What is difference between nat (inside,outside) and nat (outside,inside) in asa 8.4 ?

Posted on 2014-10-25
Medium Priority
Last Modified: 2014-11-02
HI Expert
Can you explain what is difference between nat (inside,outside) and nat (outside,inside) in asa 8.4 for the following two kind of command  ?

Here is topology -(inside10.1.1.1)------------
There are two king of static nat commands:
object network Inside
nat (inside,outside) static

object network Outside
nat (outside,inside) static

Since the static nat is bidirectional, I do not think it has difference. Do you think so ? Thank you
Question by:EESky
  • 2
LVL 29

Accepted Solution

Alan Huseyin Kayahan earned 2000 total points
ID: 40404418
host j.j.j.j
nat (interfaceX,interfaceY) static n.n.n.n

Has two functions

Replace the source IP address of any packet entering ASA from interfaceX and departing ASA from interfaceY, with the IP address n.n.n.n.
Replace the destnation IP of any packet entering ASA from interfaceY and destined to n.n.n.n, with the IP address j.j.j.j

Note that j.j.j.j is an IP with a live node whereas n.n.n.n is not and utilized by the firewall on behalf of j.j.j.j

Therefore the two examples you have given does not reflect a bidirectional behaviour of a single scenario. They are two different scenarios.

Author Comment

ID: 40404896
Thank you so much for your fast reply. The formula of Nat that you gave is very good. Usually we just use nat (inside,outside) and we barely use nat (outside,inside). Can you tell in what situation, we need to use nat (outside,inside) ?
LVL 29

Expert Comment

by:Alan Huseyin Kayahan
ID: 40405054
You are welcome!
ASA treats the subnets of VPNs terminated at outside interface as outside (unlike Cisco routers where tunnel interface maybe different to assaing to a zone in ZBF).
So if you have multiple VPN tunnels (S2S or RA) terminated at outside interface and you need a transitive connectivity (for instance you have tunnels from site A to site B and site C to site B and you want site A to be able to reach site C through site B), you need Outside NAT. This is known as U-turn. nat (outside,outside)
Outside NAT also comes handy in VPN scenarios where both sides of the S2S tunnel are the same (overlapping networks). nat (outside,inside)

Expert Comment

by:Feroz Ahmed
ID: 40407930

NAT is defined as Network Address Translation which when in the given scenario is defined as whenever inside IP address when it goes outside or Inside IP address when it travels outside network {Nat (Inside,outside)} it travels outside with a different IP Address.

Supposing the above scenario is Nat (inside,outside) and in this scenario inside ip address is and when it travels from inside to outside it travels with an ip address and this scenario is called as NATTING.

To know real inside ip address one should type below command on ASA
ASA#debug icmp
This will show when someone from outside network trying to ping the inside network pinging with different ip address .

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