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.\"	$OpenBSD: traceroute.8,v 1.76 2024/03/24 00:33:41 sthen 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:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" 3. Neither the name of the University nor the names of its contributors
.\"    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: March 24 2024 $
.Dt TRACEROUTE 8
.Os
.Sh NAME
.Nm traceroute ,
.Nm traceroute6
.Nd print the route packets take to network host
.Sh SYNOPSIS
.Nm traceroute\ \&
.Op Fl ADdIlnSvx
.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 sourceaddr
.Op Fl t Ar toskeyword
.Op Fl V Ar rtable
.Op Fl w Ar waittime
.Ar host
.Op Ar datalen
.Nm traceroute6
.Op Fl ADdIlnSv
.Op Fl f Ar first_hop
.Op Fl m Ar max_hop
.Op Fl p Ar port
.Op Fl q Ar nqueries
.Op Fl s Ar sourceaddr
.Op Fl t Ar toskeyword
.Op Fl V Ar rtable
.Op Fl w Ar waittime
.Ar host
.Op Ar datalen
.Sh DESCRIPTION
The Internet is a large and complex aggregation of
network hardware, connected together by gateways.
Tracking the route packets follow (or finding the miscreant
gateway that's discarding packets) can be difficult.
.Nm
and
.Nm traceroute6
attempt to elicit
.Dv TIME_EXCEEDED
responses from each gateway along the path to
.Ar host ,
in order to determine their route.
.Nm
is used for IPv4 networks and
.Nm traceroute6
for IPv6.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl A
Look up the AS number for each hop address.
Uses the DNS service described at
.Lk https://www.team-cymru.com/ip-asn-mapping
.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 TTL or hop limit 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 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.
This option is not available for IPv6.
.It Fl I
Use ICMP or ICMP6 ECHO instead of UDP datagrams.
.It Fl l
Display the TTL or hop limit value of the returned packet.
This is useful for checking for asymmetric routing.
.It Fl m Ar max_ttl
Set the maximum TTL or hop limit.
The default is the value of the system's
.Va net.inet.ip.ttl
or
.Va net.inet6.ip6.hlim
.Xr sysctl 8
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 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 ) .
This option is not available for IPv6.
.It Fl p Ar port
Set the base UDP
.Ar port
number used in probes.
The default is 33434.
.Nm
hopes that nothing is listening on UDP ports
.Ar base
to
.Ar base Ns + Ns Ar nhops Ns * Ns Ar nqueries Ns -1
at the destination host (so an 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 .
The default is three probes.
.It Fl S
Print how many probes were not answered for each hop.
.It Fl s Ar sourceaddr
Set the source address to transmit from, which is useful on machines
with multiple interfaces.
.It Fl t Ar toskeyword
Set the type-of-service (TOS) in probe packets.
The value may be one of
.Cm critical ,
.Cm inetcontrol ,
.Cm lowdelay ,
.Cm netcontrol ,
.Cm throughput ,
.Cm reliability ,
or one of the DiffServ Code Points:
.Cm ef ,
.Cm af11 ... af43 ,
.Cm cs0 ... cs7 ;
or a number in either hex or decimal.
The default is zero.
This option can be used to
see if different types-of-service result in different paths.
If this option is used, changes to the type-of-service in the
returned packets are displayed.
Not all values of TOS are legal or meaningful \-
see the IP spec for definitions.
Useful values are probably
.Cm lowdelay
and
.Cm throughput .
.It Fl V Ar rtable
Set the routing table to be used.
.It Fl v
Verbose output.
Received 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.
The default is 3.
.It Fl x
Print the ICMP extended headers if available.
This option is not available for IPv6.
.It Ar host
The destination host,
specified as a host name or numeric IP address.
.It Ar datalen
The probe datagram length.
The default is 40 bytes for IPv4 UDP
and 60 bytes for ICMP, IPv6 UDP and ICMP6.
.El
.Pp
The program attempts to trace the route an IP packet would follow to a
host by launching UDP probe packets with a small TTL or hop limit,
then listening for an ICMP "time exceeded" reply from a gateway.
It starts using probes with a TTL/hop limit of one
and increases by one until it gets an ICMP "port unreachable"
(which means it reached the host) or hits a maximum limit
(which defaults to 64, but can be changed using the
.Fl m
option).
Three probes (the exact number can be changed using the
.Fl q
option) are sent and a line is printed
showing the TTL or hop limit, 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 3 second timeout
interval (which can be changed using the
.Fl w
option), a "*" is printed for that
probe.
If the host cannot be reached,
.Nm
skips printing lines consisting only of * until the maximum TTL/hop limit is
reached.
.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 using the
.Fl p
option).
.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
.Bx 4.3 ) .
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:
.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 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]
.Bx
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:
.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
ICMP reply.
So, the reply will time out on the return path
(with no notice sent to anyone since ICMPs aren't sent for ICMPs)
until we probe with a TTL that's at least twice the path
length.
That is, 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 (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.
.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).
.Sy TOS=xxx!
(TOS bit in returned packet differs from last hop).
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 ping 8 ,
.Xr route 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 4.3 Reno .
The
.Nm traceroute6
command first appeared in the WIDE Hydrangea IPv6 protocol stack kit.
.Sh AUTHORS
.An -nosplit
Implemented by
.An Van Jacobson
from a suggestion by
.An Steve Deering .
Debugged
by a cast of thousands with particularly cogent suggestions or fixes from
.An C. Philip Wood ,
.An Tim Seaver ,
and
.An Ken Adelman .