glibc/resolv/res_hconf.c

/* Copyright (C) 1993-2024 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* This file provides a Linux /etc/host.conf compatible front end to
   the various name resolvers (/etc/hosts, named, NIS server, etc.).
   Though mostly compatibly, the following differences exist compared
   to the original implementation:

	- line comments can appear anywhere (not just at the beginning of
	  a line)
*/

#include <assert.h>
#include <errno.h>
#include <ctype.h>
#include <libintl.h>
#include <memory.h>
#include <stdio.h>
#include <stdio_ext.h>
#include <stdlib.h>
#include <string.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <netinet/in.h>
#include <libc-lock.h>
#include "ifreq.h"
#include "res_hconf.h"
#include <wchar.h>
#include <atomic.h>
#include <set-freeres.h>

#if IS_IN (libc)
# define fgets_unlocked __fgets_unlocked
#endif

#define _PATH_HOSTCONF	"/etc/host.conf"

/* Environment vars that all user to override default behavior:  */
#define ENV_HOSTCONF	"RESOLV_HOST_CONF"
#define ENV_TRIM_OVERR	"RESOLV_OVERRIDE_TRIM_DOMAINS"
#define ENV_TRIM_ADD	"RESOLV_ADD_TRIM_DOMAINS"
#define ENV_MULTI	"RESOLV_MULTI"
#define ENV_REORDER	"RESOLV_REORDER"

enum parse_cbs
  {
    CB_none,
    CB_arg_trimdomain_list,
    CB_arg_bool
  };

static const struct cmd
{
  const char name[11];
  uint8_t cb;
  unsigned int arg;
} cmd[] =
{
  {"order",		CB_none,		0},
  {"trim",		CB_arg_trimdomain_list,	0},
  {"multi",		CB_arg_bool,		HCONF_FLAG_MULTI},
  {"reorder",		CB_arg_bool,		HCONF_FLAG_REORDER}
};

/* Structure containing the state.  */
struct hconf _res_hconf;

/* Skip white space.  */
static const char *
skip_ws (const char *str)
{
  while (isspace (*str)) ++str;
  return str;
}


/* Skip until whitespace, comma, end of line, or comment character.  */
static const char *
skip_string (const char *str)
{
  while (*str && !isspace (*str) && *str != '#' && *str != ',')
    ++str;
  return str;
}


static const char *
arg_trimdomain_list (const char *fname, int line_num, const char *args)
{
  const char * start;
  size_t len;

  do
    {
      start = args;
      args = skip_string (args);
      len = args - start;

      if (_res_hconf.num_trimdomains >= TRIMDOMAINS_MAX)
	{
	  char *buf;

	  if (__asprintf (&buf, _("\
%s: line %d: cannot specify more than %d trim domains"),
			  fname, line_num, TRIMDOMAINS_MAX) < 0)
	    return 0;

	  __fxprintf (NULL, "%s", buf);

	  free (buf);
	  return 0;
	}
      _res_hconf.trimdomain[_res_hconf.num_trimdomains++] =
	__strndup (start, len);
      args = skip_ws (args);
      switch (*args)
	{
	case ',': case ';': case ':':
	  args = skip_ws (++args);
	  if (!*args || *args == '#')
	    {
	      char *buf;

	      if (__asprintf (&buf, _("\
%s: line %d: list delimiter not followed by domain"),
			      fname, line_num) < 0)
		return 0;

	      __fxprintf (NULL, "%s", buf);

	      free (buf);
	      return 0;
	    }
	default:
	  break;
	}
    }
  while (*args && *args != '#');
  return args;
}


static const char *
arg_bool (const char *fname, int line_num, const char *args, unsigned flag)
{
  if (__strncasecmp (args, "on", 2) == 0)
    {
      args += 2;
      _res_hconf.flags |= flag;
    }
  else if (__strncasecmp (args, "off", 3) == 0)
    {
      args += 3;
      _res_hconf.flags &= ~flag;
    }
  else
    {
      char *buf;

      if (__asprintf (&buf,
		      _("%s: line %d: expected `on' or `off', found `%s'\n"),
		      fname, line_num, args) < 0)
	return 0;

