NAT trouble (?) with Apple Macbook Pro

[Daniel <bozo@bozo.dnsalias.net>]

Greetings,

   I have been running for the past few years a Linux box to drive my DSL connection (using an USB modem), and sharing the connection to the rest of my LAN using Iptables masquerading :

    iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

I've never had the slightest trouble sharing that connection to my Windows desktop (and it's been working as well on my Sony PSP). Now I just got myself an Apple laptop (Macbook Pro), and internet access is working erratically (just for that laptop, the Windows desktop still works just fine).

The symptoms : some (not many) web sites work normally. Many others won't display, going into timeout, typically while waiting for images to load (sometimes, I can even get the page layout and text, but never get the pictures). On the other hand, I've been able to establish stable SSH connections through the NAT box.

I am pretty sure the problem doesn't come from :

- the kind of connection between the laptop and the Linux NAT box, as I get the exact same symptoms using wireless or a wired link
- MacOS, as I've also installed Windows on the laptop, and the symptoms persist
- the browser, as I've had the same trouble with Safari, Firefox and Internet Explorer
- some kind of hardware defect in the laptop, as everything works just fine when using my neighbour's internet access

Therefore, I believe the problem to come from the laptop-NAT box interaction. I tried playing around with the MTU, without success. The only explanation left to me would be trouble at the NAT level, though I can't even remotely picture why it would only affect that laptop.

The Linux box is an Ubuntu 6.10, running kernel 2.6.15(ubuntu version : -28), and Iptables 1.3.3 (ubuntu version : -2ubuntu4).

Is there any way I can trace the NAT activity to check whether the problem stems from that or not?

Thanks in advance,

    Daniel