TUN/TAP设备浅析(三) -- TUN/TAP设备的应用

上一篇文章主要讲述了TUN/TAP设备的一些原理，你可能会好奇，TUN/TAP设备究竟有什么用处呢？所以这篇文章，我想用一些实际的例子来回答这个问题。

例子源自陈硕老师的博客，博文中关于TUN/TAP设备的使用非常典型，对原文感兴趣的同学可以查看这里：http://blog.csdn.net/solstice/article/details/6579232

背景：在一台 PC 机上模拟 TCP 客户端程序发起连接请求，同时在该 PC 上创建虚拟网卡 tun0，接收连接请求

并送至 faketcp 应用程序，用于模拟 TCP 服务器端进行响应。

网络的拓扑结构如下：

拓扑结构

具体做法是：在主机 atom 上通过打开 /dev/net/tun 设备来创建一个 tun0 虚拟网卡，然后把这个网卡的地址设为192.168.0.1/24，这样 faketcp 程序就扮演了192.168.0.0/24 这个网段上的所有机器。atom 发给192.168.0.2 ～ 192.168.0.254的 IP 数据包都会发给 faketcp 程序，faketcp 程序可以模拟其中任何一个IP 给atom 发 IP 数据包。

程序分成几步来实现。

第一步：实现 icmp echo 协议，这样就能 ping 通 faketcp 了：

faketcp.h:

#include <algorithm>  // std::swap#include <assert.h>
#include <stdint.h>
#include <string.h>
#include <arpa/inet.h>  // inet_ntop
#include <net/if.h>struct SocketAddr
{uint32_t saddr, daddr;  // 源地址和目的地址uint16_t sport, dport;  // 源端口和目的端口bool operator==(const SocketAddr& rhs) const{return saddr == rhs.saddr && daddr == rhs.daddr && sport == rhs.sport && dport == rhs.dport;}bool operator<(const SocketAddr& rhs) const{return memcmp(this, &rhs, sizeof(rhs)) < 0;}
};int tun_alloc(char dev[IFNAMSIZ]);
uint16_t in_checksum(const void* buf, int len);void icmp_input(int fd, const void* input, const void* payload, int len);

faketcp.cc:

