Re: TCP flags issue

[sajjad purmohseni <spurmohseni () yahoo com>]

Thanks for replying; Now I know where is the ninth bit.
But this conditions is never taken in my code:
    if (p->tcp_header->flags & TCPHEADER_SYN){  //0x02        /*            ...
        */    }
 I believe that must be taken. Is there anything wrong?
Thank youSajad

     On Wednesday, February 4, 2015 6:59 PM, Steven Sturges <ststurge () cisco com> wrote:
   

  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> 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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