tracepath

tracepath needs no special rights and reports the path MTU: it starts with packets as large as your card allows and lowers the size each time a router answers fragmentation needed. pmtu 1500 at the end means full-size packets get through; a lower number shows which hop is the narrow one.

tracepath command usage

tracepath [-n] ADDRESS

Options

-n

addresses only, no name lookups

tracepath examples

tracepath -n 10.0.2.10

Traces the path to 10.0.2.10 and reports the path MTU at the end.

Reading the output

$ tracepath -n 10.0.2.10
 1?: [LOCALHOST]                      pmtu 1500
 1:  10.0.1.2                                             0.064ms
 2:  10.0.2.10                                            0.030ms reached
     Resume: pmtu 1500 hops 2 back 2

• pmtu 1500 on a line: from that hop on, packets up to 1500 bytes pass. A drop here (pmtu 1400) shows the narrow link.
• Resume: pmtu … hops … back …: the path MTU, hops there, and hops the replies came back over.

Pitfalls

• If a router drops the fragmentation needed message, tracepath cannot see the narrow hop either: that is the PMTU black hole.