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SysAdmin Blog

killing your network with Cisco ASA 9.x identity NAT and proxy arp

Alexander Bochmann Sunday 17 of April, 2016
I was about to prepare a longer blog post on one of the pitfalls when migrating the NAT ruleset of an older Cisco ASA to a 9.x release - but as it turns out, the problem is already documented pretty well by Cisco, if you know what to look for...

With "Twice NAT", as implemented in 9.x software versions, an ASA firewall in routed mode will automatically do proxy ARP for all addresses covered by a NAT rule, to attract traffic for them. This is usually an intended effect, unless you're configuring Identity NAT rules (used to inhibit address translation for certain source/destination pairs) that cover address space locally connected to the firewall. This was not a problem with NAT exempt rules on older ASA software, but if such a rule is used now without the no-proxy-arp parameter, the ASA will act as a blackhole for traffic on on the local network segment, by sending proxy-ARP replies for addresses it doesn't own.

In Proxy ARP Problems with Identity NAT (cache), Cisco illustrates the problem with this diagram:

image copied from vendor documentation, (c) Cisco
image copied from vendor documentation, (c) Cisco


Yeah, don't do that. Always consider whether no-proxy-arp is required for a NAT rule before it's being deployed.

(Also see ASA FAQ: Why does the ASA reply to ARP requests for other IP addresses in the subnet? (cache).)