今天调试bug时, 忘了将原始的check_sum值reset,导致发包-抓包后发现。check-sum 错误。
来看一看check-sum:简单讲就是对要计算的数据,以16bit为单元进行累加,然后取反
checksum在收包和发包时意义不一样
TCP收包时:
/* * @csum: Checksum (must include start/offset pair) * @csum_start: Offset from skb->head where checksumming should start * @csum_offset: Offset from csum_start where checksum should be stored * @ip_summed: Driver fed us an IP checksum */ struct sk_buff { union { __wsum csum; struct { __u16 csum_start; __u16 csum_offset; }; }; __u8 ip_summed:2,
skb->ip_summed
一般的取值如下 ;skb->csum:存放硬件或者软件计算的payload的checksum不包括伪头
/* Don't change this without changing skb_csum_unnecessary! */ #define CHECKSUM_NONE 0 #define CHECKSUM_UNNECESSARY 1 #define CHECKSUM_COMPLETE 2 #define CHECKSUM_PARTIAL 3
- CHECKSUM_UNNECESSARY
CHECKSUM_UNNECESSARY
表示底层硬件已经计算了CSUM;所以TCP
层在收到包后,发现skb->ip_summed
为CHECKSUM_UNNECESSARY
就不会再检查checksum:
- CHECKSUM_NONE
csum
中的校验和无效,可能有以下几种原因:设备不支持硬件校验和计算;设备计算了硬件校验和,但发现该数据帧已经损坏。部分驱动不会丢弃,而是将ip_summed
设置为CHECKSUM_NONE
,然后交给上层协议栈重新计算并处理这种错误。
- CHECKSUM_COMPLETE
网卡已经计算了L4层报头和payload的校验和,并且skb->csum
已经被赋值,此时L4层的接收者只需要加伪头并验证校验结果。
TCP发包时:
skb->ip_summed
用于L4校验和的状态,以通知底层网卡是否还需要处理校验和;此时ip_summed可以被设置的值有下面两种
- CHECKSUM_NONE
CHECKSUM_NONE
表示协议栈已经计算了校验和,设备不需要做任何事情
- CHECKSUM_PARTIAL
CHECKSUM_PARTIAL
表示使用硬件checksum ,协议栈已经计算L4层的伪头的校验和
skb->csum表示为csum_start和csum_offset,它表示硬件网卡存放将要计算的校验值的地址,和最后填充的便宜。这个域在输出包时使用,只在校验值在硬件计算的情况下才对于网卡真正有意义。硬件checksum功能只能用于非分片报文
- TCP校验和覆盖TCP首部和TCP数据,而IP首部中的校验和只覆盖IP的首部,不覆盖IP数据报中的任何数据。
- TCP的校验和是必需的,而UDP的校验和是可选的。
- TCP和UDP计算校验和时,都要加上一个12字节的伪首部。
伪首部共有12字节,包含如下信息:源IP地址、目的IP地址、保留字节(置0)、传输层协议号(TCP是6)、TCP报文长度(报头+数据)。伪首部是为了增加TCP校验和的检错能力:如检查TCP报文是否收错了(目的IP地址)、传输层协议是否选对了(传输层协议号)等。
RFC 793的TCP校验和定义
The checksum field is the 16 bit one's complement of the one's complement sum of all 16-bit words in the header and text. If a segment contains an odd number of header and text octets to be checksummed, the last octet is padded on the right with zeros to form a 16-bit word for checksum purposes. The pad is not transmitted as part of the segment. While computingthe checksum, the checksum field itself is replaced with zeros.
把伪首部、TCP报头、TCP数据分为16位的字,如果总长度为奇数个字节,则在最后增添一个位都为0的字节。把TCP报头中的校验和字段置为0;
校验和的计算与顺序无关, 可以从数据块开始计算, 也可以从未尾开始向前计算
RFC 1071的IP校验和定义
1. Adjacent octets to be checksummed are paired to form 16-bit integers, and the 1's complement sum of these 16-bit integers is formed.
