Cisco ASA 5510 I currently have a NAT for SMTP on one outside IP to an internal IP. I need to setup 2 external IPs to NAT to the same IP internally. How can I do that? ex: 25 --> 25 25 --> 25

  • 1
    What external IP do you want the internal one to respond with through the firewall? Are you needing PAT? Expound on what you are trying to accomplish (multiple ISPs, separating apps, etc.).
    – TheCleaner
    Dec 20, 2012 at 19:24
  • Could you elaborate on why you want to do this? It would be easier to multi-home your SMTP server and create a 1-to-1 nat for each of its two addresses...
    – 1.618
    Dec 20, 2012 at 19:25
  • One outside IP currently points to spam filter and another IP points to Exchange. I want both IPs to point to spam filter. I just changed providers and firewalls. My old firewall allowed this function and wasn't changing the setup really, unless I have to.
    – Keith
    Dec 21, 2012 at 3:49

4 Answers 4


You won't be able to use static PAT for this as you would break the 1:1 mapping rule. Firewall has to know what mapping to use in both directions - both in->out and out->in. In your case if originated connection from port 25 firewall would not know which global IP to use. In other words, it's not possible this way.

Easiest solution would be to assign additional IP address on the internal device and keep the NATs clean. Let's say you assign additional IP of Configuration would be:

static (inside,outside) tcp 25 25
static (inside,outside) tcp 25 25
  • This worked for me. Straight forward and I should have thought of that too. ;) Thanks for your help.
    – Keith
    Dec 21, 2012 at 14:36

With IOS 8.2 or bellow:

access-list SMTP-Services extended permit ip host host
access-list SMTP-Services2 extended permit ip host host

static (InternalInterface,ExternalInterface) access-list SMTP-Services
static (InternalInterface,ExternalInterface) access-list SMTP-Services2

Sorry, I had understood the exact opposite of what you wanted to do.

Don't forget to add an access-list on your External Interface.

access-list _outside-in_ extended permit tcp host host _YourExternalIP_ eq smtp
access-list _outside-in_ extended permit tcp host host _YourExternalIP_ eq smtp
  • There are two things, one I made a slight mistake (see edit) you need two different access-lists. Also, I think you are mixing up the IPs. When you do a static you have to make it as if your local IP wanted to NAT going outside. Since it's a static NAT, when the external IP hits your ASA, it will translate it to the local IP.
    – Alex
    Dec 21, 2012 at 4:07
  • It would also help to know what version of IOS you are running with a show version because between IOS 8.2 and IOS 8.3, NAT configuration has change a lot.
    – Alex
    Dec 21, 2012 at 4:20

First you will need to upgrade to ASA post-8.3. Create and object network with the range of IPs for the public. Then create an object network for the inside/real IP address of the server. Then add a nat statement calling the first object.

object network outside_email

object network inside_email
 nat (inside,outside) static outside_email

There is this same quesiton on another Stack Exchange site here. This works because the protocol, source ip, destination ip and port are all part of the key for this 1:1 mapping. It's also a great technique for network resilience if BGP is out of grasp.

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