Cisco – tcp-small-servers and udp-small-servers

February 19, 2009

For some truly unknown reason, Cisco’s devices still have support for “small servers” or “simple services”. Examples of these include echo, chargen, daytime and discard.  An attacker could possibly start a denial of service attack (DoS) against one or more network devices with those configured.  In this case; echo and chargen are to blame by allowing an attacker to cause  the chargen service to hit the echo services causing an endless loop of character generation and echo between the two hosts.  To disable this, simply enter the following commands:

configure terminal
no service tcp-small-servers
no service udp-small-servers

These commands can be run on nearly all IOS based Cisco equipment.

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