Configure iptables firewall to allow DHCP, DNS, HTTP, SMTP, FTP, IMAP, POP, and Squid ports – deny all other services.
1 pc for router ( place firewall) 1 pc for server ( place web,dhcp,ftp,etc)
You can use system-config-firewall if you have GUI installed on RHEL, Cent OS or Fedora systems. Then just put ticks and apply.