Network Address Translation

PIX can perform NAT for packets traversing any two of its interfaces. PIX's default behavior is to require NAT. This means that NAT must be set up for a connection state to be created, regardless of the level of the interface from which the packet originates. PIX allows NAT rules to be set up separately for various sets of interfaces. This allows flexibility in how NAT is deployed on the PIX. One method of NAT may be used for one pair of interfaces, and another may be used for another pair of interfaces. The most common use of NAT is when the private network behind the PIX uses an RFC 1918 space. PIX can convert the source addresses of the packets leaving this network into a globally routable address configured on it. PIX then maintains state information for this translation so that it can route the return traffic to the correct host on the internal network. NAT can also be used between two interfaces on the PIX, neither of which is on the public network, so they do not use a globally routable address space. In this case, NAT occurs from an RFC 1918 space to an RFC 1918 space. While the practical need for having this type of translation may be limited, PIX requires NAT to be setup in order to pass traffic between any two interfaces. PIX can do both one-to-one and one-to-many NAT. For a more detailed discussion of NAT's security aspects, see Chapter 6, "Network Address Translation and Security."

The PIX Firewall also provides static Port Address Translation (PAT). This capability can be used to send multiple inbound TCP or UDP services to different internal hosts through a single global address. The global address can be a unique address or a shared outbound PAT address or it can be shared with the external interface.

The PIX Firewall (in version 6.2 and later) also can do address translation for the source IP addresses of packets going from a low-security interface to a high-security interface. This functionality does not remove the need to have a static translation to be able to access the machines sitting on a higher security level from a lower security level.

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