To prevent a potential attack on a Cisco IOS router with the echo service enabled, what action should you take?
The echo service on a Cisco IOS router allows the router to respond to ICMP echo requests (also known as ping requests) sent to its interface IP addresses. This service can be exploited by attackers to launch denial-of-service (DoS) attacks or to probe for potential vulnerabilities.
To prevent a potential attack on a Cisco IOS router with the echo service enabled, you should disable the service with the "no ip echo" command. Option A is the correct answer.
Option B suggests using the "no echo" command to disable the service, but this command is used to disable the terminal echo feature, which is unrelated to the echo service used by the router.
Option C suggests disabling the tcp-small-servers service, which is used by the router to support the "chargen" and "daytime" services. This service is not related to the echo service and disabling it will not prevent attacks on the echo service.
Option D suggests disabling the echo service with a global access-list. While it is possible to use access-lists to block ICMP echo requests, this approach is not as effective as disabling the service altogether with the "no ip echo" command. Additionally, using an access-list to block echo requests may also block legitimate traffic, such as troubleshooting or monitoring activities.
In summary, to prevent potential attacks on a Cisco IOS router with the echo service enabled, you should use the "no ip echo" command to disable the service.