Cisco IOS Software Releases 11.0

Cisco Security Notice: Response to BugTraq - Cisco IOS Software and ICMP Redirect Issue

Document ID: 59860

Revision 1.0

For Public Release 2002 May 21


Cisco Security Procedures


This document is provided to simplify access to Cisco responses to possible product security vulnerability issues posted in public forums for Cisco customers. This does not imply that Cisco perceives each of these issues as an actual product security vulnerability. This notice is provided on an "as is" basis and does not imply any kind of guarantee or warranty. Your use of the information on the page or materials linked from this page are at your own risk. Cisco reserves the right to change or update this page without notice at any time.


Original Report: Cisco responded with the following, which is also archived at

To:  BugTraq 
Subject:  Cisco IOS ICMP redirect DoS - Cisco's response 
Date:  May 21 2002 5:45PM 
Author:  Damir Rajnovic <gaus cisco com> 
Message-ID:  <> 


We can confirm the finding made by FX phenoelit de  This issue
is assigned Cisco bug ID CSCdx32056. The fix has been developed and
it is being committed into all affected releases.

The situation in Cisco IOS 12.x code is that the redirect cache will only 
grow if "ip routing" is disabled. The Cisco IOS 11.x code will populate 
the redirect cache ignoring the state of the "ip routing". The redirect
cache is fixed in size and an entry timeout is four hours.

By filling the redirect cache the memory is consumed. If the device 
is already low on memory that may cause further irregularities in 
the device's performance. Effects can vary, some of them can be:
new routes can not be learned, new MAC entries might not be added,
Telnet session might not be established, new CDP entries might not be
added. Depending on the exact configuration and circumstances, the
device may become totally unresponsive. The device should recover by 
itself after the four hours when the entries will start to timeout.

The workaround for users running Cisco IOS 11.x code is to block all ICMP
redirect messages that are sent to the router itself. That can be
accomplished this way:

 router(config)#access-list 101 deny icmp any host <device_IP> redirect
 .... (the rest of the access-list 101)
 router(config)#interface eth0
 router(config-if)#ip access-group 101 in

This example will block all ICMP packets, sent to the router itself,
coming from the eth0 interface. All transit ICMP redirect packets will
be allowed through.

Although, Cisco IOS 12.x code is less exposed we recommend to block all 
ICMP redirect packets sent to the device itself.


Version: PGP 6.5.3

Damir Rajnovic <psirt cisco com>, PSIRT Incident Manager, Cisco Systems
<>      Telephone: +44 7715 546 033
200 Longwater Avenue, Green Park, Reading, Berkshire RG2 6GB, GB
There is no insolvable problems. 
The question is can you accept the solution?

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at

Updated: May 21, 2002 Document ID: 59860