This technique is the simplest firewall. In essence, you restrict packets that come in from the Internet to safe ports. Packet-filter firewalls are usually implemented using the filtering built into your Internet router. This means that no access is given to ports below 1024 except for certain specified ones connecting to safe services, such as SMTP, NNTP, DNS, FTP, and HTTP. The benefit is that access is denied to potentially dangerous services, such as the following:
The advantages of packet filtering are that it’s quick and easy. But there are at least two disadvantages:
Even the standard services can have bugs allowing access. Once a single machine is breached, the whole of your network is wide open. The horribly complex program sendmail is a fine example of a service that has, over the years, aided many a cracker.
Someone on the inside, cooperating with someone on the outside, can easily breach the firewall.
Another problem that can’t exactly be called a disadvantage is that if you filter packets for a particular service, then you should almost certainly not be running the service of binding it to a backend network so the Internet can’t see it — which would then make the packet filter somewhat redundant.