checksum计算方法.doc

ICMP，IP,UDP,TCP报头部分都有checksum（检验和）字段。ICMP和IP报头校验和的计算都很简单，使用RFC1071中给出的方法即可完成（如下）。/计算校验和USHORT checksum(USHORT *buffer,int size)unsigned long cksum=0;while(size1)cksum+=*buffer+;size-=sizeof(USHORT);if(size)cksum+=*(UCHAR *)buffer;/将32位数转换成16while (cksum16)cksum=(cksum16)+(cksum & 0xffff);return (USHORT) (cksum);UDP/TCP报头中的校验和的计算比较复杂的，要用到 UDP/TCP伪首部：先要填充伪首部各个字段，然后再将UDP/TCP报头以后（包括报头）的数据附加到伪首部的后面，再对位首部使用上述校验和计算，所得到的值才是UDP/TCP报头部分的校验和。位首部可以用如下的结构体表示：typedef structULONG sourceip; /源IP地址ULONG destip; /目的IP地址BYTE mbz; /置空(0)BYTE ptcl; /协议类型USHORT plen; /TCP/UDP数据包的长度(即从TCP/UDP报头算起到数据包结束的长度 单位:字节)Psd_Header;这个过程是一个很繁琐的过程，计算过几次后再也忍受不了做这样重复的工作，于是写了一个通用的计算函数。这个函数使用起来我感觉非常方便：先封装好你的数据包（完整的，包括以太头），然后将数据包的首地址作为参数，调用该函数即可。函数将帮你完成IP报头以及UDP/TCP报头部分校验和的计算。/-/ PacketCheckSum/ 计算数据包的校验和/ 参数:packet-待处理数据(将封装好的数据包的指针)/-void PacketCheckSum(unsigned char packet)Dlc_Header *pdlc_header=NULL; /以太头指针Ip_Header *pip_header=NULL; /IP头指针unsigned short attachsize=0; /传输层协议头以及附加数据的总长度pdlc_header=(Dlc_Header *)packet;/判断ethertype,如果不是IP包则不予处理if(ntohs(pdlc_header-ethertype)!=00800) return;pip_header=(Ip_Header *)(packet+14);/TCP包if(006=pip_header-proto)Tcp_Header *ptcp_header=NULL; /TCP头指针Tcp_Psd_Header *ptcp_psd_header=NULL;ptcp_header=(Tcp_Header *)(packet+14+(pip_header-ver_len)&15)*4);attachsize=ntohs(pip_header-total_len)-(pip_header-ver_len)&15)*4;ptcp_psd_header=(Tcp_Psd_Header *)malloc(attachsize+sizeof(Tcp_Psd_Header);if(!ptcp_psd_header) return;memset(ptcp_psd_header,0,attachsize+sizeof(Tcp_Psd_Header);/填充伪TCP头ptcp_psd_header-destip=pip_header-destIP;ptcp_psd_header-sourceip=pip_header-sourceIP;ptcp_psd_header-mbz=0;ptcp_psd_header-ptcl=006;ptcp_psd_header-tcpl=htons(attachsize);/计算TCP校验和ptcp_header-chksum=0;memcpy(unsigned char *)ptcp_psd_header+sizeof(Tcp_Psd_Header),(unsigned char *)ptcp_header,attachsize);ptcp_header-chksum=checksum(unsigned short *)ptcp_psd_header,attachsize+sizeof(Tcp_Psd_Header);/计算ip头的校验和pip_header-checksum=0;pip_header-checksum=checksum(unsigned short *)pip_header,20);return;/UDP包if(011=pip_header-proto)Udp_Header *pudp_header=NULL; /UDP头指针Udp_Psd_Header *pudp_psd_header=NULL;pudp_header=(Udp_Header *)(packet+14+(pip_header-ver_len)&15)*4);attachsize=ntohs(pip_header-total_len)-(pip_header-ver_len)&15)*4;pudp_psd_header=(Udp_Psd_Header *)malloc(attachsize+sizeof(Udp_Psd_Header);if(!pudp_psd_header) return;memset(pudp_psd_header,0,attachsize+sizeof(Udp_Psd_Header);/填充伪UDP头pudp_psd_header-destip=pip_header-destIP;pudp_psd_header-sourceip=pip_header-sourceIP;pudp_psd_header-mbz=0;pudp_psd_header-ptcl=011;pudp_psd_header-udpl=htons(attachsize);/计算UDP校验和pudp_header-chksum=0;memcpy(unsigned