This is one of those problems when solution is right in front of my eyes but I am so tired that I don't see even a white cat.
Anyway, my router gone out of order so I bought new one. And then, moreover, my ISP changed my static IP (some strange explanation that they had to do so because of new MAC address of my new router).
So, I changed everything I had to change in router, ISPConfig etc., etc.
And even all of my websites are visible, dns servers are functional, email is doing fine I have one BIG problem:
Apt-get can't RESOLVE. ping neither.
Now comes the strange part. I have a backup server (server200). Same Debian Etch box. Same ISPConfig. Same everything. And on backup machine everything works fine. ping, dig, apt-get etc.
Now, what I need is additional pair of eyes who are going to tell me what I done wrong.
For the beginning here is my /etc/network/interfaces (from troublesome server201). It's the same as /etc/network/interfaces file from server200 besides LAN IP address.
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
iface eth0 inet static
Debian Lenny & ISPConfig 3