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


       tracepath [-n] [-b] [-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 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 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.


       -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 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  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.


       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


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