Re: Re: Modify packet [DELPHI]

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#5582

SergeySW
Participant

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;