Comcast, DHCP and MTU

If you are a Comcast customer (particularly in Illinois) and your Voice-over-IP (or other large-UDP-packet-using) application does not work as it should, don't worry, it's their fault. :P

For some reason, they include

...
OPTION:  26 (  2) Interface MTU             576
...

in the replies from their DHCP servers, which leads your computer to throw away large UDP packets (such as SIP INVITE replies). I don't know why they'd do this since a) it breaks their customers' applications (on purpose?!) and b) it puts more load on their network.

In any case, here's the antidote: In dhclient.conf, say

# F!$#$ stupid comcast.
supersede interface-mtu 1500;

After that, everything should be working fine. If you're violating their terms of service in doing so, it's your own fault. I didn't tell you. 8)

It only cost me like two hours of my life to find and fix this. Quote from the reply from the customer service rep:

I'm sorry, but we do not have any information as to why the MTU is configured for the level that it is. That decision is likely made by the Network Department and something we would not be able to obtain or discuss.

I love you, too.

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Submitted by Anonymous on Mon, 27/07/2009 - 01:13.

Well, if you could imagine that there was a # mark in front of the bolded text..just get rid of interface-mtu and that solves the problem

Submitted by Anonymous on Mon, 27/07/2009 - 01:11.

I just ran into this.

My fix was to make dhclient not even ask for the MTU, which it is configured to do by the following line in dhclient.conf:

request subnet-mask, broadcast-address, time-offset, routers,
        domain-name, domain-name-servers, domain-search, host-name,
#       netbios-name-servers, netbios-scope, interface-mtu,
        rfc3442-classless-static-routes;

The strategically-placed # mark solves the problem (we don't care about netbios-name-servers or netbios-scope either).

This makes dhclient leave the MTU at it's default 1500.