Adding upstream version 1:9.18.19.upstream/1%9.18.19upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

6 files changed, 2798 insertions, 0 deletions

diff --git a/bin/plugins/Makefile.am b/bin/plugins/Makefile.am
new file mode 100644
index 0000000..611dd1b
--- /dev/null
+++ b/bin/plugins/Makefile.am
@@ -0,0 +1,15 @@
+include $(top_srcdir)/Makefile.top
+
+AM_CPPFLAGS +=			\
+	$(LIBISC_CFLAGS)	\
+	$(LIBDNS_CFLAGS)	\
+	$(LIBNS_CFLAGS)		\
+	$(LIBISCCFG_CFLAGS)
+
+pkglib_LTLIBRARIES = filter-aaaa.la
+pkglib_LTLIBRARIES += filter-a.la
+
+filter_aaaa_la_SOURCES = filter-aaaa.c
+filter_a_la_SOURCES = filter-a.c
+filter_aaaa_la_LDFLAGS = -avoid-version -module -shared -export-dynamic
+filter_a_la_LDFLAGS = -avoid-version -module -shared -export-dynamic
diff --git a/bin/plugins/Makefile.in b/bin/plugins/Makefile.in
new file mode 100644
index 0000000..b1d1bb8
--- /dev/null
+++ b/bin/plugins/Makefile.in
@@ -0,0 +1,847 @@
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+.SUFFIXES:
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+$(srcdir)/Makefile.in: @MAINTAINER_MODE_TRUE@ $(srcdir)/Makefile.am $(top_srcdir)/Makefile.top $(am__configure_deps)
+	@for dep in $?; do \
+	  case '$(am__configure_deps)' in \
+	    *$$dep*) \
+	      ( cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) am--refresh ) \
+	        && { if test -f $@; then exit 0; else break; fi; }; \
+	      exit 1;; \
+	  esac; \
+	done; \
+	echo ' cd $(top_srcdir) && $(AUTOMAKE) --foreign bin/plugins/Makefile'; \
+	$(am__cd) $(top_srcdir) && \
+	  $(AUTOMAKE) --foreign bin/plugins/Makefile
+Makefile: $(srcdir)/Makefile.in $(top_builddir)/config.status
+	@case '$?' in \
+	  *config.status*) \
+	    cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) am--refresh;; \
+	  *) \
+	    echo ' cd $(top_builddir) && $(SHELL) ./config.status $(subdir)/$@ $(am__maybe_remake_depfiles)'; \
+	    cd $(top_builddir) && $(SHELL) ./config.status $(subdir)/$@ $(am__maybe_remake_depfiles);; \
+	esac;
+$(top_srcdir)/Makefile.top $(am__empty):
+
+$(top_builddir)/config.status: $(top_srcdir)/configure $(CONFIG_STATUS_DEPENDENCIES)
+	cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) am--refresh
+
+$(top_srcdir)/configure: @MAINTAINER_MODE_TRUE@ $(am__configure_deps)
+	cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) am--refresh
+$(ACLOCAL_M4): @MAINTAINER_MODE_TRUE@ $(am__aclocal_m4_deps)
+	cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) am--refresh
+$(am__aclocal_m4_deps):
+
+install-pkglibLTLIBRARIES: $(pkglib_LTLIBRARIES)
+	@$(NORMAL_INSTALL)
+	@list='$(pkglib_LTLIBRARIES)'; test -n "$(pkglibdir)" || list=; \
+	list2=; for p in $$list; do \
+	  if test -f $$p; then \
+	    list2="$$list2 $$p"; \
+	  else :; fi; \
+	done; \
+	test -z "$$list2" || { \
+	  echo " $(MKDIR_P) '$(DESTDIR)$(pkglibdir)'"; \
+	  $(MKDIR_P) "$(DESTDIR)$(pkglibdir)" || exit 1; \
+	  echo " $(LIBTOOL) $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=install $(INSTALL) $(INSTALL_STRIP_FLAG) $$list2 '$(DESTDIR)$(pkglibdir)'"; \
+	  $(LIBTOOL) $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=install $(INSTALL) $(INSTALL_STRIP_FLAG) $$list2 "$(DESTDIR)$(pkglibdir)"; \
+	}
+
+uninstall-pkglibLTLIBRARIES:
+	@$(NORMAL_UNINSTALL)
+	@list='$(pkglib_LTLIBRARIES)'; test -n "$(pkglibdir)" || list=; \
+	for p in $$list; do \
+	  $(am__strip_dir) \
+	  echo " $(LIBTOOL) $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=uninstall rm -f '$(DESTDIR)$(pkglibdir)/$$f'"; \
+	  $(LIBTOOL) $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=uninstall rm -f "$(DESTDIR)$(pkglibdir)/$$f"; \
+	done
+
+clean-pkglibLTLIBRARIES:
+	-test -z "$(pkglib_LTLIBRARIES)" || rm -f $(pkglib_LTLIBRARIES)
+	@list='$(pkglib_LTLIBRARIES)'; \
+	locs=`for p in $$list; do echo $$p; done | \
+	      sed 's|^[^/]*$$|.|; s|/[^/]*$$||; s|$$|/so_locations|' | \
+	      sort -u`; \
+	test -z "$$locs" || { \
+	  echo rm -f $${locs}; \
+	  rm -f $${locs}; \
+	}
+
+filter-a.la: $(filter_a_la_OBJECTS) $(filter_a_la_DEPENDENCIES) $(EXTRA_filter_a_la_DEPENDENCIES) 
