Re: Re: Modify packet [DELPHI]

[SergeySW]

function RecalculateTCPChecksum(aLen: Integer; aSrcAddr, aDstAddr: Cardinal; buff: PByteArray): Word;

var

w16,padd: word;

i,sum: ulong;

sIP,dIP: in_addr;

begin

padd := 0;

sum := 0;

if (aLen div 2) * 2 <> aLen then begin

padd := 1;

buff[aLen] := 0;

end;

i := 0;

while i < aLen+padd do begin

w16 := ((buff shl 8)and $FF00) + (buff[i+1]and $FF);

sum := sum + dword(w16);

i := i + 2;

end;



//add IP length

sIP := in_addr(aSrcAddr);

dIP := in_addr(aDstAddr);

sum := sum + ntohs(sIP.S_un_w.s_w1) + ntohs(sIP.S_un_w.s_w2);

sum := sum + ntohs(dIP.S_un_w.s_w1) + ntohs(dIP.S_un_w.s_w2);

sum := sum + IPPROTO_TCP + word(aLen);



while (sum shr 16)>0 do

sum := (sum and $FFFF)+(sum shr 16);



sum := not sum;

sum := htons(sum);

Result := sum;

end;



aaLen := ntohs(pIPHeader.TotalLen) - ((pIPHeader.VerLen and $F)*4);

pTCPHeader.Checksum := 0;

testSum := RecalculateTCPChecksum(aaLen, pIPHeader.SourceIp, pIPHeader.DestIp, byteArray(pTCPHeader));

pTCPHeader.Checksum := testSum;