[Lop]

Here is an essay I just finished for a class of mine. Feel free to use it in whatever way you like. I know, the grammer might not be the best........


Proxy Servers
Proxy servers are a security device that sits between the internal network and an external network, usually the internet. Proxy servers are also known as Application Layer Gateways, application gateway or simply a proxy. The proxy server acts as an intermediary that actually scans all incoming and outgoing traffic. A proxy server runs on the Application layer of the OSI model.
One of the most important functions of a proxy server is to hide the internal IP address from external entities such as the internet. To better understand, imagine that you are browsing a webpage through a proxy. To get to the webpage your request must first go through the proxy server which may mean you have to log on to the proxy server first. The proxy server would then enclose your packet in a new envelope containing its’ IP address instead of the computer’s IP address that you were working with. The proxy server then forwards your packet on to the next gateway which may or may not be the Internet.
Using this security method to “hide” an internal IP address is also called sharing. Using this method the Proxy Server actually acts as a NAT. Multiple workstations can connect to the internet using only one IP address. Instead of each individual workstation receiving its’ own IP address they only obtain an internal IP address.
Often a network will have a firewall before the Internet and after the proxy server. Some proxy server packages are bundled with firewalls which may be purchased and configured to do both functions at once. For a more secure internal network a firewall is recommended along with the proxy server.
Another important function of a proxy server is to cache web pages. When a user visits a webpage the webpage is stored or saved. The next time a user in the network visits the same webpage the stored version is shown instead of fetching the entire webpage over again. This can drastically improve performance on networks and is similar to what most browsers already do with their cache.
Proxy servers may sound great but are also a lot harder to maintain than regular firewalls. By running on the application layer of the OSI model it can act more intelligently. The proxy server can actually filter out what sites a user can visit and what are declined. The actual destination can be scanned and ports can also be scanned. Several networks use proxy servers to block ports such as SMTP (25) or POP3 (110) to prevent users from sending or receiving personal email. Giving this extra ability to filter destinations requires more administration.
Proxy servers are turning up around the web. It is common to see CGI or PHP based proxy servers that users can go to and surf the web “privately”. The main emphasis behind this is to avoid any detection of where the user has been. By use the website a user can browse anywhere without anyone in the network knowing where he or she has been.
Proxy servers running on port 80 can be used to fool internet network security measures. Using proxy servers on port 80 that act as a Socks 4 or 5 proxy server a user can go around any security measure put in place to block employees from certain websites or even ports. Lists can be viewed and even purchased that contain public proxy servers with port numbers. Packages can also be bought that only function for this purpose.
One of the major drawbacks of a proxy server is that it can slow down internet access to the entire network. For small networks with only a couple of workstations connected this is not a problem. For larger, enterprise networks this server can become a bottleneck for the entire organization. Thousands of requests can hit the proxy server at any time which will cause network latency. To avoid this problem, administrators can deploy several different proxy servers. If this is an organization with many buildings it may deploy one for each building.
Another drawback of a caching proxy server is visiting pages that need to be refreshed. A web page that has a meta tag to expire will cause the proxy server to reload the page defeating the caching ability.