dirmngr: Add option --standard-resolver.

* dirmngr/dirmngr.c (oStandardResolver): New constant. (opts): New option --standard-resolver. (parse_rereadable_options): Set option. * dirmngr/dns-stuff.c: Refactor all code to support the new option. (standard_resolver): New var. (enable_standard_resolver, standard_resolver_p): New func. * dirmngr/http.c (connect_server): Remove USE_DNS_SRV build conditional. * dirmngr/ks-engine-hkp.c (map_host): Ditto. * dirmngr/server.c (cmd_getinfo) <dnsinfo>: Take care of new option * configure.ac (HAVE_ADNS_IF_TORMODE): Remove var ADNSLIB. ac_define USE_ADNS in the adns checking code. Remove options --disable-dns-srv and --disable-dns-cert. Always look for the system resolver. Print warning if no system resolver was found. (USE_DNS_CERT, USE_DNS_SRV): Remove ac_defines. (HAVE_SYSTEM_RESOLVER): New ac_define. (USE_DNS_SRV): Remove am_conditional; not used anyway. -- This option allows for runtime switching to the system's standard resolver. This is mainly useful to work around possible bugs in the optional resolver libraries (as of now ADNS). Note that on Windows there is no code to use systems's resolver and thus for full functionality dirmngr must be build with a separate resolver. This patch also does way with configure options to disable the use of CERT and SRV records. Signed-off-by: Werner Koch <wk@gnupg.org>
2025-01-03 12:11:33 +01:00 · 2016-12-01 15:45:51 +01:00 · 2016-12-01 15:45:51 +01:00 · 304566d332
commit 304566d332
parent 86efc3ee53
9 changed files with 483 additions and 391 deletions
--- a/configure.ac
+++ b/configure.ac
@ -103,6 +103,7 @@ have_sqlite=no
 have_npth=no
 have_libusb=no
 have_adns=no
+have_system_resolver=no
 gnupg_have_ldap="n/a"

 use_zip=yes
@ -1025,41 +1026,22 @@ fi
 CPPFLAGS=${_cppflags}
 LDFLAGS=${_ldflags}
 if test "$have_adns" = "yes"; then
-  ADNSLIBS="-ladns"
+  DNSLIBS="-ladns"
+  AC_DEFINE(USE_ADNS,1,[Use ADNS as resolver library.])
+else
+  DNSLIBS=""
 fi


 #
-# Now try for the resolver functions so we can use DNS for SRV, PA and CERT.
+# Check standard resolver functions.
 #
-AC_ARG_ENABLE(dns-srv,
-              AC_HELP_STRING([--disable-dns-srv],
-                             [disable the use of DNS SRV in HKP and HTTP]),
-              use_dns_srv=$enableval,use_dns_srv=yes)
-
-AC_ARG_ENABLE(dns-cert,
-   AC_HELP_STRING([--disable-dns-cert],
-	[disable the use of CERT records in DNS]),
-   use_dns_cert=$enableval,use_dns_cert=yes)
-
-if test x"$use_dns_srv" = xyes || test x"$use_dns_cert" = xyes; then
+if test "$build_dirmngr" = "yes"; then
  _dns_save_libs=$LIBS
  LIBS=""

-  if test x"$have_adns" = xyes ; then
-    # We prefer ADNS.
-    DNSLIBS="$ADNSLIBS"
-    AC_DEFINE(USE_ADNS,1,[Use ADNS as resolver library.])
-
-    if test x"$use_dns_srv" = xyes ; then
-        AC_DEFINE(USE_DNS_SRV,1)
-    fi
-
-    if test x"$use_dns_cert" = xyes ; then
-        AC_DEFINE(USE_DNS_CERT,1,[define to use DNS CERT])
-    fi
-  else
-    # With no ADNS find the system resolver.
+  # Find the system resolver which can always be enabled with
+  # the dirmngr option --standard-resolver.