#include "faketcp.h"#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <linux/if_tun.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <sys/ioctl.h>int sethostaddr(const char* dev)
{struct ifreq ifr;bzero(&ifr, sizeof(ifr));strcpy(ifr.ifr_name, dev);struct sockaddr_in addr;bzero(&addr, sizeof addr);addr.sin_family = AF_INET;inet_pton(AF_INET, "192.168.0.1", &addr.sin_addr);//addr.sin_addr.s_addr = htonl(0xc0a80001);bcopy(&addr, &ifr.ifr_addr, sizeof addr);int sockfd = socket(AF_INET, SOCK_DGRAM, 0);if (sockfd < 0)return sockfd;int err = 0;// ifconfig tun0 192.168.0.1if ((err = ioctl(sockfd, SIOCSIFADDR, (void *) &ifr)) < 0){perror("ioctl SIOCSIFADDR");goto done;}// ifup tun0 其实就是启动tun0if ((err = ioctl(sockfd, SIOCGIFFLAGS, (void *) &ifr)) < 0){perror("ioctl SIOCGIFFLAGS");goto done;}ifr.ifr_flags |= IFF_UP;if ((err = ioctl(sockfd, SIOCSIFFLAGS, (void *) &ifr)) < 0){perror("ioctl SIOCSIFFLAGS");goto done;}// ifconfig tun0 192.168.0.1/24 # 配置子网掩码inet_pton(AF_INET, "255.255.255.0", &addr.sin_addr);bcopy(&addr, &ifr.ifr_netmask, sizeof addr);if ((err = ioctl(sockfd, SIOCSIFNETMASK, (void *) &ifr)) < 0){perror("ioctl SIOCSIFNETMASK");goto done;}
done:close(sockfd);return err;
}int tun_alloc(char dev[IFNAMSIZ])
{struct ifreq ifr;int fd, err;if ((fd = open("/dev/net/tun", O_RDWR)) < 0){perror("open");return -1;}bzero(&ifr, sizeof(ifr));ifr.ifr_flags = IFF_TUN | IFF_NO_PI; // tun设备不包含以太网头部,而tap包含,仅此而已if (*dev){strncpy(ifr.ifr_name, dev, IFNAMSIZ); }if ((err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0){perror("ioctl TUNSETIFF");close(fd);return err;}strcpy(dev, ifr.ifr_name);if ((err = sethostaddr(dev)) < 0) // 设定地址等信息return err;return fd;
}uint16_t in_checksum(const void* buf, int len)
{assert(len % 2 == 0);const uint16_t* data = static_cast<const uint16_t*>(buf);int sum = 0;for (int i = 0; i < len; i+=2){sum += *data++;}while (sum >> 16)sum = (sum & 0xFFFF) + (sum >> 16);assert(sum <= 0xFFFF);return ~sum;
}void icmp_input(int fd, const void* input, const void* payload, int len)