+	$(AM_V_CCLD)$(filter_a_la_LINK) -rpath $(pkglibdir) $(filter_a_la_OBJECTS) $(filter_a_la_LIBADD) $(LIBS)
+
+filter-aaaa.la: $(filter_aaaa_la_OBJECTS) $(filter_aaaa_la_DEPENDENCIES) $(EXTRA_filter_aaaa_la_DEPENDENCIES) 
+	$(AM_V_CCLD)$(filter_aaaa_la_LINK) -rpath $(pkglibdir) $(filter_aaaa_la_OBJECTS) $(filter_aaaa_la_LIBADD) $(LIBS)
+
+mostlyclean-compile:
+	-rm -f *.$(OBJEXT)
+
+distclean-compile:
+	-rm -f *.tab.c
+
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/filter-a.Plo@am__quote@ # am--include-marker
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/filter-aaaa.Plo@am__quote@ # am--include-marker
+
+$(am__depfiles_remade):
+	@$(MKDIR_P) $(@D)
+	@echo '# dummy' >$@-t && $(am__mv) $@-t $@
+
+am--depfiles: $(am__depfiles_remade)
+
+.c.o:
+@am__fastdepCC_TRUE@	$(AM_V_CC)depbase=`echo $@ | sed 's|[^/]*$$|$(DEPDIR)/&|;s|\.o$$||'`;\
+@am__fastdepCC_TRUE@	$(COMPILE) -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< &&\
+@am__fastdepCC_TRUE@	$(am__mv) $$depbase.Tpo $$depbase.Po
+@AMDEP_TRUE@@am__fastdepCC_FALSE@	$(AM_V_CC)source='$<' object='$@' libtool=no @AMDEPBACKSLASH@
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+@am__fastdepCC_FALSE@	$(AM_V_CC@am__nodep@)$(COMPILE) -c -o $@ $<
+
+.c.obj:
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+
+.c.lo:
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+@am__fastdepCC_TRUE@	$(am__mv) $$depbase.Tpo $$depbase.Plo
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+@am__fastdepCC_FALSE@	$(AM_V_CC@am__nodep@)$(LTCOMPILE) -c -o $@ $<
+
+mostlyclean-libtool:
+	-rm -f *.lo
+
+clean-libtool:
+	-rm -rf .libs _libs
+test-local: 
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+doc-local: 
+
+ID: $(am__tagged_files)
+	$(am__define_uniq_tagged_files); mkid -fID $$unique
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+	here=`pwd`; \
+	$(am__define_uniq_tagged_files); \
+	shift; \
+	if test -z "$(ETAGS_ARGS)$$*$$unique"; then :; else \
+	  test -n "$$unique" || unique=$$empty_fix; \
+	  if test $$# -gt 0; then \
+	    $(ETAGS) $(ETAGSFLAGS) $(AM_ETAGSFLAGS) $(ETAGS_ARGS) \
+	      "$$@" $$unique; \
+	  else \
+	    $(ETAGS) $(ETAGSFLAGS) $(AM_ETAGSFLAGS) $(ETAGS_ARGS) \
+	      $$unique; \
+	  fi; \
+	fi
+ctags: ctags-am
+
+CTAGS: ctags
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+	$(am__define_uniq_tagged_files); \
+	test -z "$(CTAGS_ARGS)$$unique" \
+	  || $(CTAGS) $(CTAGSFLAGS) $(AM_CTAGSFLAGS) $(CTAGS_ARGS) \
+	     $$unique
+
+GTAGS:
+	here=`$(am__cd) $(top_builddir) && pwd` \
+	  && $(am__cd) $(top_srcdir) \
+	  && gtags -i $(GTAGS_ARGS) "$$here"
+cscopelist: cscopelist-am
+
+cscopelist-am: $(am__tagged_files)
+	list='$(am__tagged_files)'; \
+	case "$(srcdir)" in \
+	  [\\/]* | ?:[\\/]*) sdir="$(srcdir)" ;; \
+	  *) sdir=$(subdir)/$(srcdir) ;; \
+	esac; \
+	for i in $$list; do \
+	  if test -f "$$i"; then \
+	    echo "$(subdir)/$$i"; \
+	  else \
+	    echo "$$sdir/$$i"; \
+	  fi; \
+	done >> $(top_builddir)/cscope.files
+
+distclean-tags:
+	-rm -f TAGS ID GTAGS GRTAGS GSYMS GPATH tags
+distdir: $(BUILT_SOURCES)
+	$(MAKE) $(AM_MAKEFLAGS) distdir-am
+
+distdir-am: $(DISTFILES)
+	@srcdirstrip=`echo "$(srcdir)" | sed 's/[].[^$$\\*]/\\\\&/g'`; \
+	topsrcdirstrip=`echo "$(top_srcdir)" | sed 's/[].[^$$\\*]/\\\\&/g'`; \
+	list='$(DISTFILES)'; \
+	  dist_files=`for file in $$list; do echo $$file; done | \
+	  sed -e "s|^$$srcdirstrip/||;t" \
+	      -e "s|^$$topsrcdirstrip/|$(top_builddir)/|;t"`; \
+	case $$dist_files in \
+	  */*) $(MKDIR_P) `echo "$$dist_files" | \
+			   sed '/\//!d;s|^|$(distdir)/|;s,/[^/]*$$,,' | \
+			   sort -u` ;; \
+	esac; \
+	for file in $$dist_files; do \
+	  if test -f $$file || test -d $$file; then d=.; else d=$(srcdir); fi; \
+	  if test -d $$d/$$file; then \
+	    dir=`echo "/$$file" | sed -e 's,/[^/]*$$,,'`; \
+	    if test -d "$(distdir)/$$file"; then \
+	      find "$(distdir)/$$file" -type d ! -perm -700 -exec chmod u+rwx {} \;; \
+	    fi; \
+	    if test -d $(srcdir)/$$file && test $$d != $(srcdir); then \
+	      cp -fpR $(srcdir)/$$file "$(distdir)$$dir" || exit 1; \
