Re: [gentoo-user] PMTUD

[Grant <emailgrant@gmail.com>]

>> Here's my layout:
>>
>> laptop+shorewall (MTU:1500) -> hotel router (MTU:?) -> internet ->
>> Westell modem/router (MTU:1492) -> desktop+shorewall (MTU:1500)
>>
>> Shouldn't PMTUD change the desktop's MTU to 1492?
>
> Your desktop's PMTUD will get an ICMP response from Westell as it tries to
> traverse through it and it will adjust the outgoing packet size accordingly.
>
> Ditto with your laptop, when it tries to establish a connection with your
> desktop.
>
>
>> Is the fact that it
>> doesn't due to a flaw in the Westell's operation?  Should I manually
>> change the desktop's MTU to 1492 along with that of other systems on
>> its LAN?
>
> It won't harm if you do.  However, we don't know for a *fact* that the Westell
> is not returning the appropriate ICMP packets (Type 3, Code 4) to your laptop,
> or your desktop.
>
> You can use tcpdump to see what's being sent back and forth.

Ah, so PMTUD doesn't actually change the setting reported by ifconfig.
 It's done on-the-fly.

>> > If you are using Shorewall at your remote server I would expect it to
>> > behave properly and return the correct ICMP packet when it receives a
>> > DF.  However, I am not familiar with the Shorewall properties and
>> > settings, so if you suspect this as the cause of your problem it would
>> > be better if you look into it properly.
>>
>> So I'm sure I understand, this doesn't apply if ICMP is dropped at the
>> Westell?
>
> ICMP echo request may be dropped by Westell's firewall, but ICMP Type 3, Code
> 4 could well be returned when a TCP connection is being initiated by your
> laptop.  I don't know how ham-fisted Westell's firewall settings are.
>
> Meanwhile, stating the obvious, have you tried using the desktop squid proxy
> while you are inside your LAN with the same laptop to see if the problem
> remains?

That's a fine idea and I will do so once I get back there.  Thanks for
the education Mick. :)

- Grant