{const struct iphdr* iphdr = static_cast<const struct iphdr*>(input); // ip头部const struct icmphdr* icmphdr = static_cast<const struct icmphdr*>(payload); // icmp头部// const int icmphdr_size = sizeof(*icmphdr);const int iphdr_len = iphdr->ihl*4;if (icmphdr->type == ICMP_ECHO){char source[INET_ADDRSTRLEN];char dest[INET_ADDRSTRLEN];inet_ntop(AF_INET, &iphdr->saddr, source, INET_ADDRSTRLEN);inet_ntop(AF_INET, &iphdr->daddr, dest, INET_ADDRSTRLEN);printf("%s > %s: ", source, dest);printf("ICMP echo request, id %d, seq %d, length %d\n",ntohs(icmphdr->un.echo.id),ntohs(icmphdr->un.echo.sequence),len - iphdr_len);union{unsigned char output[ETH_FRAME_LEN]; // 以太网头部struct{struct iphdr iphdr;struct icmphdr icmphdr;} out;};memcpy(output, input, len);out.icmphdr.type = ICMP_ECHOREPLY;out.icmphdr.checksum += ICMP_ECHO; // FIXME: not portablestd::swap(out.iphdr.saddr, out.iphdr.daddr);write(fd, output, len);}
}

icmpecho.cc:

#include "faketcp.h"#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <linux/if_ether.h>int main()
{char ifname[IFNAMSIZ] = "tun%d";int fd = tun_alloc(ifname); // tun_alloc函数主要用于开启if (fd < 0){fprintf(stderr, "tunnel interface allocation failed\n");exit(1);}printf("allocted tunnel interface %s\n", ifname);sleep(1);for (;;){union{unsigned char buf[ETH_FRAME_LEN]; // 以太网头部struct iphdr iphdr;   // ip头部};const int iphdr_size = sizeof iphdr; // ip头部默认是20字节int nread = read(fd, buf, sizeof(buf));if (nread < 0){perror("read");close(fd);exit(1);}printf("read %d bytes from tunnel interface %s.\n", nread, ifname);const int iphdr_len = iphdr.ihl*4;if (nread >= iphdr_size&& iphdr.version == 4&& iphdr_len >= iphdr_size&& iphdr_len <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr_len) == 0){const void* payload = buf + iphdr_len;if (iphdr.protocol == IPPROTO_ICMP)  // icmp协议{icmp_input(fd, buf, payload, nread);}}else{printf("bad packet\n");for (int i = 0; i < nread; ++i){if (i % 4 == 0) printf("\n");printf("%02x ", buf[i]);}printf("\n");}}return 0;
}

