You configure a NAT rule to match a packet’s source
zone and destination zone, at a minimum. In addition to zones, you
can configure matching criteria based on the packet’s destination
interface, source and destination address, and service. You can
configure multiple NAT rules. The firewall evaluates the rules in
order from the top down. Once a packet matches the criteria of a
single NAT rule, the packet is not subjected to additional NAT rules.
Therefore, your list of NAT rules should be in order from most specific
to least specific so that packets are subjected to the most specific
rule you created for them.
Static NAT rules do not have precedence over other forms of NAT.
Therefore, for static NAT to work, the static NAT rules must be
above all other NAT rules in the list on the firewall.
NAT rules provide address translation, and are different from
security policy rules, which allow or deny packets. It is important
to understand the firewall’s flow logic when it applies NAT rules
and security policy rules so that you can determine what rules you
need, based on the zones you have defined. You must configure security
policy rules to allow the NAT traffic.
Upon ingress, the firewall inspects the packet and does a route
lookup to determine the egress interface and zone. Then the firewall determines
if the packet matches one of the NAT rules that have been defined,
based on source and/or destination zone. It then evaluates and applies
any security policies that match the packet based on the original
(pre-NAT) source and destination addresses, but the post-NAT zones.
Finally, upon egress, for a matching NAT rule, the firewall translates
the source and/or destination address and port numbers.
Keep in mind that the translation of the IP address and port
do not occur until the packet leaves the firewall. The NAT rules
and security policies apply to the original IP address (the pre-NAT
address). A NAT rule is configured based on the zone associated
with a pre-NAT IP address.
Security policies differ from NAT rules because security policies
examine post-NAT zones to determine whether the packet is allowed
or not. Because the very nature of NAT is to modify source or destination
IP addresses, which can result in modifying the packet’s outgoing interface
and zone, security policies are enforced on the post-NAT zone.
A SIP call sometimes experiences one-way audio when going
through the firewall because the call manager sends a SIP message on
behalf of the phone to set up the connection. When the message from
the call manager reaches the firewall, the SIP ALG must put the
IP address of the phone through NAT. If the call manager and the
phones are not in the same security zone, the NAT lookup of the IP
address of the phone is done using the call manager zone. The NAT
policy should take this into consideration.
No-NAT rules are configured to allow exclusion of IP addresses
defined within the range of NAT rules defined later in the NAT policy.
To define a no-NAT policy, specify all of the match criteria and
select No Source Translation in the source translation column.
You can verify the NAT rules processed by selecting
testing the traffic matches for the NAT rule. For example: