source: lib/canohost.c @ e9eee04

  /********************************************************************\
  * BitlBee -- An IRC to other IM-networks gateway                     *
  *                                                                    *
  * Copyright 2002-2017 Wilmer van der Gaast and others                *
  \********************************************************************/

/*
  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License with
  the Debian GNU/Linux distribution in /usr/share/common-licenses/GPL;
  if not, write to the Free Software Foundation, Inc., 51 Franklin St.,
  Fifth Floor, Boston, MA  02110-1301  USA
*/

/* These two functions, with some modifications, taken from OpenSSH:
 *
 * $OpenBSD: canohost.c,v 1.73 2016/03/07 19:02:43 djm Exp
 *
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Functions for returning the canonical host name of the remote site.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 */

#include "canohost.h"

char *reverse_lookup(const struct sockaddr *from_, const socklen_t fromlen_)
{
        struct sockaddr_storage from;
        socklen_t fromlen;
        struct addrinfo hints, *ai, *aitop;
        char name[NI_MAXHOST], ntop2[NI_MAXHOST];
        char ntop[INET6_ADDRSTRLEN];

        fromlen = sizeof(from);
        memset(&from, 0, sizeof(from));
        memcpy(&from, from_, fromlen_);
        ipv64_normalise_mapped(&from, &fromlen);
        if (from.ss_family == AF_INET6) {
                fromlen = sizeof(struct sockaddr_in6);
        }

        if (getnameinfo((struct sockaddr *)&from, fromlen, ntop, sizeof(ntop),
            NULL, 0, NI_NUMERICHOST) != 0) {
                return NULL;
        }

        /* Map the IP address to a host name. */
        if (getnameinfo((struct sockaddr *)&from, fromlen, name, sizeof(name),
            NULL, 0, NI_NAMEREQD) != 0) {
                /* Host name not found.  Use ip address. */
                return g_strdup(ntop);
        }

        /*
         * if reverse lookup result looks like a numeric hostname,
         * someone is trying to trick us by PTR record like following:
         *      1.1.1.10.in-addr.arpa.  IN PTR  2.3.4.5
         */
        memset(&hints, 0, sizeof(hints));
        hints.ai_socktype = SOCK_DGRAM; /*dummy*/
        hints.ai_flags = AI_NUMERICHOST;
        if (getaddrinfo(name, NULL, &hints, &ai) == 0) {
                freeaddrinfo(ai);
                return g_strdup(ntop);
        }

        /* Names are stored in lowercase. */
        char *tolower = g_utf8_strdown(name, -1);
        g_snprintf(name, sizeof(name), "%s", tolower);
        g_free(tolower);

        /*
         * Map it back to an IP address and check that the given
         * address actually is an address of this host.  This is
         * necessary because anyone with access to a name server can
         * define arbitrary names for an IP address. Mapping from
         * name to IP address can be trusted better (but can still be
         * fooled if the intruder has access to the name server of
         * the domain).
         */
        memset(&hints, 0, sizeof(hints));
        hints.ai_family = from.ss_family;
        hints.ai_socktype = SOCK_STREAM;
        if (getaddrinfo(name, NULL, &hints, &aitop) != 0) {
                return g_strdup(ntop);
        }
        /* Look for the address from the list of addresses. */
        for (ai = aitop; ai; ai = ai->ai_next) {
                if (getnameinfo(ai->ai_addr, ai->ai_addrlen, ntop2,
                    sizeof(ntop2), NULL, 0, NI_NUMERICHOST) == 0 &&
                    (strcmp(ntop, ntop2) == 0))
                        break;
        }
        freeaddrinfo(aitop);
        /* If we reached the end of the list, the address was not there. */
        if (ai == NULL) {
                /* Address not found for the host name. */
                return g_strdup(ntop);
        }
        return g_strdup(name);
}

void
ipv64_normalise_mapped(struct sockaddr_storage *addr, socklen_t *len)
{
        struct sockaddr_in6 *a6 = (struct sockaddr_in6 *)addr;
        struct sockaddr_in *a4 = (struct sockaddr_in *)addr;
        struct in_addr inaddr;
        uint16_t port;

        if (addr->ss_family != AF_INET6 ||
            !IN6_IS_ADDR_V4MAPPED(&a6->sin6_addr))
                return;

        memcpy(&inaddr, ((char *)&a6->sin6_addr) + 12, sizeof(inaddr));
        port = a6->sin6_port;

        memset(a4, 0, sizeof(*a4));

        a4->sin_family = AF_INET;
        *len = sizeof(*a4);
        memcpy(&a4->sin_addr, &inaddr, sizeof(inaddr));
        a4->sin_port = port;
}