运行方法，打开3个命令行窗口：

在第1个窗口运行sudo ./icmpecho，程序显示：

allocted tunnel interface tun0

在第2个窗口运行：

$ sudo ifconfig tun0 192.168.0.1/24 # 设定ip地址$ sudo tcpdump -i tun0  # 用tcpdump抓取通过接口tun0的数据包

在第3个窗口运行：

$ ping 192.168.0.2$ ping 192.168.0.3$ ping 192.168.0.234

发现每个192.168.0.X 的IP 都能 ping 通。

第二步：实现拒接 TCP 连接的功能，即在收到SYN TCP segment的时候发送RST segment。

rejectall.cc:

#include "faketcp.h"#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <linux/if_ether.h>void tcp_input(int fd, const void* input, const void* payload, int tot_len)
{const struct iphdr* iphdr = static_cast<const struct iphdr*>(input); // ip头部const struct tcphdr* tcphdr = static_cast<const struct tcphdr*>(payload); // tcp头部const int iphdr_len = iphdr->ihl*4; // ip头部的大小const int tcp_seg_len = tot_len - iphdr_len; // tcp报文的大小const int tcphdr_size = sizeof(*tcphdr);if (tcp_seg_len >= tcphdr_size&& tcp_seg_len >= tcphdr->doff*4){const int tcphdr_len = tcphdr->doff*4;if (tcphdr->syn) // 收到了SYN分节{char source[INET_ADDRSTRLEN];char dest[INET_ADDRSTRLEN];inet_ntop(AF_INET, &iphdr->saddr, source, INET_ADDRSTRLEN); // 将ip转化为可读的字符串inet_ntop(AF_INET, &iphdr->daddr, dest, INET_ADDRSTRLEN);printf("IP %s.%d > %s.%d: ",source, ntohs(tcphdr->source), dest, ntohs(tcphdr->dest));printf("Flags [S], seq %u, win %d, length %d\n",ntohl(tcphdr->seq), // 序列号ntohs(tcphdr->window), // 窗口大小tot_len - iphdr_len - tcphdr_len);union{unsigned char output[ETH_FRAME_LEN];struct{struct iphdr iphdr;struct tcphdr tcphdr;} out;};assert(sizeof(out) == sizeof(struct iphdr) + sizeof(struct tcphdr));int output_len = sizeof(out);bzero(&out, output_len + 4);memcpy(output, input, sizeof(struct iphdr));out.iphdr.tot_len = htons(output_len);std::swap(out.iphdr.saddr, out.iphdr.daddr);out.iphdr.check = 0;out.iphdr.check = in_checksum(output, sizeof(struct iphdr));out.tcphdr.source = tcphdr->dest;  // 源地址和目的地址对调out.tcphdr.dest = tcphdr->source;out.tcphdr.seq = 0;out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1); // 确认序列号out.tcphdr.doff = sizeof(struct tcphdr) / 4;out.tcphdr.ack = 1;out.tcphdr.rst = 1; // 注意这里的RST分节out.tcphdr.window = 0;unsigned char* pseudo = output + output_len;pseudo[0] = 0;pseudo[1] = IPPROTO_TCP;pseudo[2] = 0;pseudo[3] = sizeof(struct tcphdr);out.tcphdr.check = in_checksum(&out.iphdr.saddr, sizeof(struct tcphdr)+12);write(fd, output, output_len);}}
}int main()
{char ifname[IFNAMSIZ] = "tun%d";int fd = tun_alloc(ifname);if (fd < 0){fprintf(stderr, "tunnel interface allocation failed\n");exit(1);}printf("allocted tunnel interface %s\n", ifname);sleep(1);for (;;){union{unsigned char buf[ETH_FRAME_LEN];struct iphdr iphdr;};const int iphdr_size = sizeof iphdr;int nread = read(fd, buf, sizeof(buf));if (nread < 0){perror("read");close(fd);exit(1);}printf("read %d bytes from tunnel interface %s.\n", nread, ifname);const int iphdr_len = iphdr.ihl*4;if (nread >= iphdr_size&& iphdr.version == 4&& iphdr_len >= iphdr_size&& iphdr_len <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr_len) == 0){const void* payload = buf + iphdr_len;if (iphdr.protocol == IPPROTO_ICMP){icmp_input(fd, buf, payload, nread);}else if (iphdr.protocol == IPPROTO_TCP){tcp_input(fd, buf, payload, nread);}}else{printf("bad packet\n");for (int i = 0; i < nread; ++i){if (i % 4 == 0) printf("\n");printf("%02x ", buf[i]);}printf("\n");}}return 0;
}

