I have an old IBM Aptiva where I've been running Debian for some time now, mainly as a web server in order to get more familiar with Linux system administration. Well, I just compiled my own 2.6.16.9 kernel, having used the generic 2.4 kernel before.

It works fine except that there is a major networking problem: I can download / upload stuff with only half speed (at most), and ping spits out a weird warning every 2-6 packets or so: Warning: time of day goes back (* us), taking countermeasures, where * is something near -490000. Some packets also have a response time of 0.000 ms and others several hundred ms (way too high). This applies not only when pinging any machine on the web, but also my desktop machine (running Windows, connected through my ADSL router), and even localhost! When I ping localhost, the response time is sometimes also negative (around -400 ms)!

The time on the system has been updated with NTP and should be quite accurate.

I have not yet tried a generic (precompiled) 2.6 kernel but I will do that soon to find out whether this is a problem with all 2.6 kernels, or just my own. I fear I've left out something that I shouldn't have from the kernel config, but I cannot figure out what. I only left out support for buses and stuff I don't have / need (ISA, FireWire, sound card, etc) and some specific hardware that my machine doesn't have, as well as some file systems and all the kernel debugging stuff. I haven't touched anything in the Networking -> Networking options submenu at all, and I doubt the problem lies anywhere in Device drivers -> Network device support.

Any ideas?

No ideas?

Warning: time of day goes back
The explanation is that return traffic has a timestamp that seems so grossly out of sync with what the local system thinks time should be. Local problem definately. Here's some questions. Did said problem also occur running 2.4.x kernels? You say the box is NTP-synced, but does that mean you're running ntpd or do you sync manually or using a cronjob? If you run ntpd, how often does it throw warnings about syncing time? Is your CMOS battery OK? Did you try running a generic 2.6.x kernel?