[系统运维]用户态协议栈之协议栈设计

?在物理层上，双绞线传的是电信号，光纤传的是光信号。网卡的作用就是将物理层的光电信号转换为数字信号。在send的过程其实就是把数字信号转换为模拟信号（光电信号）。网卡即不在物理层，也不在数据链路层，而是在这两层之间做转换。然后把这个数据(通过sk_buff) 迁移到协议栈。 然后协议栈解析完数据之后将数据放入recv?buffer ，然后应用程序通过系统调用就能得到这个数据。? ?

这个过程有两次拷贝，? ? ?后来出现一种方式(DMA)，将网卡映射到内存中去(mmap)。 应用程序是可以在内存中间直接读取这块映射过来的数据的。（DMA的方式不需要通过CPU去执行指令，直接将数据放入内存）这就叫零拷贝。

?MMAP的底层实现是有DMA这种方式的支持的。需要一条总线。? ? ?当DMA传输完数据之后会给CPU引发一个中断。

?网卡驱动是运行在内核里面的，它使得网卡能够正常工作，能够正常收发数据。

有了用户态协议栈，我们对网卡的想象空间会更大

如何取到一帧完整的数据：

1：使用原生的socket? ? （RAW SOCKET)

2：利用一些开源框架? ?----? ?netmap

3：利用一些成熟的商业的框架? ?----? dpdk

柔性数组

  5 #define ETH_LEN 6
  6 
  7 
  8 struct ethhdr{
  9     unsigned char dst[ETH_LEN]; // 目的地址
 10     unsigned char src[ETH_LEN]; // 源地址
 11     unsigned short proto;       // 类型
 12 };
 13 
 14 struct iphead{
 15     unsigned char version:4,   // 版本
 16                   headlen:4;   // 首部长度
 17     unsigned char tos;         // 服务类型
 18     unsigned short totlen;     // 总长度 
 19     unsigned short id;         // 16位标识
 20     unsigned short flag:3,     // 3位标志位
 21                    offset:13,  // 13位片偏移 
 22     unsigned char ttl;         // 8位生存时间
 23     unsigned char proto;       // 8位协议，用来形容传输层用的是什么协议
 24     unsigned short check;      // 校验和
 25     
 26     unsigned int sip;          // 源地址
 27     unsigned int dip;          // 目的地址     
 28 }; 
 29 
 30 
 31 struct updhdr{ 
 32     unsigned short sport;  // 源端口号
 33     unsigned short dport;  // 目的端口号
 34     unsigned short length; // UDP长度
 35     unsigned short check;  // UDP校验
 36 };
 37 
 38 // UDP包
 39 struct udppkt{
 40     struct ethhdr eth; // 以太网头
 41     struct iphdr   ip; // IP头
 42     struct udphdr  udp;// UDP头
 43   
 44     unsigned char payload[0]; // 柔性数组(0长数组)
 45   
 46 };

长度为0的数组的主要用途是为了满足需要变长度的结构体

• 用法 : 在一个结构体的最后, 申明一个长度为0的数组, 就可以使得这个结构体是可变长的. 对于编译器来说, 此时长度为0的数组并不占用空间, 因为数组名本身不占空间, 它只是一个偏移量, 数组名这个符号本身代表了一个不可修改的地址常量

• 对于编译器而言, 数组名仅仅是一个符号, 它不会占用任何空间, 它在结构体中, 只是代表了一个偏移量, 代表一个不可修改的地址常量!

两个情况下是可以使用柔性数组的:

1：内存是已经分配好的。

2：这个柔性数组的长度我们是可以通过其他方法计算出来的。

eth0:物理网卡? ? ens33：虚拟网卡? ? ? ? ? ? ? ? ? ? ?

sudo vim /etc/default/grub? ? ? ? ?-------------->? ? GRUB_CMDLINE_LINUX=??net.ifnames=0 biosdevname=0? (将ens33改成eth0)?

