1  /* Host name canonicalization
       2  
       3     Copyright (C) 2005-2023 Free Software Foundation, Inc.
       4  
       5     Written by Derek Price <derek@ximbiot.com>.
       6  
       7     This file is free software: you can redistribute it and/or modify
       8     it under the terms of the GNU Lesser General Public License as
       9     published by the Free Software Foundation, either version 3 of the
      10     License, or (at your option) any later version.
      11  
      12     This file is distributed in the hope that it will be useful,
      13     but WITHOUT ANY WARRANTY; without even the implied warranty of
      14     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      15     GNU Lesser General Public License for more details.
      16  
      17     You should have received a copy of the GNU Lesser General Public License
      18     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      19  
      20  #include <config.h>
      21  
      22  #include "canon-host.h"
      23  
      24  #include <string.h>
      25  #include <netdb.h>
      26  
      27  /* Store the last error for the single-threaded version of this function.  */
      28  static int last_cherror;
      29  
      30  /* Single-threaded of wrapper for canon_host_r.  After a NULL return, error
      31     messages may be retrieved via ch_strerror().  */
      32  char *
      33  canon_host (const char *host)
      34  {
      35    return canon_host_r (host, &last_cherror);
      36  }
      37  
      38  /* Return a malloc'd string containing the canonical hostname associated with
      39     HOST, or NULL if a canonical name cannot be determined.  On NULL return,
      40     if CHERROR is not NULL, set *CHERROR to an error code as returned by
      41     getaddrinfo().  Use ch_strerror_r() or gai_strerror() to convert a *CHERROR
      42     value to a string suitable for error messages.
      43  
      44     WARNINGS
      45       HOST must be a string representation of a resolvable name for this host.
      46       Strings containing an IP address in dotted decimal notation will be
      47       returned as-is, without further resolution.
      48  
      49       The use of the word "canonical" in this context is unfortunate but
      50       entrenched.  The value returned by this function will be the end result
      51       of the resolution of any CNAME chains in the DNS.  There may only be one
      52       such value for any given hostname, though the actual IP address
      53       referenced by this value and the device using that IP address may each
      54       actually have any number of such "canonical" hostnames.  See the POSIX
      55       getaddrinfo spec
      56       <https://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html>,
      57       RFC 1034 <https://www.ietf.org/rfc/rfc1034.txt>, & RFC 2181
      58       <https://www.ietf.org/rfc/rfc2181.txt> for more on what this confusing
      59       term really refers to. */
      60  char *
      61  canon_host_r (char const *host, int *cherror)
      62  {
      63    char *retval = NULL;
      64    static struct addrinfo hints;
      65    struct addrinfo *res = NULL;
      66    int status;
      67  
      68    hints.ai_flags = AI_CANONNAME;
      69    status = getaddrinfo (host, NULL, &hints, &res);
      70    if (!status)
      71      {
      72        /* https://lists.gnu.org/r/bug-coreutils/2006-09/msg00300.html
      73           says Darwin 7.9.0 getaddrinfo returns 0 but sets
      74           res->ai_canonname to NULL.  */
      75        retval = strdup (res->ai_canonname ? res->ai_canonname : host);
      76        if (!retval && cherror)
      77          *cherror = EAI_MEMORY;
      78        freeaddrinfo (res);
      79      }
      80    else if (cherror)
      81      *cherror = status;
      82  
      83    return retval;
      84  }
      85  
      86  /* Return a string describing the last error encountered by canon_host.  */
      87  const char *
      88  ch_strerror (void)
      89  {
      90    return gai_strerror (last_cherror);
      91  }