hecksumSize := iUdpChecksumSize + length(strMessage);

cksum := checksum(buf, iUdpChecksumSize);
udpHdr.udp_checksum := cksum;

//
// 现在 IP 和 UDP 头OK了，我们可以把它发送出去。
//
FillChar(Buf, SizeOf(Buf), 0);
Ptr := @Buf[0];

Move(ipHdr, ptr^, SizeOf(ipHdr)); IncPtr(SizeOf(ipHdr));
Move(udpHdr, ptr^, SizeOf(udpHdr)); IncPtr(SizeOf(udpHdr));
Move(StrMessage, ptr^, length(StrMessage));


remote.sin_family := AF_INET;
remote.sin_port := htons(iToPort);
remote.sin_addr.s_addr := dwToIP;
end;

procedure TForm1.SendIt;
Var
sh : TSocket;
bOpt : Integer;
ret : Integer;
Buf : TPacketBuffer;
Remote : TSockAddr;
Local : TSockAddr;
iTotalSize : Word;
wsdata : TWSAdata;

begin
// Startup Winsock 2
ret := WSAStartup($0002, wsdata);
if ret<>0 then begin
memo1.lines.add(''WSA Startup failed.'');
exit;
end;
with memo1.lines do begin
add(''WSA Startup:'');
add(''Desc.: ''+wsData.szDescription);
add(''Status: ''+wsData.szSystemStatus);
end;

try
// Create socket
sh := Socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
if (sh = INVALID_SOCKET) then begin
memo1.lines.add(''Socket() failed: ''+IntToStr(WSAGetLastError));
exit;
end;
Memo1.lines.add(''Socket Handle = ''+IntToStr(sh));

// Option: Header Include
bOpt := 1;
ret := SetSockOpt(sh, IPPROTO_IP, IP_HDRINCL, @bOpt, SizeOf(bOpt));
if ret = SOCKET_ERROR then begin
Memo1.lines.add(''setsockopt(IP_HDRINCL) failed: ''+IntToStr(WSAGetLastError));
exit;
end;

// Build the packet
BuildHeaders( SrcIP, SrcPort,
DestIP, DestPort,
''THIS IS A TEST PACKET'',
Buf, Remote, iTotalSize );

// Send the packet
ret := SendTo(sh, buf, iTotalSize, 0, Remote, SizeOf(Remote));
if ret = SOCKET_ERROR then
Memo1.Lines.Add(''sendto() failed: ''+IntToStr(WSAGetLastError))
else
Memo1.Lines.Add(''send ''+IntToStr(ret)+'' bytes.'');

// Close socket
CloseSocket(sh);
finally
// Close Winsock 2
WSACleanup;
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
SendIt;
end;

end.