connection socket pool

转载注：测试代码和tcpdump的内容稍微改动了一下。

Linux connect(2)还有一个不怎么为人所知的属性：如果地址的family是AF_UNSPEC，对于TCP连接将通过发送RST包关闭连接，对于UDP连接将去掉目的地址关联，共同的结果是留下一个未连接的socket，这个socket和刚通过socket(2)创建的socket一样。这使得我们可以通过connect(2)到family是AF_UNSPEC的地址来替代close(2)关闭一个连接，并把这个连接的socket放到一个pool里面，以备后用，因为这能省去一个socket(2)调用的开销。

测试代码如下：

#include

#include

#include

#include

#include

#include

#include

#define xconnect(fd, addr, len) \

    do { \

        if (connect(fd, addr, len) < 0) { \

            fprintf(stderr, "connect error with: %s\n", \

                    strerror(errno)); \

            exit(EXIT_FAILURE); \

        } \

    } while (0)

int main(int argc, char *argv[])

{

        int retval;

        struct sockaddr_in addr;

        socklen_t len;

if(argc!=2)

{

printf("Usage: xconnect xx.xx.xx.xx\n");

return 0;

}

char *addr_str = argv[1];

        retval = socket(AF_INET, SOCK_STREAM, 0);

        memset(&addr, 0, sizeof(&addr));

        addr.sin_family = AF_INET;

        addr.sin_addr.s_addr = inet_addr(addr_str);

        addr.sin_port = htons(22);

        len = sizeof(addr);

        xconnect(retval, (struct sockaddr*)&addr, len);

        getsockname(retval, (struct sockaddr*)&addr, &len);

        printf("Old Port: %d\n", ntohs(addr.sin_port));

        addr.sin_family = AF_UNSPEC;

        xconnect(retval, (struct sockaddr*)&addr, len);

        addr.sin_family = AF_INET;

        addr.sin_addr.s_addr = inet_addr(addr_str);

        addr.sin_port = htons(22);

        xconnect(retval, (struct sockaddr*)&addr, len);

        getsockname(retval, (struct sockaddr*)&addr, &len);

        printf("New Port: %d\n", ntohs(addr.sin_port));

        return 0;

}

tcpdump的抓包如下：

debian-wangyao:/home/wangyao# tcpdump -i lo -nvv tcp

tcpdump: listening on lo, link-type EN10MB (Ethernet), capture size 96 bytes

13:27:48.919082 IP (tos 0x0, ttl 64, id 52187, offset 0, flags [DF], proto TCP (6), length 60)

    127.0.0.1.39206 > 127.0.0.1.22: Flags [S], cksum 0x0338 (correct), seq 629688948, win 32792, options [mss 16396,sackOK,TS val 24019375 ecr 0,nop,wscale 6], length 0

13:27:48.919108 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)

    127.0.0.1.22 > 127.0.0.1.39206: Flags [S.], cksum 0x51bd (correct), seq 637536356, ack 629688949, win 32768, options [mss 16396,sackOK,TS val 24019375 ecr 24019375,nop,wscale 6], length 0

13:27:48.919135 IP (tos 0x0, ttl 64, id 52188, offset 0, flags [DF], proto TCP (6), length 52)

    127.0.0.1.39206 > 127.0.0.1.22: Flags [.], cksum 0x38e0 (correct), seq 1, ack 1, win 513, options [nop,nop,TS val 24019375 ecr 24019375], length 0

--------------------------------------------------------------

第一次三次握手

--------------------------------------------------------------

13:27:48.919661 IP (tos 0x0, ttl 64, id 52189, offset 0, flags [DF], proto TCP (6), length 52)

    127.0.0.1.39206 > 127.0.0.1.22: Flags [R.], cksum 0x38dc (correct), seq 1, ack 1, win 513, options [nop,nop,TS val 24019375 ecr 24019375], length 0

--------------------------------------------------------------

RST中断第一个连接

--------------------------------------------------------------

13:27:48.919687 IP (tos 0x0, ttl 64, id 19416, offset 0, flags [DF], proto TCP (6), length 56)

    127.0.0.1.39207 > 127.0.0.1.22: Flags [S], cksum 0x9740 (correct), seq 629721719, win 32792, options [mss 16396,sackOK,TS val 24019375 ecr 0], length 0

13:27:48.919704 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)

    127.0.0.1.22 > 127.0.0.1.39207: Flags [S.], cksum 0xc014 (correct), seq 631516785, ack 629721720, win 32768, options [mss 16396,sackOK,TS val 24019375 ecr 24019375], length 0

13:27:48.919723 IP (tos 0x0, ttl 64, id 19417, offset 0, flags [DF], proto TCP (6), length 52)

    127.0.0.1.39207 > 127.0.0.1.22: Flags [.], cksum 0x1513 (correct), seq 1, ack 1, win 32792, options [nop,nop,TS val 24019375 ecr 24019375], length 0

--------------------------------------------------------------

第二次三次握手

--------------------------------------------------------------

13:27:48.920877 IP (tos 0x0, ttl 64, id 19418, offset 0, flags [DF], proto TCP (6), length 52)

    127.0.0.1.39207 > 127.0.0.1.22: Flags [F.], cksum 0x1512 (correct), seq 1, ack 1, win 32792, options [nop,nop,TS val 24019375 ecr 24019375], length 0

13:27:48.924008 IP (tos 0x0, ttl 64, id 45021, offset 0, flags [DF], proto TCP (6), length 52)

    127.0.0.1.22 > 127.0.0.1.39207: Flags [.], cksum 0x1529 (correct), seq 1, ack 2, win 32768, options [nop,nop,TS val 24019376 ecr 24019375], length 0

13:27:48.933517 IP (tos 0x0, ttl 64, id 45022, offset 0, flags [DF], proto TCP (6), length 84)

    127.0.0.1.22 > 127.0.0.1.39207: Flags [P.], seq 1:33, ack 2, win 32768, options [nop,nop,TS val 24019378 ecr 24019375], length 32

13:27:48.933539 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)

    127.0.0.1.39207 > 127.0.0.1.22: Flags [R], cksum 0x289f (correct), seq 629721721, win 0, length 0

--------------------------------------------------------------

最后只是由于程序关闭，用FIN关闭连接，接受到server端响应，由于程序已经关闭，协议栈响应RST

--------------------------------------------------------------

我们尽量在收到对方的FIN包，也就是read(2)等返回0的时候再调用connect(AF_UNSPEC)关闭连接，防止造成误会。

可能的应用：反向HTTP代理中，由代理服务器发往HTTP服务器的连接。