Event Manager question. Monitoring DNS entry.

Question

Hello everybody,
I  want to use Cisco Embedded Event Manager in a router for monitoring a  DNS entry that normally resolves 1.1.1.1 but if it changes to 2.2.2.2 the  router has to change some access list automatically in order to permit different  routes.
I  know how to configure de "action" sentences for change the ACLs but I  dont know how to configure a track or a SLA in order to check the DNS  entry and react if it changes.
Someone knows how to do that?
event manager applet ChangeRoutesWhenDNSEntryChanges
event track 21 state ??????????????????????????????????????????????????<----------
action 1.0 cli command "enable"
action 2.0 cli command "configure terminal"
action 3.0 cli command "ip access...
Thank you in advance!!
Marcos.

Answer 1

Indeed, I just provided the above example to demonstrate how we can force a DNS lookup and parse the IP address...
I was actually thinking about using the IP SLA DNS probe, but I could not find a way to get the IP address from the results... It just shows the response time.
In order to make the script work, we would most likely need to use some timer to trigger it periodically.
This is an adaptation of my previous example to actually accomplish a periodic check:
event manager applet CHECK-DNS
event timer watchdog time 60
action 1.0 cli command "ena"
action 1.1 cli command "ping host1 repeat 1 timeout 0"
action 2.0 regexp "ICMP Echos to (.*), timeout is 0 seconds:" "$_cli_result" _match _ip
action 3.0 if $_ip ne 1.1.1.1
action 4.0  syslog msg "host1 is now $_ip"
action 5.0 end
If you want to do something as a reaction to detecting the change, that would have to go into the "4.0" block (you can use 4.1, 4.2 etc)
Another thing to consider is that this script would run every minute, so as long as the DNS query resolves to anything but 1.1.1.1 we would re-apply the config changes, which is not that good...
A way to solve this can be seen in the next example:
event manager applet HOST1-NOT-1.1.1.1
event timer watchdog time 60
action 1.0 cli command "ena"
action 1.1 cli command "ping host1 repeat 1 timeout 0"
action 2.0 regexp "ICMP Echos to (.*), timeout is 0 seconds:" "$_cli_result" _match _ip
action 2.1 track read 100
action 3.0 if $_ip eq 1.1.1.1
action 4.0  if $_track_state eq down
action 4.1   track set 100 state up
action 4.2   syslog msg "host1 is now 1.1.1.1 again"
action 4.9  end
action 5.0 else
action 6.1  if $_track_state eq up
action 6.2   track set 100 state down
action 6.3   syslog msg "host1 is not 1.1.1.1, new ip is $_ip"
action 6.9  end
action 7.0 end
Basically we are using a stub tracking object to maintain state. Track object 100 would be up if we know host1=1.1.1.1, but if we detect it is something else we change it to down. Only after we detect that host1=1.1.1.1 again we change the track object back to up, which would enabled detecting another change...
Any actions you want to take should go into section "6" and any clean up (when host1=1.1.1.1 again) has to go into section "4".
It is possible to make this detect any change and not just have a static 1.1.1.1 value by assigning the newly detected value to a variable and basically look for a change... Not sure what is your requirement.
The output below shows how this works...
Router#show run | inc ip host
ip host host1 1.1.1.1
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#ip hos
Router(config)#ip host host1 2.2.2.2
Router(config)#^Z
Router#
*Nov  9 18:23:18.009: %TRACKING-5-STATE: 100 stub   Up->Down
*Nov  9 18:23:18.021: %HA_EM-6-LOG: HOST1-NOT-1.1.1.1: host1 is not 1.1.1.1, new ip is 2.2.2.2
Router#
*Nov  9 18:23:18.065: %SYS-5-CONFIG_I: Configured from console by console
Router#
Router#
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#ip host host1 1.1.1.1
Router(config)#^Z
Router#
*Nov  9 18:23:42.805: %SYS-5-CONFIG_I: Configured from console by console
Router#
*Nov  9 18:24:18.025: %TRACKING-5-STATE: 100 stub   Down->Up
Router#
*Nov  9 18:24:18.033: %HA_EM-6-LOG: HOST1-NOT-1.1.1.1: host1 is now 1.1.1.1 again