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ASA 9.1 NAT basic understanding

Posted on 2014-11-26
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Last Modified: 2015-01-19
Hi,

on ios 9.1 - im confused about  nat - I have a statement for example that forwards (port forward in a sense) port 80 that hits an "outside" public IP object - forwards to an inside object (inside LAN) Please disregard the poor security design of doing this - the question is for my understanding of nat on 9.1

eg   object network insidewebserver
 nat (inside,outside) static outsideobject service tcp www www

it works   - however my confusion in understanding is the "nat(inside,outside) order.  It has inside first and then outside. Why?  the traffic would be coming from the outside first - Joe Smith on the internet would hit the "outside on http and then forward to the inside object.  I thought the order of (inside,outside) meant that traffic flow means "in" on inside first and then "out" on the outside. Or is that old way pre 8.3 ios thinking. -  my question is really to understand the order of nat (inside,outside) and whether it is important for nat on asa 9.1 hardware  model 5515 thanks.
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Question by:philb19
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kevinhsieh earned 500 total points
ID: 40469362
Normally NAT is on an IP basis and doesn't care about which port, service, or direction the flow starts in. I only include the service if there is something special going on like you need to have 1 public IP point to different private IP depending on the port, or you need to translate 1 port to another. For classic NAT the NAT rule sets up the translation and an ACL controls the traffic. Even with your NAT rule you still need to configure the ACL on the outside interface to permit the tcp/80 traffic inbound.

The order of the objects in the NAT is from the higher security interface to the lower security interface, and is not related to where you expect the flow to originate from. I think it has been that way for at least 15 years. One possible issue with your NAT statement is that unless you have another NAT statement covering your server your server can't access the Internet unless the origination port is tcp/80. If there is another NAT you would see that the webserver has a different public IP than the one in your example because that only covers port 80.
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by:philb19
ID: 40469878
Thanks for comment. I did some reading to find answer myself. Basically what I found is that the NAT on ASA is bidirectional.(basically what you have said :)   ) Also the order is "real" and then "mapped"  so real is the inside   and then mapped is the outside.  For IP address's this is reversed in the NAT statement. - So i think biderectional is the key. - if i was on the internal webserver itself surfing the net on 80 then its internal IP wold be natted to the outside IP specified in the NAT for port 80.

I got on to the internal webserver and checked this out
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