nefu (network fidelity utility) is a Unix daemon that monitors services over the network. It uses a "no false alarms" fault verification algorithm, and understands network dependancies. Natively-monitored protocols include ICMP echo (ping), SSH, IPP, DNS, HTTP, POP, NTP, IMAP, SMTP, and LDAP, as well as having facilities to execute external programs. Status pages are available via finger or the Web.
links2world Firewall is an iptables script generator for Linux 2.4.x or newer kernels. The configuration file is very human readable and easy to understand and write. It does not matter if you have one, two, three, or twenty network interfaces; it is able to generate stateful iptables rule sets that control the packet flows between all the networks your machine is attached to.