Debian logs a lot of stuff to the console, esp. when you are working with firewalls. This section describes what you can do to solve this.
To make dmesg behave nicely and prevent it from sending everything it receives to the console, change the bootmisc.sh script in /etc/init.d to read like this:
# # Save kernel messages in /var/log/dmesg # dmesg -s 65536 > /var/log/dmesg dmesg -n1
After this only panic messages will be printed to the console, which is mostly what you want.