Detect DHCP client address change with EEM applet

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By Ivan Pepelnjak

Symptom Change in the client IP address assigned by a DHCP server to a Cisco router is hard to detect due to lack of consistent DHCP-related syslog message.

Cisco IOS generates a DHCP-6-ADDRESS_ASSIGN syslog message when an interface configured with ip address dhcp acquires its initial address through DHCP, but not when the address is subsequently changes due to a DHCP NAK response from the DHCP server.

Solution Change in an interface IP address always generates a change in the IP routing table: the connected route to the subnet of the old IP address is removed and new connected route is inserted in the IP routing table. The change in the IP routing table can be detected with the IP routing event detector introduced in EEM 3.0.

The following EEM applet requires the IP routing event detector, programming logic and regular expression support available in Embedded Event Manager 3.0 (first released with Cisco IOS release 12.4(22)T).

This topic and numerous other EEM topics are covered in the EEM Workshop

Applet source code

event manager applet DHCPAddressChange
 event routing network type add protocol connected ge 1
 action 100 cli command "show ip interface brief | include $_routing_lastinterface"
 action 110 regexp "YES\s+DHCP" "$_cli_result"
 action 200 if $_regexp_result eq 1
 action 220  set ipaddress "unknown"
 action 230  regexp " ([0-9.]+) " "$_cli_result" match ipaddress
 action 240  info type routername
 action 250  mail server "$_mail_smtp" to "$_mail_rcpt" from "$_info_routername@$_mail_domain" →
             subject "DHCP address on $_routing_lastinterface changed to $ipaddress" →
             body "\n$_cli_result"
 action 299 end 

The event routing network type add protocol connected event detector detects all additions of connected routes (the mask indicates we want to catch all changes regardless of the actual IP prefix).

Connected routes could change after interface flaps or manual configuration changes. The show ip interface brief command is thus used to inspect the IP address allocation method of the affected interface (the interface name is available in the $_routing_lastinterface EEM variable).

The presence of “YES\s+DHCP” pattern in the show ip interface brief command output indicates the interface is operational and got its IP address via DHCP. In this case, the actual IP address is extracted from the show ip interface brief command output and sent in an e-mail to the network operator.

Additional configuration

The EEM applet expects several EEM environmental variables described in the Send e-mail from EEM applets article. A sample configuration is included in the following printout:

event manager environment _mail_smtp
event manager environment _mail_domain
event manager environment _mail_rcpt
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