[Snort-users] Sobre las reglas snort

Hutchinson, Andrew Andrew.Hutchinson at ...3639...
Fri Jul 5 09:34:02 EDT 2002

Okay, my Spanish is marginal at best, so I'm replying in English.

The "A+" refers to the tcp flags on the packet in question.  In the alert you provided, the "A+" means that the ACK flag MUST be set, and any other flags MAY be set.  Thus, an ACK, SYN-ACK, FIN-ACK, or an XMAS packet with everything set would all match this rule.  The ACK flag is a must, the other flags can go either way.  If the rule were written with only "flags: A", then the rule would only match if ONLY the ACK flag were set (i.e. a SYN-ACK would not match).

Here are some other examples:

flags: SF    # Matches ONLY SYN-FIN
flags: SF+   # Would match SYN-FIN, SYN-FIN-ACK, XMAS, would NOT match FIN-ACK, SYN-ACK, SYN, ACK, RST
flags: SF*   # Would match SYN-FIN, SYN-FIN-ACK, XMAS, FIN-ACK, SYN, and SYN-ACK, would NOT match ACK, RST

So, in essence the "+" is like a logical "AND" (with the side-effect that non-included flags can be set and still match), while the "*" is like a non-exclusive "OR".  

Hope that this helps,


-----Original Message-----
From: fon Al [mailto:Alfon at ...6253...] 
Sent: Friday, July 05, 2002 11:10 AM
To: snort-users at lists.sourceforge.net
Subject: [Snort-users] Sobre las reglas snort


Soy nuevo en la lista queria hacer una pregunta.

En la creación de reglas Snort, que significa 'flags: A+;'

me refiero al significado del +A 

por ejemplo:

alert tcp $EXTERNAL_NET any -> $HOME_NET 113 (msg:"SCAN ident 
version"; flags: A+; content: "VERSION|0A|"; depth: 
16;reference:arachnids,303; classtype:attempted-recon; sid:616; 

Según el manual dice que:

+ ALL flag, match on all specified flags plus any others
* ANY flags, match.......

Pero es que no tengo ni idea de que significa y que tiene que ver 
con ACK.

Muchas gracias.


Ps. Ya tienes tu correo @maestrosdelweb.com
Consiguelo en http://www.maestrosdelweb.com

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