c - 为什么 UDP 组播服务器没有响应？

Question

我正在尝试在 Linux 上实现基本的 UDP 多播客户端和服务器。服务器根据客户端发送的消息，应该用系统参数进行回复(有点像 SNMP)。现在，我正在使用一台服务器进行测试。在不同的终端上运行客户端和服务器后，我向服务器发送了一个 3 个字符的请求，但服务器似乎无法继续，只是保持挂起状态，等待客户端。代码如下:

客户端:

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

char* createheader(int , int , int , char , int );

#define PORT 15002 
#define MAXLINE 10000 
char* MULTICAST = "224.0.0.3";
char* myIP = "127.0.0.1";

// Driver code 
int main(int argc, char *argv[]) 
{    
    char buffer[10000];
    char message[10000]; 
    char *msg;
    int sockfd, n;
    char c = '0';
    int req1, req2, req3;

    int ctr = 0;
    int prev = 0, curr = 0;

    char tp = 'Q';
    int seq = 0, len;
    int yes = 1;
    short int resendflag = 0;
    struct timeval time1, time2, tv={2,0}; // structures that can take time in seconds and micro seconds.

    struct sockaddr_in servaddr;   
    bzero(&servaddr, sizeof(servaddr)); 

    servaddr.sin_addr.s_addr = inet_addr(MULTICAST); 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_family = AF_INET;

    struct in_addr interface_addr;
    interface_addr.s_addr = inet_addr(myIP);


    // create datagram socket  
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (int*) &yes, sizeof(yes));

       if (sockfd < 0) {
     perror("Error: socket");
     exit(1);
   }

    /*if(connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) 
        { 
            printf("\n Error : Connect Failed \n"); 
            exit(0); 
        }*/

    setsockopt(sockfd,SOL_SOCKET,SO_RCVTIMEO,(char *)&tv,sizeof(struct timeval));

    //u_char loop;
    //setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

    //Create Tx Header

    while(1)
    {
        prev = seq; 
        seq++;

        printf("\nEnter the 3 request characters, each followed by newline\n");
        scanf("%d%d%d", &req1, &req2, &req3);    
        msg = createheader(req1, req2, req3, tp, seq);


        while(c!='\0')
        {
            c = *(msg+ctr);
            //puts(&c);
            message[ctr] = c;
            ctr++;
        }   
        //printf("\nsize of %d",sizeof(c));
        c = '0';
        ctr = 0;

        //msg = NULL; 

        // connect to server 


        // request to send datagram 
        // connect stores the peers IP and port

        sendto(sockfd, message, sizeof(message), 0, (struct sockaddr*)&servaddr, sizeof(servaddr)); 
        puts(message);
        // waiting for response
        n = recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr*)&servaddr, &len);

此后的部分特定于我的应用程序:除非您想测试它，否则可能会被忽略。

        curr = buffer[4]-'0' + (buffer[5]-'0')*256 + (buffer[6]-'0')*65536 + (buffer[7]-'0')*65536*256;

        if (prev == curr || n == -1)
            {   
                resendflag = 1;     
                while(resendflag)
                    {
                        printf("No Response Recieved. Resending...\n");                       
                        sendto(sockfd, message, strlen(message), 0, (struct sockaddr*)&servaddr, sizeof(servaddr));
                        n = recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr*)NULL, NULL); 
                        curr = buffer[4]-'0' + (buffer[5]-'0')*256 + (buffer[6]-'0')*65536 + (buffer[7]-'0')*65536*256;

                        if (prev == curr || n == -1)
                            {
                                resendflag = 1;
                            }
                        else
                            {
                                resendflag = 0;
                                puts(buffer+12);
                            }

                        sleep(1);
                     }
            }
        else
            puts(buffer+12);

        //tp = 'A';        
        //msg = createheader(req1, req2, req3, tp, seq);
        // close the descriptor

    } 
    close(sockfd);
} 

char* createheader(int req1, int req2, int req3, char tp, int seq)
{
    static char msg1[1000];
    int len;
    //char req;
    msg1[0] = 'A';
    msg1[1] = tp;
    msg1[3] = '0';
    msg1[4] = seq%256+'0';
    msg1[5] = (seq/256)%256+'0';
    msg1[6] = (seq/65536)%256+'0';
    msg1[7] = (seq/(65536*256))%256+'0';
    msg1[8] = req1+'0';
    msg1[9] = req2+'0';
    msg1[10] = req3+'0';   
    msg1[12] = '\0';
    len = strlen(msg1);

    msg1[2] = len +'0';


    return msg1;   

