Firewalls protect computers and networks by blocking incoming connections. However, some organizations, like Web hosts, need to allow these connections through to fulfill their business purposes. These situations require a proxy server, which creates a higher level of security.
A proxy server is a computer that acts on behalf of other computers. It doesn't pass traffic through but retransmits the contents of incoming messages. Proxy servers guard networks and resources by receiving all incoming traffic and responding to it with data retrieved from servers behind it. This type of proxy server is called a reverse proxy server.
A firewall either blocks incoming connections or passes them through. The operators of the firewall set up a list of address that are permitted to make connections to the protected computer or network. This is called an exceptions list. The firewall examines the headers of data packets to check their sources but doesn't look at the contents of the packet.
Computers and networks can be harmed by malicious programs, called malware. These programs download onto computers and perform some function not intended by the computer owner. Examples of the activities of these programs are destroying files, using the computer's memory and reporting activity back to the malware producer. A proxy server stops malware from getting onto the network by blocking all incoming traffic. Any malicious programs get no further than the proxy. If it arrives with a seemingly innocent request, the proxy responds to the request but discards any program content of the incoming data packets. The proxy server examines the data contained in arriving data packets, as well as the packet header.
A network that requires different levels of security for different machines is split into two sections. Any computers that need to allow incoming connections are separated from the business's regular network. These are placed in an outer network, called a Demilitarized Zone, which has different security procedures. A reverse proxy server sits between the outer network and the Internet, receiving requests on behalf of the server. Traffic meant for the inner network goes through a second gateway server. This machine may also be a proxy server. The double proxy configuration also helps an organization circumvent a firewall for specific types of traffic like Voice over IP, which would be impeded by a firewall and is judged to be low-risk.
The weakness of any security system involving firewalls and proxy servers is the ability to override settings and allow traffic through. Human intervention always outranks machines and so access to the configuration of both types of security facilities should be controlled. Antivirus software is another line of defense, reinforcing the work of firewalls and proxy servers.
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