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tcp/ip:连接服务器失败(错误原因:connection refused)-ag真人游戏

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 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
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 connection
    return 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:255.255.255.0
          inet 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 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#define backlog 16
int main()
{
    // socket
    int 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);
    }
    // bind
    struct sockaddr_in server_addr;
    server_addr.sin_family = af_inet; // ipv4
    server_addr.sin_port = htons(12500); // port
    server_addr.sin_addr.s_addr = htonl(inaddr_any); // ip
    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);
    }
    // listen
    if (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");
    // accept
    int 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 eof
    while (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:255.255.255.0
          inet 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):

    // bind
    struct 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:255.255.255.0
          inet 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:255.255.255.0
          inet 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:255.255.255.0
          inet 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,^_^。

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