Cisco ASA - Twice NAT
Twice NAT allows you to NAT both the source and destination within a single rule.
A scenario where this type of configuration would be required is shown below. To ensure that any traffic originating from the Internet isn't sent back out to its default gateway (asymmetrically routed) the source IP is translated to an IP within the range of the inside segment (192.168.100.0/24). In turn ensuring the return traffic is sent back via the the Cisco ASA.
Within this example we will perform both Static PAT along with Dynamic PAT to ensure that traffic to our SMTP (192.168.100.200) server is not asymmetrically routed.
Here are some more details :
- Static PAT - A static NAT is configured for the real server 192.168.100.200 to a translated address of 220.127.116.11 on port TCP/25 (SMTP).
- Dynamic PAT - A dynamic PAT is configured which translates all traffic coming in from the outside interface to the source IP address of 192.168.100.100.
Below shows the required syntax.
object network SERVER-18.104.22.168
object network SERVER-192.168.100.200
object service SMTP-SERVICE
service tcp destination eq 25
object network PAT-ADDRESS-100
nat (outside,inside) 1 source dynamic any PAT-ADDRESS-100 destination static SERVER-22.214.171.124 SERVER-192.168.100.200 service SMTP-SERVICE SMTP-SERVICE