+	      find "$(distdir)/$$file" -type d ! -perm -700 -exec chmod u+rwx {} \;; \
+	    fi; \
+	    cp -fpR $$d/$$file "$(distdir)$$dir" || exit 1; \
+	  else \
+	    test -f "$(distdir)/$$file" \
+	    || cp -p $$d/$$file "$(distdir)/$$file" \
+	    || exit 1; \
+	  fi; \
+	done
+check-am: all-am
+check: check-am
+all-am: Makefile $(LTLIBRARIES)
+installdirs:
+	for dir in "$(DESTDIR)$(pkglibdir)"; do \
+	  test -z "$$dir" || $(MKDIR_P) "$$dir"; \
+	done
+install: install-am
+install-exec: install-exec-am
+install-data: install-data-am
+uninstall: uninstall-am
+
+install-am: all-am
+	@$(MAKE) $(AM_MAKEFLAGS) install-exec-am install-data-am
+
+installcheck: installcheck-am
+install-strip:
+	if test -z '$(STRIP)'; then \
+	  $(MAKE) $(AM_MAKEFLAGS) INSTALL_PROGRAM="$(INSTALL_STRIP_PROGRAM)" \
+	    install_sh_PROGRAM="$(INSTALL_STRIP_PROGRAM)" INSTALL_STRIP_FLAG=-s \
+	      install; \
+	else \
+	  $(MAKE) $(AM_MAKEFLAGS) INSTALL_PROGRAM="$(INSTALL_STRIP_PROGRAM)" \
+	    install_sh_PROGRAM="$(INSTALL_STRIP_PROGRAM)" INSTALL_STRIP_FLAG=-s \
+	    "INSTALL_PROGRAM_ENV=STRIPPROG='$(STRIP)'" install; \
+	fi
+mostlyclean-generic:
+
+clean-generic:
+
+distclean-generic:
+	-test -z "$(CONFIG_CLEAN_FILES)" || rm -f $(CONFIG_CLEAN_FILES)
+	-test . = "$(srcdir)" || test -z "$(CONFIG_CLEAN_VPATH_FILES)" || rm -f $(CONFIG_CLEAN_VPATH_FILES)
+
+maintainer-clean-generic:
+	@echo "This command is intended for maintainers to use"
+	@echo "it deletes files that may require special tools to rebuild."
+clean: clean-am
+
+clean-am: clean-generic clean-libtool clean-pkglibLTLIBRARIES \
+	mostlyclean-am
+
+distclean: distclean-am
+		-rm -f ./$(DEPDIR)/filter-a.Plo
+	-rm -f ./$(DEPDIR)/filter-aaaa.Plo
+	-rm -f Makefile
+distclean-am: clean-am distclean-compile distclean-generic \
+	distclean-tags
+
+doc: doc-am
+
+doc-am: doc-local
+
+dvi: dvi-am
+
+dvi-am:
+
+html: html-am
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+html-am:
+
+info: info-am
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+info-am:
+
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+
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+install-exec-am: install-pkglibLTLIBRARIES
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+install-html: install-html-am
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+install-html-am:
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+install-pdf-am:
+
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+
+install-ps-am:
+
+installcheck-am:
+
+maintainer-clean: maintainer-clean-am
+		-rm -f ./$(DEPDIR)/filter-a.Plo
+	-rm -f ./$(DEPDIR)/filter-aaaa.Plo
+	-rm -f Makefile
+maintainer-clean-am: distclean-am maintainer-clean-generic
+
+mostlyclean: mostlyclean-am
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+mostlyclean-am: mostlyclean-compile mostlyclean-generic \
+	mostlyclean-libtool
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+pdf: pdf-am
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+
+test-am: test-local
+
+uninstall-am: uninstall-pkglibLTLIBRARIES
+
+unit: unit-am
+
+unit-am: unit-local
+
+.MAKE: install-am install-strip
+
+.PHONY: CTAGS GTAGS TAGS all all-am am--depfiles check check-am clean \
+	clean-generic clean-libtool clean-pkglibLTLIBRARIES \
+	cscopelist-am ctags ctags-am distclean distclean-compile \
+	distclean-generic distclean-libtool distclean-tags distdir \
+	doc-am doc-local dvi dvi-am html html-am info info-am install \
+	install-am install-data install-data-am install-dvi \
+	install-dvi-am install-exec install-exec-am install-html \
+	install-html-am install-info install-info-am install-man \
+	install-pdf install-pdf-am install-pkglibLTLIBRARIES \
+	install-ps install-ps-am install-strip installcheck \
+	installcheck-am installdirs maintainer-clean \
+	maintainer-clean-generic mostlyclean mostlyclean-compile \
+	mostlyclean-generic mostlyclean-libtool pdf pdf-am ps ps-am \
+	tags tags-am test-am test-local uninstall uninstall-am \
+	uninstall-pkglibLTLIBRARIES unit-am unit-local
+
+.PRECIOUS: Makefile
+
+
+# Tell versions [3.59,3.63) of GNU make to not export all variables.
