traceroute(8)

NAME
traceroute - print the route packets take to network host

SYNOPSIS
traceroute [-A] [-a] [-c stoptime] [-f] [-g gateway] [-h server] [-i initial_ttl] [-k] [-l] [-m maxhops] [-N] [-n] [-p port] [-Q maxquit] [-q nqueries] [-r] [-S] [-s source_addr] [-t tos] [-v] [-w waittime] 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 packets) 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 38 bytes, but this may be increased by specifying a packet size (in bytes) after the destination host name. This becomes especially interesting when the -f option is given for MTU discovery along the route. One should start with the maximum size for your own network interface (if the given value is even bigger, traceroute will try to select a more appropriate value).

Other options are:

OPTIONS

-A  Look up the AS-number (Autonomous System) for each hop's network address at the whois server specified by the -h option.

-a  If the destination host has multiple addresses, traceroute will probe all addresses if this option is set. Normally only the first address as returned by the resolver is attempted.

-c stoptime
    Specify a delay (in seconds) to pause between probe packets. This may be necessary if the final destination is a Cisco router which does not accept undeliverable packets in bursts.

-f  Disable IP fragmentation. If the given packetsize is too big to be handled unfragmented by a machine along the route, a ``fragmentation needed'' status is returned. The indicator !F is printed. Some routers will return the value of the proper MTU size to be used. traceroute will trim down the packet size automatically to this new value. If the proper MTU size is not returned, a shorter packet size will be chosen according to the suggestions in RFC 1191. (Note that this option has no effect on those platforms which do not allow raw ip socket manipulation).

-g gateway
    Enable the IP LSRR (Loose Source Record Route) option. This is useful for asking how somebody else, at the specified gateway, reaches a particular target.

-h server
    Specify the name or IP address of the whois server that will be contacted for the AS-number lookup, if the -A option is given.

-i initial_ttl
    Set the starting time-to-live value to initial_ttl, to override the default value 1. Effectively this skips processing for those intermediate hosts which are less than initial_ttl hops away.

-k  Keep the connection to the whois server permanently open. This will speedup lookups considerably, since otherwise a connection needs to be set up for each individual lookup. Unfortunately, not all whois servers support this feature.

-l  Print the value of the ttl field in each received packet (this can be used to help detect asymmetric routing).

-m maxhops
    Set the max time-to-live (max number of hops) used in outgoing probe packets. The default is 30 hops (the same default used for TCP connections).

-N  Display the network name for each hop. This will be useful only if you have linked traceroute with the BIND 4.9.3 resolver library, and the network names have been stored in DNS according to RFC 1101. If this is not the case, network names are retrieved just from the /etc/networks file.

-n  Print hop addresses numerically rather than both symbolically and numerically (saves a nameserver address-to-name lookup for each gateway found on the path). It also prevents a reverse lookup for numeric dotted quad addresses given on the command line (destination host, or -g gateway addresses).

-p port
    Set the base UDP 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 maxquit
    Stop probing this hop after maxquit consecutive timeouts are detected. The default value is 5. Useful in combination with -S if you have specified a big nqueries probe count.

-q nqueries
    Set the desired number of probe queries. The default is 3.

-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(8C)).

-S  Print a per-hop min/avg/max rtt statistics summary. This suppresses the per-probe rtt and ttl reporting. For better statistics you need to increase the default nqueries probe count. See also -Q.

-s source_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.

-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 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 `-t 16' (low delay) and `-t 8' (high throughput).

-v  Verbose output. Received ICMP packets other than "time exceeded" and "unreachable" are listed.

-w waittime
    Set the time (in seconds) to wait for a response to a probe (default 3 seconds).

DEFAULT OPTIONS
Default options and parameters can be preset in an environment variable TRACEROUTE_DEFAULTS using the same syntax as on the command line. They will be evaluated before the command line arguments.

CUSTOMIZING AS-number LOOKUPS
The built-in defaults for AS-number lookups can be overruled by defining three environment variables.

AS_SERVER_HOST
    Defines the name of the whois server to be queried. The command line option -h always takes precedence. If not set, and if no server is given on the command line, lookups will not be performed.

AS_SERVER_PORT
    Can be used to overrule the default whois port 43. There is no command line option to define an alternative whois port.

AS_SERVER_OPTIONS
    Gives special options to be sent to the whois server for each lookup. By default, no special options are defined.

INTERNALS
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 (change with -i flag) 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 and can be changed with the -m flag). Three probes (change 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 3 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).

EXAMPLES
A sample use and output might be:
    [yak 71]% traceroute nis.nsf.net.
    traceroute to nis.nsf.net (35.1.1.48), 30 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
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 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.

A more interesting example is:
    [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
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

SPECIAL ANNOTATIONS

!H  Host is unreachable.

!N  Network is unreachable.

!P  Protocol is unreachable.

!F  Fragmentation needed. This indicator may show up if the -f command line option is being used, and the associated gateway requires further fragmentation. In case the desired new MTU size is known, it is indicated.

!S  Source route failed. Should not occur under normal circumstances and the associated gateway is busted if you see one.

!T  Host or network is unreachable for the given tos.

!U  Destination is unreachable. This indicator is printed for some of the new unreachable subcodes as defined per RFC 1812.

!A  Some routers fail to generate an ICMP "port unreachable" message, but send an ICMP "time exceeded" message instead, if they are the target host. The indicator is printed if this is detected.

!G  Other routers erroneously generate ICMP "port unreachable" instead of "time exceeded", if they are specified as loose source route gateway hosts. The indicator is printed if this is detected.

If all the probes result in some kind of unreachable, traceroute will give up and exit.

TTL INDICATION

(ttl=n!)
    indicates that the ttl value in the ICMP "time exceeded" packet that we received was "unexpected". What we expect is that the value will be (some initial value - the number of routers between us). In other words, if the path from hop 5 to us is the same as the path from us to hop 5, we expect to receive a ttl value of (some initial value - 4). Unfortunately, there are several common "initial value"s for ICMP ttls. The most common are 255, 60, 59, 30, 29. (4.3 tahoe BSD and cisco routers use 255, Proteon routers use either 59 or 29 depending on software release, several other implementations use 60 and 30.) traceroute checks against all of these, making it hard to detect some "off by one" routing asymmetries. If you want to see all the ttl values in all the packets, use the "-l" option.

OTHER EXAMPLES
    traceroute -g 10.3.0.5 128.182.0.0

will show the path from the Cambridge Mailbridge to PSC while

    traceroute -g 192.5.146.4 -g 10.3.0.5 35.0.0.0

shows how the Cambridge Mailbridge reaches Merit, by using PSC to reach the Mailbridge.

WARNING
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.

Rewritten by Eric Wassenaar, Nikhef-H, <e07@nikhef.nl>

SEE ALSO
netstat(1), ping(8)