      __fxprintf (NULL, "%s", buf);

      free (buf);
      return 0;
    }
  return args;
}


static void
parse_line (const char *fname, int line_num, const char *str)
{
  const char *start;
  const struct cmd *c = 0;
  size_t len;
  size_t i;

  str = skip_ws (str);

  /* skip line comment and empty lines: */
  if (*str == '\0' || *str == '#') return;

  start = str;
  str = skip_string (str);
  len = str - start;

  for (i = 0; i < sizeof (cmd) / sizeof (cmd[0]); ++i)
    {
      if (__strncasecmp (start, cmd[i].name, len) == 0
	  && strlen (cmd[i].name) == len)
	{
	  c = &cmd[i];
	  break;
	}
    }
  if (c == NULL)
    {
      char *buf;

      if (__asprintf (&buf, _("%s: line %d: bad command `%s'\n"),
		      fname, line_num, start) < 0)
	return;

      __fxprintf (NULL, "%s", buf);

      free (buf);
      return;
    }

  /* process args: */
  str = skip_ws (str);

  if (c->cb == CB_arg_trimdomain_list)
    str = arg_trimdomain_list (fname, line_num, str);
  else if (c->cb == CB_arg_bool)
    str = arg_bool (fname, line_num, str, c->arg);
  else
    /* Ignore the line.  */
    return;

  if (!str)
    return;

  /* rest of line must contain white space or comment only: */
  while (*str)
    {
      if (!isspace (*str)) {
	if (*str != '#')
	  {
	    char *buf;

	    if (__asprintf (&buf,
			    _("%s: line %d: ignoring trailing garbage `%s'\n"),
			    fname, line_num, str) < 0)
	      break;

	    __fxprintf (NULL, "%s", buf);

	    free (buf);
	  }
	break;
      }
      ++str;
    }
}


static void
do_init (void)
{
  const char *hconf_name;
  int line_num = 0;
  char buf[256], *envval;
  FILE *fp;

  memset (&_res_hconf, '\0', sizeof (_res_hconf));

  hconf_name = getenv (ENV_HOSTCONF);
  if (hconf_name == NULL)
    hconf_name = _PATH_HOSTCONF;

  fp = fopen (hconf_name, "rce");
  if (fp)
    {
      /* No threads using this stream.  */
      __fsetlocking (fp, FSETLOCKING_BYCALLER);

      while (fgets_unlocked (buf, sizeof (buf), fp))
	{
	  ++line_num;
	  *__strchrnul (buf, '\n') = '\0';
	  parse_line (hconf_name, line_num, buf);
	}
      fclose (fp);
    }

  envval = getenv (ENV_MULTI);
  if (envval)
    arg_bool (ENV_MULTI, 1, envval, HCONF_FLAG_MULTI);

  envval = getenv (ENV_REORDER);
  if (envval)
    arg_bool (ENV_REORDER, 1, envval, HCONF_FLAG_REORDER);

  envval = getenv (ENV_TRIM_ADD);
  if (envval)
    arg_trimdomain_list (ENV_TRIM_ADD, 1, envval);

  envval = getenv (ENV_TRIM_OVERR);
  if (envval)
    {
      _res_hconf.num_trimdomains = 0;
      arg_trimdomain_list (ENV_TRIM_OVERR, 1, envval);
    }

  /* See comments on the declaration of _res_hconf.  */
  atomic_store_release (&_res_hconf.initialized, 1);
}


/* Initialize hconf datastructure by reading host.conf file and
   environment variables.  */
void
_res_hconf_init (void)
{
  __libc_once_define (static, once);

  __libc_once (once, do_init);
}


#if IS_IN (libc)
# if defined SIOCGIFCONF && defined SIOCGIFNETMASK
/* List of known interfaces.  */
static struct netaddr *ifaddrs;
weak_alias (ifaddrs, __libc_resolv_res_hconf_freemem_ptr)
# endif

/* Reorder addresses returned in a hostent such that the first address
   is an address on the local subnet, if there is such an address.
   Otherwise, nothing is changed.

