[AusNOG] Exemption to a NAT rule for a particular destination

[Scott Anderson]

If I understand correctly, you’re terminating the tunnel outside of your NAT’d local network and you want to in effect “extend” that private network out by one hop to reach the router, but only for cases where the traffic is bound for the other side of the tunnel. Is that right?

If you had a private address space between the ASA and the router you could simply add a “nat (interface) 0” entry (the 0 entry defines what traffic to NOT translate, a global exclude if you will, but the ACL should contain permit statements) to the ASA that’s tied to an ACL that specifies a source address of your internal subnet and a destination of the subnet at the other end of the tunnel.

However, from what you described you have an internet routable address on the ASA and so by doing that you would be routing private address space on the public internet (which, if there’s no other devices between, should technically work as it’ll just hop in to the tunnel and on its way without ever going out in to the big swamp but it’s poor form).

Personally, I’d save myself the pain and move the IPSEC termination to the ASA and just use the aforementioned nat 0 entry to stop it translating tunnel bound traffic.


Scott


From: AusNOG [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of Geordie Guy
Sent: Thursday, 1 May 2014 2:59 PM
To: Craig Askings
Cc: <ausnog at lists.ausnog.net>
Subject: Re: [AusNOG] Exemption to a NAT rule for a particular destination

Router does the tunnel and is at the edge, firewall is inside and doing the NAT.

On Thu, May 1, 2014 at 2:49 PM, Craig Askings <craig at askings.com.au<mailto:craig at askings.com.au>> wrote:
Sorry you have lost me here. Is the ASA doing all the NAT + the ipsec tunnel or is the upstream cisco router doing NAT and the ASA doing the ipsec tunnel?

On 1 May 2014, at 2:45 pm, Geordie Guy <elomis at gmail.com<mailto:elomis at gmail.com>> wrote:


Sorry guys, it's an ASA 5500 firewall making the decision to NAT, and cutting the upstream Cisco router out of making the decision to forward it into the tunnel.  More reading seems to reveal what I want to do is configure a higher priority NAT rule that NATs traffic to that destination by rewriting the source and destination traffic with the same original info, thereby cutting out the PAT for the public IP.  Does this make sense? (it seems to, in a weird way)

On Thu, May 1, 2014 at 2:37 PM, Karl Auer <kauer at biplane.com.au<mailto:kauer at biplane.com.au>> wrote:
On Thu, 2014-05-01 at 14:15 +1000, Geordie Guy wrote:
> Is there a way of exempting a particular IP
> address or providing some other criteria for a NAT rule?
Almost certainly, but how to do it depends on what system you are using.
Tell us what you are trying to do it *with* and someone who uses that
system will probably be able to help.

For MikroTik, for example, you add an "accept" rule to the srcnat chain
in "/ip firewall nat", limiting it to specific source or destination
addresses. Make sure such rules are placed before any masquerade actions
involving the same sources or destinations, of course.

> PS: (*%&*$ing NAT.

What you said.

Regards, K.

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