Hello everybody!

I'm only slightly good with Linux, but I've managed to set up a working Debian server, with SSH, Apache (With SSL, PHP and Mono), MySQL, FTP, IRC, Samba and TeamSpeak3. All these services are working as they should.

I'm not sure to which extend it's important, but the server is connected to a router, which is configured to forward the ports for SSH, HTTP, HTTPS, FTP, IRC and TS3 to the Debian server. The router is also connected to two network printers and a switch. The switch is connected to two Windows PCs and a WLAN access point, through which up to three laptops and an XBox usually connect.

Everything was well quite some time, but recently (I'm not sure since when, but at least since a few weeks ago) I noticed that HTTP connections as well as SSH and IRC connections to the server fail on the first attempt (On the local network, with timeouts). When I wait a few seconds and try again, everything works just fine. TS3 also takes unusually long to connect.
I haven't noticed these problems on FTP (With SSL) and Samba connections. Also, I'm only connecting to MySQL through PHP so that's not applicable.

While connecting over HTTP (first attempt) and following the traffic with Wireshark, I noticed a [TCP Dup ACK], so a package apparently gets duplicated. Immediately after that, the connection is closed.

As I tested this just now with my PC (One connected to the switch), I noticed that my Laptop apparently doesn't have these connection problems. The strange thing is, however, the PC also doesn't have any issues connecting to other computers (local and remote), so I'm not quite sure anymore if it's a problem with the Debian server or my Windows PC.

I've tried to restart Apache and restarted the whole server, which did not help. Since I'm not that good with Linux or networking, I don't know where to start looking for the problem.

Any suggestions and/or help?

You mentioned TCP DUP ACK which can only suggest that packets are being dropped somewhere. I have a hunch that its due to your CPU load on your Debian Server (or maybe a crappy network card- have you tried disabling TCP Checksum offload?) but without any logs or any configuration information, I cannot help you. What have you tried?

Thanks for the reply!

It could be that the CPU load is at fault, the server is a rather old computer, by todays standards (AMD Athlon, 1,9GHz), although it shouldn't get much traffic besides from my connections. The network card should be a built in card on the mainboard, so not the best either.

As I don't really know how to trace errors, all I have tried is switching the services and the server off and on again. I didn't even know you could enable/disable TCP Checksum offload, so I haven't tried that.
If you need any logs, just ask which ones/where they are and I'll post them.

After further testing with Wireshark, I noticed that packets don't get sent at all on the first attempt. I've tried this with SSH, FTP and HTTP, the effect is the same across different Programs. I've come to the conclusion that this is a Windows specific problem after all (I still don't know where that other TCP DUP ACK packet came from, I probably misread the address field...). I'll reask this question on a Windows related forum.

Thanks to everyone who read about my problem and thought about it!