   Note that this function currently only handles IPv4 addresses.  */

void
_res_hconf_reorder_addrs (struct hostent *hp)
{
#if defined SIOCGIFCONF && defined SIOCGIFNETMASK
  int i, j;
  /* Number of interfaces.  Also serves as a flag for the
     double-checked locking idiom.  */
  static int num_ifs = -1;
  /* Local copy of num_ifs, for non-atomic access.  */
  int num_ifs_local;
  /* We need to protect the dynamic buffer handling.  The lock is only
     acquired during initialization.  Afterwards, a positive num_ifs
     value indicates completed initialization.  */
  __libc_lock_define_initialized (static, lock);

  /* Only reorder if we're supposed to.  */
  if ((_res_hconf.flags & HCONF_FLAG_REORDER) == 0)
    return;

  /* Can't deal with anything but IPv4 for now...  */
  if (hp->h_addrtype != AF_INET)
    return;

  /* This load synchronizes with the release MO store in the
     initialization block below.  */
  num_ifs_local = atomic_load_acquire (&num_ifs);
  if (num_ifs_local <= 0)
    {
      struct ifreq *ifr, *cur_ifr;
      int sd, num, i;
      /* Save errno.  */
      int save = errno;

      /* Initialize interface table.  */

      /* The SIOCGIFNETMASK ioctl will only work on an AF_INET socket.  */
      sd = __socket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
      if (sd < 0)
	return;

      /* Get lock.  */
      __libc_lock_lock (lock);

      /* Recheck, somebody else might have done the work by now.  No
	 ordering is required for the load because we have the lock,
	 and num_ifs is only updated under the lock.  Also see (3) in
	 the analysis below.  */
      num_ifs_local = atomic_load_relaxed (&num_ifs);
      if (num_ifs_local <= 0)
	{
	  /* This is the only block which writes to num_ifs.  It can
	     be executed several times (sequentially) if
	     initialization does not yield any interfaces, and num_ifs
	     remains zero.  However, once we stored a positive value
	     in num_ifs below, this block cannot be entered again due
	     to the condition above.  */
	  int new_num_ifs = 0;

	  /* Get a list of interfaces.  */
	  __ifreq (&ifr, &num, sd);
	  if (!ifr)
	    goto cleanup;

	  ifaddrs = malloc (num * sizeof (ifaddrs[0]));
	  if (!ifaddrs)
	    goto cleanup1;

	  /* Copy usable interfaces in ifaddrs structure.  */
	  for (cur_ifr = ifr, i = 0; i < num;
	       cur_ifr = __if_nextreq (cur_ifr), ++i)
	    {
	      union
	      {
		struct sockaddr sa;
		struct sockaddr_in sin;
	      } ss;

	      if (cur_ifr->ifr_addr.sa_family != AF_INET)
		continue;

	      ifaddrs[new_num_ifs].addrtype = AF_INET;
	      ss.sa = cur_ifr->ifr_addr;
	      ifaddrs[new_num_ifs].u.ipv4.addr = ss.sin.sin_addr.s_addr;

	      if (__ioctl (sd, SIOCGIFNETMASK, cur_ifr) < 0)
		continue;

	      ss.sa = cur_ifr->ifr_netmask;
	      ifaddrs[new_num_ifs].u.ipv4.mask = ss.sin.sin_addr.s_addr;

	      /* Now we're committed to this entry.  */
	      ++new_num_ifs;
	    }
	  /* Just keep enough memory to hold all the interfaces we want.  */
	  ifaddrs = realloc (ifaddrs, new_num_ifs * sizeof (ifaddrs[0]));
	  assert (ifaddrs != NULL);

	cleanup1:
	  __if_freereq (ifr, num);

	cleanup:
	  /* Release lock, preserve error value, and close socket.  */
	  errno = save;

