firewall : Computer Hardware Buyers’ Glossary

by Roedy Green ©1997-2008 Canadian Mind Products
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firewall

A computer that examines traffic coming and going to the Internet, and dynamically filters out messages from certain IP addresses. It can prevent people inside from hooking up to various outside computers and vice versa. The firewall rules allow you to specify which applications and services can send or receive packets via TCP or UDP protocols. Usually all UDP protocols such as NNTP are blocked, since you tend to burst security wide open if you open it up a little for UDP. Often a firewall is implemented with Linux. Using two separate operating systems tends to make life more difficult for the vandals. The most secure sort of firewall is a dedicted router box that is incapable of running any programs other than the firewall firmware. These are now under $150.00 USD . They are simple to install, but lack the flexibility of a Linux-based firewall. Hardware firewalls are good at keeping the outside world out. Software firewalls are good at preventing trojans running on your machine from accessing the net and divulging data.

There are four schemes for tunneling through firewalls NAT, SPI, SOCKS and explicit proxy servers.

I am not too clear on this, but there appear to be several configuring protocols UPnP-NAT(PC), NAT-PMP(Mac) and SOHO. And of course many routers are not configurable,

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