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.\" $Id: traceroute.8,v 1.19 2000/09/21 08:44:19 leres Exp $
.\" $FreeBSD$
.\"
.Dd November 25, 2020
.Dt TRACEROUTE 8
.Os
.Sh NAME
.Nm traceroute
.Nd "print the route packets take to network host"
.Sh SYNOPSIS
.Nm
.Bk -words
.Op Fl adDeFISnrvx
.Op Fl f Ar first_ttl
.Op Fl g Ar gateway
.Op Fl M Ar first_ttl
.Op Fl m Ar max_ttl
.Op Fl P Ar proto
.Op Fl p Ar port
.Op Fl q Ar nprobes
.Op Fl s Ar src_addr
.Op Fl t Ar tos
.Op Fl w Ar waittime
.Op Fl A Ar as_server
.Op Fl z Ar pausemsecs
.Ar host
.Op Ar packetlen
.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
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:
.Bl -tag -width Ds
.It Fl a
Turn on AS# lookups for each hop encountered.
.It Fl A Ar as_server
Turn on AS# lookups and use the given server instead of the
default.
.It Fl e
Firewall evasion mode.
Use fixed destination ports for UDP, UDP-Lite, TCP and SCTP probes.
The destination port does NOT increment with each packet sent.
.It Fl f Ar first_ttl
Set the initial time-to-live used in the first outgoing probe packet.
.It Fl F
Set the "don't fragment" bit.
.It Fl d
Enable socket level debugging.
.It Fl D
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.
.It Fl g Ar gateway
Specify a loose source route gateway (8 maximum).
.It Fl i Ar iface
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
.Fl s
flag for another way to do this.)
.It Fl I
Use ICMP ECHO instead of UDP datagrams. (A synonym for "-P icmp").
.It Fl M Ar first_ttl
Set the initial time-to-live value used in outgoing probe packets.
The default is 1, i.e., start with the first hop.
.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
.Va net.inet.ip.ttl
.Xr sysctl 8
(the same default used for TCP
connections).
.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
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
.Nm
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.
.It Fl p Ar port
Protocol specific. For UDP, UDP-Lite, TCP and SCTP, sets
the base
.Ar port
number used in probes (default is 33434).
Traceroute hopes that nothing is listening on UDP ports (or UDP-Lite ports
if used by
.Nm
and supported by the peer)
.Em port + 1
to
.Em port + (max_ttl - first_ttl + 1) * 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.
.It Fl q Ar nprobes
Set the number of probes per hop (default is 3,
unless
.Fl D
is specified,
when it is 1).
.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 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
.Xr routed 8 .
.It Fl s Ar src_addr
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
.Fl i
flag for another way to do this.)
.It Fl S
Print a summary of how many probes were not answered for each hop.
.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. The upper six
bits are the Differentiated Services Codepoint (RFC4594). The lower two
bits are the Explicit Congestion Notification field (RFC3168).
.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 (default 5
sec.).
.It Fl x
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
.Fl x
causes them to be calculated). Note that checksums are usually required
for the last hop when using ICMP ECHO probes
.Pq Fl I .
So they are always calculated when using ICMP.
.It Fl z Ar pausemsecs
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 second).
.El
.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 the amount of hops specified by the
.Va net.inet.ip.ttl
.Xr sysctl 8
and can be changed with the
.Fl m
flag). Three
probes (change 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 sec. 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 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 4n
% traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 64 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
.Ed
.Pp
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.
.Pp
A more interesting example is:
.Bd -literal -offset 4n
% 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]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:
.Bd -literal -offset 4n
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 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
.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:
.Bl -hang -offset indent -width 12n
.It Sy !H
Host unreachable.
.It Sy !N
Network unreachable.
.It Sy !P
Protocol unreachable.
.It Sy !S
Source route failed.
.It Sy !F\-<pmtu>
Fragmentation needed.
The RFC1191 Path MTU Discovery value is displayed.
.It Sy !U
Destination network unknown.
.It Sy !W
Destination host unknown.
.It Sy !I
Source host is isolated.
.It Sy !A
Communication with destination network administratively prohibited.
.It Sy !Z
Communication with destination host administratively prohibited.
.It Sy !Q
For this ToS the destination network is unreachable.
.It Sy !T
For this ToS the destination host is unreachable.
.It Sy !X
Communication administratively prohibited.
.It Sy !V
Host precedence violation.
.It Sy !C
Precedence cutoff in effect.
.It Sy !<num>
ICMP unreachable code <num>.
.El
.Pp
These are defined by RFC1812 (which supersedes RFC1716).
If almost all the probes result in some kind of unreachable,
.Nm
will give up and exit.
.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 ,
.Xr traceroute6 8 .
.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.
.Sh BUGS
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,
.Nm
should listen for a RST from the destination host (or an intermediate
router that's filtering packets), but this is not implemented yet.
.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.