+# Otherwise a system limit (for SysV at least) may be exceeded.
+.NOEXPORT:
diff --git a/bin/plugins/filter-a.c b/bin/plugins/filter-a.c
new file mode 100644
index 0000000..8556cb7
--- /dev/null
+++ b/bin/plugins/filter-a.c
@@ -0,0 +1,879 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+/*! \file */
+
+/* aliases for the exported symbols */
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <string.h>
+
+#include <isc/buffer.h>
+#include <isc/hash.h>
+#include <isc/ht.h>
+#include <isc/log.h>
+#include <isc/mem.h>
+#include <isc/netaddr.h>
+#include <isc/result.h>
+#include <isc/types.h>
+#include <isc/util.h>
+
+#include <dns/acl.h>
+#include <dns/db.h>
+#include <dns/enumtype.h>
+#include <dns/log.h>
+#include <dns/message.h>
+#include <dns/rdataset.h>
+#include <dns/types.h>
+#include <dns/view.h>
+
+#include <isccfg/aclconf.h>
+#include <isccfg/cfg.h>
+#include <isccfg/grammar.h>
+
+#include <ns/client.h>
+#include <ns/hooks.h>
+#include <ns/log.h>
+#include <ns/query.h>
+#include <ns/types.h>
+
+#define CHECK(op)                              \
+	do {                                   \
+		result = (op);                 \
+		if (result != ISC_R_SUCCESS) { \
+			goto cleanup;          \
+		}                              \
+	} while (0)
+
+/*
+ * Possible values for the settings of filter-a-on-v6 and
+ * filter-a-on-v4: "no" is NONE, "yes" is FILTER, "break-dnssec"
+ * is BREAK_DNSSEC.
+ */
+typedef enum { NONE = 0, FILTER = 1, BREAK_DNSSEC = 2 } filter_a_t;
+
+/*
+ * Persistent data for use by this module. This will be associated
+ * with client object address in the hash table, and will remain
+ * accessible until the client object is detached.
+ */
+typedef struct filter_data {
+	filter_a_t mode;
+	uint32_t flags;
+} filter_data_t;
+
+typedef struct filter_instance {
+	ns_plugin_t *module;
+	isc_mem_t *mctx;
+
+	/*
+	 * Hash table associating a client object with its persistent data.
+	 */
+	isc_ht_t *ht;
+	isc_mutex_t hlock;
+
+	/*
+	 * Values configured when the module is loaded.
+	 */
+	filter_a_t v4_a;
+	filter_a_t v6_a;
+	dns_acl_t *a_acl;
+} filter_instance_t;
+
+/*
+ * Per-client flags set by this module
+ */
+#define FILTER_A_RECURSING 0x0001 /* Recursing for AAAA */
+#define FILTER_A_FILTERED  0x0002 /* A was removed from answer */
+
+/*
+ * Client attribute tests.
+ */
+#define WANTDNSSEC(c)  (((c)->attributes & NS_CLIENTATTR_WANTDNSSEC) != 0)
+#define RECURSIONOK(c) (((c)->query.attributes & NS_QUERYATTR_RECURSIONOK) != 0)
+
+/*
+ * Forward declarations of functions referenced in install_hooks().
+ */
+static ns_hookresult_t
+filter_qctx_initialize(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_respond_begin(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_respond_any_found(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_prep_response_begin(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_query_done_send(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_qctx_destroy(void *arg, void *cbdata, isc_result_t *resp);
+
+/*%
+ * Register the functions to be called at each hook point in 'hooktable', using
+ * memory context 'mctx' for allocating copies of stack-allocated structures
+ * passed to ns_hook_add().  Make sure 'inst' will be passed as the 'cbdata'
+ * argument to every callback.
