Arch Linux

reztho

Member

Registered: 2007-12-16

Posts: 43

CDC_Ether module with MTU of 576

Hi,

   I was trying to make a firewall machine with internet sharing using arch linux. I'm sharing an internet connection with a cable modem which requires the CDC_Ether module. The internal machines had connectivity in a strange way. They can ping and traceroute external IP's but they cannot browse the web. It never loads the web page. So using tcpdump i saw the path MTU discovery protocol was messing the thing.

  It seems by default, connections using the CDC_Ether the MTU is at 576. It's wrong. It must be 1500. When I changed the MTU to 1500 (using ifconfig) i had the internal machines working perfectly. Even more, if i make a speed test (this time in the firewall machine) with the MTU at 576 i get the half of my broadband bandwidth, so it must be changed definitely to the correct value (just for reference, in Ubuntu i had a MTU of 1500).

  So i don't know if this must be changed in the kernel module directly or externally (udev?) but at the moment i think i will make an udev rule or just putting the ifconfig at boot.

NOTE: I'm not using any variant of the kernel, just the regular kernel.

Last edited by reztho (2008-02-07 10:28:38)