菜鸟笔记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 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 kernelwireshark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 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!