NAME

       tracepath - traces path to a network host discovering MTU along this path

SYNOPSIS

       tracepath [-4] [-6] [-n] [-b] [-l pktlen] [-m max_hops] [-p port] [-V] {destination}

DESCRIPTION

       It traces the network path to destination discovering MTU along this path. It uses UDP
       port port or some random port. It is similar to traceroute. However, it does not require
       superuser privileges and has no fancy options.

       tracepath -6 is a good replacement for traceroute6 and classic example of application of
       Linux error queues. The situation with IPv4 is worse, because commercial IP routers do not
       return enough information in ICMP error messages. Probably, it will change, when they are
       updated. For now it uses Van Jacobson's trick, sweeping a range of UDP ports to maintain
       trace history.

OPTIONS

       -4
           Use IPv4 only.

       -6
           Use IPv6 only.

       -n
           Print primarily IP addresses numerically.

       -b
           Print both: Host names and IP addresses.

       -l
           Sets the initial packet length to pktlen instead of 65535 for IPv4 or 128000 for IPv6.

       -m
           Set maximum hops (or maximum TTLs) to max_hops instead of 30.

       -p
           Sets the initial destination port to use.

       -V
           Print version and exit.

OUTPUT

           root@mops:~ # tracepath -6 3ffe:2400:0:109::2
            1?: [LOCALHOST]                              pmtu 1500
            1:  dust.inr.ac.ru                   0.411ms
            2:  dust.inr.ac.ru        asymm  1   0.390ms pmtu 1480
            2:  3ffe:2400:0:109::2               463.514ms reached
                Resume: pmtu 1480 hops 2 back 2

       The first column shows the TTL of the probe, followed by colon. Usually the value of TTL
       is obtained from the reply from the network, but sometimes it does not contain the
       necessary information and we have to guess it. In this case the number is followed by ?.

       The second column shows the network hop which replied to the probe. It is either the
       address of the router or the word [LOCALHOST], if the probe was not sent to the network.

       The rest of the line shows miscellaneous information about the path to the corresponding
       network hop. It contains the value of RTT, and additionally it can show Path MTU when it
       changes. If the path is asymmetric or the probe finishes before it reaches the prescribed
       hop, the difference between number of hops in forward and return direction is shown next
       to the keyword "async". This information is not reliable, e.g. the third line shows
       asymmetry of 1. This is because the first probe with TTL of 2 was rejected at the first
       hop due to Path MTU Discovery.

       The last line summarizes information about all the paths to the destination. It shows
       detected Path MTU, amount of hops to the destination and our guess about the number of
       hops from the destination to us, which can be different when the path is asymmetric.

SEE ALSO

       traceroute(8), traceroute6(8), ping(8).

AUTHOR

       tracepath was written by Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>.

SECURITY

       No security issues.

       This lapidary deserves to be elaborated.  tracepath is not a privileged program, unlike
       traceroute, ping and other beasts of their kind.  tracepath may be executed by everyone
       who has enough access to the network to send UDP datagrams to the desired destination
       using the given port.

AVAILABILITY

       tracepath is part of iputils package.