//  [c]
host_name_lookup(void)
{
int old_pool, rc;
int sep = 0;
uschar *save_hostname;
uschar **aliases;
uschar *ordername;
const uschar *list = host_lookup_order;
dns_answer * dnsa = store_get_dns_answer();
dns_scan dnss;

sender_host_dnssec = host_lookup_deferred = host_lookup_failed = FALSE;

HDEBUG(D_host_lookup)
  debug_printf("looking up host name for %s\n", sender_host_address);

/* For testing the case when a lookup does not complete, we have a special
reserved IP address. */

if (f.running_in_test_harness &&
    Ustrcmp(sender_host_address, "99.99.99.99") == 0)
  {
  HDEBUG(D_host_lookup)
    debug_printf("Test harness: host name lookup returns DEFER\n");
  host_lookup_deferred = TRUE;
  return DEFER;
  }

/* Do lookups directly in the DNS or via gethostbyaddr() (or equivalent), in
the order specified by the host_lookup_order option. */

while ((ordername = string_nextinlist(&list, &sep, NULL, 0)))
  {
  if (strcmpic(ordername, US"bydns") == 0)
    {
    uschar * name = dns_build_reverse(sender_host_address);

    dns_init(FALSE, FALSE, FALSE);    /* dnssec ctrl by dns_dnssec_ok glbl */
    rc = dns_lookup_timerwrap(dnsa, name, T_PTR, NULL);

    /* The first record we come across is used for the name; others are
    considered to be aliases. We have to scan twice, in order to find out the
    number of aliases. However, if all the names are empty, we will behave as
    if failure. (PTR records that yield empty names have been encountered in
    the DNS.) */

    if (rc == DNS_SUCCEED)
      {
      uschar **aptr = NULL;
      int ssize = 264;
      int count = 1;  /* need 1 more for terminating NULL */
      int old_pool = store_pool;

      sender_host_dnssec = dns_is_secure(dnsa);
      DEBUG(D_dns)
        debug_printf("Reverse DNS security status: %s\n",
            sender_host_dnssec ? "DNSSEC verified (AD)" : "unverified");

      store_pool = POOL_PERM;        /* Save names in permanent storage */

      for (dns_record * rr = dns_next_rr(dnsa, &dnss, RESET_ANSWERS);
           rr;
           rr = dns_next_rr(dnsa, &dnss, RESET_NEXT)) if (rr->type == T_PTR)
	count++;

      /* Get store for the list of aliases. For compatibility with
      gethostbyaddr, we make an empty list if there are none. */

      aptr = sender_host_aliases = store_get(count * sizeof(uschar *), FALSE);

      /* Re-scan and extract the names */

      for (dns_record * rr = dns_next_rr(dnsa, &dnss, RESET_ANSWERS);
           rr;
           rr = dns_next_rr(dnsa, &dnss, RESET_NEXT)) if (rr->type == T_PTR)
        {
        uschar * s = store_get(ssize, TRUE);	/* names are tainted */

        /* If an overlong response was received, the data will have been
        truncated and dn_expand may fail. */

        if (dn_expand(dnsa->answer, dnsa->answer + dnsa->answerlen,
             US (rr->data), (DN_EXPAND_ARG4_TYPE)(s), ssize) < 0)
          {
          log_write(0, LOG_MAIN, "host name alias list truncated for %s",
            sender_host_address);
          break;
          }

        store_release_above(s + Ustrlen(s) + 1);
        if (!s[0])
          {
          HDEBUG(D_host_lookup) debug_printf("IP address lookup yielded an "
            "empty name: treated as non-existent host name\n");
          continue;
          }
        if (!sender_host_name) sender_host_name = s;
	else *aptr++ = s;
        while (*s) { *s = tolower(*s); s++; }
        }

      *aptr = NULL;            /* End of alias list */
      store_pool = old_pool;   /* Reset store pool */

      /* If we've found a name, break out of the "order" loop */

      if (sender_host_name) break;
      }

    /* If the DNS lookup deferred, we must also defer. */

    if (rc == DNS_AGAIN)
      {
      HDEBUG(D_host_lookup)
        debug_printf("IP address PTR lookup gave temporary error\n");
      host_lookup_deferred = TRUE;
      return DEFER;
      }
    }

  /* Do a lookup using gethostbyaddr() - or equivalent */

  else if (strcmpic(ordername, US"byaddr") == 0)
    {
    HDEBUG(D_host_lookup)
      debug_printf("IP address lookup using gethostbyaddr()\n");
    rc = host_name_lookup_byaddr();
    if (rc == DEFER)
      {
      host_lookup_deferred = TRUE;
      return rc;                       /* Can't carry on */
      }
    if (rc == OK) break;               /* Found a name */
    }
  }      /* Loop for bydns/byaddr scanning */

