虚拟网卡TUN/TAP设备使用实例

文章出处：http://blog.csdn.net/solstice/article/details/6579232

转载渊源：这篇文章源自陈硕老师的博客，原文讨论的主题是在绕开操作系统协议栈的情况下，对tcp并发连接数的支持情况；因为其中对TUN / TAP设备的使用非常典型，而且讲解清晰，所以特部分转载过来作为资料留存；

IBM developerworks上有一篇文章对tun / tap设备进行了详细的介绍，也是一篇非常好的参考资料，另附连接如下：http://www.ibm.com/developerworks/cn/linux/l-tuntap/

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；

拓扑结构如下：



具体做法是：在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 packet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 packet；

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；

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);

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
{
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;
}

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 tun_alloc(char *dev)
{
struct ifreq ifr;
int fd, err;

if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
{
perror("open");
return -1;
}

memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = IFF_TUN | IFF_NO_PI;

if (*dev)
{
strncpy(ifr.ifr_name, dev, IFNAMSIZ);
}

if ((err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0)
{
perror("ioctl");
close(fd);
return err;
}
strcpy(dev, ifr.ifr_name);

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);
const struct icmphdr* icmphdr = static_cast<const struct icmphdr*>(payload);
// 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];
struct
{
struct iphdr iphdr;
struct icmphdr icmphdr;
} out;
};

memcpy(output, input, len);
out.icmphdr.type = ICMP_ECHOREPLY;
out.icmphdr.checksum += ICMP_ECHO; // FIXME: not portable
std::swap(out.iphdr.saddr, out.iphdr.daddr);
write(fd, output, len);
}
}

运行方法，打开3个命令行窗口：

1. 在第1个窗口运行 sudo ./icmpecho，程序显示：

allocted tunnel interface tun0

2. 在第2个窗口运行：

$ sudo ifconfig tun0 192.168.0.1/24

$ sudo tcpdump -i tun0

3. 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);
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;

if (tcphdr->syn)
{
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 [S], seq %u, win %d, length %d\n",
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.seq = 0;
out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);
out.tcphdr.doff = sizeof(struct tcphdr) / 4;
out.tcphdr.ack = 1;
out.tcphdr.rst = 1;
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

3. 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);
out.tcphdr.syn = 1;
out.tcphdr.ack = 1;
response = true;
}
else if (tcphdr->fin)
{
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);
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)
{
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);
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，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)
{
out.tcphdr.seq = htonl(passive ? 123456 : 123457);
out.tcphdr.ack_seq = htonl(ntohl(tcphdr->seq)+1);
if (passive)
{
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);
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);
}
}
}

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;
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;
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);
}

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);

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_input(fd, buf, payload, nread, false);
}
}

return true;
}

void connect_many(int fd, const char* ipstr, int port, int count)
{
uint32_t destip;
inet_pton(AF_INET, ipstr, &destip);

uint32_t srcip = ntohl(destip)+1;
int srcport = 1024;

for (int i = 0; i < count; ++i)
{
connect_one(fd, destip, htons(port), htonl(srcip), htons(srcport));
srcport++;
if (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];
int port = atoi(argv[2]);
int count = atoi(argv[3]);
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);

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);
}
}
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。

1. 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

2. 在第二个窗口运行

$ sudo ifconfig tun0 192.168.0.1/24

$ sudo tcpdump -i tun0

3. 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。

4. 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