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TRACEPATH(8)           System Manager's Manual: iputils           TRACEPATH(8)

       tracepath, tracepath6 - traces path to a network host discovering MTU along this path

       tracepath [ -n]  [ -l pktlen]  destination [ port]

       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 not require superuser privileges and has no fancy options.

       tracepath6 is good replacement for traceroute6 and classic example of application of Linux error  queues.   The
       situation with tracepath 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.

       -n     Do not look up host names.  Only print IP addresses numerically.

       -l     Sets the initial packet length to pktlen instead of 65536 for tracepath or 128000 for tracepath6.

       root@mops:~ # tracepath6 3ffe:2400:0:109::2
        1?: [LOCALHOST]                              pmtu 1500
        1:                   0.411ms
        2:        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 correspinding hetwork hop. As rule  it  con-
       tains  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 direc-
       tion is shown folloing 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 differ-
       ent when the path is asymmetric.

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

       tracepath was written by Alexey Kuznetsov <>.

       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.

       tracepath  is  part  of  iputils  package  and  the  latest  versions  are   available  in   source   form   at

iputils-071127                   22 March 2017                    TRACEPATH(8)