运行方法，打开3个命令行窗口，头两个窗口的操作与前面相同，运行的faketcp 程序是 ./rejectall。

在第3个窗口运行


$ nc 192.168.0.2  2000$ nc 192.168.0.2  3333$ nc 192.168.0.7  5555

发现向其中任意一个 IP 发起的 TCP 连接都被拒接了。

第三步：实现接受 TCP 连接的功能，即在接收到SYN TCP segment的时候发回 SYN + ACK。这个程序同时处理了连接断开的情况，即在收到FIN segment的时候发回 FIN + ACK。

acceptall.cc:

#include "faketcp.h"#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <linux/if_ether.h>void tcp_input(int fd, const void* input, const void* payload, int tot_len)
{const struct iphdr* iphdr = static_cast<const struct iphdr*>(input);const struct tcphdr* tcphdr = static_cast<const struct tcphdr*>(payload);const int iphdr_len = iphdr->ihl*4;const int tcp_seg_len = tot_len - iphdr_len;const int tcphdr_size = sizeof(*tcphdr);if (tcp_seg_len >= tcphdr_size&& tcp_seg_len >= tcphdr->doff*4){const int tcphdr_len = tcphdr->doff*4;char source[INET_ADDRSTRLEN];char dest[INET_ADDRSTRLEN];inet_ntop(AF_INET, &iphdr->saddr, source, INET_ADDRSTRLEN);inet_ntop(AF_INET, &iphdr->daddr, dest, INET_ADDRSTRLEN);printf("IP %s.%d > %s.%d: ",source, ntohs(tcphdr->source), dest, ntohs(tcphdr->dest));printf("Flags [%c], seq %u, win %d, length %d\n",tcphdr->syn ? 'S' : (tcphdr->fin ? 'F' : '.'),ntohl(tcphdr->seq),ntohs(tcphdr->window),tot_len - iphdr_len - tcphdr_len);union{unsigned char output[ETH_FRAME_LEN];struct{struct iphdr iphdr;struct tcphdr tcphdr;} out;};assert(sizeof(out) == sizeof(struct iphdr) + sizeof(struct tcphdr));int output_len = sizeof(out);bzero(&out, output_len + 4);memcpy(output, input, sizeof(struct iphdr));out.iphdr.tot_len = htons(output_len);std::swap(out.iphdr.saddr, out.iphdr.daddr);out.iphdr.check = 0;out.iphdr.check = in_checksum(output, sizeof(struct iphdr));out.tcphdr.source = tcphdr->dest;out.tcphdr.dest = tcphdr->source;out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);out.tcphdr.doff = sizeof(struct tcphdr) / 4;out.tcphdr.window = htons(5000);bool response = false;if (tcphdr->syn){out.tcphdr.seq = htonl(123456); // 序列号随机产生out.tcphdr.syn = 1; // SYNout.tcphdr.ack = 1; // ACKresponse = true;}else if (tcphdr->fin) // 对于对方发送的FIN也需要接收是吧!{out.tcphdr.seq = htonl(123457);out.tcphdr.fin = 1;out.tcphdr.ack = 1;response = true;}unsigned char* pseudo = output + output_len;pseudo[0] = 0;pseudo[1] = IPPROTO_TCP;pseudo[2] = 0;pseudo[3] = sizeof(struct tcphdr);out.tcphdr.check = in_checksum(&out.iphdr.saddr, sizeof(struct tcphdr)+12);if (response){write(fd, output, output_len);}}
}int main()
{char ifname[IFNAMSIZ] = "tun%d";int fd = tun_alloc(ifname);if (fd < 0){fprintf(stderr, "tunnel interface allocation failed\n");exit(1);}printf("allocted tunnel interface %s\n", ifname);sleep(1);for (;;){union{unsigned char buf[ETH_FRAME_LEN];struct iphdr iphdr;};const int iphdr_size = sizeof iphdr;int nread = read(fd, buf, sizeof(buf));if (nread < 0){perror("read");close(fd);exit(1);}printf("read %d bytes from tunnel interface %s.\n", nread, ifname);const int iphdr_len = iphdr.ihl*4;if (nread >= iphdr_size&& iphdr.version == 4&& iphdr_len >= iphdr_size&& iphdr_len <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr_len) == 0){const void* payload = buf + iphdr_len;if (iphdr.protocol == IPPROTO_ICMP){icmp_input(fd, buf, payload, nread);}else if (iphdr.protocol == IPPROTO_TCP){tcp_input(fd, buf, payload, nread);}}else{printf("bad packet\n");for (int i = 0; i < nread; ++i){if (i % 4 == 0) printf("\n");printf("%02x ", buf[i]);}printf("\n");}}return 0;
}