2. To generate a checksum, the checksum field itself is cleared, the 16-bit 1's complement sum is computed over the octets concerned, and the 1's complement of this sum is placed in the checksum field.
3. To check a checksum, the 1's complement sum is computed over the same set of octets, including the checksum field. If the result is all 1 bits (-0 in 1's complement arithmetic), the check succeeds.
内核协议栈中:
为了提高计算效率, TCP包的校验和并不一次算出,而是采用32位部分累加和(sk->csum)进行增量计算.
csum_partial()用来计算数据块的32位部分累加和, 累加和可以用csum_fold()折叠为16位校验和.csum_partial_copy_nocheck()可在拷贝用户数据的同时计算出它的部分累加和. 为了加快执行速度, csum_partial()将8个32位字分为一组用分立的指令进行32位累加,这样可加长循环体中指令长度, 提高CPU指令流水线的效率.
TCP包接收校验的初始化 static __sum16 tcp_v4_checksum_init(struct sk_buff *skb) { const struct iphdr *iph = ip_hdr(skb); //如果TCP包本身的校验已经完成 if (skb->ip_summed == CHECKSUM_COMPLETE) { if (!tcp_v4_check(skb->len, iph->saddr, iph->daddr, skb->csum)) { //附加伪头进行校验 skb->ip_summed = CHECKSUM_UNNECESSARY; return 0; } } //生成包含伪头的累加和 skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, skb->len, IPPROTO_TCP, 0); if (skb->len <= 76) { return __skb_checksum_complete(skb); //计算数据部分校验和 } return 0; } 附加伪头进行校验 static inline __sum16 tcp_v4_check(int len, __be32 saddr, __be32 daddr, __wsum base) { return csum_tcpudp_magic(saddr, daddr, len, IPPROTO_TCP, base); } static inline __sum16 csum_tcpudp_magic(__be32 saddr, __be32 daddr, unsigned short len, unsigned short proto, __wsum sum) { return csum_fold(csum_tcpudp_nofold(saddr, daddr, len, proto, sum)); } 生成包含伪头的累加和(源,目的,长度,协议号) static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr, unsigned short len, unsigned short proto, __wsum sum) { __asm__( "addl %1, %0 ; " //addl 加法 "adcl %2, %0 ; " //adcl 带进位的加法 "adcl %3, %0 ; " "adcl $0, %0 ; " //如果有进位,进行累加 : "=r" (sum) : "g" (daddr), "g"(saddr), "g"((len + proto) << 8), "0"(sum) ); return sum; } 将32位累加和折叠成16位校验和 static inline __sum16 csum_fold(__wsum sum) { __asm__( "addl %1, %0 ; " "adcl $0xffff, %0 ; " : "=r" (sum) : "r" ((__force u32)sum << 16), "0" ((__force u32)sum & 0xffff0000) ); return (__force __sum16)(~(__force u32)sum >> 16); } 基于伪头累加和,完成全包校验 static __inline__ int tcp_checksum_complete(struct sk_buff *skb) { return skb->ip_summed != CHECKSUM_UNNECESSARY && __tcp_checksum_complete(skb); } __sum16 __skb_checksum_complete(struct sk_buff *skb) { return __skb_checksum_complete_head(skb, skb->len); } __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len) { __sum16 sum; sum = csum_fold(skb_checksum(skb, 0, len, skb->csum)); if (likely(!sum)) { if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE)) netdev_rx_csum_fault(skb->dev); skb->ip_summed = CHECKSUM_UNNECESSARY; } return sum; } __wsum skb_checksum(const struct sk_buff *skb, int offset, int len, __wsum csum) { int start = skb_headlen(skb); int i, copy = start - offset; int pos = 0; /* Checksum header. */ if (copy > 0) { if (copy > len) copy = len; csum = csum_partial(skb->data + offset, copy, csum); if ((len -= copy) == 