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"Backwards" NAT Rules on a Cisco ASA 5505

Posted on 2014-12-12
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Last Modified: 2014-12-22
Take a look at the 2 NAT rules on my Cisco ASA 5505. They exist to allow port 4430 and port 8001 traffic to travel from the the outside interface on the firewall to the server (10.20.2.3). They both work perfectly, and yet they both look completely backwards.

Note the inside"interface specified in the top left. It's the outside interface on the firewall that traffic is hitting and these NAT rules exist to handle.
Similarly, the Original/Source values and the Translated/Interface values seem ass-backwards, yet they work.

NAT Rules
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Question by:jdana
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Jan Springer earned 500 total points
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It looks backwards because you are mapping the inside to the outside.
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by:jdana
ID: 40496398
Jan Springer, please elaborate.
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by:Jan Springer
ID: 40496410
using cli where outside subnet is 172.16.34.0/24 without port forwarding):

static (outside,inside) 192.168.10.1 172.16.34.72 netmask 255.255.255.255

means maps the outside interface to the inside IP using the outside IP to the inside interface.

it would make more sense to have the inside to inside and outside to outside.
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by:jdana
ID: 40496714
Jan, Forgive my ignorance, I'm not a network guy. Is it reasonable to simplify this construct with the following "laymanish" statement, "The NAT Rule creates a "pathway" between the firewall's outside interface and the network interface of my server. As long as matching firewall rules exist allowing traffic, the NAT Rule is bi-directional. That is, it facilitates traffic both ways." A NAT rule based on the outside firewall interface would work as well.
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by:Jan Springer
ID: 40496770
The NAT rule takes a packet destined to an IP address, translates it and forwards the packet to the translated address.

And yes, a rule needs to exist both ways for consistent traffic flow.
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by:jdana
ID: 40512757
THANKS
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