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 65535 for
tracepath or 128000 for tracepath6.
-m Set maximum hops (or maximum TTLs) to max_hops instead of 30.
-p Sets the initial destination port to use.
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
correspinding 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
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.
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.
This page is part of the iputils (IP utilities) project. Information
about the project can be found at ⟨http://www.skbuff.net/iputils/⟩.
If you have a bug report for this manual page, send it to
firstname.lastname@example.org, email@example.com. This page was obtained
from the project's upstream Git repository
⟨git://git.linux-ipv6.org/gitroot/iputils.git⟩ on 2016-07-16. If you
discover any rendering problems in this HTML version of the page, or
you believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
iputils-151218 16 July 2016 TRACEPATH(8)