missing LAN traffic slows down internet traffic
I have the following problem:
Two computers are connected to my Router, one running Debian (stable) and one running Windows, both connected via cable. The Debian machine doesn't have a GUI (old Thinkpad with broken display), I log in to it with putty via ssh. The Windows machine always can download from the internet with full speed (4-5mb/sec), the Debian machine not (100-200kb/sec). That's strange but I would have explanations for this, the really strange thing is that the Debian machine downloads with full speed when I transfer data between both machines (in my LAN) via scp. So "no traffic in LAN" slows down internet speed of my Debian machine by a factor of 20-50. That behaviour is weird and completely reproducible. I use wget, and the data is really on Hd, so wget doesn't miscalculate speed and I don't measure LAN speed either, it's really the internet speed that goes up when transferring data in LAN. Another machine reaches full speed when using the same cable the Debian machine uses.
I already tries the following:
-installed Ubuntu 12.04 server
-did a fresh install of debian stable
-tried all ports of my Router (can't be the reason, cause another machine does work on the same cable)
-sysctl -w net.ipv4.tcp_congestion_control=reno
-sysctl -w net.ipv4.tcp_congestion_control=cubic
-sysctl -w net.core.wmem_max=12582912
-sysctl -w net.core.rmem_max=12582912
-sysctl -w net.ipv4.tcp_window_scaling=1
-sysctl -w net.core.netdev_max_backlog=5000
Nothing fixed my problem, so I reset these values to default afterwards.
Ping times are constant regardless of downloading or not. CPU load is under 30%.
Does anybody have an idea of what could be the issue? Or is there something like /dev/null for network? I could always copy data there as a workaround ;).
Details about my machine can be viewed in attachment.
I've been using debian for many years, but I'm no expert by far and don't have an idea what I could do, so any hint is appreciated.
Thanks in advance!