[Gllug] MTU discovery (was Re: Pidgin)

[Alistair Mann]

Ben Whyte wrote:
> Afternoon
>
> I am having a odd problem and I think its been around for a while.  I am 
> unable to connect to the website for Pidgin.  Specifically 
> www.pidgin.im.  It appears to have dissapeared.
>
> Does anyone know why or what has happened ?
>   
Works fine here, and worked fine when I looked at it last week. Single 
sites that are down for "a while" often resolve to MTU problems, IME, 
something that's usually platform independent - ie, the router's fault.

On linux, you can work out the Maximum Transmission Unit between you and 
a website with:

    ping -s 1472 -M do -c 1 www.pidgin.im

On a working connection, you should receive a pong back as per normal. 
If the MTU is too low, you'll receive something akin to

    "From 192.168.0.100 icmp_seq=1 Frag needed and DF set (mtu = 1500)"

Modify 1472 up and down; at some value it'll work, and at the next value 
larger, it won't. That last working number is your MTU.

Use traceroute twixt you and the website, and repeat the above test for 
each router that responds en route: when the MTU suddenly drops, the 
problem is the router you last looked at. (Unless, of course, you are 
checking the first router, when it could equally be your box that's the 
problem.) Most routers have an admin bit where you can change the MTU: I 
would suggest changing it even if the value it has appears to be fine.

Or you might investigate whether you've been mischiefed by someone with 
access to /etc/hosts ;-)

Just a couple ideas,
-- 
Alistair Mann



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