If you install a firewall in Directadmin it is recommended on opening the following default ports:

TCP: 20, 21, 22, 25, 53, 80, 110, 110, 143, 443, 587,993, 995, 2222, 3306, 35000-35999
UDP: 53

Here's an explenation of what each port is for:

20, 21: FTP

Note that ftp will use a 'random high port number' if the client is in PORT mode, so you may need to add a port range into your /etc/proftpd.conf file to allow ftp connections, eg:

PassivePorts 35000 35999

and then open this port range as well in your firewall.

22: ssh access

25, 587: smtp for exim to recieve email

53: dns (named), so your sites resolve; open both TCP and UDP

80, 443: apache traffic; http and https

110, 995: client pop email access

143, 993: clients imap email access

2222: DirectAdmin access

Optional:

3306: mysql access. You don't need to open this port if you don't want to allow remote mysql access, as most mysql scripts are all accessed locally.