Re: firewalls and MTU - using ping?

Adam Hardy on 22/10/10 13:36, wrote:
> I have a small LAN at home connected up to the net via a DSL modem on a 
> gateway machine running lenny and iptables.
> 
> I have a problem with some software running on a windows machine on the 
> LAN where the company who wrote the sofware say that there must be 
> something wrong with my internet connection, but can't help any further.
> 
> I'd like to know what tests I can run to verify that my config and my 
> firewall are OK, especially regarding PMTU.
> 
> I had an issue before this which I solved by adding some ICMP related 
> rules to iptables, and by disabling the firewall on my DSL modem, which 
> is undocumented by the manufacturer (and denied by their Support). It's 
> iptables running on an embedded linux and resets itself every power 
> cycle. I think it does ICMP clamping but I can telnet onto the modem and 
> drop the rules (as long as I remember).
> 
> What can I do with ping? This ping command gives suspicious results:
> 
>  ping -s 1473 mktgw1.ibllc.com
> 
> One byte less and it works. Fragmentation also works for normal websites 
> like news.bbc.co.uk.
> 
> I'd massively appreciate any help with this! I'm stuck with either my 
> own mistake, or until I can prove I haven't made one.

The main thing to prove that my gateway, firewall, LAN, modem and all are OK is 
to find a command, e.g. ping, which will demonstrate that PMTU isn't working 
between me and mktgw1.ibllc.com - or maybe that something else is bust, not PMTU.

I mean, maybe with the concentration on firewalls and all that here, maybe this 
list isn't the place to ask and I need to ask somewhere else - suggestions welcome.

Thanks
Adam