  # the double underscore thing is a glibc-ism?
  AC_SEARCH_LIBS(res_query,resolv bind,,
@ -1116,23 +1098,18 @@ if test x"$use_dns_srv" = xyes || test x"$use_dns_cert" = xyes; then
  fi

  if test x"$have_resolver" = xyes ; then
-      DNSLIBS=$LIBS
-
-      if test x"$use_dns_srv" = xyes ; then
-         AC_DEFINE(USE_DNS_SRV,1,[define to use DNS SRV])
-      fi
-
-      if test x"$use_dns_cert" = xyes ; then
-        AC_DEFINE(USE_DNS_CERT,1,[define to use DNS CERT])
-      fi
-
+    AC_DEFINE(HAVE_SYSTEM_RESOLVER,1,[The system's resolver is usable.])
+    DNSLIBS="$DNSLIBS $LIBS"
    if test x"$need_compat" = xyes ; then
      AC_DEFINE(BIND_8_COMPAT,1,[an Apple OSXism])
    fi
  else
-      use_dns_srv=no
-      use_dns_cert=no
-    fi
+    AC_MSG_WARN([[
+***
+*** The system's DNS resolver is not usable.
+*** Dirmngr functionality is limited.
+***]])
+   show_tor_support="${show_tor_support} (no system resolver)"
  fi

  LIBS=$_dns_save_libs
@ -1140,8 +1117,6 @@ fi

 AC_SUBST(DNSLIBS)

-AM_CONDITIONAL(USE_DNS_SRV, test x"$use_dns_srv" = xyes)
-

 #
 # Check for LDAP
@ -1963,7 +1938,6 @@ echo "
        Dirmngr auto start:  $dirmngr_auto_start
        Readline support:    $gnupg_cv_have_readline
        LDAP support:        $gnupg_have_ldap
-        DNS SRV support:     $use_dns_srv
        TLS support:         $use_tls_library
        TOFU support:        $use_tofu
        Tor support:         $show_tor_support
--- a/dirmngr/dirmngr.c
+++ b/dirmngr/dirmngr.c
@ -140,6 +140,7 @@ enum cmd_and_opt_values {
  oKeyServer,
  oNameServer,
  oDisableCheckOwnSocket,
+  oStandardResolver,
  aTest
 };

@ -236,6 +237,7 @@ static ARGPARSE_OPTS opts[] = {
  ARGPARSE_s_s (oHTTPWrapperProgram, "http-wrapper-program", "@"),
  ARGPARSE_s_n (oHonorHTTPProxy, "honor-http-proxy", "@"),
  ARGPARSE_s_s (oIgnoreCertExtension,"ignore-cert-extension", "@"),
+  ARGPARSE_s_n (oStandardResolver, "standard-resolver", "@"),

  ARGPARSE_group (302,N_("@\n(See the \"info\" manual for a complete listing "
                         "of all commands and options)\n")),
@ -543,6 +545,7 @@ parse_rereadable_options (ARGPARSE_ARGS *pargs, int reread)
      FREE_STRLIST (opt.keyserver);
      /* Note: We do not allow resetting of opt.use_tor at runtime.  */
      disable_check_own_socket = 0;
+      enable_standard_resolver (0);
      return 1;
    }

@ -617,6 +620,8 @@ parse_rereadable_options (ARGPARSE_ARGS *pargs, int reread)

    case oUseTor: opt.use_tor = 1; break;

+    case oStandardResolver: enable_standard_resolver (1); break;
+
    case oKeyServer:
      if (*pargs->r.ret_str)
        add_to_strlist (&opt.keyserver, pargs->r.ret_str);
--- a/dirmngr/dns-stuff.c
+++ b/dirmngr/dns-stuff.c
@ -1,6 +1,6 @@
 /* dns-stuff.c - DNS related code including CERT RR (rfc-4398)
 * Copyright (C) 2003, 2005, 2006, 2009 Free Software Foundation, Inc.
- * Copyright (C) 2005, 2006, 2009, 2015 Werner Koch
+ * Copyright (C) 2005, 2006, 2009, 2015. 2016 Werner Koch
 *
 * This file is part of GnuPG.
 *
@ -36,9 +36,11 @@
 # endif
 # include <windows.h>
 #else
+# if HAVE_SYSTEM_RESOLVER
 #  include <netinet/in.h>
 #  include <arpa/nameser.h>
 #  include <resolv.h>
+# endif
 # include <netdb.h>
 #endif
 #include <string.h>
@ -101,6 +103,8 @@
 /* The default nameserver used with ADNS in Tor mode.  */
 #define DEFAULT_NAMESERVER "8.8.8.8"

