NetBSD/usr.sbin/traceroute/traceroute.8

.\" $NetBSD: traceroute.8,v 1.28 2006/02/17 21:31:18 rpaulo Exp $
.\"
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.\"	Header: traceroute.8,v 1.11 97/04/22 13:45:13 leres Exp
.\"
.TH TRACEROUTE 8 "17 February 2006"
.UC 6
.SH NAME
traceroute \- print the route packets take to network host
.SH SYNOPSIS
.na
.B traceroute
[
.B \-aDFPIdlMnrvx
] [
.B \-f
.I first_ttl
]
.br
.ti +8
[
.B \-g
.I gateway
] [
.B \-i
.I iface
] [
.B \-m
max_ttl
]
.br
.ti +8
[
.B \-p
.I port
] [
.B \-q
.I nqueries
] [
.B \-s
.I src_addr
]
.br
.ti +8
[
.B \-t
.I tos
] [
.B \-w
.I waittime
] [
.B \-A
.I as_server
]
.br
.ti +8
.I host
[
.I packetlen
]
.ad
.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.
.I Traceroute
uses 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.
.PP
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.
.PP
Other options are:
.TP
.B \-a
Turn on AS# lookups for each hop encountered.
.TP
.B \-A
Turn on AS# lookups and use the given server instead of the default.
.TP
.B \-d
Turn on socket-level debugging.
.TP
.B \-D
Dump the packet data to standard error before transmitting it.
.TP
.B \-f
Set the initial time-to-live used in the first outgoing probe packet.
.TP
.B \-F
Set the "don't fragment" bit.
.TP
.B \-g
Specify a loose source route gateway (8 maximum).
.TP
.B \-i
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 the
.B \-s
flag for another way to do this.)
.TP
.B \-I
Use ICMP ECHO instead of UDP datagrams.
.TP
.B \-l
Display the ttl value of the returned packet.  This is useful for
checking for asymmetric routing.
.TP
.B \-m
Set the max time-to-live (max number of hops) used in outgoing probe
packets.  The default value is taken from the
.I net.inet.ip.ttl
sysctl(3) variable.
.TP
.B \-M
If found, show the MPLS Label and the Experimental (EXP) bit for the hop.
.TP
.B \-n
Print hop addresses numerically rather than symbolically and numerically
(saves a nameserver address-to-name lookup for each gateway found on the
path).
.TP
.B \-p
Set the base UDP port number used in probes (default is 33434).
Traceroute hopes that nothing is listening on UDP ports
.I base
to
.I base + nhops \- 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.
.TP
.B \-P
Set the "don't fragment" bit, and use the next hop mtu each time we get
the "need fragmentation" error, thus probing the path MTU.
.TP
.B \-q
Set the number of probe packets sent for each hop.  By default, traceroute
sends three probe packets.
.TP
.B \-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 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
.IR routed (8)).
.TP
.B \-s
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
.B \-i
flag for another way to do this.)
.TP
.B \-t
Set the
.I 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
.RB ` -t
.IR 16 '
(low delay) and
.RB ` -t
.IR 8 '
(high throughput).
.TP
.B \-v
Verbose output.  Received ICMP packets other than TIME_EXCEEDED and
UNREACHABLEs are listed.
.TP
.B \-w
Set the time (in seconds) to wait for a response to a probe (default 5
sec.).
.TP
.B \-x
Toggle checksums. Normally, this prevents traceroute from calculating
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
.B \-x
causes them to be calculated). Note that checksums are usually required
for the last hop when using ICMP ECHO probes
.RB ( \-I ).
.PP
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 30 hops \*[Am] can be changed with the
.B \-m
flag).  Three
probes (change with
.B \-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
.B \-w
flag), a "*" is printed for that
probe.
.PP
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
.B \-p
flag).
.PP
A sample use and output might be:

.RS
.nf
[yak 71]% traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 30 hops max, 38 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
.fi
.RE

Note that lines 2 \*[Am] 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.
.PP
A more interesting example is:

.RS
.nf
[yak 72]% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.26.0.115), 30 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
.fi
.RE

Note that the gateways 12, 14, 15, 16 \*[Am] 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.
.PP
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:

.RS
.nf
 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 !
.fi
.RE

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
.BR !H ,
.BR !N ,
or
.B !P
(got a host, network or protocol unreachable, respectively),
.B !S
or
.B !F
(source route failed or fragmentation needed \- neither of these should
ever occur and the associated gateway is busted if you see one),
.B !X
(communication administratively prohibited), or
.B !\*[Lt]N\*[Gt]
(ICMP unreachable code N).
If almost all the probes result in some kind of unreachable, traceroute
will give up and exit.

.RS
.nf
traceroute \-g 10.3.0.5 128.182.0.0
.fi
.RE

will show the path from the Cambridge Mailbridge to PSC, while

.RS
.nf
traceroute \-g 192.5.146.4 \-g 10.3.0.5 35.0.0.0
.fi
.RE

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
.I traceroute
during normal operations or from automated scripts.
.SH SEE ALSO
netstat(1), ping(8)
.SH AUTHOR
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.
.LP
The current version is available via anonymous ftp:
.LP
.RS
.I ftp://ftp.ee.lbl.gov/traceroute.tar.Z
.RE
.SH BUGS
Please send bug reports to traceroute@ee.lbl.gov.
.PP
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.