help about NAT and ISP - developing a kernel module

[Giacomo]

Good morning, I'm Giacomo Strangolino from Italy.

I finished developing an ipv4 packet filter with NAT/MASQUERADING and have 
been
testing it
for some time with success connecting from home to my ISP named "libero".

Then i changed ISP to another one, called "telecom" and with great surprise
i discovered that
images from sites and also sites failed to load.

So now, when i call an ISP all works fine, when i call the other, things go
wrong.

I NAT machines behind my firewall changing only ips and ports, and
recalculating checksum (ip and tcp/udp)
to adjust such changes.
I do not touch any other field as window size or seq number or ack, since
the only things i manipulate are
addresses and ports.

I was wondering what i could do to solve, since iptables and ipfw+natd on
freeBSD or winXP sp2 work fine
with this ISP...

Tweaking with ethereal i found that probably sometimes a tcp segment gets
lost.

My firewall is a 2.6.12 kernel module which registers with netfilter hooks.
A userspace program sends rules to
kernel via netlink.

I thank you if you could help me find the way to fix the problem or
understand what could be wrong with an
ISP network and anyway work fine with the other.

Also any indication of where in iptables source is solved such problem
would be appreciated.

I have been consulting news for many days and until now i tried to resolve 
the issue
without success in the following ways:

- reducing MTU on both gateway and internal hosts.
- trying with the option --clamp-tcpmss-to-pmtu.

Both failed and problem persisted.


Thanks a lot in advance.

Giacomo S. Udine, Italy