+/* If set force the use of the standard resolver.  */
+static int standard_resolver;

 /* If set Tor mode shall be used.  */
 static int tor_mode;
@ -114,6 +118,25 @@ static char tor_nameserver[40+20];
 static char tor_credentials[50];
 #endif

+
+/* Calling this function with YES set to True forces the use of the
+ * standard resolver even if dirmngr has been built with support for
+ * an alternative resolver.  */
+void
+enable_standard_resolver (int yes)
+{
+  standard_resolver = yes;
+}
+
+
+/* Return true if the standard resolver is used.  */
+int
+standard_resolver_p (void)
+{
+  return standard_resolver;
+}
+
+
 /* Sets the module in Tor mode.  Returns 0 is this is possible or an
   error code.  */
 gpg_error_t
@ -121,7 +144,7 @@ enable_dns_tormode (int new_circuit)
 {
  (void) new_circuit;

-#if defined(USE_DNS_CERT) && defined(USE_ADNS)
+#ifdef USE_ADNS
 # if HAVE_ADNS_IF_TORMODE
   if (!*tor_credentials || new_circuit)
     {
@ -367,7 +390,6 @@ resolve_name_adns (const char *name, unsigned short port,
 #endif /*USE_ADNS*/


-#ifndef USE_ADNS
 /* Resolve a name using the standard system function.  */
 static gpg_error_t
 resolve_name_standard (const char *name, unsigned short port,
@ -472,7 +494,6 @@ resolve_name_standard (const char *name, unsigned short port,
    *r_dai = daihead;
  return err;
 }
-#endif /*!USE_ADNS*/


 /* Resolve an address using the standard system function.  */
@ -552,12 +573,12 @@ resolve_dns_name (const char *name, unsigned short port,
                  dns_addrinfo_t *r_ai, char **r_canonname)
 {
 #ifdef USE_ADNS
+  if (!standard_resolver)
    return resolve_name_adns (name, port, want_family, want_socktype,
                              r_ai, r_canonname);
-#else
+#endif
  return resolve_name_standard (name, port, want_family, want_socktype,
                                r_ai, r_canonname);
-#endif
 }


@ -565,11 +586,7 @@ gpg_error_t
 resolve_dns_addr (const struct sockaddr *addr, int addrlen,
                  unsigned int flags, char **r_name)
 {
-#ifdef USE_ADNS_disabled_for_now
-  return resolve_addr_adns (addr, addrlen, flags, r_name);
-#else
  return resolve_addr_standard (addr, addrlen, flags, r_name);
-#endif
 }


@ -654,23 +671,13 @@ is_onion_address (const char *name)
 }


-/* Returns 0 on success or an error code.  If a PGP CERT record was
-   found, the malloced data is returned at (R_KEY, R_KEYLEN) and
-   the other return parameters are set to NULL/0.  If an IPGP CERT
-   record was found the fingerprint is stored as an allocated block at
-   R_FPR and its length at R_FPRLEN; an URL is is allocated as a
-   string and returned at R_URL.  If WANT_CERTTYPE is 0 this function
-   returns the first CERT found with a supported type; it is expected
-   that only one CERT record is used.  If WANT_CERTTYPE is one of the
-   supported certtypes only records with this certtype are considered
-   and the first found is returned.  (R_KEY,R_KEYLEN) are optional. */
-gpg_error_t
-get_dns_cert (const char *name, int want_certtype,
+#ifdef USE_ADNS
+/* ADNS version of get_dns_cert.  */
+static gpg_error_t
+get_dns_cert_adns (const char *name, int want_certtype,
                   void **r_key, size_t *r_keylen,
                   unsigned char **r_fpr, size_t *r_fprlen, char **r_url)
 {
-#ifdef USE_DNS_CERT
-#ifdef USE_ADNS
  gpg_error_t err;
  int ret;
  adns_state state;
@ -678,14 +685,6 @@ get_dns_cert (const char *name, int want_certtype,
  unsigned int ctype;
  int count;

-  if (r_key)
-    *r_key = NULL;
-  if (r_keylen)
-    *r_keylen = 0;
-  *r_fpr = NULL;
-  *r_fprlen = 0;
-  *r_url = NULL;
-
  err = my_adns_init (&state);
  if (err)
    return err;
@ -812,22 +811,22 @@ get_dns_cert (const char *name, int want_certtype,
  adns_free (answer);
  adns_finish (state);
  return err;
+}
+#endif /*!USE_ADNS */

-#else /*!USE_ADNS*/

+/* Standard resolver version of get_dns_cert.  */
+static gpg_error_t
+get_dns_cert_standard (const char *name, int want_certtype,
+                       void **r_key, size_t *r_keylen,
+                       unsigned char **r_fpr, size_t *r_fprlen, char **r_url)
+{
+#ifdef HAVE_SYSTEM_RESOLVER
  gpg_error_t err;
  unsigned char *answer;
  int r;
  u16 count;

-  if (r_key)
-    *r_key = NULL;
-  if (r_keylen)
-    *r_keylen = 0;
-  *r_fpr = NULL;
-  *r_fprlen = 0;
-  *r_url = NULL;
-
  /* Allocate a 64k buffer which is the limit for an DNS response.  */
  answer = xtrymalloc (65536);
  if (!answer)
@ -1004,9 +1003,36 @@ get_dns_cert (const char *name, int want_certtype,
  xfree (answer);
  return err;

-#endif /*!USE_ADNS */
-#else /* !USE_DNS_CERT */
+#else /*!HAVE_SYSTEM_RESOLVER*/
+
  (void)name;
+  (void)want_certtype;
+  (void)r_key;
+  (void)r_keylen;
+  (void)r_fpr;
+  (void)r_fprlen;
+  (void)r_url;
+  return gpg_error (GPG_ERR_NOT_SUPPORTED);
+
+#endif /*!HAVE_SYSTEM_RESOLVER*/
+}
+
+
+/* Returns 0 on success or an error code.  If a PGP CERT record was
+   found, the malloced data is returned at (R_KEY, R_KEYLEN) and
+   the other return parameters are set to NULL/0.  If an IPGP CERT
+   record was found the fingerprint is stored as an allocated block at
+   R_FPR and its length at R_FPRLEN; an URL is is allocated as a
+   string and returned at R_URL.  If WANT_CERTTYPE is 0 this function
+   returns the first CERT found with a supported type; it is expected
+   that only one CERT record is used.  If WANT_CERTTYPE is one of the
+   supported certtypes only records with this certtype are considered
+   and the first found is returned.  (R_KEY,R_KEYLEN) are optional. */
+gpg_error_t
+get_dns_cert (const char *name, int want_certtype,
+              void **r_key, size_t *r_keylen,
+              unsigned char **r_fpr, size_t *r_fprlen, char **r_url)
+{
  if (r_key)
    *r_key = NULL;
  if (r_keylen)
@ -1015,11 +1041,16 @@ get_dns_cert (const char *name, int want_certtype,
  *r_fprlen = 0;
  *r_url = NULL;

-  return gpg_error (GPG_ERR_NOT_SUPPORTED);
-#endif
+#ifdef USE_ADNS
+  if (!standard_resolver)
+    return get_dns_cert_adns (name, want_certtype, r_key, r_keylen,
+                              r_fpr, r_fprlen, r_url);
+#endif /*!USE_ADNS */
+  return get_dns_cert_standard (name, want_certtype, r_key, r_keylen,
+                                r_fpr, r_fprlen, r_url);
 }

