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POsetting the maximum segment size in the tcp header
 

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  1. POsetting the maximum segment size in the tcp header
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    <p>I am putting together a port scanner as a learning exercise. My problem is I'm trying to set the maximum segment size option(MSS) in the TCP header. I had a look at tcp.h, but I'm having trouble figuring out how to set it. I was hoping there would be an option like this:</p> <p><code>tcp_header-&gt;mss(32000);</code></p> <p>Something similar to the above was in tcp.h but not in the right struct. Admittedly, I'm still fairly new to reading struct definitions and I couldn't make much sense out of tcp.h so in the end I tried just tacking on the necessary bytes to the end of the TCP header:</p> <pre><code>struct tcphdr *CreateTcpHeader() { struct tcphdr *tcp_header; tcp_header = (struct tcphdr *)malloc(sizeof(struct tcphdr)+4*sizeof(int)); tcp_header-&gt;source = htons(SRC_PORT); tcp_header-&gt;dest = htons(DST_PORT); tcp_header-&gt;seq = htonl(0); tcp_header-&gt;ack_seq = htonl(0); tcp_header-&gt;res1 = 0; tcp_header-&gt;doff = (sizeof(struct tcphdr))/4; tcp_header-&gt;syn = 1; tcp_header-&gt;window = htons(4096); tcp_header-&gt;check = 0; /* Will calculate the checksum with pseudo-header later */ tcp_header-&gt;urg_ptr = 0; /*memcpy the mss data onto the end of the tcp header. */ int mssCode = 2; int mssLength = 4; uint16_t mss = htonl(32000); int offset = sizeof(struct tcphdr); memcpy( (tcp_header+offset), &amp;mssCode, 1 ); memcpy( (tcp_header+offset+1), &amp;mssLength, 1 ); memcpy( (tcp_header+offset+2), &amp;mss, 2); return (tcp_header); } </code></pre> <p>But after I wrote it it was clear that is wasn't a real solution, plus it still doesn't work :P So is there a better way?</p>
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    1. USCalumMcCall
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    1. COHow are you using that header? Are you sending it with libnet?
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      1. USJonathan
    2. COI'm just using raw sockets. Although even with the new mss option, my packet is still ignored, despite it being practically identical to nmap's. Any ideas anyone?
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      1. USCalumMcCall
    3. CODepending on what you are learning - C or networking, you might want to look at http://www.secdev.org/projects/scapy/
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      1. USAndrew Y
 

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