linux - 如果在客户机上运行，​​getservbyname 如何获取服务器端口信息？

Question

以下客户端程序尝试连接到服务器并查找该服务器上的当前时间和日期。

/*  Start with the usual includes and declarations.  */

#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    char *host;
    int sockfd;
    int len, result;
    struct sockaddr_in address;
    struct hostent *hostinfo;
    struct servent *servinfo;
    char buffer[128];

    if(argc == 1)
        host = "localhost";
    else
        host = argv[1];

/*  Find the host address and report an error if none is found.  */

    hostinfo = gethostbyname(host);
    if(!hostinfo) {
        fprintf(stderr, "no host: %s\n", host);
        exit(1);
    }

/*  Check that the daytime service exists on the host.  */

    servinfo = getservbyname("daytime", "tcp");
    if(!servinfo) {
        fprintf(stderr,"no daytime service\n");
        exit(1);
    }
    printf("daytime port is %d\n", ntohs(servinfo -> s_port));

/*  Create a socket.  */

    sockfd = socket(AF_INET, SOCK_STREAM, 0);

/*  Construct the address for use with connect...  */

    address.sin_family = AF_INET;
    address.sin_port = servinfo -> s_port;
    address.sin_addr = *(struct in_addr *)*hostinfo -> h_addr_list;
    len = sizeof(address);

/*  ...then connect and get the information.  */

    result = connect(sockfd, (struct sockaddr *)&address, len);
    if(result == -1) {
        perror("oops: getdate");
        exit(1);
    }

    result = read(sockfd, buffer, sizeof(buffer));
    buffer[result] = '\0';
    printf("read %d bytes: %s", result, buffer);

    close(sockfd);
    exit(0);
}

问题:

我们在客户端机器上运行上面的程序，函数getservbyname是如何得到在参数列表中没有引用服务器机器的服务器信息？

Answer 1

它检查 /etc/services 以查找具有给定服务名称和协议(protocol)的条目。

$ grep "^daytime\s.*/tcp" /etc/services 
daytime         13/tcp