Internet Firewall: Proxy Servers
Rated: 
Proxy services can provide additional security in addition to firewalls. Inspecting the way they work reveals they may also be considered firewalls themselves.
According to Cisco, "proxy services are special-purpose programs that manage traffic through a firewall for a specific service, such as HTTP or FTP. Proxy services are specific to the protocol that they are designed to forward, and they can provide increased access control, careful detailed checks for valid data, and generate audit records about the traffic that they transfer."
A proxy service consists of a proxy server and a proxy client. Rather than having a client computer communicating directly with other computers on the Internet, it must talk to the proxy server. A proxy client is an application installed on the client computer to be able to talk to that proxy server. As a go-between, a proxy server transfers requests to and from the original parties involved. These two parties perceive to be talking to each other directly; hence, proxy services are usually considered transparent.
So how do proxies act as firewalls? Proxy services can inspect network packets for any possible threats. They typically work at a lower networking level compared to application layer firewalls, more like network layer firewalls.
One tradeoff you might have to deal with when installing proxies is speed. They are not as easy to install as your typical internet firewall since as mentioned above, they work for a specific protocol or service, like HTTP, FTP, SMTP, SOCKS, S-HTTP, Gopher, WAIS, etc. Packets are also forced to pass through several lower layers so that information inside them may be inspected thoroughly, and they move up to the application layer, down to the kernel, then down the stack to be distributed. This happens every session for every packet.
Aside from filtering packets, proxies have a host of other uses. One common function includes caching. By saving web pages onto a proxy server's cache, response times for client requests are increased. Caching also helps conserve bandwidth and improve reliability, since pages may still be accessed in case network lines go offline.
Here are some of the more popular types of proxies:
Web Proxy
A caching web proxy provides requested web pages more quickly by caching them to the proxy server. They can also filter web page content are typically implementations of censorware programs. CGI and PHP web proxies may be able to override the blocking proxies in a network as well as hide the IP address being used.
S-HTTP and HTTPS/SSL Proxy
Secure proxies support the S-HTTP and SSL protocols.
Transparent Proxy
Also known as an intercepting proxy or a forced proxy, a transparent proxy combines a server with Network Address Translation (NAT). Common uses for this type include implementing acceptable use policies (AUP) for networks.
Open Proxy
Open proxies are accessible by an IP address or user. Malware such as viruses have been known to exploit host computers by running open proxy servers on them.
Reverse Proxy
A reverse proxy focuses on traffic that comes from the Internet (rather than requests that come from client computers). Everything that arrives from the external network (relative to that proxy server) must pass through that proxy.
Split Proxy
A split proxy consists of two proxy servers that run on separate computers, often for addressing slow connections.
For a list of pros and cons regarding using proxy servers, Cisco has also listed down several of them.
Print Article
Send to a friend
Save as PDF