TCP/IP：连接服务器失败（错误原因：Connection refused）

Linux中，通过系统调用（system call） connect 连接指定服务器建立TCP连接。

connect 最常见的失败原因是 Connection refused。

假设服务器IP是192.168.44.148，且并未有进程监听端口是12500时：

若有TCP连接请求包到达192.168.44.148，则192.168.44.148的内核将 回复RST包 给客户端。

此时，在客户端一侧看来就是connect连接失败，被服务端拒绝连接。

Code：

客户端：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>int main()
{int client_fd = socket(AF_INET, SOCK_STREAM, 0);if (client_fd < 0){fprintf(stderr, "create socket error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}struct sockaddr_in server_addr;server_addr.sin_family = AF_INET;server_addr.sin_port = htons(12500);if (inet_pton(AF_INET, "192.168.44.148", &server_addr.sin_addr) <= 0){fprintf(stderr, "inet_pton error!!!\n");exit(1);}if (connect(client_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0){fprintf(stderr, "socket connect error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}fprintf(stdout, "connect to server ok!\n");close(client_fd); // free connectionreturn 0;
}

编译：

[jiang@localhost client]$ gcc -o client client.c
[jiang@localhost client]$ ll
total 16
-rwxrwxr-x. 1 jiang jiang 8237 Jun 10 09:41 client
-rw-rw-r--. 1 jiang jiang  906 Jun 10 09:29 client.c

运行：

服务器主机：[test1280@localhost ~]$ ifconfig | grep "inet addr"inet addr:192.168.44.148  Bcast:192.168.44.255  Mask:255.255.255.0inet addr:127.0.0.1  Mask:255.0.0.0
[test1280@localhost ~]$ netstat -an | grep 12500客户端主机：[jiang@localhost client]$ ./client
socket connect error=111(Connection refused)!!!

tcpdump抓取协议包：

[root@localhost ~]# tcpdump tcp port 12500 -i eth0 -s 0 -w jiang.cap
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
^C2 packets captured
2 packets received by filter
0 packets dropped by kernel

wireshark分析jiang.cap：

1）

客户端调用 connect，发起TCP连接建立请求到服务端。

2）

服务器内核收到要连接本机12500端口的请求，发现并未有对应的监听SOCKET，于是回复RST到客户端。

3）

客户端调用 connect 失败，错误原因：Connection refused。

服务器Code：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>#define BACKLOG 16int main()
{// socketint listen_fd = socket(AF_INET, SOCK_STREAM, 0);if (listen_fd < 0){fprintf(stderr, "create socket error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}int flag = 1;if (setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &flag, sizeof(flag)) < 0){fprintf(stderr, "socket setsockopt error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}// bindstruct sockaddr_in server_addr;server_addr.sin_family = AF_INET; // IPv4server_addr.sin_port = htons(12500); // Portserver_addr.sin_addr.s_addr = htonl(INADDR_ANY); // IPif (bind(listen_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0){fprintf(stderr, "socket bind error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}// listenif (listen(listen_fd, BACKLOG) < 0){fprintf(stderr, "socket listen error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}fprintf(stdout, "server init ok, start to accept new connect...\n");// acceptint client_fd = accept(listen_fd, NULL, NULL);if (client_fd < 0){fprintf(stderr, "socket accept error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}fprintf(stdout, "accept one new connect!!!\n");char msg[1024] = "";// read client-FIN=1 EOFwhile (read(client_fd, msg, sizeof(msg)-1) == 0)break;close(client_fd);return 0;
}

编译：

[test1280@localhost server]$ gcc -o server server.c
[test1280@localhost server]$ ll
total 16
-rwxrwxr-x. 1 test1280 test1280 9616 Jun 10 09:51 server
-rw-r--r--. 1 test1280 test1280 1555 Jun 10 09:50 server.c

运行：

[test1280@localhost server]$ ./server
server init ok, start to accept new connect...
accept one new connect!!!

[test1280@localhost ~]$ ifconfig | grep "inet addr"inet addr:192.168.44.148  Bcast:192.168.44.255  Mask:255.255.255.0inet addr:127.0.0.1  Mask:255.0.0.0
[test1280@localhost ~]$ netstat -an | grep 12500
tcp        0      0 0.0.0.0:12500               0.0.0.0:*                   LISTEN

[jiang@localhost client]$ ./client
connect to server ok!

即使当前主机有进程监听12500端口，connect依然有可能被拒绝连接

略微修改下服务器代码（不使用地址通配INADDR_ANY）：

    // bindstruct sockaddr_in server_addr;server_addr.sin_family = AF_INET;server_addr.sin_port = htons(12500); // Port// if (inet_aton("0.0.0.0", &(server_addr.sin_addr)) != 1)// if (inet_aton("192.168.44.148", &(server_addr.sin_addr)) != 1)if (inet_aton("127.0.0.1", &(server_addr.sin_addr)) != 1){fprintf(stderr, "inet_aton error!!!\n");exit(1);}if (bind(listen_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0){fprintf(stderr, "socket bind error=%d(%s)!!!\n", errno, strerror(errno));exit(1);}

编译 && 运行：

[test1280@localhost server]$ gcc -o server server.c
[test1280@localhost server]$ ll
total 16
-rwxrwxr-x. 1 test1280 test1280 9620 Jun 10 10:12 server
-rw-r--r--. 1 test1280 test1280 1737 Jun 10 10:12 server.c
[test1280@localhost server]$ ./server
server init ok, start to accept new connect...

[test1280@localhost ~]$ ifconfig | grep "inet addr"inet addr:192.168.44.148  Bcast:192.168.44.255  Mask:255.255.255.0inet addr:127.0.0.1  Mask:255.0.0.0
[test1280@localhost ~]$ netstat -an | grep 12500
tcp        0      0 127.0.0.1:12500             0.0.0.0:*                   LISTEN

此时，在服务器上虽然有进程监听12500端口，但是监听套接字的IP地址是127.0.0.1（在127.0.0.1地址上监听12500端口）。

如果我们在客户端主机（192.168.44.144）上向【192.168.44.148:12500】发起TCP连接请求，将会被拒绝：

[jiang@localhost client]$ ifconfig | grep "inet addr"inet addr:192.168.44.144  Bcast:192.168.44.255  Mask:255.255.255.0inet addr:127.0.0.1  Mask:255.0.0.0
[jiang@localhost client]$ ./client
socket connect error=111(Connection refused)!!!

被拒绝原因是在148服务器上不存在监听套接字【192.168.44.148:12500】，只有监听套接字【127.0.0.1:12500】。

如果我们在服务端主机（192.168.44.148）上向【127.0.0.1:12500】发起TCP连接请求，将会被允许：

修改客户端代码：

        struct sockaddr_in server_addr;server_addr.sin_family = AF_INET;server_addr.sin_port = htons(12500);if (inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr) <= 0){fprintf(stderr, "inet_pton error!!!\n");exit(1);}

编译 && 运行：

[test1280@localhost client]$ ifconfig | grep "inet addr"inet addr:192.168.44.148  Bcast:192.168.44.255  Mask:255.255.255.0inet addr:127.0.0.1  Mask:255.0.0.0
[test1280@localhost client]$ gcc -o client client.c
[test1280@localhost client]$ ./client
connect to server ok!

总结：

connect 发起TCP连接请求被拒绝是由于目标服务器上无对应的监听套接字（IP && PORT）。

在哪个IP上监听哪个端口，^_^。