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

#include <sys/poll.h>
#include <arpa/inet.h>


#define NETMAP_WITH_LIBS

#include <net/netmap_user.h> 
#pragma pack(1)



#define ETH_ALEN	6
#define PROTO_IP	0x0800
#define PROTO_ARP	0x0806

#define PROTO_UDP	17
#define PROTO_ICMP	1
#define PROTO_IGMP	2

struct ethhdr {
	unsigned char h_dest[ETH_ALEN];
	unsigned char h_source[ETH_ALEN];
	unsigned short h_proto;
};



struct iphdr {
	unsigned char version;
	unsigned char tos;
	unsigned short tot_len;
	unsigned short id;
	unsigned short flag_off;
	unsigned char ttl;
	unsigned char protocol;
	unsigned short check;
	unsigned int saddr;
	unsigned int daddr;
};


struct udphdr {
	unsigned short source;
	unsigned short dest;
	unsigned short len;
	unsigned short check;
};


struct udppkt {
	struct ethhdr eh;
	struct iphdr ip;
	struct udphdr udp;
	unsigned char body[128];
};


struct arphdr {
	unsigned short h_type;
	unsigned short h_proto;
	unsigned char h_addrlen;
	unsigned char protolen;
	unsigned short oper;
	unsigned char smac[ETH_ALEN];
	unsigned int sip;
	unsigned char dmac[ETH_ALEN];
	unsigned int dip;
};

struct arppkt {
	struct ethhdr eh;
	struct arphdr arp;
};


struct icmphdr {
	unsigned char type;
	unsigned char code;
	unsigned short check;
	unsigned short identifier;
	unsigned short seq;
	unsigned char data[32];
};

struct icmppkt {
	struct ethhdr eh;
	struct iphdr ip;
	struct icmphdr icmp;
};

void print_mac(unsigned char *mac) {
	int i = 0;
	for (i = 0;i < ETH_ALEN-1;i ++) {
		printf("%02x:", mac[i]);
	}
	printf("%02x", mac[i]);
}

void print_ip(unsigned char *ip) {
	int i = 0;

	for (i = 0;i < 3;i ++) {
		printf("%d.", ip[i]);
	}
	printf("%d", ip[i]);
}


void print_arp(struct arppkt *arp) {
	print_mac(arp->eh.h_dest);
	printf(" ");

	print_mac(arp->eh.h_source);
	printf(" ");

	printf("0x%04x ", ntohs(arp->eh.h_proto));
	printf("  ");
	
}

int str2mac(char *mac, char *str) {

	char *p = str;
	unsigned char value = 0x0;
	int i = 0;

	while (p != '\0') {
		
		if (*p == ':') {
			mac[i++] = value;
			value = 0x0;
		} else {
			
			unsigned char temp = *p;
			if (temp <= '9' && temp >= '0') {
				temp -= '0';
			} else if (temp <= 'f' && temp >= 'a') {
				temp -= 'a';
				temp += 10;
			} else if (temp <= 'F' && temp >= 'A') {
				temp -= 'A';
				temp += 10;
			} else {	
				break;
			}
			value <<= 4;
			value |= temp;
		}
		p ++;
	}

	mac[i] = value;

	return 0;
}

void echo_arp_pkt(struct arppkt *arp, struct arppkt *arp_rt, char *hmac) {

	memcpy(arp_rt, arp, sizeof(struct arppkt));

	memcpy(arp_rt->eh.h_dest, arp->eh.h_source, ETH_ALEN);
	str2mac(arp_rt->eh.h_source, hmac);
	arp_rt->eh.h_proto = arp->eh.h_proto;

	arp_rt->arp.h_addrlen = 6;
	arp_rt->arp.protolen = 4;
	arp_rt->arp.oper = htons(2);
	
	str2mac(arp_rt->arp.smac, hmac);
	arp_rt->arp.sip = arp->arp.dip;
	
	memcpy(arp_rt->arp.dmac, arp->arp.smac, ETH_ALEN);
	arp_rt->arp.dip = arp->arp.sip;

}


void echo_udp_pkt(struct udppkt *udp, struct udppkt *udp_rt) {

	memcpy(udp_rt, udp, sizeof(struct udppkt));

	memcpy(udp_rt->eh.h_dest, udp->eh.h_source, ETH_ALEN);
	memcpy(udp_rt->eh.h_source, udp->eh.h_dest, ETH_ALEN);

	udp_rt->ip.saddr = udp->ip.daddr;
	udp_rt->ip.daddr = udp->ip.saddr;

	udp_rt->udp.source = udp->udp.dest;
	udp_rt->udp.dest = udp->udp.source;

