gethostbyname.c

/*
Print the IPs of given hostnames.

A few hostnames are hardcoded and always queried, but you can add more like:

    ./gethostbyname.out example.com apple.com

# gethostbyname

    Obsoleted by `getaddrinfo` and `getnameinfo`.

    Give a hostname string ("localhost", "john") and get info on that host,
    including its IP.

        struct hostent *gethostbyname(const char *name);

    Return value:

        struct hostent {
            char *h_name;       // name of the host
            char **h_aliases;   // list of aliases (nicknames)
            int h_addrtype;     // address type
            int h_length;       // length in bytes of the address
            char **h_addr_list  // list of address (network order)
        };

    `NULL` on error.

    TODO: this does not seem to actually do DNS queries, it is too fast.
    Must be using some Linux local cache?

# gethostbyaddr

    Obsoleted by `getaddrinfo` and `getnameinfo`.

    Same as gethostbyname but by address.
*/

#include "common.h"

void print_ip(char *hostname) {
    char **addrs;
    char **names;
    struct hostent *hostent;

    hostent = gethostbyname(hostname);
    if (hostent == NULL) {
        fprintf(stderr, "gethostbyname failed for hostname = %s\n", hostname);
    } else {
        printf("hostname: %s\n", hostent -> h_name);
        printf("  aliases:\n");
        names = hostent -> h_aliases;
        while (*names) {
            printf("    %s\n", *names);
            names++;
        }
        /* Assert that it is an inet address. */
        if (hostent -> h_addrtype != AF_INET) {
            printf("host is not AF_INET\n");
            exit(EXIT_FAILURE);
        }

        /* Show addresses. */
        printf("  IPs:\n");
        addrs = hostent->h_addr_list;
        while (*addrs) {
            /*
            # inet_ntoa

                Converts integer representation of ip (4 bytes) to a string.

                Takes network byte ordering into consideration.
            */
            printf("    %s\n", inet_ntoa(*(struct in_addr*)*addrs));
            addrs++;
        }
        printf("\n");
    }
}

int main(int argc, char **argv) {
    int i;
    enum CONSTEXPR { MAX_HOSTNAME_LENGTH = 256 };
    char hostnames[][MAX_HOSTNAME_LENGTH] = {
        "", /* Will be set to the localhost. */
        "www.google.com",
        "www.amazon.com"
    };
    enum CONSTEXPR2 { N_HOSTNAMES = sizeof(hostnames) / sizeof(hostnames[0]) };

    if (gethostname(hostnames[0], MAX_HOSTNAME_LENGTH) == -1) {
        perror("gethostname");
        exit(EXIT_FAILURE);
    }

    for (i = 0; i < N_HOSTNAMES; i++) {
        print_ip(hostnames[i]);
    }
    for (i = 1; i < argc; i++) {
        print_ip(argv[i]);
    }
    return EXIT_SUCCESS;
}