+ */
+static void
+install_hooks(ns_hooktable_t *hooktable, isc_mem_t *mctx,
+	      filter_instance_t *inst) {
+	const ns_hook_t filter_init = {
+		.action = filter_qctx_initialize,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_respbegin = {
+		.action = filter_respond_begin,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_respanyfound = {
+		.action = filter_respond_any_found,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_prepresp = {
+		.action = filter_prep_response_begin,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_donesend = {
+		.action = filter_query_done_send,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_destroy = {
+		.action = filter_qctx_destroy,
+		.action_data = inst,
+	};
+
+	ns_hook_add(hooktable, mctx, NS_QUERY_QCTX_INITIALIZED, &filter_init);
+	ns_hook_add(hooktable, mctx, NS_QUERY_RESPOND_BEGIN, &filter_respbegin);
+	ns_hook_add(hooktable, mctx, NS_QUERY_RESPOND_ANY_FOUND,
+		    &filter_respanyfound);
+	ns_hook_add(hooktable, mctx, NS_QUERY_PREP_RESPONSE_BEGIN,
+		    &filter_prepresp);
+	ns_hook_add(hooktable, mctx, NS_QUERY_DONE_SEND, &filter_donesend);
+	ns_hook_add(hooktable, mctx, NS_QUERY_QCTX_DESTROYED, &filter_destroy);
+}
+
+/**
+** Support for parsing of parameters and configuration of the module.
+**/
+
+/*
+ * Support for parsing of parameters.
+ */
+static const char *filter_a_enums[] = { "break-dnssec", NULL };
+
+static isc_result_t
+parse_filter_a(cfg_parser_t *pctx, const cfg_type_t *type, cfg_obj_t **ret) {
+	return (cfg_parse_enum_or_other(pctx, type, &cfg_type_boolean, ret));
+}
+
+static void
+doc_filter_a(cfg_printer_t *pctx, const cfg_type_t *type) {
+	cfg_doc_enum_or_other(pctx, type, &cfg_type_boolean);
+}
+
+static cfg_type_t cfg_type_filter_a = {
+	"filter_a",   parse_filter_a,  cfg_print_ustring,
+	doc_filter_a, &cfg_rep_string, filter_a_enums,
+};
+
+static cfg_clausedef_t param_clauses[] = {
+	{ "filter-a", &cfg_type_bracketed_aml, 0 },
+	{ "filter-a-on-v6", &cfg_type_filter_a, 0 },
+	{ "filter-a-on-v4", &cfg_type_filter_a, 0 },
+};
+
+static cfg_clausedef_t *param_clausesets[] = { param_clauses, NULL };
+
+static cfg_type_t cfg_type_parameters = { "filter-a-params", cfg_parse_mapbody,
+					  cfg_print_mapbody, cfg_doc_mapbody,
+					  &cfg_rep_map,	     param_clausesets };
+
+static isc_result_t
+parse_filter_a_on(const cfg_obj_t *param_obj, const char *param_name,
+		  filter_a_t *dstp) {
+	const cfg_obj_t *obj = NULL;
+	isc_result_t result;
+
+	result = cfg_map_get(param_obj, param_name, &obj);
+	if (result != ISC_R_SUCCESS) {
+		return (ISC_R_SUCCESS);
+	}
+
+	if (cfg_obj_isboolean(obj)) {
+		if (cfg_obj_asboolean(obj)) {
+			*dstp = FILTER;
+		} else {
+			*dstp = NONE;
+		}
+	} else if (strcasecmp(cfg_obj_asstring(obj), "break-dnssec") == 0) {
+		*dstp = BREAK_DNSSEC;
+	} else {
+		result = ISC_R_UNEXPECTED;
+	}
+
+	return (result);
+}
+
+static isc_result_t
+check_syntax(cfg_obj_t *fmap, const void *cfg, isc_mem_t *mctx, isc_log_t *lctx,
+	     void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	const cfg_obj_t *aclobj = NULL;
+	dns_acl_t *acl = NULL;
+	filter_a_t f4 = NONE, f6 = NONE;
+
+	cfg_map_get(fmap, "filter-a", &aclobj);
+	if (aclobj == NULL) {
+		return (result);
+	}
+
+	CHECK(cfg_acl_fromconfig(aclobj, (const cfg_obj_t *)cfg, lctx,
+				 (cfg_aclconfctx_t *)actx, mctx, 0, &acl));
+
+	CHECK(parse_filter_a_on(fmap, "filter-a-on-v6", &f6));
+	CHECK(parse_filter_a_on(fmap, "filter-a-on-v4", &f4));
+
+	if ((f4 != NONE || f6 != NONE) && dns_acl_isnone(acl)) {
+		cfg_obj_log(aclobj, lctx, ISC_LOG_WARNING,
+			    "\"filter-a\" is 'none;' but "
+			    "either filter-a-on-v6 or filter-a-on-v4 "
+			    "is enabled");
+		result = ISC_R_FAILURE;
+	} else if (f4 == NONE && f6 == NONE && !dns_acl_isnone(acl)) {
+		cfg_obj_log(aclobj, lctx, ISC_LOG_WARNING,
+			    "\"filter-a\" is set but "
+			    "neither filter-a-on-v6 or filter-a-on-v4 "
+			    "is enabled");
+		result = ISC_R_FAILURE;
+	}
+
+cleanup:
+	if (acl != NULL) {
+		dns_acl_detach(&acl);
+	}
+
+	return (result);
+}
+
