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In a case I can use only UDP and ICMP protocols, how can I discover, in bytes, the path MTU for packet transfer from my computer to a destination IP?

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  • Do you want to solve this programmatically or with ping? Ping and either shell programming or combined with traceroute are options for example. Also do you want ipv4 or ipv6?
    – Max
    Commented Dec 30, 2012 at 17:46
  • @Max: shell programming it sounds good , have an idea for that ?
    – URL87
    Commented Dec 30, 2012 at 17:58
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    I thought about using ping -s $SIZE -c 1 -M dont and lowering that SIZE value till it works, or just having a look at the ICMP returns via tcpdump if we are talking ipv6 but traceroute has this built in I just realized.
    – Max
    Commented Dec 30, 2012 at 18:06

3 Answers 3

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I believe what you are looking for, is easiest gotten via traceroute --mtu <target>; maybe with a -6 switch thrown in for good measure depending on your interests. Linux traceroute uses UDP as a default, if you believe your luck is better with ICMP try also -I.

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A quick search with apropos mtu on CentOS5 turned out:

tracepath traces path to a network host discovering MTU along this path

This command even works unprivileged in user space.

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  • I prefer this over traceroute since it returns a simple result at the end and I don't have to look for the last hop that returned a different MTU.
    – comfreak
    Commented Aug 14, 2017 at 17:22
  • Alas, tracepath doesn't let you choose which network interface to use. You can choose with traceroute (using -i).
    – Ian D. Allen
    Commented Feb 14, 2020 at 19:35
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Under Python, you are just one install away:

pip install ofunctions.network

Then you can run the following python code

from ofunctions.network import probe_mtu

print(probe_mtu(target_IP))

This is the simplest syntax, but you may also force ICMPv6 or ICMPv4 discovery, and limit the minimum and maximum probe settings in order for the probing to happen faster.

Disclaimer: I am the author of ofunctions ;)

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  • I guessed that you meant print(probe_mtu("142.250.187.206")), but that just gave me ValueError: Both sides of the argument list are unexpected results. Did I misunderstand your instructions?
    – Robin Green
    Commented Dec 12, 2021 at 21:57
  • I just tried that code again. Works for me. What version of python and ofunctions.network do you use ?
    – Orsiris de Jong
    Commented Dec 17, 2021 at 10:04
  • Python 3.9.9 and ofunctions.network-1.0.0
    – Robin Green
    Commented Dec 17, 2021 at 16:48
  • I've just tried with Python 3.9.9 x64 on Win10 x64 without any trouble. There may be two solutions, or you don't have internet while trying this command (or at least ECHO request filtering somewhere), or your ping command is not standard. Can you give more details on your environment in order to replicate the issue ?
    – Orsiris de Jong
    Commented Dec 20, 2021 at 8:26
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    It's pretty slow, but it works. If you want to see some progress, run tcpdump along side and see it binary searching (i believe) the value for the path mtu. Rough command: sudo tcpdump -i any icmp -n
    – Valer
    Commented Feb 7, 2022 at 15:39

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