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

       tracepath -6 is 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 will
       be 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 of 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 TTL of the probe, followed by colon. Usually value of TTL is
       obtained from reply from network, but sometimes reply does not contain 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 address
       of router or word [LOCALHOST], if the probe was not sent to the network.

       The rest of line shows miscellaneous information about path to the corresponding network
       hop. As rule it contains value of RTT. Additionally, it can show Path MTU, when it
       changes. If the path is asymmetric or the probe finishes before it reach prescribed hop,
       difference between number of hops in forward and backward direction is shown following
       keyword async. This information is not reliable. F.e. the third line shows asymmetry of 1,
       it 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 path to the destination, it shows
       detected Path MTU, amount of hops to the destination and our guess about amount 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 this kind.  tracepath may be executed by everyone who
       has some access to network, enough to send UDP datagrams to investigated destination using
       given port.

AVAILABILITY

       tracepath is part of iputils package.