+static isc_result_t
+parse_parameters(filter_instance_t *inst, const char *parameters,
+		 const void *cfg, const char *cfg_file, unsigned long cfg_line,
+		 isc_mem_t *mctx, isc_log_t *lctx, void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	cfg_parser_t *parser = NULL;
+	cfg_obj_t *param_obj = NULL;
+	const cfg_obj_t *obj = NULL;
+	isc_buffer_t b;
+
+	CHECK(cfg_parser_create(mctx, lctx, &parser));
+
+	isc_buffer_constinit(&b, parameters, strlen(parameters));
+	isc_buffer_add(&b, strlen(parameters));
+	CHECK(cfg_parse_buffer(parser, &b, cfg_file, cfg_line,
+			       &cfg_type_parameters, 0, &param_obj));
+
+	CHECK(check_syntax(param_obj, cfg, mctx, lctx, actx));
+
+	CHECK(parse_filter_a_on(param_obj, "filter-a-on-v6", &inst->v6_a));
+	CHECK(parse_filter_a_on(param_obj, "filter-a-on-v4", &inst->v4_a));
+
+	result = cfg_map_get(param_obj, "filter-a", &obj);
+	if (result == ISC_R_SUCCESS) {
+		CHECK(cfg_acl_fromconfig(obj, (const cfg_obj_t *)cfg, lctx,
+					 (cfg_aclconfctx_t *)actx, mctx, 0,
+					 &inst->a_acl));
+	} else {
+		CHECK(dns_acl_any(mctx, &inst->a_acl));
+	}
+
+cleanup:
+	if (param_obj != NULL) {
+		cfg_obj_destroy(parser, &param_obj);
+	}
+	if (parser != NULL) {
+		cfg_parser_destroy(&parser);
+	}
+	return (result);
+}
+
+/**
+** Mandatory plugin API functions:
+**
+** - plugin_destroy
+** - plugin_register
+** - plugin_version
+** - plugin_check
+**/
+
+/*
+ * Called by ns_plugin_register() to initialize the plugin and
+ * register hook functions into the view hook table.
+ */
+isc_result_t
+plugin_register(const char *parameters, const void *cfg, const char *cfg_file,
+		unsigned long cfg_line, isc_mem_t *mctx, isc_log_t *lctx,
+		void *actx, ns_hooktable_t *hooktable, void **instp) {
+	filter_instance_t *inst = NULL;
+	isc_result_t result = ISC_R_SUCCESS;
+
+	isc_log_write(lctx, NS_LOGCATEGORY_GENERAL, NS_LOGMODULE_HOOKS,
+		      ISC_LOG_INFO,
+		      "registering 'filter-a' "
+		      "module from %s:%lu, %s parameters",
+		      cfg_file, cfg_line, parameters != NULL ? "with" : "no");
+
+	inst = isc_mem_get(mctx, sizeof(*inst));
+	memset(inst, 0, sizeof(*inst));
+	isc_mem_attach(mctx, &inst->mctx);
+
+	if (parameters != NULL) {
+		CHECK(parse_parameters(inst, parameters, cfg, cfg_file,
+				       cfg_line, mctx, lctx, actx));
+	}
+
+	isc_ht_init(&inst->ht, mctx, 16, ISC_HT_CASE_SENSITIVE);
+	isc_mutex_init(&inst->hlock);
+
+	/*
+	 * Set hook points in the view's hooktable.
+	 */
+	install_hooks(hooktable, mctx, inst);
+
+	*instp = inst;
+
+cleanup:
+	if (result != ISC_R_SUCCESS) {
+		plugin_destroy((void **)&inst);
+	}
+
+	return (result);
+}
+
+isc_result_t
+plugin_check(const char *parameters, const void *cfg, const char *cfg_file,
+	     unsigned long cfg_line, isc_mem_t *mctx, isc_log_t *lctx,
+	     void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	cfg_parser_t *parser = NULL;
+	cfg_obj_t *param_obj = NULL;
+	isc_buffer_t b;
+
+	CHECK(cfg_parser_create(mctx, lctx, &parser));
+
+	isc_buffer_constinit(&b, parameters, strlen(parameters));
+	isc_buffer_add(&b, strlen(parameters));
+	CHECK(cfg_parse_buffer(parser, &b, cfg_file, cfg_line,
+			       &cfg_type_parameters, 0, &param_obj));
+
+	CHECK(check_syntax(param_obj, cfg, mctx, lctx, actx));
+
+cleanup:
+	if (param_obj != NULL) {
+		cfg_obj_destroy(parser, &param_obj);
+	}
+	if (parser != NULL) {
+		cfg_parser_destroy(&parser);
+	}
+	return (result);
+}
+
+/*
+ * Called by ns_plugins_free(); frees memory allocated by
+ * the module when it was registered.
+ */
+void
+plugin_destroy(void **instp) {
+	filter_instance_t *inst = (filter_instance_t *)*instp;
+
+	if (inst->ht != NULL) {
+		isc_ht_destroy(&inst->ht);
+		isc_mutex_destroy(&inst->hlock);
+	}
+	if (inst->a_acl != NULL) {
+		dns_acl_detach(&inst->a_acl);
+	}
+
+	isc_mem_putanddetach(&inst->mctx, inst, sizeof(*inst));
+	*instp = NULL;
+
+	return;
+}
+
+/*
+ * Returns plugin API version for compatibility checks.
+ */
+int
+plugin_version(void) {
+	return (NS_PLUGIN_VERSION);
+}
+
+/**
+** "filter-a" feature implementation begins here.
