Arch Linux

Gullible Jones

Member

Registered: 2004-12-29

Posts: 4,863

Utilities for MTU resolution?

My router does not support MTU resolution, and it looks rather tedious (if actually possible) to find the MTU(s) that my DNS servers are using via the 'ping' command. Are there any utilities in the repos that would allow me to determine MTUs without much hassle?

cactus

Taco Eater

From: t͈̫̹ͨa͖͕͎̱͈ͨ͆ć̥̖̝o̫̫̼s͈̭̱̞͍̃!̰

Registered: 2004-05-25

Posts: 4,622

Website

Re: Utilities for MTU resolution?

ping -M do 192.168.1.1 -s 2000 -c 1

this sets the do not fragment bit, and should return an icmp fragmentation needed packet, specifying the largest size packet possible for the current hop (or any hop in between that is too large).

Lower the packet size accordingly. realize that icmp headers add 28 bytes.. pretty sure..
so a ping -s 1472, would translate to a 1500 byte frame

---

"Be conservative in what you send; be liberal in what you accept." -- Postel's Law
"tacos" -- Cactus' Law
"t̥͍͎̪̪͗a̴̻̩͈͚ͨc̠o̩̙͈ͫͅs͙͎̙͊ ͔͇̫̜t͎̳̀a̜̞̗ͩc̗͍͚o̲̯̿s̖̣̤̙͌ ̖̜̈ț̰̫͓ạ̪͖̳c̲͎͕̰̯̃̈o͉ͅs̪ͪ ̜̻̖̜͕" -- -̖͚̫̙̓-̺̠͇ͤ̃ ̜̪̜ͯZ͔̗̭̞ͪA̝͈̙͖̩L͉̠̺͓G̙̞̦͖O̳̗͍

Gullible Jones

Member

Registered: 2004-12-29

Posts: 4,863

Re: Utilities for MTU resolution?

Thanks... FWIW, the man page says something about a 56-byte header IIRC.

Gullible Jones

Member

Registered: 2004-12-29

Posts: 4,863

Re: Utilities for MTU resolution?

As it happens, ping also tells you what the MTU is if there's an error... Again, thanks.