运行方法，打开3个命令行窗口，步骤与前面相同，运行的 faketcp 程序是 ./acceptall。

这次会发现 nc 能和192.168.0.X中的每一个 IP 每一个 PORT 都能连通。还可以在第4个窗口中运行 netstat -tpn，以确认连接确实建立起来了。

如果在 nc 中输入数据，数据会堆积在操作系统中，表现为netstat 显示的发送队列（Send-Q）的长度增加。

第四步：在第三步接受TCP连接的基础上，实现接收数据，即在收到包含 payload 数据的 TCP segment时发回ACK。

discardall.cc:

#include "faketcp.h"#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <linux/if_ether.h>void tcp_input(int fd, const void* input, const void* payload, int tot_len)
{const struct iphdr* iphdr = static_cast<const struct iphdr*>(input);const struct tcphdr* tcphdr = static_cast<const struct tcphdr*>(payload);const int iphdr_len = iphdr->ihl*4;const int tcp_seg_len = tot_len - iphdr_len;const int tcphdr_size = sizeof(*tcphdr);if (tcp_seg_len >= tcphdr_size&& tcp_seg_len >= tcphdr->doff*4){const int tcphdr_len = tcphdr->doff*4;const int payload_len = tot_len - iphdr_len - tcphdr_len;char source[INET_ADDRSTRLEN];char dest[INET_ADDRSTRLEN];inet_ntop(AF_INET, &iphdr->saddr, source, INET_ADDRSTRLEN); // 将ip地址变得可读inet_ntop(AF_INET, &iphdr->daddr, dest, INET_ADDRSTRLEN);printf("IP %s.%d > %s.%d: ",source, ntohs(tcphdr->source), dest, ntohs(tcphdr->dest));printf("Flags [%c], seq %u, win %d, length %d\n",tcphdr->syn ? 'S' : (tcphdr->fin ? 'F' : '.'),ntohl(tcphdr->seq),ntohs(tcphdr->window),payload_len);union{unsigned char output[ETH_FRAME_LEN];struct{struct iphdr iphdr;struct tcphdr tcphdr;} out;};assert(sizeof(out) == sizeof(struct iphdr) + sizeof(struct tcphdr));int output_len = sizeof(out);bzero(&out, output_len + 4);memcpy(output, input, sizeof(struct iphdr));out.iphdr.tot_len = htons(output_len);std::swap(out.iphdr.saddr, out.iphdr.daddr);out.iphdr.check = 0;out.iphdr.check = in_checksum(output, sizeof(struct iphdr));out.tcphdr.source = tcphdr->dest; // 目的地址和源地址倒换out.tcphdr.dest = tcphdr->source;out.tcphdr.doff = sizeof(struct tcphdr) / 4;out.tcphdr.window = htons(5000);bool response = false;if (tcphdr->syn){out.tcphdr.seq = htonl(123456);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);out.tcphdr.syn = 1;out.tcphdr.ack = 1;response = true;}else if (tcphdr->fin){out.tcphdr.seq = htonl(123457);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);out.tcphdr.fin = 1;out.tcphdr.ack = 1;response = true;}else if (payload_len > 0){out.tcphdr.seq = htonl(123457);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+payload_len); // 确认的序列号out.tcphdr.ack = 1; // ack,不发送数据,仅发送确认号response = true;}unsigned char* pseudo = output + output_len;pseudo[0] = 0;pseudo[1] = IPPROTO_TCP;pseudo[2] = 0;pseudo[3] = sizeof(struct tcphdr);out.tcphdr.check = in_checksum(&out.iphdr.saddr, sizeof(struct tcphdr)+12);if (response){write(fd, output, output_len);}}
}int main()
{char ifname[IFNAMSIZ] = "tun%d";int fd = tun_alloc(ifname);if (fd < 0){fprintf(stderr, "tunnel interface allocation failed\n");exit(1);}printf("allocted tunnel interface %s\n", ifname);sleep(1);for (;;){union{unsigned char buf[ETH_FRAME_LEN];struct iphdr iphdr;};const int iphdr_size = sizeof iphdr;int nread = read(fd, buf, sizeof(buf));if (nread < 0){perror("read");close(fd);exit(1);}printf("read %d bytes from tunnel interface %s.\n", nread, ifname);const int iphdr_len = iphdr.ihl*4;if (nread >= iphdr_size&& iphdr.version == 4&& iphdr_len >= iphdr_size&& iphdr_len <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr_len) == 0){const void* payload = buf + iphdr_len;if (iphdr.protocol == IPPROTO_ICMP){icmp_input(fd, buf, payload, nread);}else if (iphdr.protocol == IPPROTO_TCP){tcp_input(fd, buf, payload, nread);}}else{printf("bad packet\n");for (int i = 0; i < nread; ++i){if (i % 4 == 0) printf("\n");printf("%02x ", buf[i]);}printf("\n");}}return 0;
}

运行方法，打开3个命令行窗口，步骤与前面相同，运行的faketcp程序是./acceptall。

这次会发现 nc 能和192.168.0.X中的每一个IP 每一个PORT 都能连通，数据也能发出去。还可以在第4个窗口中运行netstat -tpn，以确认连接确实建立起来了，并且发送队列的长度为0；

这一步已经解决了前面的问题2，扮演任意 TCP 服务端。

第五步：解决前面的问题1，扮演客户端向atom 发起任意多的连接。

connectmany.cc:

