C客户端服务器连接被拒绝

Question

I have a laptop running windows 10 and intalled ubuntu in Oracle VM Virtualbox Machine. I have the following server and and client source code. I run the server first then the client. But it gave me a connection refused. Tried netstat -ntlp then found out that there is actually no port listening to the specific port I typed.

server.c

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/utsname.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<netdb.h>
#include<unistd.h>

int _GetHostName(char *buffer, int lenght);

const char MESSAGE[]="Hello, World!\n";
const int BACK_LOG=5;

int main(int argc, char *argv[]){
    int serverSocket=0, on=0, port=0, status=0, childPid=0;
    struct hostent *hostPtr=NULL;
    char hostname[80]="";
    struct sockaddr_in serverName={0};

    if(2!= argc){
        fprintf(stderr, "Usage : %s <port>\n", argv[0]);
        exit(1);
    }
    port=atoi(argv[1]);
    serverSocket=socket(PF_INET,SOCK_STREAM, IPPROTO_TCP);
    if(-1==serverSocket){
        perror("socket()");
        exit(1);
    }

    on=1;
    status=setsockopt(serverSocket, SOL_SOCKET, SO_REUSEADDR, (const char *)&on, sizeof(on));
    if(-1==status){
        perror("setsockopt(...,SO_REUSEADDRE,...)");
    }

    {
        struct linger linger={0};
        linger.l_onoff=1;
        linger.l_linger=30;
        status=setsockopt(serverSocket, SOL_SOCKET, SO_LINGER, (const char*)&linger, sizeof(linger));
        if(-1==status){
            perror("setsockopt(...,SO_LINGER,...)");
        }
    }

    status=_GetHostName(hostname, sizeof(hostname));
    if(-1==status){
        perror("_GetHostName()");
        exit(1);
    }

    hostPtr=gethostbyname(hostname);
    if(NULL==hostPtr){
        perror("gethostbyname()");
        exit(1);
    }

    (void)memset(&serverName,0,sizeof(serverName));
    (void)memcpy(&serverName.sin_addr, hostPtr->h_addr,hostPtr->h_length);

    serverName.sin_family=AF_INET;
    serverName.sin_port=htons(port);

    status=bind(serverSocket, (struct sockaddr*)&serverName,sizeof(serverName));
    if(-1==status){
        perror("bind");
        exit(1);
    }


    status=listen(serverSocket, BACK_LOG);
    if(-1==status){
        perror("listen()");
        exit(1);
    }

    for(;;){
        struct sockaddr_in clientName={0};
        int slaveSocket, clientLength=sizeof(clientName);

        (void)memset(&clientName,0,sizeof(clientName));

        slaveSocket=accept(serverSocket,(struct sockaddr*)&clientName, & clientLength);
        if(-1==slaveSocket){
            perror("accept()");
            exit(1);
        }

        childPid=fork();

        switch(childPid){
            case -1:perror("fork()");
            exit(1);
            case 0: close(serverSocket);
            if(-1==getpeername(slaveSocket, (struct sockaddr*)&clientName, &clientLength)){
                perror("getpeername()");
            }else{
                printf("Connection request from %s \n", inet_ntoa(clientName.sin_addr));
            }

            /*
                Server Application specific code here
                response to client
            */
            write(slaveSocket,MESSAGE,strlen(MESSAGE));
            close(slaveSocket);
            exit(0);
            default:close(slaveSocket);
        }
    }
    return 0;
}

int _GetHostName(char *buffer,int length){
    struct utsname sysname={0};
    int status=0;

    status=uname(&sysname);
    if(-1!=status){
        strncpy(buffer,sysname.nodename,length);
    }
    return(status);
}

client.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <string.h>

int main(int argc, char *argv[]){
    int clientSocket, remotePort, status=0;
    struct hostent *hostPtr=NULL;
    struct sockaddr_in serverName={0};
    char buffer[256]="";
    char *remoteHost=NULL;

    if(3!=argc){
        fprintf(stderr, "Usage: %s <serverHost> <serverPort> \n",argv[0]);
        exit(1);
    }
    remoteHost=argv[1];
    remotePort=atoi(argv[2]);
    clientSocket=socket(PF_INET,SOCK_STREAM,IPPROTO_TCP);
    if(-1==clientSocket){
        perror("socket()");
        exit(1);
    }

    hostPtr=gethostbyname(remoteHost);
    if(NULL==hostPtr){
        hostPtr=gethostbyaddr(remoteHost,strlen(remoteHost), AF_INET);
        if(NULL==hostPtr){
            perror("Error resolving server address ");
            exit(1);
        }
    }
    serverName.sin_family=AF_INET;
    serverName.sin_port=htons(remotePort);
    (void)memcpy(&serverName.sin_addr,hostPtr->h_addr,hostPtr->h_length);
    status=connect(clientSocket,(struct sockaddr*)&serverName,sizeof(serverName));
    if(-1==status){
        perror("connect()");
        exit(1);
    }
    //CLIENT SPECIFIC HERE

    while(0 < (status=read(clientSocket, buffer,sizeof(buffer)-1)))
        printf("%d: %s", status, buffer);
        if(-1==status)perror("read()");
        close(clientSocket);
        return 0;

}

Answer 1

As you can see in the server console, the call to gethostbyname is failing. This function sets h_errno, not errno, so you can't use perror to output the error. This is why you see it claim "success".

I don't see the point of using gethostbyname here though, most servers default to using INADDR_ANY when the address isn't specified by the user.

Answer 2

As Joni already pointed out, your program fails at gethostbyname() in server.c and then exits. Therefore no port gets bound since your program isn't running anymore.

Just don't use the gethostbyname() function if it's not really necessary and set sin_addr directly like in one of the following examples:

// let the kernel fill in the address of your interface for you automatically
serverName.sin_addr.s_addr = htonl(INADDR_ANY);

---

// use a loopback address (127.0.0.1) = localhost
serverName.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

---

// use whatever address you like
serverName.sin_addr.s_addr = inet_addr("127.1.1.1");

---

Don't forget to remove the line

memcpy(&serverName.sin_addr, hostPtr->h_addr,hostPtr->h_length);

in server.c when you set sin_addr like in the examples above.

Answer 3

Just figured it out. There is actually no problem in the code. The reason why I got an error particularly in the following line is that because just after I installed ubuntu in the virtual box, I changed the device name of the ubuntu without changing the etc/hosts .

hostPtr=gethostbyname(hostname);
if(NULL==hostPtr){
    perror("gethostbyname()");
    exit(1);
}