The content keyword is one of the more important features of Snort. It allows the user to set rules that search for specific content in the packet payload and trigger response based on that data. Whenever a content option pattern match is performed, the Boyer-Moore pattern match function is called and the (rather computationally expensive) test is performed against the packet contents. If data exactly matching the argument data string is contained anywhere within the packet's payload, the test is successful and the remainder of the rule option tests are performed. Be aware that this test is case sensitive.
The option data for the content keyword is somewhat complex; it can contain mixed text and binary data. The binary data is generally enclosed within the pipe () character and represented as bytecode. Bytecode represents binary data as hexadecimal numbers and is a good shorthand method for describing complex binary data. The example below shows use of mixed text and binary data in a Snort rule.
Note that multiple content rules can be specified in one rule. This allows rules to be tailored for less false positives.
If the rule is preceded by a !, the alert will be triggered on packets that do not contain this content. This is useful when writing rules that want to alert on packets that do not match a certain pattern
Also note that the following characters must be escaped inside a content rule:
; \ "