Chapter1 Cisco SDM Express
Supplementary Help
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Cisco SDM Express
OL-7141-04
The configuration that will be delivered to the router to disable PAD is as follows:
no service pad
You can undo this fix using the CiscoSD M Security Audit feature. To learn how,
see the Securi ty Audit online h elp in Cisco SDM. For more information, click
CiscoRouter and Security Device Manager.
Disable TCP Small Servers Service
Cisco SDM Express disables small services whenever possible. By default, Cisco
devices running Cisco IOS release 11.3 or earlier offer the “small servi ces”: echo,
chargen, and discard. (Small services are disabled by default in Cisco IOS
software release12.0 and later.) These services, especially their User Datagram
Protocol (UDP) versions, are infrequently used for legitimate purposes, but they
can be used to launch Denial of Service (DoS) and other attacks that would
otherwise be prevented by packet filtering.
For example, an attacker might send a Domain Name System (DNS) packet,
falsifying the source address to be a DNS server that would otherwise be
unreachable, and falsifying the source port to be the DNS service port (port 53).
If such a packet were sent to the router UDP echo port, the result would be the
router sending a DNS packet to the server in question. No ou tgoing access list
checks would be applied to this packet because it would be considered to be
locally generated by the router itself.
Although most abuses of the small services can be avoided or made less dangerous
by antispoofing access lists, the services should almost always be disabled in any
router which is part of a firewall or lies in a security-critical part of the network.
Because the services are rarely used, the best policy is usually to disable them on
all routers of any description.
The configuration that will be delivered to the router to disable TCP small servers
is as follows:
no service tcp-small-servers
You can undo this fix using the CiscoSD M Security Audit feature. To learn how,
see the Securi ty Audit online h elp in Cisco SDM. For more information, click
CiscoRouter and Security Device Manager.