I host an e-commerce website for a client who suddenly is unable to access it from his computer. He can ping the server, he can SSH in, but cannot load the website in any browser we've tried: IE, Firefox, Chrome, Opera. Other machines at this location can connect ok. DNS resolves fine, accessing direct with IP address does not work. Using Wireshark on the server side I can see the requests come in, and the responses leave our switch. Wireshark on the client end sees no reply.
Tried so far:
I find nothing in the client's router settings that would block the site for this one computer. He's had his public IP reassigned, and he's tried a different private IP to no avail.
This all seems to point to malware or an otherwise busted Windows 7 install.
The kicker: carry the machine off-site to a different Internet connection, and it works fine. I have never heard of malware that would deny only return responses from one specific server and only while working from a
certain public IP address specific physical location (public IP changed, no difference).
In my experience when some sort of filter is the cause, either all the machines behind a single connection would be blocked, or a single machine would be blocked behind all connections. Here it seems neither applies.
What could be the problem? What do we test next?
I noticed the initial redirect to
www. was working before the hang, and that the favicon was coming through. Sure enough, both these requests were under 1k and it lead us to check the MTU values on the client and server. But why were they changed?