raw socket syn scan nodejs

syn.js

/*

tcpdump -i lo -n ip

nmap   :19:54:07.502287 IP 127.0.0.1.56239 > 127.0.0.1.23: Flags [S], seq 411956820, win 1024, options [mss 1460], length 0
syn.js :21:26:57.830349 IP 127.0.0.1.54444 > 127.0.0.1.23: Flags [S], seq 234156285, win 1024, options [mss 1460], length 0

--> sound good but no ack+syn in reply, let's go verbose mode with tcpdump


tcpdump -vvv -i lo -n ip

OK: NMAP result (nmap -sS localhost -p 23)

21:50:20.794277 IP (tos 0x0, ttl 40, id 56063, offset 0, flags [none], proto TCP (6), length 44)
    127.0.0.1.56649 > 127.0.0.1.23: Flags [S], cksum 0x17c8 (correct), seq 1809790236, win 1024, options [mss 1460], length 0
21:50:20.794292 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 44)
    127.0.0.1.23 > 127.0.0.1.56649: Flags [S.], cksum 0xfe20 (incorrect -> 0xa280), seq 2375108310, ack 1809790237, win 43690, options [mss 65495], length 0
21:50:20.794297 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
    127.0.0.1.56649 > 127.0.0.1.23: Flags [R], cksum 0x3381 (correct), seq 1809790237, win 0, length 0

KO: syn.js result :
1:51:01.309365 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 48)
    127.0.0.1.35643 > 127.0.0.1.23: Flags [S], cksum 0x0000 (incorrect -> 0x01db), seq 234156285:234156289, win 1024, options [mss 1460], length 4

*/

var raw = require ("raw-socket");

var options = {
    protocol: raw.Protocol.TCP,
    addressFamily: raw.AddressFamily.IPv4,
    generateChecksums: true,
    //checksumOffset: 40 do i really need to provide that ?
};

var socket = raw.createSocket(options);

var buffer = new Buffer ([
    0x00,0x00, // src port
    0x00,0x00, // dst port
    0x0d,0xf4,0xf0,0xfd, // sequence number (should be random)
    0x0f,0x00,0xf2,0x00, // acquitement number
    0x69, // header length
    0x02, // flags (fin=1,syn=2,rst=4,psh=8,ack=16,urg=32)
    0x04,0x00, // window 1024
    0x00,0x00, // crc
    0x00,0x00, // ptr urgent
    0x02,0x04,0x05,0xb4, // options and padding (mss=1460)
    0x00,0x00,0x00,0x00 // ??
]);


var setPortDst = function(buf,port) {
    buffer[0x02] = (port >> 8) & 0xff;
    buffer[0x02 + 1] = (port & 0xff);
    return buf;
}

var setPortSrc = function(buf,port) {
    buffer[0x00] = (port >> 8) & 0xff;
    buffer[0x00 + 1] = (port & 0xff);
    return buf;
}

setPortDst(buffer,23);
setPortSrc(buffer,54444); // port src should be random, not static

var target = '127.0.0.1';

socket.on ("close", function () {
	console.log ("socket closed");
	process.exit (-1);
});

socket.on ("error", function (error) {
	console.log ("error: " + error.toString ());
	process.exit (-1);
});

socket.on ("message", function (buffer, source) {

	//console.log ("received " + buffer.length + " bytes from " + source);

    if (buffer[0x09] == 0x06) {
        // TCP flag found
        if (buffer[0x21] == 0x21) {
            // SYN flag found
            process.stdout.write('\nYOU WIN, must send RST packet here');
        } else {
            process.stdout.write('.');
        }
    } else {
        process.stdout.write('.');
    }

});

var level = raw.SocketLevel.IPPROTO_IP;
var option = raw.SocketOption.IP_TTL;

socket.send (buffer, 0, buffer.length, '127.0.0.1', function (error, bytes) {
    if (error) {
        console.log (error.toString ());
    } else {
        console.log ("sent " + bytes + " bytes to " + target);
    }
});

Hey ! I can't find any doc but I really want to do the same with UDP datagram can you help me ?