Cisco ASA 8.4 vs. Typical NAT/PAT Configuration

In computer networking, network address translation (NAT) is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device.

Here we will focus on NAT and PAT, as well as the related access control list changes. This only addresses typical static and dynamic source address translation scenarios. Policy based NAT and DMZ configuration will be address in future articles. This is an area of significant change in ASA 8.4.

For this configuration challenge, we will meet following configuration requirements:

  • 192.168.1.x/24 should use the outside interface IP for Dynamic PAT
  • 192.168.1.2 TCP Port 80 will have a static PAT translation to 1.1.1.3 Port 80
  • 192.168.1.3 will have a static NAT (one-to-one) translation to 1.1.1.3

Note: The last two requirements overlap. The desire is that only traffic to TCP port 80 on 1.1.1.3 be delivered to 192.168.1.2. All other traffic coming in to 1.1.1.3 will go to 192.168.1.3.

Cisco asa 8.4 vs. typical natpat configuration

 

For those familiar with ASA version 8.2 and earlier, the relevant configuration excerpts are found below.

nat (inside) 1 0.0.0.0 0.0.0.0
global (outside) 1 interface
static (inside,outside) tcp 1.1.1.3 www 192.168.1.2 www netmask 255.255.255.255
static (inside,outside) 1.1.1.3 192.168.1.3 netmask 255.255.255.255

 

//ACL Entries–Note the translated addresses

 

access-list outside_access_in extended permit tcp any host 1.1.1.3 eq www
access-list outside_access_in extended permit tcp any host 1.1.1.3 eq smtp
access-list outside_access_in extended permit tcp any host 1.1.1.3 eq https
access-list outside_access_in extended permit icmp any host 1.1.1.3

access-group outside_access_in in interface outside

If you reproduce this configuration, it is imperative that the static translations be entered in the exact order. By doing so, the static NAT configuration is implemented with the exception of the the previously entered static PAT entry.

 

ciscoasa(config)# static (inside,outside) tcp 1.1.1.3 80 192.168.1.2 80
ciscoasa(config)# static (inside,outside) 1.1.1.3 192.168.1.3
WARNING: mapped-address conflict with existing static
TCP inside:192.168.1.2/80 to outside:1.1.1.3/80 netmask 255.255.255.255

Comparing NAT and access-list configuration to the 8.4 equivalent, major changes are apparent. After performing an upgrade of the 8.2 configuration, the following is an excerpt that represents the 8.4 NAT and ACL configuration.

//object definitions

 

object network obj_any
subnet 0.0.0.0 0.0.0.0
object network obj-192.168.1.2
host 192.168.1.2
object network obj-192.168.1.3
host 192.168.1.3

 

//NAT Assignments

 

object network obj_any
nat (inside,outside) dynamic interface
object network obj-192.168.1.2
nat (inside,outside) static 1.1.1.3 service tcp www www
object network obj-192.168.1.3
nat (inside,outside) static 1.1.1.3

 

//ACL Interface Binding

 

access-group outside_access_in in interface outside

 

//ACL Entries–Note the real IP address

 

access-list outside_access_in extended permit tcp any host 192.168.1.2 eq www
access-list outside_access_in extended permit tcp any host 192.168.1.3 eq smtp
access-list outside_access_in extended permit tcp any host 192.168.1.3 eq https
access-list outside_access_in extended permit icmp any host 192.168.1.3

 

As you can quickly see ASA 8.4 radically changes the NAT configuration. A good way to get a grasp of the differences is to go through the upgrade process between 8.2 and 8.4 with known working configurations.

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