0) return csum; offset += copy; pos = copy; } ...... } 计算32位中间累加和 unsigned int csum_partial(const unsigned char * buff, int len, unsigned int sum) { //arch/x86/lib/checksum_32.S 汇编文件 } 基于TCP用户数据的中间累加和, 生成TCP包校验码 void tcp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb) { struct inet_sock *inet = inet_sk(sk); struct tcphdr *th = tcp_hdr(skb); if (skb->ip_summed == CHECKSUM_PARTIAL) { th->check = ~tcp_v4_check(len, inet->saddr, inet->daddr, 0); //附加伪头进行校验 skb->csum_start = skb_transport_header(skb) - skb->head; skb->csum_offset = offsetof(struct tcphdr, check); } else { //完整的tcp校验和计算方法 th->check = tcp_v4_check(len, inet->saddr, inet->daddr, csum_partial((char *)th, th->doff << 2, skb->csum)); } } 在拷贝用户数据时同时计算累加和 unsigned int csum_partial_copy_nocheck(const char *src, char *dst, int len, int sum) { return csum_partial_copy_generic(src, dst, len, sum, NULL, NULL); // arch/x86/lib/checksum_32.S } ip头校验和计算 static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl) { unsigned int sum; __asm__ __volatile__( "movl (%1), %0 ; " "subl $4, %2 ; " "jbe 2f ; " "addl 4(%1), %0 ; " //sum = sum + *(iph+4) "adcl 8(%1), %0 ; " //sum = sum + *(iph+8) + carry "adcl 12(%1), %0 ; " //sum = sum + *(iph+12) + carry "1: adcl 16(%1), %0 ; " //sum = sum + *(iph+16) + carry "lea 4(%1), %1 ; " //iph = iph + 4 "decl %2 ; " "jne 1b ; " "adcl $0, %0 ; " "movl %0, %2 ; " "shrl $16, %0 ; " "addw %w2, %w0 ; " "adcl $0, %0 ; " "notl %0 ; " "2: ; " /* Since the input registers which are loaded with iph and ihl are modified, we must also specify them as outputs, or gcc will assume they contain their original values. */ : "=r" (sum), "=r" (iph), "=r" (ihl) : "1" (iph), "2" (ihl) : "memory" ); return (__force __sum16)sum; } 递减ip->ttl,更新校验和 static inline int ip_decrease_ttl(struct iphdr *iph) { u32 check = (__force u32)iph->check; check += (__force u32)htons(0x0100); iph->check = (__force __sum16)(check + (check>=0xFFFF)); return --iph->ttl; } static inline __wsum csum_add(__wsum csum, __wsum addend) { u32 res = (__force u32)csum; res += (__force u32)addend; return (__force __wsum)(res + (res < (__force u32)addend)); } static inline __wsum csum_sub(__wsum csum, __wsum addend) { return csum_add(csum, ~addend); } static inline __wsum csum_block_add(__wsum csum, __wsum csum2, int offset) { u32 sum = (__force u32)csum2; if (offset & 1) sum = ((sum & 0xFF00FF)<<8) + ((sum>>8) & 0xFF00FF); return csum_add(csum, (__force __wsum)sum); } static inline __wsum csum_block_sub(__wsum csum, __wsum csum2, int offset) { u32 sum = (__force u32)csum2; if (offset & 1) sum = ((sum & 0xFF00FF)<<8) + ((sum>>8) & 0xFF00FF); return csum_sub(csum, (__force __wsum)sum); } [/函数实现] 转载:https://www.cnblogs.com/super-king/p/3284884.html
https://hustcat.github.io/checksum-in-kernel/
https://www.kernel.org/doc/Documentation/networking/checksum-offloads.txt
https://w180112.pixnet.net/blog/post/200083785
http://blog.chinaunix.net/uid-25518484-id-5709671.html