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[Full-disclosure] Content detection in html payload with snort ?

hi list,
I could not found an answer to my problem, so I ask the list :

I use snort to detect attackers playing with my web application.
I try to detect some specific text in html response, like "Bad User" ou " Warning Mysql Error". But snort stay blind.

Sample :
1 - Attacker   -> web-server : http://server/script.asp?param=' or 1=1--
2 - web-server -> attacker : 200 OK, ......<html>......datatype error....

I try to catch the string "datatype error" with a snort rule like that :

alert tcp $HTTP_SERVERS $HTTP_PORTS -> any any (msg:"web-server attack"; flow:from_server,established; content:"datatype error"; classtype:web-application-attack; sid:80005; rev:1;) 

But Snort never detects that.

I try with binary mode, same.
When I sniff with ethereal, the packet I try to catch is like that :

attcker    -> web-webser  : HTTP : GET  http://server/script.asp?param='
web-server -> attacker : HTTP : HTTP/1.1 304 Not Modified    
web-server -> attacker : HTTP : Continuation or non-HTTP traffic (*HERE)

If anyone have an idea ?

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