#include "faketcp.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <linux/if_ether.h>void tcp_input(int fd, const void* input, const void* payload, int tot_len, bool passive)
{const struct iphdr* iphdr = static_cast<const struct iphdr*>(input);const struct tcphdr* tcphdr = static_cast<const struct tcphdr*>(payload);const int iphdr_len = iphdr->ihl*4;const int tcp_seg_len = tot_len - iphdr_len;const int tcphdr_size = sizeof(*tcphdr);if (tcp_seg_len >= tcphdr_size&& tcp_seg_len >= tcphdr->doff*4){const int tcphdr_len = tcphdr->doff*4;const int payload_len = tot_len - iphdr_len - tcphdr_len;char source[INET_ADDRSTRLEN];char dest[INET_ADDRSTRLEN];inet_ntop(AF_INET, &iphdr->saddr, source, INET_ADDRSTRLEN);inet_ntop(AF_INET, &iphdr->daddr, dest, INET_ADDRSTRLEN);printf("IP %s.%d > %s.%d: ",source, ntohs(tcphdr->source), dest, ntohs(tcphdr->dest));printf("Flags [%c], seq %u, win %d, length %d\n",tcphdr->syn ? 'S' : (tcphdr->fin ? 'F' : '.'),ntohl(tcphdr->seq),ntohs(tcphdr->window),payload_len);union{unsigned char output[ETH_FRAME_LEN];struct{struct iphdr iphdr;struct tcphdr tcphdr;} out;};assert(sizeof(out) == sizeof(struct iphdr) + sizeof(struct tcphdr));int output_len = sizeof(out);bzero(&out, output_len + 4);memcpy(output, input, sizeof(struct iphdr));out.iphdr.tot_len = htons(output_len);std::swap(out.iphdr.saddr, out.iphdr.daddr);out.iphdr.check = 0;out.iphdr.check = in_checksum(output, sizeof(struct iphdr));out.tcphdr.source = tcphdr->dest;out.tcphdr.dest = tcphdr->source;out.tcphdr.doff = sizeof(struct tcphdr) / 4;out.tcphdr.window = htons(5000);bool response = false;if (tcphdr->syn) // 对方发起连接,或者对方发送了确认的syn和ack{out.tcphdr.seq = htonl(passive ? 123456 : 123457);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);if (passive) // passive==true表示被动接收连接,表示对方连过来{out.tcphdr.syn = 1;}// 否则的话,表示自己主动发送的连接,接收到了对方的syn和ack,我们只需要发送一个ack即可out.tcphdr.ack = 1; response = true;}else if (tcphdr->fin) // 对方关闭连接{out.tcphdr.seq = htonl(123457);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);out.tcphdr.fin = 1;out.tcphdr.ack = 1;response = true;}else if (payload_len > 0){out.tcphdr.seq = htonl(123457);out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+payload_len);out.tcphdr.ack = 1;response = true;}unsigned char* pseudo = output + output_len;pseudo[0] = 0;pseudo[1] = IPPROTO_TCP;pseudo[2] = 0;pseudo[3] = sizeof(struct tcphdr);out.tcphdr.check = in_checksum(&out.iphdr.saddr, sizeof(struct tcphdr)+12);if (response){write(fd, output, output_len);}}
}// connect_one 发起一个tcp连接?
bool connect_one(int fd, uint32_t daddr, int dport, uint32_t saddr, int sport)
{{union{unsigned char output[ETH_FRAME_LEN];struct{struct iphdr iphdr;struct tcphdr tcphdr;} out;};bzero(&out, (sizeof out)+4);out.iphdr.version = IPVERSION;out.iphdr.ihl = sizeof(out.iphdr)/4;out.iphdr.tos = 0;out.iphdr.tot_len = htons(sizeof(out));out.iphdr.id = 55564;out.iphdr.frag_off |= htons(IP_DF);out.iphdr.ttl = IPDEFTTL;out.iphdr.protocol = IPPROTO_TCP;out.iphdr.saddr = saddr;out.iphdr.daddr = daddr;out.iphdr.check = in_checksum(output, sizeof(struct iphdr));out.tcphdr.source = sport; // 端口号out.tcphdr.dest = dport;out.tcphdr.seq = htonl(123456);out.tcphdr.ack_seq = 0;out.tcphdr.doff = sizeof(out.tcphdr)/4;out.tcphdr.syn = 1;       // 主动发起连接out.tcphdr.window = htons(4096);unsigned char* pseudo = output + sizeof out;pseudo[0] = 0;pseudo[1] = IPPROTO_TCP;pseudo[2] = 0;pseudo[3] = sizeof(struct tcphdr);out.tcphdr.check = in_checksum(&out.iphdr.saddr, sizeof(struct tcphdr)+12);write(fd, output, sizeof out); // 发送连接}union{unsigned char buf[ETH_FRAME_LEN];struct iphdr iphdr;};const int iphdr_size = sizeof iphdr;int nread = read(fd, buf, sizeof(buf)); // 接收到回复之后if (nread < 0){perror("read");close(fd);exit(1);}// printf("read %d bytes from tunnel interface %s.\n", nread, ifname);if (nread >= iphdr_size&& iphdr.version == 4&& iphdr.ihl*4 >= iphdr_size&& iphdr.ihl*4 <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr.ihl*4) == 0){const void* payload = buf + iphdr.ihl*4;if (iphdr.protocol == IPPROTO_ICMP){icmp_input(fd, buf, payload, nread);}else if (iphdr.protocol == IPPROTO_TCP) // tcp 报文{tcp_input(fd, buf, payload, nread, false); // 注意到这里的false,表示是自己主动发起连接}}return true;
}void connect_many(int fd, const char* ipstr, int port, int count)
{uint32_t destip;inet_pton(AF_INET, ipstr, &destip); // 连接到目的ipuint32_t srcip = ntohl(destip)+1;int srcport = 1024; // 端口从1024开始for (int i = 0; i < count; ++i){connect_one(fd, destip, htons(port), htonl(srcip), htons(srcport));srcport++; // 源端口在不断加1if (srcport > 0xFFFF){srcport = 1024;srcip++;}}
}void usage()
{
}int main(int argc, char* argv[])
{if (argc < 4){usage();return 0;}char ifname[IFNAMSIZ] = "tun%d";int fd = tun_alloc(ifname);if (fd < 0){fprintf(stderr, "tunnel interface allocation failed\n");exit(1);}const char* ip = argv[1]; // ipint port = atoi(argv[2]); // 端口int count = atoi(argv[3]); // 数量printf("allocted tunnel interface %s\n", ifname);printf("press enter key to start connecting %s:%d\n", ip, port);getchar();connect_many(fd, ip, port, count); // 发起连接for (;;){union{unsigned char buf[ETH_FRAME_LEN];struct iphdr iphdr;};const int iphdr_size = sizeof iphdr;int nread = read(fd, buf, sizeof(buf));if (nread < 0){perror("read");close(fd);exit(1);}printf("read %d bytes from tunnel interface %s.\n", nread, ifname);const int iphdr_len = iphdr.ihl*4;if (nread >= iphdr_size&& iphdr.version == 4&& iphdr_len >= iphdr_size&& iphdr_len <= nread&& iphdr.tot_len == htons(nread)&& in_checksum(buf, iphdr_len) == 0){const void* payload = buf + iphdr_len;if (iphdr.protocol == IPPROTO_ICMP){icmp_input(fd, buf, payload, nread);}else if (iphdr.protocol == IPPROTO_TCP){tcp_input(fd, buf, payload, nread, true); // 注意到这里的true,表示是被动接收连接}}else{printf("bad packet\n");for (int i = 0; i < nread; ++i){if (i % 4 == 0) printf("\n");printf("%02x ", buf[i]);}printf("\n");}}return 0;
}

这一步的运行方法与前面不同，打开4个命令行窗口。

在第1个窗口运行sudo ./connectmany 192.168.0.1 2007 1000，表示将向192.168.0.1：2007 发起1000个并发连接。程序显示:

allocated tunnel interface tun0
press enter key to start connecting 192.168.0.1  2007

在第二个窗口运行

$ sudo ifconfig tun0 192.168.0.1/24$ sudo tcpdump -i tun0

在第3个窗口运行一个能接收并发TCP 连接的服务程序，可以是httpd，也可以是muduo 的echo 或 discard 示例，程序应listen 2007端口。
回到第1个窗口敲回车，然后在第4个窗口中用netstat -tpn来观察并发连接。

文中代码目录连接：https://github.com/chenshuo/recipes/tree/master/faketcp

作者：Yihulee
链接：https://www.jianshu.com/p/14f9340d940d
来源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

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