gethostbyaddr_r - Linux

Overview

gethostbyaddr_r is a C library function used to retrieve host information from an IPv4 or IPv6 address. This function is part of the Berkeley Software Distribution (BSD) Socket API and is widely used in networking applications to resolve IP addresses to hostnames or other related information.

Syntax

int gethostbyaddr_r(const void *addr, socklen_t addrlen, int type,
                   struct hostent *result, char *buf, size_t buflen,
                   struct hostent **result_ptr);

Options/Flags

None

Examples

Retrieve hostname from IPv4 address:

#include <netdb.h>
#include <stdlib.h>

int main() {
    struct hostent *host;
    char buffer[256];
    int ret;

    ret = gethostbyaddr_r("192.168.1.1", 4, AF_INET, &host, buffer, sizeof(buffer), &host);
    if (ret == 0) {
        printf("Hostname: %s\n", host->h_name);
    } else {
        fprintf(stderr, "Error: %s\n", hstrerror(ret));
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}

Retrieve host information from IPv6 address:

#include <netdb.h>
#include <stdlib.h>

int main() {
    struct hostent *host;
    char buffer[256];
    int ret;

    ret = gethostbyaddr_r("::1", 16, AF_INET6, &host, buffer, sizeof(buffer), &host);
    if (ret == 0) {
        printf("Hostname: %s\n", host->h_name);
        printf("IP Addresses:\n");
        for (int i = 0; host->h_addr_list[i] != NULL; i++) {
            printf("  %s\n", inet_ntop(host->h_addrtype, host->h_addr_list[i], buffer, sizeof(buffer)));
        }
    } else {
        fprintf(stderr, "Error: %s\n", hstrerror(ret));
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}

Common Issues

• Ensure that the addr argument points to a valid IP address in the correct format (IPv4 or IPv6).
• Verify that the addrlen parameter matches the length of the IP address (4 for IPv4 and 16 for IPv6).
• Check if the type parameter is set to AF_INET or AF_INET6, depending on the IP address type.
• Make sure that the buf and buflen parameters are of sufficient size to hold the host information.
• Handle error codes returned by gethostbyaddr_r using hstrerror().

Integration

gethostbyaddr_r can be used in conjunction with other networking functions to obtain detailed information about a remote host:

#include <netdb.h>
#include <arpa/inet.h>

int main() {
    struct hostent *host;
    struct sockaddr_in addr;
    char buffer[256];

    inet_aton("192.168.1.1", &addr.sin_addr);
    host = gethostbyaddr_r(&addr.sin_addr, sizeof(addr.sin_addr), AF_INET, &host, buffer, sizeof(buffer), &host);

    if (host != NULL) {
        printf("Hostname: %s\n", host->h_name);
        printf("IP Address: %s\n", inet_ntoa(addr.sin_addr));
        printf("Aliases:\n");
        for (int i = 0; host->h_aliases[i] != NULL; i++) {
            printf("  %s\n", host->h_aliases[i]);
        }
    }

    return EXIT_SUCCESS;
}

Related Commands

• gethostbyaddr(): Retrieves host information for IPv4 addresses (deprecated).
• gethostbyname_r(): Retrieves host information for hostnames.
• gethostname(): Obtains the hostname of the local machine.
• dnslookup: Alternative command-line utility for host resolution.