NAT/routing help on ASA 5520

Unanswered Question
Apr 15th, 2009


We have 2 email servers with 2 Nics. One has a public IP (e.g'ed to it's private IP for inbound email, so inbound email is fine. Now our ISP only accept smtp traffic from and the issue I have is the server with the NAT can send emails to them, but the server that doesn't have the NAT servers it's smtp traffic outbound via the ASA's default gateway and gets refused.

Is it possible to route this server with no NAT to route outbound via the so both servers go outbound via this and not the ASA's default gateway?


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whiteford Wed, 04/15/2009 - 02:50

Hmm I've never tried that before, plus I only use the ASDM, but do have access tot he CLI and have basic experience with this.

How would I do this then?

Say and wanted to go outbound via (public IP)?

andrew.prince@m... Wed, 04/15/2009 - 03:01

I do not use the ASDM, not even sure if it is capable to configure something like this in the ASDM, via cli

global (outside) 99

access-list email-servers extended permit tcp host any eq smtp

access-list email-servers extended permit tcp host any eq smtp

nat (inside) 99 access-list email-servers

Basically the above instructs the ASA to NAT servers & .2 when they try an access anything out on the internet using smtp and NAT them to external IP


whiteford Wed, 04/15/2009 - 04:19


As one of the email servers has the public IP ( assigned to it via NAT, can I just do the above method to just the server that needs it?

Also, are there any useful commands that will show me that it is using this new public IP?

andrew.prince@m... Wed, 04/15/2009 - 04:21

Sorry - I am no confused, if you are performing a specific static NAT - you WILL have to change it from a 1:1 to a 1:Many

You cannot just add an internal server, you will have to configure either a static 1:Many NAT based on an access-list or a policy based NAT.


whiteford Wed, 04/15/2009 - 06:42

Sorry to confuse:

Could I:

Get the 2 email servers send outbound as public IP on smtp

But only nat inbound to one of the email servers on smtp?

andrew.prince@m... Wed, 04/15/2009 - 07:06

No - the ASA will send return traffic back to either server.

So if you have an environment where only 1 email server is receving/delivering email - but both servers are sending email out to the internet, you need to look at your email environment.


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