As neat seems to be currently broken due to python being broken, how can
I set a DNS server address? ifcfg eth0 xxx.xxx.xxx.xxx gives me an error
that eth0 cannot be contacted (or something like that)

For set an ip address:

into a command line type:

ifconfig eth0 111.222.333.444/24 up
or
ifconfig eth0 111.222.333.444 netmask 255.255.255.0 up

route add default gw 111.222.333.444

For Setting DNS
edit the /etc/resolv.conf and add a line like this

nameserver 111.222.333.444