Connection sharing with 3G modem, *very* weird problem. Subsequent despair.

[Tobia Tesan]

Hello everybody, I have a problem that's driving me insane.

I have a Thinkpad with 12.04 (hereafter, t-rex) and an iBook with OS X 
10.4.11, freshly installed (hereafter, persephone).

I'm trying to share t-rex's internet connection, achieved via 3G USB 
modem, with persephone.
I did the usual "new wired connection -> ipv4 -> shared with other 
computers" dance on t-rex, connected the two with a ethernet cable and 
expected it to work.

Little did I know, persephone obtains an IP, can ssh to t-rex, can ping 
t-rex and various external servers, can dig, can send and receive mail, 
albeit *very* slowly, but *cannot* browse with Safari nor curl, unless 
it is very short responses (301 redirect and stuff like that).

I tried toying around with the MTA (1400, then 700) with no result.

The interesting bit is that if I remove the 3G modem and tether t-rex to 
my cellphone instead, everything works fine, so I'm assuming the problem 
is between the ethernet cable and the modem (and in fact both laptops 
work just fine when connected to the office's LAN).

Any hints please?

Thanks a lot.

-- 
Tobia Tesan
<tobia.tesan at gmail.com>
It is much easier to suggest solutions when you know nothing about the 
problem.