When I activate default firewall in ISPConfig3 I got following connection errors when using apt-get update:
Err http://ftp.us.debian.org stable Release.gpg
Could not resolve 'ftp.us.debian.org'
Err http://security.debian.org stable/updates Release.gpg
Could not resolve 'security.debian.org'
Err http://ftp.debian.org stable Release.gpg
Could not resolve 'ftp.debian.org'
Reading package lists... Done
When I ping domains it also didn't work, but when I ping IP it works... so I think this could be related with server dns...
The issue is that when I deactivate the ispconfig firewall all works!
This server is a openvz vps, debian 5 with following firewall config:
Open TCP ports: 20,21,222,25,53,80,110,143,443,3306,8080,10000
Open UDP ports: 53,3306