}

这是服务器:

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

#define PORT 15002 
#define MAXLINE 10000
char* MULTICAST = "224.0.0.3";
char* myIP = "127.0.0.2";

char* createheader(int , int , int , char , int , int);
char* sysfunc(int , int , int); 

// Driver code 
int main(int argc, char *argv[]) 
{    
    setbuf(stdout, NULL);
    printf("lololol5"); //Just some indicators to see the progress
    char buffer[10000]; 
    char *message = "Hello Client";
    char msg[10000];
    char *msg1;
    char tp = 'R';
    int yes = 1; 
    int listenfd, len, l=0, seq = 0, req1, req2, req3, i, curr = 0, exc = 0;
    const char* syscl= NULL;

    int drop;
    srand(time(NULL));

    FILE* fp;

    struct sockaddr_in servaddr, cliaddr; 
    bzero(&servaddr, sizeof(servaddr)); 
    printf("lololol4");  
    // Create a UDP Socket 
    listenfd = socket(AF_INET, SOCK_DGRAM, 0);
       if (listenfd < 0) {
     perror("socket");
     exit(1);
   }

    if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (int*)&yes, sizeof(yes))<0)
        {
            perror("socket");
            exit(1);
        }

    printf("lololol3");

    servaddr.sin_addr.s_addr = INADDR_ANY; 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_family = AF_INET;

    char *ip = inet_ntoa(servaddr.sin_addr);
    printf("\nip is %s\n", ip); 

    // bind server address to socket descriptor 
    if (bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {        
         perror("bind");
     exit(1);
      }

     printf("lololol2");   

    struct ip_mreq mreq;
    mreq.imr_multiaddr.s_addr = inet_addr(MULTICAST);
    mreq.imr_interface.s_addr = inet_addr(myIP);

    char *ip1 = inet_ntoa(mreq.imr_multiaddr);
    printf("\nmulip is %s\n", ip1); 

    if (setsockopt(listenfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)) < 0)
    {
        perror("setsockopt");
        return 1;
    } 
    while(1)
    {   
        //receive the datagram 
        len = sizeof(cliaddr);
        int n = recvfrom(listenfd, (char*) buffer, sizeof(buffer), 0, (struct sockaddr*)&cliaddr, &len); //receive message from client 

和以前一样，应用程序特定部分。

printf("\nHi\n"); 
        puts(buffer); //display message
        printf("\n");
        curr = buffer[4]-'0' + (buffer[5]-'0')*256 + (buffer[6]-'0')*65536 + (buffer[7]-'0')*65536*256;//acquire client seq. no

        req1 = buffer[8]; req2 = buffer[9]; req3 = buffer[10];//extract request bytes      
        syscl = sysfunc(req1-48, req2-48, req3-48); //Function call to get system corresponding system command string

        l = 12; //After 12 bytes of header
        puts(syscl);

        if (strcmp(syscl, "\nInvalid Command...\n\0"))
            {
                system(syscl); //The system call with the string gotten
                fp = fopen("sysstat.txt","r"); //Open the file where system output is written    
                while(!feof(fp))
                    {
                        msg[l] = fgetc(fp); //write file into an array
                        l++;
                    }
                fclose(fp);
                msg[l] = '\0'; //Append string with end of text char

            }
        else
            {
                char c1 = '0';
                while(c1 != '\0')
                    {
                        c1 = *(syscl+l-12); //write file into an array
                        //printf("\n %c", msg[l]);
                        msg[l] = c1; 
                        l++;
                    }
            }



        msg1 = createheader(req1, req2, req3, tp, curr, l); //Create Header  

        for (i=0; i<12; i++)
           {
                msg[i] = *(msg1+i);