+**/
+
+/*%
+ * Structure describing the filtering to be applied by process_section().
+ */
+typedef struct section_filter {
+	query_ctx_t *qctx;
+	filter_a_t mode;
+	dns_section_t section;
+	const dns_name_t *name;
+	dns_rdatatype_t type;
+	bool only_if_aaaa_exists;
+} section_filter_t;
+
+/*
+ * Check whether this is an IPv4 client.
+ */
+static bool
+is_v4_client(ns_client_t *client) {
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET) {
+		return (true);
+	}
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET6 &&
+	    IN6_IS_ADDR_V4MAPPED(&client->peeraddr.type.sin6.sin6_addr))
+	{
+		return (true);
+	}
+	return (false);
+}
+
+/*
+ * Check whether this is an IPv6 client.
+ */
+static bool
+is_v6_client(ns_client_t *client) {
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET6 &&
+	    !IN6_IS_ADDR_V4MAPPED(&client->peeraddr.type.sin6.sin6_addr))
+	{
+		return (true);
+	}
+	return (false);
+}
+
+static filter_data_t *
+client_state_get(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state = NULL;
+	isc_result_t result;
+
+	LOCK(&inst->hlock);
+	result = isc_ht_find(inst->ht, (const unsigned char *)&qctx->client,
+			     sizeof(qctx->client), (void **)&client_state);
+	UNLOCK(&inst->hlock);
+
+	return (result == ISC_R_SUCCESS ? client_state : NULL);
+}
+
+static void
+client_state_create(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state;
+	isc_result_t result;
+
+	client_state = isc_mem_get(inst->mctx, sizeof(*client_state));
+
+	client_state->mode = NONE;
+	client_state->flags = 0;
+
+	LOCK(&inst->hlock);
+	result = isc_ht_add(inst->ht, (const unsigned char *)&qctx->client,
+			    sizeof(qctx->client), client_state);
+	UNLOCK(&inst->hlock);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+}
+
+static void
+client_state_destroy(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result;
+
+	if (client_state == NULL) {
+		return;
+	}
+
+	LOCK(&inst->hlock);
+	result = isc_ht_delete(inst->ht, (const unsigned char *)&qctx->client,
+			       sizeof(qctx->client));
+	UNLOCK(&inst->hlock);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+
+	isc_mem_put(inst->mctx, client_state, sizeof(*client_state));
+}
+
+/*%
+ * Mark 'rdataset' and 'sigrdataset' as rendered, gracefully handling NULL
+ * pointers and non-associated rdatasets.
+ */
+static void
+mark_as_rendered(dns_rdataset_t *rdataset, dns_rdataset_t *sigrdataset) {
+	if (rdataset != NULL && dns_rdataset_isassociated(rdataset)) {
+		rdataset->attributes |= DNS_RDATASETATTR_RENDERED;
+	}
+	if (sigrdataset != NULL && dns_rdataset_isassociated(sigrdataset)) {
+		sigrdataset->attributes |= DNS_RDATASETATTR_RENDERED;
+	}
+}
+
+/*%
+ * Check whether an RRset of given 'type' is present at given 'name'.  If
+ * it is found and either it is not signed or the combination of query
+ * flags and configured processing 'mode' allows it, mark the RRset and its
+ * associated signatures as already rendered to prevent them from appearing
+ * in the response message stored in 'qctx'.  If 'only_if_aaaa_exists' is
+ * true, an RRset of type AAAA must also exist at 'name' in order for the
+ * above processing to happen.
+ */
+static bool
+process_name(query_ctx_t *qctx, filter_a_t mode, const dns_name_t *name,
+	     dns_rdatatype_t type, bool only_if_aaaa_exists) {
+	dns_rdataset_t *rdataset = NULL, *sigrdataset = NULL;
+	isc_result_t result;
+	bool modified = false;
+
+	if (only_if_aaaa_exists) {
+		CHECK(dns_message_findtype(name, dns_rdatatype_aaaa, 0, NULL));
+	}
+
+	(void)dns_message_findtype(name, type, 0, &rdataset);
+	(void)dns_message_findtype(name, dns_rdatatype_rrsig, type,
+				   &sigrdataset);
+
+	if (rdataset != NULL &&
+	    (sigrdataset == NULL || !WANTDNSSEC(qctx->client) ||
+	     mode == BREAK_DNSSEC))
+	{
+		/*
+		 * An RRset of given 'type' was found at 'name' and at least
+		 * one of the following is true:
+		 *
+		 *   - the RRset is not signed,
+		 *   - the client did not set the DO bit in its request,
+		 *   - configuration allows us to tamper with signed responses.
+		 *
+		 * This means it is okay to filter out this RRset and its
+		 * signatures, if any, from the response.
+		 */
+		mark_as_rendered(rdataset, sigrdataset);
+		modified = true;
+	}
+
+cleanup:
+	return (modified);
+}
+
+/*%
+ * Apply the requested section filter, i.e. prevent (when possible, as
+ * determined by process_name()) RRsets of given 'type' from being rendered
+ * in the given 'section' of the response message stored in 'qctx'.  Clear
+ * the AD bit if the answer and/or authority section was modified.  If
+ * 'name' is NULL, all names in the given 'section' are processed;
+ * otherwise, only 'name' is.  'only_if_aaaa_exists' is passed through to
+ * process_name().
