[PLUG] Cannot set mtu in profile.d on Ubuntu 8.10

Abhijit Bhopatkar bain at devslashzero.com
Tue Apr 13 12:39:33 IST 2010

> 1. Even though I understand that the VPN server adds headers and hence the
> need to lower the MTU, I cannot understand why communication stalls when I
> talk to CVS in such a situation, and it does not stall with regular internet
> traffic.

The cvs will have a huge bulk traffic going in and out. The main
problem with MTU is not at server or client side, but is usually in
the middle, the routers. The problem is that some routers on the
network are not able to handle large packets very well. They start
building up huge chains of backlog packets and eventually cause TCP
retransmissions. The problem keeps aggravating very quickly and within
seconds the network traffic for that particular connection starts
grounding to halt. Addition of VPN on top of CVS might just be tipping
the sizes beyond the trigger point for your network setup. The problem
is typically intense when a cheap router is being used to share a
broadband connection on LAN.

This is just a guess, all the network guru's thrash me  if i am wrong :p

> 2. I tried adding a script to my profile.d which would set the MTU for wlan0
> and eth0 to 1200. However, this does not seem to have any effect. So I have
> to set the MTU manually everytime I boot into the machine. Could this be
> becasue the network interfaces are not in a state to be configured at the
> time scripts in profile.d are run?

The Network manager might be resetting the interfaces, you can add
scripts in  /etc/NetworkManager/dispatcher.d instead to set the MTU on
connection UP or DOWN for every interface.
Take a look at 01ifupdown script as an example you should add 02MTU or
something like that.

BTW you could also use the if-updown scripts as being invoked from
01ifupdown. I never used them so don't know how those work.


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