Re: NAT and ISP problem
From: CJT (abujlehc_at_prodigy.net)
Date: 08/31/05
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Date: Wed, 31 Aug 2005 14:15:38 GMT
Giacomo wrote:
> Good morning, I'm Giacomo Strangolino from Italy.
>
> I posted some days ago a problema about NAT and Internet Service Provider
> problems:
>
> I finished developing an ipv4 forewall 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.
>
> * Thanks to news help, i tried to lower MTU and to disable ECN, but the
> problem persists. *
>
> 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 anyone who 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.
>
> Thanks a lot
>
> Giacomo S. Udine, Italy
>
>
>
Did you try the second ISP _without_ the firewall? Maybe there's a DNS
problem or something silly like that which is causing problems with
some images.
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