Thank you all who gave advice on /etc/hosts.deny. I just put in the line
portmap: ALL
which I found in the NFS HOWTO and it worked. I tried that before I asked for
advice and it didn't work. Maybe I had a bad day.
Now I can't mount anything on this machine from the other one on the LAN, and
I hope this will prevent anyone from the outside world to break into my
system, because that's why I do it.
To allow the other machine on the LAN access to mine again I put the line
portmap: 192.168.1.0/255.255.255.0
in /etc/hosts.allow
but the other machine still can't mount anything. It gets the error message
mount: RPC: program not registered
just like with an empty /etc/hosts.allow.
I took this from the NFS HOWTO, section 6.3 again.
192.168.1.0 is the network and 255.255.255.0 is the netmask in
/etc/network/interfaces under eth0 on both machines.
So how can I allow the other machine in my network to access mine without
allowing anyone from the internet to break in?