Firewalls are mainly used as a means to protect an organization’s internal network from those on the outside (internet). It
is used to keep outsiders from gaining information to secrets or from doing damage to internal computer systems.
Firewalls are also used to limit the access of individuals on the internal network to services on the internet along with
keeping track of what is done through the firewall. Please note the difference between firewalls and routers as described
in the second paragraph in the IP Masquerading section.
Types of Firewalls
1. Packet Filtering – Blocks selected network packets.
2. Circuit Level Relay – SOCKS is an example of this type of firewall. This type of proxy is not aware of
applications but just cross links your connects to another outside connection. It can log activity, but not as
detailed as an application proxy. It only works with TCP connections, and doesn’t provide for user authentication.
3. Application Proxy Gateway – The users connect to the outside using the proxy. The proxy gets the information
and returns it to the user. The proxy can record everything that is done. This type of proxy may require a user
login to use it. Rules may be set to allow some functions of an application to be done and other functions denied.
The “get” function may be allowed in the FTP application, but the “put” function may not.