How to change window scale in TCP packet in linux?

Question

I need to change window size in TCP header, but when I changing it and recalculate the checksum the final window is multiplying by window scale option. Where and in which structure I can find it and change?

Answer 1

You can't change the window scale after it has been negotiated during connection establishment. You need to set a large receive buffer on the socket prior to connecting it. In the case of a server, you need to set the receive buffer size on the listening socket, from whence it will be inherited by the accepted socket.

Answer 2

Yes, I can't change the window scale after it has been negotiated during connection establishment. I want to post a piece of code which I wrote to get tcp options(mss and window scale):

typedef struct {
    unsigned char kind;
    unsigned char size;
} tcp_option_t;
unsigned char* tmp = tcp_header;    
if (tcp_header->doff > 5) {
    unsigned char* opt = tmp + sizeof(struct tcphdr);
    while( *opt != 0 ) {
        tcp_option_t* _opt = (tcp_option_t*)opt;
        if( _opt->kind == 1 ) { //NOP
                        ++opt;  // NOP is one byte;
                         continue;
                }
        if( _opt->kind == 2 ) { //MSS
            unsigned int* mss_opt = (unsigned int*)(opt + sizeof(tcp_option_t));
            unsigned int mss = htons(*mss_opt);
            f->mss = mss;
        }
        if( _opt->kind == 3 ) { //wnd_scale
            integer24* wnd_opt = (integer24*)(opt + sizeof(tcp_option_t));
            integer24 wnd_scale = *wnd_opt;
            f->wnd_scale = wnd_scale.data;
        }
        opt += _opt->size;

        if (_opt->size == 0) {
            //pr_info("Very strange situation: zero size\n");
            break;
        }       
    }