How to extract part of “content” and print in “msg” of a Snort Alert

[Heshan Perera <anthonyheshanperera () gmail com>]

I am trying to write a Snort rule that will allow me to print the name of a
file being downloaded via FTP.

The following is the rule I have so far...

alert tcp any any <> any any (content:"RETR:";msg:"A file is being
downloaded.";sid:1000004;)

While this rule works, I can't figure out how to print the name of the file
in the "msg" component of the alert. For example I would want the output of
the alert to be something like...

*"A file is being downloaded. The file name is foo.txt".*

The file name is available in the content of the FTP traffic (RETR: /foo.txt
)

I just cannot figure out how to extract that content and print it as a part
of the message.

Any help on this would be highly appreciated.

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