tracepath, tracepath6 - traces path to a network host discovering MTU along this
] [-l pktlen
] [-p 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.
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.
- Print primarily IP addresses numerically.
- Print both of host names and IP addresses.
- Sets the initial packet length to pktlen instead of
65535 for tracepath or 128000 for tracepath6.
- Set maximum hops (or maximum TTLs) to max_hops
instead of 30.
- 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
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 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.
was written by Alexey Kuznetsov <email@example.com>.
No security issues.
This lapidary deserves to be elaborated. tracepath
is not a privileged
program, unlike traceroute
and other beasts of this kind.
may be executed by everyone who has some access to network,
enough to send UDP datagrams to investigated destination using given port.
is part of iputils
package and the latest versions are
available in source form at