Accessing TCP flags in TCP packets on Linux using C !!

vishamr2000 · 10-14-2006, 01:52 PM

Hi to all,

i would like some help regarding the following:

I'm trying myself at network programming and I want to access the TCP flags in a TCP packet. I know that in C programming language, the TCP header can be accessed in the following manner:

Code:

const struct ethernet_header *ethernet;
const struct tcp_header *tcp;
const u_char *packet;
ethernet = (struct ethernet_header*) (packet)
tcp = (struct tcp_header*) (packet + size_ethernet + size_ip)

We kind of skip past the ethernet frame and IP packet headers. But I'm having problems with how to access the TCP flags (SYN, ACK, URG, FIN...).

Can anyone help me with this? Any help will be very much appreciated.

Warm regards,
Visham

wwwdev · 10-14-2006, 06:24 PM

hi vishamr2000,

Quote:

Originally Posted by vishamr2000

We kind of skip past the ethernet frame and IP packet headers. But I'm having problems with how to access the TCP flags (SYN, ACK, URG, FIN...).

Can anyone help me with this? Any help will be very much appreciated.

Warm regards,
Visham

struct tcpheader {
unsigned short int th_sport;
unsigned short int th_dport;
unsigned int th_seq;
unsigned int th_ack;
unsigned char th_x2:4, th_off:4;
unsigned char th_flags; // TCP flags (SYN, ACK,...)
unsigned short int th_win;
unsigned short int th_sum;
unsigned short int th_urp;
};

See here http://linux.sys-con.com/read/34589_1.htm or google "raw sockets programming".

vishamr2000 · 10-16-2006, 09:46 AM

Many thx for the reply wwwdev..i'll try it and see how it goes.

Warm regards,
Visham