}

unsigned short in_cksum(unsigned short *addr, int len)
{
	register int nleft = len;
	register unsigned short *w = addr;
	register int sum = 0;
	unsigned short answer = 0;

	while (nleft > 1)  {
		sum += *w++;
		nleft -= 2;
	}

	if (nleft == 1) {
		*(u_char *)(&answer) = *(u_char *)w ;
		sum += answer;
	}

	sum = (sum >> 16) + (sum & 0xffff);	
	sum += (sum >> 16);			
	answer = ~sum;
	
	return (answer);

}


void echo_icmp_pkt(struct icmppkt *icmp, struct icmppkt *icmp_rt) {

	memcpy(icmp_rt, icmp, sizeof(struct icmppkt));

	icmp_rt->icmp.type = 0x0; //
	icmp_rt->icmp.code = 0x0; //
	icmp_rt->icmp.check = 0x0;

	icmp_rt->ip.saddr = icmp->ip.daddr;
	icmp_rt->ip.daddr = icmp->ip.saddr;

	memcpy(icmp_rt->eh.h_dest, icmp->eh.h_source, ETH_ALEN);
	memcpy(icmp_rt->eh.h_source, icmp->eh.h_dest, ETH_ALEN);

	icmp_rt->icmp.check = in_cksum((unsigned short*)&icmp_rt->icmp, sizeof(struct icmphdr));
	
}


int main() {
	
	struct ethhdr *eh;
	struct pollfd pfd = {0};
	struct nm_pkthdr h;
	unsigned char *stream = NULL;

	struct nm_desc *nmr = nm_open("netmap:eth0", NULL, 0, NULL);
	if (nmr == NULL) {
		return -1;
	}

	pfd.fd = nmr->fd;
	pfd.events = POLLIN;

	while (1) {
		int ret = poll(&pfd, 1, -1);
		if (ret < 0) continue;
		
		if (pfd.revents & POLLIN) {
			stream = nm_nextpkt(nmr, &h);
			eh = (struct ethhdr*)stream;

			if (ntohs(eh->h_proto) == PROTO_IP) {

				struct udppkt *udp = (struct udppkt*)stream;
				if (udp->ip.protocol == PROTO_UDP) {

					struct in_addr addr;
					addr.s_addr = udp->ip.saddr;

					int udp_length = ntohs(udp->udp.len);
					printf("%s:%d:length:%d, ip_len:%d --> ", inet_ntoa(addr), udp->udp.source, 
						udp_length, ntohs(udp->ip.tot_len));

					udp->body[udp_length-8] = '\0';
					printf("udp --> %s\n", udp->body);
#if 1
					struct udppkt udp_rt;
					echo_udp_pkt(udp, &udp_rt);
					nm_inject(nmr, &udp_rt, sizeof(struct udppkt));
#endif
#if 0
				} else if (udp->ip.protocol == PROTO_ICMP) {
					
					struct icmppkt *icmp = (struct icmppkt*)stream;

					printf("icmp ---------- --> %d, %x\n", icmp->icmp.type, icmp->icmp.check);
					if (icmp->icmp.type == 0x08) {
						struct icmppkt icmp_rt = {0};
						echo_icmp_pkt(icmp, &icmp_rt);

						//printf("icmp check %x\n", icmp_rt.icmp.check);
						nm_inject(nmr, &icmp_rt, sizeof(struct icmppkt));
					}
#endif	
				} else if (udp->ip.protocol == PROTO_IGMP) {

				} else {
					printf("other ip packet");
				}
#if 0
			}  else if (ntohs(eh->h_proto) == PROTO_ARP) {

				struct arppkt *arp = (struct arppkt *)stream;
				struct arppkt arp_rt;

				if (arp->arp.dip == inet_addr("192.168.2.217")) {
					echo_arp_pkt(arp, &arp_rt, "00:50:56:33:1c:ca");
					nm_inject(nmr, &arp_rt, sizeof(struct arppkt));
				}
#endif
			}
		} 
	}
}