traceroute.8   [plain text]

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.Dd September 21, 2000
.Os BSD 4.3
.Nm traceroute
.Nd print the route packets take to network host
.Nm traceroute
.Op Fl dFISdNnrvx
.Op Fl f Ar first_ttl
.Op Fl g Ar gateway
.Op Fl i Ar iface
.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 nqueries
.Op Fl s Ar src_addr
.Op Fl t Ar tos
.Op Fl w Ar waittime
.Op Fl z Ar pausemsecs
.Ar host
.Op Ar packetsize
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 Traceroute
utilizes the IP protocol `time to live' field and attempts to elicit an
response from each gateway along the path to some
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 size (in bytes) after the destination host
Other options are:
.Bl -tag -width Ds
.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 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
ECHO instead of 
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
.Em net.inet.ip.ttl
hops (the same default used for 
.It Fl N
Disables the increment of the destination port (see -p). Useful for using tcpdump through firewalls.  
.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
.It Fl P Ar proto
Send packets of specified IP protocol. The currently supported protocols
Other protocols may also be specified (either by name or by number), though
.Nm traceroute
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 
.Tn TCP, 
sets the base
.Ar port
number used in probes (default is 33434).
.Nm Traceroute
hopes that nothing is listening on
.Em base
.Em base+nhops-1
at the destination host (so an
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
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 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, 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.  (If you
are not running a
.Bx 4.4
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
are legal or
meaningful \- see the IP spec for definitions.  Useful values are
.Ql \-t 16
(low delay) and
.Ql \-t 8
(high throughput).
.It Fl v
Verbose output.  Received
packets other than
are listed.
.It Fl w
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 
ECHO probes (
.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).
This program attempts to trace the route an IP packet would follow to some
internet host by launching
packets with a small ttl (time to live) then listening for an
"time exceeded" reply from a gateway.  We start our probes
with a ttl of one and increase by one until we get an
"port unreachable"
(which means we got to "host") or hit a max (which
defaults to
.Em net.inet.ip.ttl
hops & 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 sec. timeout
interval (changed with the
.Fl w
flag), a "*" is printed for that
We don't want the destination
host to process the
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
A sample use and output might be:
.Bd -literal
[yak 71]% traceroute
traceroute to (, 64 hops max, 38 byte packet
1 (  19 ms  19 ms  0 ms
2  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
3  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
4  ccngw-ner-cc.Berkeley.EDU (  39 ms  40 ms  39 ms
5  ccn-nerif22.Berkeley.EDU (  39 ms  39 ms  39 ms
6 (  40 ms  59 ms  59 ms
7 (  59 ms  59 ms  59 ms
8 (  99 ms  99 ms  80 ms
9 (  139 ms  239 ms  319 ms
10 (  220 ms  199 ms  199 ms
11 (  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 \- \- 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
doesn't supply address-to-name translations for its
.Tn NSS Ns es .
A more interesting example is:
.Bd -literal
[yak 72]% traceroute
traceroute to (, 64 hops max
1 (  0 ms  0 ms  0 ms
2  lilac-dmc.Berkeley.EDU (  19 ms  19 ms  19 ms
3  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  19 ms
4  ccngw-ner-cc.Berkeley.EDU (  19 ms  39 ms  39 ms
5  ccn-nerif22.Berkeley.EDU (  20 ms  39 ms  39 ms
6 (  59 ms  119 ms  39 ms
7 (  59 ms  59 ms  39 ms
8 (  80 ms  79 ms  99 ms
9 (  139 ms  139 ms  159 ms
10 (  199 ms  180 ms  300 ms
11 (  300 ms  239 ms  239 ms
12  * * *
13 (  259 ms  499 ms  279 ms
14  * * *
15  * * *
16  * * *
17  * * *
18  ALLSPICE.LCS.MIT.EDU (  339 ms  279 ms  279 ms

Note that the gateways 12, 14, 15, 16 & 17 hops away
either don't send
"time exceeded" messages or send them
with a ttl too small to reach us.  14 \- 17 are running the
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]
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
"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
1 (  0 ms  0 ms  0 ms
2  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  39 ms
3  lilac-dmc.Berkeley.EDU (  19 ms  39 ms  19 ms
4  ccngw-ner-cc.Berkeley.EDU (  39 ms  40 ms  19 ms
5  ccn-nerif35.Berkeley.EDU (  39 ms  39 ms  39 ms
6  csgw.Berkeley.EDU (  39 ms  59 ms  39 ms
7  * * *
8  * * *
9  * * *
10  * * *
11  * * *
12  * * *
13  rip.Berkeley.EDU (  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
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 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
software, expect to see this problem
frequently and/or take care picking the target host of your
Other possible annotations after the time are
.Sy !H ,
.Sy !N ,
.Sy !P
(host, network or protocol unreachable),
.Sy !S
(source route failed),
.Sy !F
(fragmentation needed \- the RFC1191 Path MTU Discovery value is displayed),
.Sy !X
(communication administratively prohibited),
.Sy !V
(host precedence violation),
.Sy !C
(precedence cutoff in effect), or
.Sy !<num>
(ICMP unreachable code <num>).
These are defined by RFC1812 (which supersedes RFC1716).
If almost all the probes result in some kind of unreachable,
.Nm traceroute
will give up and exit.
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 traceroute
during normal operations or from automated scripts.
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.
.Xr netstat 1 ,
.Xr ping 8
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,
should listen for a RST from the destination host (or an intermediate
router that's filtering packets), but this is not implemented yet.