            }

        drop = rand()%10+1; //to simulate dropped packets

        if (drop > 2) //Drop with a given prob (i.e (x-1)/10)
        {
            sendto(listenfd, &msg, MAXLINE, 0,(struct sockaddr*)&cliaddr, sizeof(cliaddr));
            puts(msg);
        }

        l = 0; 
    }
}

char* createheader(int req1, int req2, int req3, char tp, int seq, int len) //header
{
    static char msg1[10000];
    //int len;
    //char req;
    msg1[0] = '$'; //Start Char
    msg1[1] = tp; //Type of req.
    //msg1[3] = '0';
    msg1[4] = seq%256+'0'; //4-7: seq no in little endian
    msg1[5] = (seq/256)%256+'0';
    msg1[6] = (seq/65536)%256+'0';
    msg1[7] = (seq/(65536*256))%256+'0';
    msg1[8] = req1;
    msg1[9] = req2;
    msg1[10] = req3;   
    msg1[11] = '0';//Reserved Byte, Also for alignment
    //len = strlen(msg1);

    msg1[2] = (len +'0')%256+'0'; //2-3:Length in lil' endian
    msg1[3] = ((len+'0')/256)%256+'0';

    return msg1;   

}

char* sysfunc(int req1, int req2, int req3)
{
    static char syscl[100];
    //printf("\ncomm %d\n", req2);
    //printf("\ncomm %d\n", req3);

    switch (req2)
    {
        case 1: //Hardware
            {
                switch (req3)
                {
                    case 1: strcpy(syscl, "lscpu > sysstat.txt\0"); //CPU
                    break;

                    case 2: strcpy(syscl, "lsmem > sysstat.txt\0"); //Memoru=y
                    break;

                    case 3: strcpy(syscl, "lsblk > sysstat.txt\0"); //HDDs
                    break;

                    case 4: strcpy(syscl, "lspci > sysstat.txt\0"); //PCI Add-Ons
                    break;

                    default: strcpy(syscl, "\nInvalid Command...\n\0"); //Default
                    break;
                }

            }
        break;

        case 2: //OS
            {
                 switch (req3)
                {
                    case 1: strcpy(syscl, "hostname > sysstat.txt\0");//Hostname
                    break;

                    case 2: strcpy(syscl, "hostnamectl > sysstat.txt\0");//OS and Kernel
                    break;

                    case 3: strcpy(syscl, "uptime > sysstat.txt\0");//Uptime
                    break;

                    default: strcpy(syscl, "\nInvalid Command...\n\0");
                    break;
                }
            }
        break;

        case 3: //Network
            {
                 switch (req3)
                {
                    case 1: strcpy(syscl, "ip link show > sysstat.txt\0");//Ifs
                    break;

                    case 2: strcpy(syscl, "ifconfig | grep ether > sysstat.txt\0");//Ethernet
                    break;

                    case 3: strcpy(syscl, "ifconfig > sysstat.txt\0");//IP
                    break;

                    case 4: strcpy(syscl, "route -n > sysstat.txt\0");//Routing Table
                    break;

                    default: strcpy(syscl, "\nInvalid Command...\n\0");
                    break;
                }
            }
        break;

        default: strcpy(syscl, "\nInvalid Command...\n\0");
        break;
    }

    return syscl;            
}

已经搞了2天了。无法弄清楚我哪里出错了。由于服务器没有显示任何反应，我假设初始部分(服务器端的recvfrom)存在问题，因此以这种方式分割代码。抱歉，如果我犯了一些明显的菜鸟错误。

Answer 1

看来您在服务器中选择了错误的接口(interface)本地IP地址(您从哪里获得“127.0.0.2”？) - 如果我更改

    mreq.imr_interface.s_addr = inet_addr(myIP);

至

    mreq.imr_interface.s_addr = INADDR_ANY;

服务器开始接收。

Aren't 127.0.0.0/8 block of local addresses?

这些是环回地址(请参阅 What is the rest of the 127.0.0.0/8 address space used for? )。 struct ip_mreq定义中的注释接口(interface)的本地IP地址可能有点误导，因为它可以提醒localhost，但它实际上是指从外部看到的分配给本地主机接口(interface)的IP地址。

It is still not working with INADDR_ANY.

通常，“不工作”是一个不充分的问题描述。您可以收集更多信息。 G。通过使用 strace -enetwork ... 运行服务器。我建议先把不必要的复杂性(例如 Mininet)放在一边，直到程序在纯网络上运行为止。