Policy NAT 8.6(1)2 Windows Server Cluster

Question

We have 2 email servers in a cluster on the network.  I have the cluster IP address configured for Object static NAT.  This works great for email coming into our organization.  However, when either of these 2 email servers send mail, they send using their configured IP address which is different from the cluster IP address.  Thus, the NAT'd address is different than for incoming.  It hasn't been an issue to this point, but I would like to be able to send SMTP from either server and have it NAT to the same IP used for the cluster IP.  This way, any reverse DNS lookups on the internet would show a consistent IP to name mapping for our mail servers.  I've attached a diagram.  If there is a way to force the cluster servers to use the cluster address on the Windows server side, that could be an option as well.
Thanks,
Andrew

Answer 1

Hi,
The actual NAT configuration used depends on how your Dynamic PAT rule for all the users of the network is configured at the moment. Mainly is it Auto NAT or Manual NAT.
Though naturally I can give you an example that includes both Dynamic PAT for all users and Dynamic PAT for the Mail servers and the Static NAT for incoming mail.
MAIL SERVER STATIC NAT
object network MAIL-SERVER
host 10.0.0.1
nat (inside,outside) static 10.10.10.140
The above configuration is the basic Static NAT configuration for a host using Auto NAT / Network Object NAT. It could be done with Manual NAT / Twice NAT also but I prefer Auto NAT / Network Object NAT
MAIL SERVER DYNAMIC PAT
object-group network MAIL-PAT-SOURCE
network-object host 10.0.0.1
network-object host 10.0.0.2
network-object host 10.0.0.3
object network MAIL-SERVER-PUBLIC
host 10.10.10.140
nat (inside,outside) after-auto source dynamic MAIL-PAT-SOURCE MAIL-SERVER-PUBLIC
The above is a normal Dynamic PAT configuration (no Policy elements involved).
The key thing to notice here is that we are entering this to the ASA before the next Dynamic PAT that catches all the rest of the source IP address. One thing to notice also is that its a Section 3 NAT rule (the lowest priority) so that it wont override any other NAT rules like the above Static NAT.
I you had your existing Dynamic PAT for all users already with a similiar configuration than last configuration example then you would have to add a line number to the NAT configuration like this
nat (inside,outside) after-auto 1 source dynamic MAIL-PAT-SOURCE MAIL-SERVER-PUBLIC
DEFAULT DYNAMIC PAT FOR USERS
nat (inside,outside) after-auto source dynamic any interface
The above is just an Dynamic PAT configuration that catches all source addresses from behind the "inside" interface and does Dynamic PAT for them when connecting to networks behind "outside". As this is inserted to the configuration after the above command it will be at a lower priority and wont apply for the 3 source hosts we specified above.
I wonder if I made this out to be more complicated than it needs to be
I guess the easiest way to determine the configuration you will need/want would be to see the current NAT configuration on the ASA
Hope this helps
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Feel free to ask more if needed
- Jouni