[heilong]

I not absolutely understand a question, therefore I shall answer how I have understood it.

...

var

ReadRequest: ETH_REQUEST;

Buffer: INTERMEDIATE_BUFFER;

pEthernetHeader: PETHERNET_HDR;

pIpHeader: PIP_HDR;

pTCPHeader: PTCP_HDR;

pData: Pbyte;

DataSize: Integer;

...



const

HSIZE = sizeof(ETHERNET_HDR) + sizeof(IP_HDR)+ sizeof(ICMP_HDR);



...



ReadRequest.EthPacket.Buffer := @Buffer;

ReadRequest.hAdapterHandle := hAdapter;



...



if ReadPacket (hFilt, @ReadRequest) <> 0 then

begin

...

pEthernetHeader := @ReadRequest.EthPacket.Buffer.m_IBuffer;

pIPHeader := Pointer(integer(pEthernetHeader)+sizeof(ETHERNET_HDR));

pTCPHeader := Pointer(integer(pIPHeader)+sizeof(IP_HDR));

DataSize := ReadRequest.EthPacket.Buffer.m_Length - HSIZE;



PData := Pointer(integer(pEthernetHeader)+HSIZE);

// or so

//PData := Pointer(integer(pTCPHeader)+sizeof(TCP_HDR));

...



// Change Port Destination to 4032



pTCPHeader.Destination := htons(4032);



// Tne IPHeader checksum it is not recalculated therefore as the size of a package has not changed



pTCPHeader.CheckSum := 00;

pTCPHeader.Checksum := htons(Checksum(Pword(pTCPHeader), (sizeof(TCP_HDR) + DataSize) div 2));



//If the size of a packet has changed, it is necessary to align a packet to word and recalc IP Header and accordingly TCP Header

...



SendPacketToAdapter(hFilt, @ReadRequest);

end;

...

In a code there can be mistakes, for what beforehand apologize.

[heilong]

Checksum = calculate (pointer, (header+data) div 2), as for ICMP as TCP

Example:

Packet = ETH_HDR+IP_HDR+TCP_HDR+Data;

IP_HDR.Checksum := 00;

IP_HDR.Checksum := htons(Checksum(Pword(IP_HDR), sizeof(IP_HDR) div 2));



TCP.Checksum := 00;

TCP_HDR.Checksum := htons(Checksum(Pword(TCP_HDR), sizeof((TCP_HDR+Length(Data)) div 2));

For more: See ICMP example in previous reply

[heilong]

Try it. It is universal function.

p – pointer to begin of header.
Size – size of header in words.
Attention: Field Checksum must be = 0 before call function.

function Checksum(p: PWORD; Size: word): WORD;
var
i: Integer;
Sum: DWORD;
begin
Sum := 0;
for i := 0 to Size – 1 do
begin
Sum := Sum + ntohs(p^);
inc(p);
end;
while (Sum shr 16) > 0 do
Sum := (Sum and $FFFF) + (Sum shr 16);
Sum := not Sum;
Result := Sum;
end;

Examples:
1. Calc checksum for IPHeader. For uses without IPOptions.

IPHeader.Checksum := 00;
IPHeader.Checksum := htons(Checksum(Pword(IPHeader), sizeof(IP_HDR) div 2));

2. Calc checksum for ICMPHeader.
IcmpHeader.Checksum := 00;
IcmpHeader.Checksum := htons(Checksum(Pword(IcmpHeader),
(sizeof(ICMP_HDR)+Length(Data)) div 2));

Don`t forget call htons for result of function Checksum.
Sorry for my English.

[Author]

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