TRACEROUTE(8) BSD System Manager's Manual TRACEROUTE(8)
NAME
traceroute -- print the route packets take to network host
SYNOPSIS
traceroute [-dFISdNnrvx] [-f first_ttl] [-g gateway] [-i iface] [-M first_ttl] [-m max_ttl] [-P proto]
[-p port] [-q nqueries] [-s src_addr] [-t tos] [-w waittime] [-z pausemsecs] host
[packetsize]
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 pack-ets) packets)
ets) can be difficult. Traceroute 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.
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 destina-tion destination
tion host name.
Other options are:
-f first_ttl
Set the initial time-to-live used in the first outgoing probe packet.
-F Set the "don't fragment" bit.
-d Enable socket level debugging.
-g gateway
Specify a loose source route gateway (8 maximum).
-i 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 -s flag for another way to do this.)
-I Use ICMP ECHO instead of UDP datagrams. (A synonym for "-P icmp").
-M 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.
-m max_ttl
Set the max time-to-live (max number of hops) used in outgoing probe packets. The default is
net.inet.ip.ttl hops (the same default used for TCP connections).
-N Disables the increment of the destination port (see -p). Useful for using tcpdump through fire-walls. firewalls.
walls.
-n Print hop addresses numerically rather than symbolically and numerically (saves a nameserver
address-to-name lookup for each gateway found on the path).
-P proto
Send packets of specified IP protocol. The currently supported protocols are: UDP , TCP , GRE
and ICMP Other protocols may also be specified (either by name or by number), though 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.
-p port
Protocol specific. For UDP and TCP, sets the base port number used in probes (default is
33434). Traceroute hopes that nothing is listening on UDP ports base to 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.
-q nqueries
Set the number of probes per ``ttl'' to nqueries (default is three probes).
-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 routed(8)).
-s 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 inter-face interface
face 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 -i flag for another way to do
this.)
-S Print a summary of how many probes were not answered for each hop.
-t tos Set the 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 typesof-service
of-service result in different paths. (If you are not running a 4.4BSD or later system, 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 `-t 16' (low delay) and `-t 8' (high throughput).
-v Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs are listed.
-w Set the time (in seconds) to wait for a response to a probe (default 5 sec.).
-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 -x causes them
to be calculated). Note that checksums are usually required for the last hop when using ICMP
ECHO probes ( -I ). So they are always calculated when using ICMP.
-z 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
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 net.inet.ip.ttl hops & can
be changed with the -m flag). Three probes (changed with -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 -w flag), a "*" is printed for
that probe.
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 -p
flag).
A sample use and output might be:
[yak 71]% 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
Note that lines 2 & 3 are the same. This is due to a buggy kernel on the 2nd hop system - lbl-csam.arpa lblcsam.arpa
csam.arpa - that forwards packets with a zero ttl (a bug in the distributed version of 4.3 BSD). 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.
A more interesting example is:
[yak 72]% 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
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.
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:
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 !
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 (HPUX) 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 !H, !N, or !P (host, network or protocol unreachable), !S
(source route failed), !F (fragmentation needed - the RFC1191 Path MTU Discovery value is displayed),
!X (communication administratively prohibited), !V (host precedence violation), !C (precedence cutoff
in effect), or !<num> (ICMP unreachable code <num>). These are defined by RFC1812 (which supersedes
RFC1716). If almost all the probes result in some kind of unreachable, 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 traceroute during normal operations or from automated scripts.
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.
SEE ALSO
netstat(1), ping(8)
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, traceroute should listen for a RST from
the destination host (or an intermediate router that's filtering packets), but this is not implemented
yet.
4.3 Berkeley Distribution September 21, 2000 4.3 Berkeley Distribution
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