The following are the most important linux port numbers you should know if you are running a linux vps or dedicated server. Make sure that you dont block these services with your firewall.
21 => FTP
22 => SSH
23 => Telnet
25 => SMTP Mail Transfer
43 => WHOIS service
53 => name server (DNS)
80 => HTTP (Web server)
110 => POP protocol (for email)
995 => POP over SSL/TLS
9999 => Urchin
111 => rpcbind
953 => rndc
143 => IMAP Protocol (for email)
993 => IMAP Secure
443 => HTTP Secure (SSL for https:// )
3306 = > MysQL Server
4643 => Virtuosso Power Panel
2082 => CPANEL
2083 => CPANEL - Secure/SSL
2086 => CPANEL WHM
2087 => CPANEL WHM - Secure/SSL
2095 => cpanel webmail
2096 => cpanel webmail - secure/SSL
Plesk Control Panel => 8443
DirectAdmin Control Panel => 2222
Webmin Control Panel => 10000
1. How to find which ports are open?
You can find the ports in your linux server with the nmap command
netstat -nap --tcp
2. How to investigate a port and kill suspicious process?
A good tutorial is here
3. Where do i find a complete list of linux ports for reference?
You can find the ports list: here
4. Which firewall is best for linux servers?
I would recommend to install APF firewall. You can find a good tutorial here: http://www.mysql-apache-php.com/apf-firewall.htm
Warning: Make sure that you dont block the important ports with the firewall.
Back to home