host unreachable for TOS),
.Sy !<code>
(other ICMP unreachable code).
If almost all the probes result in some kind of unreachable,
.Nm
will give up and exit.
.Pp
.Dl $ traceroute -g 10.3.0.5 128.182.0.0
.Pp
will show the path from the Cambridge Mailbridge to PSC, while
.Pp
.Dl $ traceroute -g 192.5.146.4 -g 10.3.0.5 35.0.0.0
.Pp
will show the path from the Cambridge Mailbridge to Merit, using PSC to
reach the Mailbridge.
.Pp
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
.Nm
during normal operations or from automated scripts.
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr ping 8
.Sh HISTORY
The very first
.Nm
(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.
.Nm
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.
.Pp
The
.Nm
command first appeared in
.Bx 4.4 .
.Sh 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.