-#ifdef USE_DNS_SRV
+
 static int
 priosort(const void *a,const void *b)
 {
@ -1033,17 +1064,14 @@ priosort(const void *a,const void *b)
 }


-int
-getsrv (const char *name,struct srventry **list)
-{
-  int srvcount=0;
-  u16 count;
-  int i, rc;
-
-  *list = NULL;
-
 #ifdef USE_ADNS
-  {
+/* ADNS based helper for getsrv.  */
+static int
+getsrv_adns (const char *name, struct srventry **list)
+{
+  int srvcount = 0;
+  u16 count;
+  int rc;
  adns_state state;
  adns_answer *answer = NULL;

@ -1084,7 +1112,11 @@ getsrv (const char *name,struct srventry **list)

      newlist = xtryrealloc (*list, (srvcount+1)*sizeof(struct srventry));
      if (!newlist)
-          goto fail;
+        {
+          xfree (*list);
+          *list = NULL;
+          return -1;
+        }
      *list = newlist;
      memset (&(*list)[srvcount], 0, sizeof(struct srventry));
      srv = &(*list)[srvcount];
@ -1098,9 +1130,17 @@ getsrv (const char *name,struct srventry **list)

  adns_free (answer);
  adns_finish (state);
-  }
-#else /*!USE_ADNS*/
-  {
+
+  return srvcount;
+}
+#endif /*USE_ADNS*/
+
+
+/* Standard resolver based helper for getsrv.  */
+static int
+getsrv_standard (const char *name, struct srventry **list)
+{
+#ifdef HAVE_SYSTEM_RESOLVER
  union {
    unsigned char ans[2048];
    HEADER header[1];
@ -1108,8 +1148,10 @@ getsrv (const char *name,struct srventry **list)
  unsigned char *answer = res.ans;
  HEADER *header = res.header;
  unsigned char *pt, *emsg;
-    int r;
+  int r, rc;
  u16 dlen;
+  int srvcount=0;
+  u16 count;

  /* Do not allow a query using the standard resolver in Tor mode.  */
  if (tor_mode)
@ -1134,37 +1176,37 @@ getsrv (const char *name,struct srventry **list)

  while (count-- > 0 && pt < emsg)
    {
-        struct srventry *srv=NULL;
-        u16 type,class;
+      struct srventry *srv = NULL;
+      u16 type, class;
      struct srventry *newlist;

      newlist = xtryrealloc (*list, (srvcount+1)*sizeof(struct srventry));
      if (!newlist)
        goto fail;
      *list = newlist;
-        memset(&(*list)[srvcount],0,sizeof(struct srventry));
-        srv=&(*list)[srvcount];
+      memset (&(*list)[srvcount], 0, sizeof(struct srventry));
+      srv = &(*list)[srvcount];
      srvcount++;

-        rc = dn_skipname(pt,emsg); /* the name we just queried for */
+      rc = dn_skipname (pt, emsg); /* The name we just queried for.  */
      if (rc == -1)
        goto fail;
-        pt+=rc;
+      pt += rc;

      /* Truncated message? */
-        if((emsg-pt)<16)
+      if ((emsg-pt) < 16)
        goto fail;

      type = buf16_to_u16 (pt);
      pt += 2;
      /* We asked for SRV and got something else !? */
-        if(type!=T_SRV)
+      if (type != T_SRV)
        goto fail;

      class = buf16_to_u16 (pt);
      pt += 2;
      /* We asked for IN and got something else !? */
-        if(class!=C_IN)
+      if (class != C_IN)
        goto fail;

      pt += 4; /* ttl */
@ -1179,9 +1221,8 @@ getsrv (const char *name,struct srventry **list)
      pt += 2;

