我正在构建一个网络设备。我需要支持NAT和IP数据包碎片。当我更改UDP数据包的源或目标地址时，我必须更正UDP校验和（以及IP校验和，但这是微不足道的）。当数据包碎片化时，我必须收集所有碎片以重新计算校验和。我知道旧地址和新地址。我想：
这个过程并不总是有效。有没有办法更新校验和而不必从头开始重新计算？
我试过了：
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);
}
谢谢！
编辑：添加下面的单元测试代码
#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;
}
你是对的，上面的解决方案只适用于某些情况，但我有一个新的实现，适用于所有类型的数据包（碎片或非碎片，UDP，TCP，IP）。这是实现：
/* incremental checksum update */
static inline void
cksum_update(uint16_t *csum, uint32_t from, uint32_t to)
{
uint32_t sum, csum_c, from_c, res, res2, ret, ret2;
csum_c = ~((uint32_t)*csum);
from_c = ~from;
res = csum_c + from_c;
ret = res + (res < from_c);
res2 = ret + to;
ret2 = res2 + (res2 < to);
sum = ret2;
sum = (sum & 0xffff) + (sum >> 16);
sum = (sum & 0xffff) + (sum >> 16);
*csum = (uint16_t)~sum;
}
您现在可以在翻译包地址时使用此功能，然后再发送：
/* Update L4 checksums on all packet a part from [2nd, n] fragment */
switch (IS_FRAG(ipv4_hdr) ? 0 : ipv4_hdr->next_proto_id) {
case IPPROTO_TCP:
{
struct tcp_hdr *tcp_hdr = tcp_header(pkt);
/* Compute TCP checksum using incremental update */
cksum_update(&tcp_hdr->cksum, old_ip_addr, *address);
break;
}
case IPPROTO_UDPLITE:
case IPPROTO_UDP:
{
struct udp_hdr *udp_hdr = udp_header(pkt);
/* Compute UDP checksum using incremental update */
cksum_update(&udp_hdr->dgram_cksum, old_ip_addr, *address);
break;
}
default:
break;
}
您必须减去旧的IP地址并在udp校验和上添加新的IP地址，这里是伪代码：
udp_hdr->dgram_cksum -= old_ipv4_addr & 0xffff;
udp_hdr->dgram_cksum -= old_ipv4_addr >> 16 & 0xffff;
udp_hdr->dgram_cksum += new_ipv4_addr & 0xffff;
udp_hdr->dgram_cksum += new_ipv4_addr >>16 & 0xffff;
这应该处理IP片段的UDP校验和。