# tracepath

tracepath

8 iputils

tracepath

traces path to a network host discovering MTU along this path

          tracepath

            -4

            -6

            -n

            -b

            -l
            pktlen

            -m
            max_hops

            -p
            port

            -V

          destination

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.

tracepath -6 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.

`-4`

- Use IPv4 only..

`-6`

- Use IPv6 only..

`-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 IPv4 or 128000 for IPv6.

`-m`

- Set maximum hops (or maximum TTLs) to max_hops instead of 30.

`-p`

- Sets the initial destination port to use.

`-V`

- Print version and exit.

## OUTPUT

    root@mops:~ # tracepath -6 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 corresponding 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.

## SEE ALSO

traceroute

8 ,

traceroute6

8 ,

ping

8 .

## AUTHOR

`tracepath` was written by Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>.

## SECURITY

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.

## AVAILABILITY

`tracepath` is part of iputils package.