dns_resolvers.c

#include <netinet/in.h>
#include <string.h>
#include <netdb.h>

/*
 *  Functions: get_ipaddr_by_name(), get_name_by_ipaddr()
 *  are wrappers over getaddrinfo() and getnameinfo()
 *  respectively.
 *
 */


/*
 * Function: get_ipaddr_by_name()
 * ------------------------------
 *  Makes a DNS resolution of domain name.
 *
 *  name - domain name (e.g. google.com)
 *
 *  out - pointer to an in_addr_t to `return`, returns the first entry
 *          of  getaddrinfo() list.
 *
 *  canon_name - pointer to string for canon_name, will be filled if not NULL
 *
 *  canon_name_size - size out output buffer
 *
 *  returns:    0 - success
 *              OTHER - error, use gai_strerror()
 *              to discover a particular error
 *
 */

int get_ipaddr_by_name(const char *name, in_addr_t *out,
            char *canon_name, size_t canon_name_size) {
    struct addrinfo hints, *result;
    int errcode;

    memset (&hints, 0, sizeof (hints));
    hints.ai_family = AF_INET;
    hints.ai_flags |= AI_CANONNAME;

    errcode = getaddrinfo(name, NULL, &hints, &result);
    if (errcode != 0) {
        return errcode;
    }

    *out = ((struct sockaddr_in *)result->ai_addr)->sin_addr.s_addr;
    if (canon_name) {
        strncpy(canon_name, result->ai_canonname, canon_name_size);
    }

    freeaddrinfo(result);

    return 0;
}

/*
 * Function: get_name_by_ipaddr()
 * ------------------------------
 *  Resolve a hostname by IPv4. Caches up to 10 results.
 *
 *  ip - input IPv4 address
 *
 *  host - pointer to ouput hostname buffer
 *
 *  host_len - output buffer len
 *
 *  returns:    0 - success
 *              OTHER - error, use gai_strerror()
 *              to discover a particular error
 *
 */

int get_name_by_ipaddr(in_addr_t ip, char *host, size_t host_len) {
    struct help {
        in_addr_t addr;
        char      hostname[NI_MAXHOST];
    };
    static struct help storage[10] = { 0 }; /* No lru, man, no lru */
    static int count = 0;

    // Lookup in cache
    for (int i = 0; i < count; i++) {
        if (storage[i].addr == ip) {
            // Found in cache
            strncpy(host, storage[i].hostname, host_len);
            return 0;
        }
    }

    struct sockaddr_in addr;
    int ret_code;
    memset(&addr, 0, sizeof addr);
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = ip;

    ret_code = getnameinfo((struct sockaddr*)&addr,
            sizeof addr,
            host, host_len,
            NULL, 0, 0);

    if (ret_code != 0) {
        return ret_code;
    }

    // Append to cache
    if (count < 10) {
        storage[count].addr = ip;
        strncpy(storage[count].hostname, host, NI_MAXHOST);
        count++;
    }

    return 0;
}