Re: TCP flags issue

[Steven Sturges <ststurge () cisco com>]

Per updates to the TCP specification, there are 8 bits of flags in the 
TCP flags section.
The additional "flag" is really part of the adjacent 8 bits that cover 
the Data Offset &
Reserved fields.

The ECN Nonce is "flag" 9, aka bit 1 of that adjacent octet.

Use bit 1 of offset_reserved in TCPHeader if you need to check that in a 
dynamic
module that uses sf_snort_packet.h.

On 2/4/15 3:10 AM, Balasubramaniam Natarajan wrote:
> On Wed, Feb 4, 2015 at 3:12 AM, sajjad purmohseni 
> <spurmohseni () yahoo com <mailto:spurmohseni () yahoo com>> wrote:
>
>     Hi all
>
>
>
>     As you know TCP has 9 flags. But as you see TCP flags of snort is
>     8 bits defined in *sf_snort_packet.h* file:
>
> For all I know TCP has just 6 Flags in use currently.
>
>     typedef struct _TCPHeader
>     {
>     ...
>     uint8_t flags;
>     ...
>     } TCPHeader;
>
>     also predefinded flags are 9 bits as must be:
>
>     #define TCPHEADER_FIN  0x01
>     #define TCPHEADER_SYN  0x02
>     ...
>
>     Now I am confused if I want to check TCP_SYN flag; how to do that.
>     I have used this but it doesn't return correct answer:
>
> I believe the 13 offset of TCP header should be 0x03 for SYN&FIN.
>
>     if (packet->tcp_header->flags & TCPHEADER_SYN){
>         ...
>     }
>     Can anybody guide me about this issue? Thank you.
>
>     ----
>     kind regards;
>     Sajad Pourmohseni
>     -- 
>
> Regards,
> Balasubramaniam Natarajan
> http://blog.etutorshop.com
>
>
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