      /* Get the name.  2782 doesn't allow name compression, but
-           dn_expand still works to pull the name out of the
-           packet. */
-        rc = dn_expand(answer,emsg,pt,srv->target, sizeof srv->target);
+       * dn_expand still works to pull the name out of the packet. */
+      rc = dn_expand (answer, emsg, pt, srv->target, sizeof srv->target);
      if (rc == 1 && srv->target[0] == 0) /* "." */
        {
          xfree(*list);
@ -1195,8 +1236,43 @@ getsrv (const char *name,struct srventry **list)
      if (dlen != rc+6)
        goto fail;
    }
-  }
+
+  return srvcount;
+
+ fail:
+  xfree (*list);
+  *list = NULL;
+  return -1;
+
+#else /*!HAVE_SYSTEM_RESOLVER*/
+
+  (void)name;
+  (void)list;
+  return -1;
+
+#endif /*!HAVE_SYSTEM_RESOLVER*/
+}
+
+
+int
+getsrv (const char *name, struct srventry **list)
+{
+  int srvcount;
+  int i;
+
+  *list = NULL;
+
+  if (0)
+    ;
+#ifdef USE_ADNS
+  else if (!standard_resolver)
+    srvcount = getsrv_adns (name, list);
 #endif /*!USE_ADNS*/
+  else
+    srvcount = getsrv_standard (name, list);
+
+  if (srvcount <= 0)
+    return srvcount;

  /* Now we have an array of all the srv records. */

@ -1272,25 +1348,16 @@ getsrv (const char *name,struct srventry **list)
    }

  return srvcount;
-
- fail:
-  xfree(*list);
-  *list=NULL;
-  return -1;
 }
-#endif /*USE_DNS_SRV*/


+#ifdef USE_ADNS
+/* ADNS version of get_dns_cname.  */
 gpg_error_t
-get_dns_cname (const char *name, char **r_cname)
+get_dns_cname_adns (const char *name, char **r_cname)
 {
  gpg_error_t err;
  int rc;
-
-  *r_cname = NULL;
-
-#ifdef USE_ADNS
-  {
  adns_state state;
  adns_answer *answer = NULL;

@ -1328,9 +1395,17 @@ get_dns_cname (const char *name, char **r_cname)
  adns_free (answer);
  adns_finish (state);
  return err;
-  }
-#else /*!USE_ADNS*/
-  {
+}
+#endif /*USE_ADNS*/
+
+
+/* Standard resolver version of get_dns_cname.  */
+gpg_error_t
+get_dns_cname_standard (const char *name, char **r_cname)
+{
+#ifdef HAVE_SYSTEM_RESOLVER
+  gpg_error_t err;
+  int rc;
  union {
    unsigned char ans[2048];
    HEADER header[1];
@ -1391,6 +1466,26 @@ get_dns_cname (const char *name, char **r_cname)
      return err;
    }
  return 0;
-  }
-#endif /*!USE_ADNS*/
+
+#else /*!HAVE_SYSTEM_RESOLVER*/
+
+  (void)name;
+  (void)r_cname;
+  return -1;
+
+#endif /*!HAVE_SYSTEM_RESOLVER*/
+}
+
+
+gpg_error_t
+get_dns_cname (const char *name, char **r_cname)
+{
+  *r_cname = NULL;
+
+#ifdef USE_ADNS
+  if (!standard_resolver)
+    return get_dns_cname_adns (name, r_cname);
+#endif /*!USE_ADNS*/
+
+  return get_dns_cname_standard (name, r_cname);
 }
--- a/dirmngr/dns-stuff.h
+++ b/dirmngr/dns-stuff.h
@ -92,6 +92,14 @@ struct srventry
 };


+/* Calling this function with YES set to True forces the use of the
+ * standard resolver even if dirmngr has been built with support for
+ * an alternative resolver.  */
+void enable_standard_resolver (int yes);
+
+/* Return true if the standard resolver is used.  */
+int standard_resolver_p (void);
+
 /* Calling this function switches the DNS code into Tor mode if
   possibe.  Return 0 on success.  */
 gpg_error_t enable_dns_tormode (int new_circuit);
--- a/dirmngr/http.c
+++ b/dirmngr/http.c
@ -2323,7 +2323,6 @@ connect_server (const char *server, unsigned short port,
 #endif /*!HASSUAN_SOCK_TOR*/
    }

