A while ago a reader has asked me whether you could modify an IP access-list when the interface IP address changes. While that's definitely doable with Tcl and Embedded Event Manager, it's not a trivial task, so I've tried to understand why he would need such a functionality.
The answer was quite interesting: he's running NTP on his firewall router and thus needs to accept incoming NTP responses from an external NTP server. While that could be easily achieved with the following configuration (only the relevant bits-and-pieces are shown), he didn't want to make the access-list too generic (allowing NTP from the external server to any IP address).
ip inspect name DEFAULT100 tcpThis problem nicely illustrates a broader issues: the router does not inspect it's own traffic and thus does not prepare conduits for the return packets; you have to specify all the return traffic you're expecting in the incoming access list. This drawback has been fixed in IOS release 12.3(14)T with the introduction of the Inspection of Router-Generated Traffic feature. In our scenario you only need to change the inspect rules:
ip inspect name DEFAULT100 udp
ip access-group 102 in
ip inspect DEFAULT100 out
access-list 102 remark #### Dialer0 incoming ####
access-list 102 remark #### non-relevant lines deleted
access-list 102 permit udp host 188.8.131.52 eq ntp any eq ntp
ip inspect name DEFAULT100 tcp router-traffic... and the router synchronizes to an external NTP server:
ip inspect name DEFAULT100 udp router-traffic
sp#show ip inspect sessionsNote: This article is part of You've asked for it series.
Session 474032B4 (192.168.1.3:123)=>(10.0.0.1:123) udp SIS_OPEN
01:04:34: %NTP-5-PEERSYNC: NTP synced to peer 10.0.0.1
01:04:34: %NTP-6-PEERREACH: Peer 10.0.0.1 is reachable