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Inspect router-generated traffic

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 tcp
ip inspect name DEFAULT100 udp
interface Dialer0
description $FW_OUTSIDE$
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 eq ntp any eq ntp
This 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 tcp router-traffic
ip inspect name DEFAULT100 udp router-traffic
... and the router synchronizes to an external NTP server:
sp#show ip inspect sessions
Established Sessions
Session 474032B4 (>( udp SIS_OPEN
01:04:34: %NTP-5-PEERSYNC: NTP synced to peer
01:04:34: %NTP-6-PEERREACH: Peer is reachable
Note: This article is part of You've asked for it series.


  1. Is this possible with ZBF? SDM does not seems to provide only for limited router-generated inspection.

  2. Is it not right that you have to configure policy in both directions...out zone-self zone and self-out in order to sync the router with external ntp server as there is no inspect option only pass. 12.4(24)T...great site btw.


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