Program Statement

Write a program that retrieves (a) host information, (b) network information, (c) the protocol table and (d) the local domain name.

/* lookup.c — (a) host (b) network (c) protocols (d) domain */
int main(int argc, char **argv) {
    struct hostent *h = gethostbyname(argv[1]);            /* (a) host   */
    if (h) {
        printf("official name: %s\n", h->h_name);
        for (char **p = h->h_aliases; *p; p++) printf("alias: %s\n", *p);
        for (char **p = h->h_addr_list; *p; p++) {
            char ip[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, *p, ip, sizeof(ip));
            printf("address: %s\n", ip);
        }
    }
    struct protoent *pr;                                    /* (c) protocols */
    while ((pr = getprotoent()) != NULL)
        printf("protocol %-12s number %d\n", pr->p_name, pr->p_proto);
    endprotoent();

    struct servent *se = getservbyname("http", "tcp");      /* services      */
    printf("http/tcp is port %d\n", ntohs(se->s_port));

    char domain[256];                                       /* (d) domain    */
    getdomainname(domain, sizeof(domain));
    printf("NIS/local domain: %s\n", domain[0] ? domain : "(none)");
    /* (b) network: getnetbyname("loopback") — largely historical (/etc/networks) */
    return 0;
}

Theory Link

Unit 3 "Name & Address Conversions".

Extension

Redo part (a) with the modern, protocol-independent getaddrinfo and compare the two APIs in your record — gethostbyname is IPv4-only and not thread-safe; getaddrinfo handles both IPv4 and IPv6.