North American Network Operators Group

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Re: Blocking nimda probes with a content-layer switch

  • From: Lincoln Dale
  • Date: Wed Sep 19 03:06:08 2001

At 12:32 AM 19/09/2001 -0400, Joe Abley wrote:
Has anybody tried to block nimda HTTP GET probes using URL
pattern matches in a "layer-4-7"[1] switch?
note that this worm uses multiple methods for spreading - email and HTTP. these methods are only addressing the HTTP side of things:

NBAR (Network Based Application Recognition) in IOS:
existing CodeRed NBAR settings will block the HTTP probes.

Cisco Content Engine:
rule enable
rule block url-regex http://.*/cmd\.exe
rule block url-regex http://.*/root\.exe

Cisco CSS11K (Content Services switch):
service dummy
ip address
keepalive type none
header-field-group cmd.exe
header-field cmd.exe request-line contain "cmd.exe"
header-field-group root.exe
header-field root.exe request-line contain "root.exe"
content block_cmd.exe
protocol tcp
port 80
url "/*"
header-field-rule cmd.exe
add service dummy
content block_root.exe
protocol tcp
port 80
url "/*"
header-field-rule root.exe
add service dummy

The ideal result is to prevent nimda GET probes from ever
reaching the destination address, but causing the session
to be reset towards the server after the open handshake but
before the GET can be sent to the server would be acceptably
note that only the CSS and the CE in the above will actually prevent the 'tcp syn' from propogating, and they'll establish the tcp 3-way handshake, receive the HTTP headers and then match the request to be an "attack".
NBAR will only match the headers on connections that successfully establish a 3-way handshake. (NBAR won't cause the router to spoof the TCP setup - so-called delayed-binding).