Assigning Varying Security Levels to Interfaces

PIX Firewall allows varying security levels to be assigned to its various interfaces. This key technique allows for the implementation of a security policy calling for varying levels of security control in a network's different segments. These segments are usually called security zones.

A PIX Firewall can have from two to ten interfaces. Each interface can be assigned a level from 0 to 100. A lower level number means that an interface belongs to a relatively lesssecure part of the network as compared to an interface that has a higher level number. Typically, the interface connected to the public network has a 0 level assigned to it. This describes a very low level of security. The interface sitting on the private network has a security level of 100, meaning that it is the most secure. DMZ interfaces have security levels between 0 and 100. By default, traffic can flow freely from a high security level interface to a low security level interface, provided that a network address translation (xlate) is built for the traffic's source IP addresses. PIX keeps track of the connections for this traffic and allows the return traffic through. However, for traffic to flow from a low security level to a high security level, rules need to be explicitly defined on the PIX Firewall, allowing this traffic to go through. This is done using access control lists or conduit statements. These rules are in addition to the creation of static xlates for this type of traffic when going from a lower to a higher security level. You will see examples of how to implement these in the case studies at the end of this chapter.

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