Cisco IOS Cookbook, 2nd Edition

Logging Unauthorized SNMP Attempts

Problem

You want to log unauthorized SNMP attempts.

Solution

Use the following commands to configure your router to log unauthorized SNMP requests:

Router#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#access-list 99 permit 172.25.1.0 0.0.0.255
Router(config)#access-list 99 permit host 10.1.1.1
Router(config)#access-list 99 deny any log
Router(config)#snmp-server community ORARO ro 99
Router(config)#snmp-server community ORARW rw 99
Router(config)#end
Router#

Discussion

If you are concerned about unauthorized access to SNMP services on your router, it can be quite useful to configure the router to maintain detailed records of every failed request. These verbose log messages can provide information on incorrectly configured management servers as well as malicious (or just plain nosy) users.

Simply adding the keyword log to the deny any line in your access-list instructs the router to log all unauthorized SNMP attempts.

The following command will display the status of your SNMP access-list:

Router#show access-list 99
Standard IP access list 99
    permit 10.1.1.1  (1293 matches)
    permit 172.25.1.0, wildcard bits 0.0.0.255 (630 matches)
    deny   any log (17 matches)
Router#

Unlike the example shown in Recipe 17.6, the show access-list output now includes the log keyword on the deny any line. The router will now send information on every unauthorized SNMP request to the logging facility (see Chapter 18 for more information ...