I'm building a network appliance. I need to support NAT and IP packet fragmentation. When I change the source or destination address of a UDP packet, I have to correct the UDP checksum (and the IP checksum too, but that's trivial). When the packet is fragmented, I'd have to collect all the fragments to recalculate the checksum. I know the old address and the new address. I'd like to:
- Un-negate the checksum
- Subtract the old address
- Add the new address
- Re-reduce the sum and negate
This process doesn't always work. Is there any way to update the checksum versus having to recalculate it from scratch?
I've tried:
long CalcCheckSumAdd(unsigned char *pbHeader, int iSize, long lInitial){
long lSum = lInitial;
while (iSize > 1){
lSum += *((unsigned short*)pbHeader);
pbHeader += 2;
iSize -= 2;
}
if (iSize > 0) lSum += *pbHeader;
return lSum;
}
long CalcCheckSumSubract(unsigned char *pbHeader, int iSize, long lInitial){
long lSum = lInitial;
while (iSize > 1){
lSum -= *((unsigned short*)pbHeader);
pbHeader += 2;
iSize -= 2;
}
if (iSize > 0) lSum -= *pbHeader;
return lSum;
}
unsigned short CalcCheckSumFinish(long lSum){
while (lSum >> 16){
lSum = (lSum & 0xFFFF) + (lSum >> 16);
}
return (unsigned short)(~lSum);
}
long CalcCheckSumUnfinish(unsigned short usSum){
// Can't totally undo lossy finish logic
return ~usSum;
}
unsigned short CalcCheckSumUpdateAddress(unsigned short usOldSum, unsigned long ulOldAddress, unsigned long ulNewAddress){
long lSumFixed = CalcCheckSumUnfinish(usOldSum);
lSumFixed = CalcCheckSumSubract((unsigned char*)&ulOldAddress,sizeof(ulOldAddress),lSumFixed);
lSumFixed = CalcCheckSumAdd((unsigned char*)&ulNewAddress,sizeof(ulNewAddress),lSumFixed);
return CalcCheckSumFinish(lSumFixed);
}
Thanks!
EDIT: Added unit test code below
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
long CalcCheckSumAdd(unsigned char *pbHeader, int iSize, long lInitial){
long lSum = lInitial;
while (iSize > 1){
lSum += *((unsigned short*)pbHeader);
pbHeader += 2;
iSize -= 2;
}
if (iSize > 0) lSum += *pbHeader;
return lSum;
}
unsigned short CalcCheckSumFinish(long lSum){
while (lSum >> 16){
lSum = (lSum & 0xFFFF) + (lSum >> 16);
}
return (unsigned short)(~lSum);
}
void Randomize(unsigned char *pucPacket, unsigned long ulSize){
for (unsigned long ulByte = 0; ulByte < ulSize; ulByte++){
pucPacket[ulByte] = (unsigned char)(255 * rand() / RAND_MAX);
}
}
unsigned short Calc(unsigned char *pucPacket, unsigned long ulSize){
long lSum = CalcCheckSumAdd(pucPacket,ulSize,0);
return CalcCheckSumFinish(lSum);
}
unsigned short Fix(unsigned short usOrig, unsigned int uiOld, unsigned int uiNew){
// TODO: Replace this with something that makes main never fail
usOrig -= uiOld & 0xffff;
usOrig -= uiOld >> 16 & 0xffff;
usOrig += uiNew & 0xffff;
usOrig += uiNew >>16 & 0xffff;
return usOrig;
}
void Break(unsigned char *pucPacket, unsigned int *puiOld, unsigned int *puiNew){
unsigned int *puiChange = (unsigned int*)pucPacket;
*puiOld = *puiChange;
Randomize((unsigned char*)puiNew,sizeof(unsigned int));
*puiChange = *puiNew;
}
void PrintBuffer(const char *szName, unsigned char *pucBuff, unsigned int uiSize){
printf("%s: ",szName);
for (unsigned int uiByte = 0; uiByte < uiSize; uiByte++){
printf("%02X",(unsigned int)pucBuff[uiByte]);
}
printf("\n");
}
void PrintTestCase(unsigned char *pucOrig, unsigned char *pucChanged, unsigned int uiSize, unsigned short usOrig, unsigned short usChanged, unsigned short usFixed){
PrintBuffer("Original Buffer",pucOrig,uiSize);
PrintBuffer("Changed Buffer ",pucChanged,uiSize);
printf("Orig checksum: %04X\n",(unsigned int)usOrig);
printf("Changed checksum: %04X\n",(unsigned int)usChanged);
printf("Fixed checksum: %04X\n",(unsigned int)usFixed);
}
int main(){
srand((unsigned int)time(nullptr));
unsigned char pucDataOrig[100];
unsigned char pucDataChanged[100];
bool bTestFailed = false;
while (!bTestFailed){
Randomize(pucDataOrig,sizeof(pucDataOrig));
memcpy(pucDataChanged,pucDataOrig,sizeof(pucDataOrig));
unsigned short usOrig = Calc(pucDataOrig,sizeof(pucDataOrig));
unsigned int uiOld = 0,
uiNew = 0;
Break(pucDataChanged,&uiOld,&uiNew);
unsigned short usFixed = Fix(usOrig,uiOld,uiNew);
unsigned short usChanged = Calc(pucDataChanged,sizeof(pucDataChanged));
if (usChanged == usFixed){
printf(".");
}else{
printf("\nTest case failed\n");
PrintTestCase(pucDataOrig,pucDataChanged,sizeof(pucDataOrig),usOrig,usChanged,usFixed);
bTestFailed = true;
}
}
return 0;
}
