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.
[email protected]:~ # 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 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
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
different when the path is asymmetric.
tracepath was written by
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