	  /* Advertise successful initialization if new_num_ifs is
	     positive (and no updates to ifaddrs are permitted after
	     that).  Otherwise, num_ifs remains unchanged, at zero.
	     This store synchronizes with the initial acquire MO
	     load.  */
	  atomic_store_release (&num_ifs, new_num_ifs);
	  /* Keep the local copy current, to save another load.  */
	  num_ifs_local = new_num_ifs;
	}

      __libc_lock_unlock (lock);

      __close (sd);
    }

  /* num_ifs_local cannot be negative because the if statement above
     covered this case.  It can still be zero if we just performed
     initialization, but could not find any interfaces.  */
  if (num_ifs_local == 0)
    return;

  /* The code below accesses ifaddrs, so we need to ensure that the
     initialization happens-before this point.

     The actual initialization is sequenced-before the release store
     to num_ifs, and sequenced-before the end of the critical section.

     This means there are three possible executions:

     (1) The thread that initialized the data also uses it, so
         sequenced-before is sufficient to ensure happens-before.

     (2) The release MO store of num_ifs synchronizes-with the acquire
         MO load, and the acquire MO load is sequenced before the use
         of the initialized data below.

     (3) We enter the critical section, and the relaxed MO load of
         num_ifs yields a positive value.  The write to ifaddrs is
         sequenced-before leaving the critical section.  Leaving the
         critical section happens-before we entered the critical
         section ourselves, which means that the write to ifaddrs
         happens-before this point.

     Consequently, all potential writes to ifaddrs (and the data it
     points to) happens-before this point.  */

  /* Find an address for which we have a direct connection.  */
  for (i = 0; hp->h_addr_list[i]; ++i)
    {
      struct in_addr *haddr = (struct in_addr *) hp->h_addr_list[i];

      for (j = 0; j < num_ifs_local; ++j)
	{
	  uint32_t if_addr    = ifaddrs[j].u.ipv4.addr;
	  uint32_t if_netmask = ifaddrs[j].u.ipv4.mask;

	  if (((haddr->s_addr ^ if_addr) & if_netmask) == 0)
	    {
	      void *tmp;

	      tmp = hp->h_addr_list[i];
	      hp->h_addr_list[i] = hp->h_addr_list[0];
	      hp->h_addr_list[0] = tmp;
	      return;
	    }
	}
    }
#endif /* defined(SIOCGIFCONF) && ... */
}


/* If HOSTNAME has a postfix matching any of the trimdomains, trim away
   that postfix.  Notice that HOSTNAME is modified inplace.  Also, the
   original code applied all trimdomains in order, meaning that the
   same domainname could be trimmed multiple times.  I believe this
   was unintentional.  */
void
_res_hconf_trim_domain (char *hostname)
{
  size_t hostname_len, trim_len;
  int i;

  hostname_len = strlen (hostname);

  for (i = 0; i < _res_hconf.num_trimdomains; ++i)
    {
      const char *trim = _res_hconf.trimdomain[i];

      trim_len = strlen (trim);
      if (hostname_len > trim_len
	  && __strcasecmp (&hostname[hostname_len - trim_len], trim) == 0)
	{
	  hostname[hostname_len - trim_len] = '\0';
	  break;
	}
    }
}


/* Trim all hostnames/aliases in HP according to the trimdomain list.
   Notice that HP is modified inplace!  */
void
_res_hconf_trim_domains (struct hostent *hp)
{
  int i;

  if (_res_hconf.num_trimdomains == 0)
    return;

  _res_hconf_trim_domain (hp->h_name);
  for (i = 0; hp->h_aliases[i]; ++i)
    _res_hconf_trim_domain (hp->h_aliases[i]);
}
#endif