[Snort-users] Flags rule option

Matt Kettler mkettler at ...4108...
Thu Sep 26 18:06:05 EDT 2002


Unlike content, I don't think snort allows multiple flags sections in a 
rule. Offhand I think flags: AR! will do what you want, which appears to be 
detecting anything to a client port which is initiated by an outside 
server. This will match any reasonable syn packet, along with synfin, etc. 
but won't trigger on fin-ack, reset, synack, etc.

Also beware, any active-mode ftp transfers will trigger this rule (or your 
original one), since that causes an outside server to syn (connect to) 
a >1024  port on your machine.




At 05:21 PM 9/26/2002 -0700, Bill McCarty wrote:
>Hi all,
>
>I have the following Snort rule:
>
>alert tcp any any -> XXX.XXX.XXX.XXX 1024:65535 (flags:S+; flags:A!; 
>msg:"Local suspicious inbound"; )
>
>This rule matches the following datagram:
>
>09/26/02-15:49:14.256645 YYY.YYY.YYY.YYY:80 -> XXX.XXX.XXX.XXX:24247
>TCP TTL:237 TOS:0x0 ID:0 IpLen:20 DgmLen:40 DF
>*****R** Seq: 0x2ED4C858 Ack: 0x0 Win: 0x0 TcpLen: 20
>
>I think that both flags options must be matched in order for the rule as a 
>whole to match. Yet, the matching datagram does not have the S flag set. 
>What am I missing?
>
>Thanks!
>
>---------------------------------------------------
>Bill McCarty
>
>
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