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TRACEROUTE(8) System Manager's Manual TRACEROUTE(8)

traceroute, traceroute6print the route packets take to network host

traceroute  [-ADdIlnSvx] [-f first_ttl] [-g gateway_addr] [-m max_ttl] [-P proto] [-p port] [-q nqueries] [-s sourceaddr] [-t toskeyword] [-V rtable] [-w waittime] host [datalen]

traceroute6 [-ADdIlnSv] [-f first_hop] [-m max_hop] [-p port] [-q nqueries] [-s sourceaddr] [-t toskeyword] [-V rtable] [-w waittime] host [datalen]

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. traceroute and traceroute6 attempt to elicit TIME_EXCEEDED responses from each gateway along the path to host, in order to determine their route. traceroute is used for IPv4 networks and traceroute6 for IPv6.

The options are as follows:

Look up the AS number for each hop address. Uses the DNS service described at
Dump the packet data to standard error before transmitting it.
Turn on socket-level debugging.
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 traceroute. The default is 1 (skip no hosts).
Add 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.
Use ICMP or ICMP6 ECHO instead of UDP datagrams.
Display the TTL or hop limit value of the returned packet. This is useful for checking for asymmetric routing.
Set the maximum TTL or hop limit. The default is the value of the system's net.inet.ip.ttl or net.inet6.ip6.hlim sysctl(8) variable, which defaults to 64.
Print hop addresses numerically rather than symbolically and numerically (saves a nameserver address-to-name lookup for each gateway found on the path).
Change the protocol being used from UDP to a numeric protocol or a name as specified in /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 ping(8)). This option is not available for IPv6.
Set the base UDP port number used in probes. The default is 33434. traceroute hopes that nothing is listening on UDP ports base to base+nhops*nqueries-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.
Set the number of probes per TTL to nqueries. The default is three probes.
Print how many probes were not answered for each hop.
Set the source address to transmit from, which is useful on machines with multiple interfaces.
Set the type-of-service (TOS) in probe packets. The value may be one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, af11 ... af43, 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 lowdelay and throughput.
Set the routing table to be used.
Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs are listed.
Set the time, in seconds, to wait for a response to a probe. The default is 3.
Print the ICMP extended headers if available. This option is not available for IPv6.
The destination host, specified as a host name or numeric IP address.
The probe datagram length. The default is 40 bytes for IPv4 UDP and 60 bytes for ICMP, IPv6 UDP and ICMP6.

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 -m option). Three probes (the exact number can be changed using the -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 -w option), a "*" is printed for that probe. If the host cannot be reached, traceroute skips printing lines consisting only of * until the maximum TTL/hop limit is reached.

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 -p option).

A sample use and output might be:

$ traceroute
traceroute to (, 64 hops max, 56 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 4.3BSD). 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:

$ 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 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 (  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 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. 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 (HP-UX) 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 , , (got a host, network or protocol unreachable, respectively), , (access to the network or host, respectively, is prohibited), (communication administratively prohibited by filtering), or (source route failed or fragmentation needed - neither of these should ever occur and the associated gateway is busted if you see one), (destination network or host unknown), (destination network or host unreachable for TOS), (other ICMP unreachable code). (TOS bit in returned packet differs from last hop). If almost all the probes result in some kind of unreachable, traceroute will give up and exit.

$ traceroute -g

will show the path from the Cambridge Mailbridge to PSC, while

$ traceroute -g -g

will show the path from the Cambridge Mailbridge to Merit, using PSC to reach the Mailbridge.

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.

ping(8), route(8)

The very first traceroute (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. traceroute 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.

The traceroute command first appeared in 4.3BSD-Reno. The traceroute6 command first appeared in the WIDE Hydrangea IPv6 protocol stack kit.

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.

March 24, 2024 OpenBSD-current