Inspection of router-generated traffic does not recognize DHCP client traffic

After I've published a post on how you can use the new router-traffic keyword to minimize the Internet-facing access list you use with CBAC, Euphrates Greene pointed out to me that this feature does not work for client DHCP traffic (if the router is acting as a DHCP client, for example, when connected to a MAN Ethernet environment).

Once you start thinking about what's really going on, it all becomes obvious: as the router has no IP address when it sends the DHCP request, and it sends the DHCP request to a broadcast address (as it doesn't know the IP address of the upstream DHCP server), there is no session that could be entered into the CBAC session table. So you still have to allow all DHCP traffic to your router with an access-list similar to this one:
ip access-list extended Internet
 permit udp any eq bootps any eq bootpc
 deny ip any any

Note: Replace the highlighted any keyword with the actual DHCP server's IP adress if you have it available and you want to have an even more secure IP access-list.

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