What is a Firewall?

A firewall is a computer that links two logical networks together and re-routes data between the two networks as required. The firewall computer also contains a filter. This filter only allows data to pass through it that is requesting a particular service (using a variety of filtering methods defined by the firewall administrator).

A typical use of a firewall is for Web servers. A Web server needs to be accessed by remote computers outside the logical network so they can access the web service. However, these computers should not be able to access other services on that server that are more likely to be a security risk.

Firewalls can vary in the way that they restrict access to the networks. They can:

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only allow access to and from certain computers.

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analyze the port number the client is requesting and compare it to a list of port numbers allowed.

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analyze the data that is being sent and only allow it through if it conforms to some pre-determined rules that have been set up.

Within the data being sent (known as packets), many firewalls can obtain the RPC number requested for RPC calls and only allow data through if it is requesting a particular RPC number and therefore a particular RPC service.