We're hosting a customer's Debian Linux web server. It's running a PHP based web application.

The server is sitting behind our firewall with it's own virtual interface and port 80 is forwarded internally to a machine sitting in the DMZ.

The issue we're having is that when data is posted to the server it seems to be being cut short for some users. It's reproducable for some users on the same box. But the same user sending the same data on the same lan on another PC it works.

The data gets cut to around 1140 bytes I'm told.

Any idea why this might be happening? The customer is blaming our firewall, but then surely we'd have issues with other services. I'm suspecting it's a problem with the website itself. Suggestions on how to isolate the problem would be of help.

Our firewall is Astaro.

EDIT: A customer has set the ethernet frame size temporarily to 500bytes on the server. This made it work for now! I know some of the customers are using an internet provider that runs PPPoE