/* If we have failed to find a name, return FAIL and log when required.
NB host_lookup_msg must be in permanent store.  */

if (!sender_host_name)
  {
  if (host_checking || !f.log_testing_mode)
    log_write(L_host_lookup_failed, LOG_MAIN, "no host name found for IP "
      "address %s", sender_host_address);
  host_lookup_msg = US" (failed to find host name from IP address)";
  host_lookup_failed = TRUE;
  return FAIL;
  }

HDEBUG(D_host_lookup)
  {
  uschar **aliases = sender_host_aliases;
  debug_printf("IP address lookup yielded \"%s\"\n", sender_host_name);
  while (*aliases != NULL) debug_printf("  alias \"%s\"\n", *aliases++);
  }

/* We need to verify that a forward lookup on the name we found does indeed
correspond to the address. This is for security: in principle a malefactor who
happened to own a reverse zone could set it to point to any names at all.

This code was present in versions of Exim before 3.20. At that point I took it
out because I thought that gethostbyaddr() did the check anyway. It turns out
that this isn't always the case, so it's coming back in at 4.01. This version
is actually better, because it also checks aliases.

The code was made more robust at release 4.21. Prior to that, it accepted all
the names if any of them had the correct IP address. Now the code checks all
the names, and accepts only those that have the correct IP address. */

save_hostname = sender_host_name;   /* Save for error messages */
aliases = sender_host_aliases;
for (uschar * hname = sender_host_name; hname; hname = *aliases++)
  {
  int rc;
  BOOL ok = FALSE;
  host_item h = { .next = NULL, .name = hname, .mx = MX_NONE, .address = NULL };
  dnssec_domains d =
    { .request = sender_host_dnssec ? US"*" : NULL, .require = NULL };

  if (  (rc = host_find_bydns(&h, NULL, HOST_FIND_BY_A | HOST_FIND_BY_AAAA,
	  NULL, NULL, NULL, &d, NULL, NULL)) == HOST_FOUND
     || rc == HOST_FOUND_LOCAL
     )
    {
    HDEBUG(D_host_lookup) debug_printf("checking addresses for %s\n", hname);

    /* If the forward lookup was not secure we cancel the is-secure variable */

    DEBUG(D_dns) debug_printf("Forward DNS security status: %s\n",
	  h.dnssec == DS_YES ? "DNSSEC verified (AD)" : "unverified");
    if (h.dnssec != DS_YES) sender_host_dnssec = FALSE;

    for (host_item * hh = &h; hh; hh = hh->next)
      if (host_is_in_net(hh->address, sender_host_address, 0))
        {
        HDEBUG(D_host_lookup) debug_printf("  %s OK\n", hh->address);
        ok = TRUE;
        break;
        }
      else
        HDEBUG(D_host_lookup) debug_printf("  %s\n", hh->address);

    if (!ok) HDEBUG(D_host_lookup)
      debug_printf("no IP address for %s matched %s\n", hname,
        sender_host_address);
    }
  else if (rc == HOST_FIND_AGAIN)
    {
    HDEBUG(D_host_lookup) debug_printf("temporary error for host name lookup\n");
    host_lookup_deferred = TRUE;
    sender_host_name = NULL;
    return DEFER;
    }
  else
    HDEBUG(D_host_lookup) debug_printf("no IP addresses found for %s\n", hname);

  /* If this name is no good, and it's the sender name, set it null pro tem;
  if it's an alias, just remove it from the list. */

  if (!ok)
    {
    if (hname == sender_host_name) sender_host_name = NULL; else
      {
      uschar **a;                              /* Don't amalgamate - some */
      a = --aliases;                           /* compilers grumble */
      while (*a != NULL) { *a = a[1]; a++; }
      }
    }
  }

/* If sender_host_name == NULL, it means we didn't like the name. Replace
it with the first alias, if there is one. */

if (sender_host_name == NULL && *sender_host_aliases != NULL)
  sender_host_name = *sender_host_aliases++;

/* If we now have a main name, all is well. */

if (sender_host_name != NULL) return OK;

/* We have failed to find an address that matches. */

HDEBUG(D_host_lookup)
  debug_printf("%s does not match any IP address for %s\n",
    sender_host_address, save_hostname);

/* This message must be in permanent store */

old_pool = store_pool;
store_pool = POOL_PERM;
host_lookup_msg = string_sprintf(" (%s does not match any IP address for %s)",
  sender_host_address, save_hostname);
store_pool = old_pool;
host_lookup_failed = TRUE;
return FAIL;
}