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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: 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 cor-
respinding hetwork hop. As rule it contains value of RTT. Addition-
ally, 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 fol-
loing 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 destina-
tion, 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
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 http://www.skbuff.net/iputils/iputils-cur-
iputils-071127 23 July 2015 TRACEPATH(8)