char *)pudp_psd_header+sizeof(Udp_Psd_Header),(unsigned char *)pudp_header,attachsize);pudp_header-chksum=checksum(unsigned short *)pudp_psd_header,attachsize+sizeof(Udp_Psd_Header);/计算ip头的校验和pip_header-checksum=0;pip_header-checksum=checksum(unsigned short *)pip_header,20);return;return;需要几个头文件，以及库：#include #include #include “packet.h”#pragma comment(lib,”ws2_32.lib”)如果要自己填充IP数据报，那么计算Checksum是必不可少的一步，算法如下。按16位一组，取补码相加，然后对和取补码USHORT Checksum(USHORT *buffer, int size)unsigned long cksum=0;while(size 1)cksum+=*buffer+;size -=sizeof(USHORT);if(size)cksum += *(UCHAR*)buffer;cksum = (cksum 16) + (cksum & 0xffff);cksum += (cksum 16);return (USHORT)(cksum);此算法根据linux中快速计算ip数据包的chksum算法改写而成，运算速度快，使用方便。此算法也可以应用到其他计算chksum的地方，但是要注意计算条件要求：数据长度是4的倍数。几年前改写而成，这里共享出来。/* ip_fast_csum Compute the IPv4 header checksum efficiently.* iph: ipv4 header* ihl: length of header / 4*/*static inline unsigned short ip_fast_csum(unsigned char * iph, unsigned int ihl)unsigned int sum;_asm_ _volatile_( “movl (%1), %0 ;n” “subl $4, %2 ;n” “jbe 2f ;n” “addl 4(%1), %0 ;n” “adcl 8(%1), %0 ;n” “adcl 12(%1), %0 ;n”“1: adcl 16(%1), %0 ;n” “lea 4(%1), %1 ;n” “decl %2 ;n” “jne 1b ;n” “adcl $0, %0 ;n” “movl %0, %2 ;n” “shrl $16, %0 ;n” “addw %w2, %w0 ;n” “adcl $0, %0 ;n” “notl %0 ;n”“2: ;n”: “=r” (sum), “=r” (iph), “=r” (ihl): “1 (iph), “2 (ihl): “memory”);return(sum);*/*jonathan 改写于linux; linux的code 见上 */static unsigned shortip_fast_csum( IN unsigned char* iph, IN unsigned int ihl)unsigned int cksum = 0;_asm push eax push ebx push ecx mov ebx , dword ptr iph mov edx, ihl mov eax, dword ptr ebx sub edx, 004 jbe EXIT add eax, dword ptr ebx + 4 adc eax, dword ptr ebx + 8 adc eax, dword ptr ebx + 12BEGIN: adc eax, dword ptr ebx + 16 lea ebx, dword ptr ebx + 4 dec edx jne BEGIN adc eax, 000 mov edx, eax shr eax, 010 add ax, dx adc eax, 000 not eaxEXIT: mov cksum , eax pop ecx pop ebx pop eaxreturn (unsigned short)cksum;UDP checksum计算方法：/*For udp checksum, sophie add, 20100524*/struct fake_head u_int sIp4; u_int dIp4; u_int len; u_short reserved1; u_char reserved2; u_char nextHead;u_short in_cksum_v6 (u_char *mdata, u_int len, u_int ck) u_int sum = 0; int i; if (len % 2) return 0; for (i = 0; i len; i = i + 2) sum += (mdatai 16) sum = (sum 16) + (sum & 0xffff); return (sum & 0xffff);u_short udp_check_sum_v6(char *udp, const struct in6_addr *sIp, const struct in6_addr *dIp, u_short payload_length, u_char next_protocol) u_short *pFakeHead = NULL; struct fake_head fakeHead; u_int i,ck = 0; struct udphdr *udpHdr; udpHdr = (struct udphdr *)udp; memcpy(char *)fakeHead.sIp, (char *)sIp, 16); memcpy(char *)fakeHead.