-#ifdef USE_DNS_SRV
  /* Do the SRV thing */
  if (srvtag)
    {
@ -2347,10 +2346,6 @@ connect_server (const char *server, unsigned short port,
            }
 	}
    }
-#else
-  (void)flags;
-  (void)srvtag;
-#endif /*USE_DNS_SRV*/

  if (!serverlist)
    {
--- a/dirmngr/ks-engine-hkp.c
+++ b/dirmngr/ks-engine-hkp.c
@ -426,11 +426,9 @@ map_host (ctrl_t ctrl, const char *name, int force_reselect,
      int refidx;
      int is_pool = 0;
      char *cname;
-#ifdef	USE_DNS_SRV
      char *srvrecord;
      struct srventry *srvs;
      int srvscount;
-#endif	/* USE_DNS_SRV */

      reftblsize = 100;
      reftbl = xtrymalloc (reftblsize * sizeof *reftbl);
@ -447,7 +445,6 @@ map_host (ctrl_t ctrl, const char *name, int force_reselect,
        }
      hi = hosttable[idx];

-#ifdef USE_DNS_SRV
      if (!is_ip_address (name))
        {
          /* Check for SRV records.  */
@ -488,7 +485,6 @@ map_host (ctrl_t ctrl, const char *name, int force_reselect,
              xfree (srvs);
            }
        }
-#endif /* USE_DNS_SRV */

      /* Find all A records for this entry and put them into the pool
         list - if any.  */
--- a/dirmngr/server.c
+++ b/dirmngr/server.c
@ -2309,13 +2309,19 @@ cmd_getinfo (assuan_context_t ctx, char *line)
    }
  else if (!strcmp (line, "dnsinfo"))
    {
+      if (standard_resolver_p ())
+        assuan_set_okay_line
+          (ctx, "- Forced use of System resolver (w/o Tor support)");
+      else
+        {
 #if USE_ADNS && HAVE_ADNS_IF_TORMODE
          assuan_set_okay_line (ctx, "- ADNS with Tor support");
 #elif USE_ADNS
          assuan_set_okay_line (ctx, "- ADNS w/o Tor support");
 #else
-      assuan_set_okay_line (ctx, "- System resolver w/o Tor support");
+          assuan_set_okay_line (ctx, "- System resolver (w/o Tor support)");
 #endif
+        }
      err = 0;
    }
  else
--- a/dirmngr/t-dns-stuff.c
+++ b/dirmngr/t-dns-stuff.c
@ -66,6 +66,7 @@ main (int argc, char **argv)
                 "Options:\n"
                 "  --verbose           print timings etc.\n"
                 "  --debug             flyswatter\n"
+                 "  --standard-resolver use the system's resolver\n"
                 "  --use-tor           use Tor\n"
                 "  --new-circuit       use a new Tor circuit\n"
                 "  --bracket           enclose v6 addresses in brackets\n"
@ -96,6 +97,11 @@ main (int argc, char **argv)
          opt_new_circuit = 1;
          argc--; argv++;
        }
+      else if (!strcmp (*argv, "--standard-resolver"))
+        {
+          enable_standard_resolver (1);
+          argc--; argv++;
+        }
      else if (!strcmp (*argv, "--bracket"))
        {
          opt_bracket = 1;
--- a/doc/dirmngr.texi
+++ b/doc/dirmngr.texi
@ -244,6 +244,13 @@ this still leaks the DNS queries; e.g. to lookup the hosts in a
 keyserver pool.  Certain other features are disabled if this mode is
 active.

+@item --standard-resolver
+@opindex standard-resolver
+This option forces the use of the system's standard DNS resolver code.
+This is mainly used for debugging.  Note that on Windows a standard
+resolver is not used and all DNS access will return the error ``Not
+Implemented'' if this function is used.
+
@item --allow-version-check
@opindex allow-version-check
 Allow Dirmngr to connect to @code{https://versions.gnupg.org} to get