Date: Tue, 21 May 2002 18:45:40 +0100
From: Damir Rajnovic <gaus@cisco.com.>
To: [email protected]Subject: Cisco IOS ICMP redirect DoS - Cisco's response
Cc: [email protected]
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Hello,
We can confirm the finding made by [email protected]. 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.
Gaus
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==============
Damir Rajnovic <psirt@cisco.com.>, PSIRT Incident Manager, Cisco Systems
<http://www.cisco.com/go/psirt>; 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?
