NAT & forwarding : only partial connections

[Francesco Bochicchio <bockman@virgilio.it>]

Hi all,

I have a laptop with installed Sarge and a PC with installed Sid. I would
like to implement this schema:

   Laptop <-wireless-> PC+ADSL Modem< phone lines> Provider<..>Internet

I had it working in the past, but after a reinstallation of SID on the
PC, something broke.  

All the connections works: I can connect to internet from the PC and I can
connect the laptop and the PC via wireless.

What I do is this:
	On the PC : iptables -t NAT -A POSTROUTING -o ppp0 -j MASQUERADE
	            echo 1  > /proc/sys/net/ipv4/ip_forward
	On the laptop:
		    route add default gw pc-ip # where pc-ip is the 
                                               # IP of the PC wlan0

What I get : partial connections. From the laptop, I can ping internet
addresses like www.debian.org and I can resolve host names using my
provider DNS, but this is all. Browsers can 'connect' but do not get the
pages. Fetchmail can 'see' the mail on the provider server, but fails to
download it. And so on. Clearly some packet goes through, some is blocked.
But there is no other iptables rule on either laptop or PC.

Even more strange, if I invert the machines and use the laptop as gateway, 
everything works fine with the same setup. So, it has to be some
difference in the machine configuration. I have compared network
configurations, installed the same kernel version  (both standard
kernel-image debian packeges), manually loaded the same modules, but
without success. 

Any clue, enybody?

Ciao
-----
FB