+ */
+static void
+process_section(const section_filter_t *filter) {
+	query_ctx_t *qctx = filter->qctx;
+	filter_a_t mode = filter->mode;
+	dns_section_t section = filter->section;
+	const dns_name_t *name = filter->name;
+	dns_rdatatype_t type = filter->type;
+	bool only_if_aaaa_exists = filter->only_if_aaaa_exists;
+
+	dns_message_t *message = qctx->client->message;
+	isc_result_t result;
+
+	for (result = dns_message_firstname(message, section);
+	     result == ISC_R_SUCCESS;
+	     result = dns_message_nextname(message, section))
+	{
+		dns_name_t *cur = NULL;
+		dns_message_currentname(message, section, &cur);
+		if (name != NULL && !dns_name_equal(name, cur)) {
+			/*
+			 * We only want to process 'name' and this is not it.
+			 */
+			continue;
+		}
+
+		if (!process_name(qctx, mode, cur, type, only_if_aaaa_exists)) {
+			/*
+			 * Response was not modified, do not touch the AD bit.
+			 */
+			continue;
+		}
+
+		if (section == DNS_SECTION_ANSWER ||
+		    section == DNS_SECTION_AUTHORITY)
+		{
+			message->flags &= ~DNS_MESSAGEFLAG_AD;
+		}
+	}
+}
+
+/*
+ * Initialize filter state, fetching it from a memory pool and storing it
+ * in a hash table keyed according to the client object; this enables us to
+ * retrieve persistent data related to a client query for as long as the
+ * object persists.
+ */
+static ns_hookresult_t
+filter_qctx_initialize(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state;
+
+	*resp = ISC_R_UNSET;
+
+	client_state = client_state_get(qctx, inst);
+	if (client_state == NULL) {
+		client_state_create(qctx, inst);
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Determine whether this client should have A filtered or not, based on
+ * the client address family and the settings of filter-a-on-v6 and
+ * filter-a-on-v4.
+ */
+static ns_hookresult_t
+filter_prep_response_begin(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result;
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state == NULL) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (inst->v4_a != NONE || inst->v6_a != NONE) {
+		result = ns_client_checkaclsilent(qctx->client, NULL,
+						  inst->a_acl, true);
+		if (result == ISC_R_SUCCESS && inst->v4_a != NONE &&
+		    is_v4_client(qctx->client))
+		{
+			client_state->mode = inst->v4_a;
+		} else if (result == ISC_R_SUCCESS && inst->v6_a != NONE &&
+			   is_v6_client(qctx->client))
+		{
+			client_state->mode = inst->v6_a;
+		}
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Hide A rrsets if there is a matching AAAA. Trigger recursion if
+ * necessary to find out whether an AAAA exists.
+ *
+ * (This version is for processing answers to explicit A queries; ANY
+ * queries are handled in filter_respond_any_found().)
+ */
+static ns_hookresult_t
+filter_respond_begin(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result = ISC_R_UNSET;
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state == NULL) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (client_state->mode != BREAK_DNSSEC &&
+	    (client_state->mode != FILTER ||
+	     (WANTDNSSEC(qctx->client) && qctx->sigrdataset != NULL &&
+	      dns_rdataset_isassociated(qctx->sigrdataset))))
+	{
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (qctx->qtype == dns_rdatatype_a) {
+		dns_rdataset_t *trdataset;
+		trdataset = ns_client_newrdataset(qctx->client);
+		result = dns_db_findrdataset(
+			qctx->db, qctx->node, qctx->version, dns_rdatatype_aaaa,
+			0, qctx->client->now, trdataset, NULL);
+		if (dns_rdataset_isassociated(trdataset)) {
+			dns_rdataset_disassociate(trdataset);
+		}
+		ns_client_putrdataset(qctx->client, &trdataset);
+
+		/*
+		 * We found an A. If we also found an AAAA, then the A
+		 * must not be rendered.
+		 *
+		 * If the AAAA is not in our cache, then any result other than
+		 * DNS_R_DELEGATION or ISC_R_NOTFOUND means there is no AAAAA,
+		 * and so AAAAs are okay.
+		 *
+		 * We assume there is no AAAA if we can't recurse for this
+		 * client. That might be the wrong answer, but what else
+		 * can we do?  Besides, the fact that we have the A and
+		 * are using this mechanism in the first place suggests
+		 * that we care more about AAAAs than As, and would have
+		 * cached an AAAA if it existed.
+		 */
+		if (result == ISC_R_SUCCESS) {
+			mark_as_rendered(qctx->rdataset, qctx->sigrdataset);
+			qctx->client->message->flags &= ~DNS_MESSAGEFLAG_AD;
+			client_state->flags |= FILTER_A_FILTERED;
+		} else if (!qctx->authoritative && RECURSIONOK(qctx->client) &&
+			   (result == DNS_R_DELEGATION ||
+			    result == ISC_R_NOTFOUND))
+		{
+			/*
+			 * This is an ugly kludge to recurse
+			 * for the AAAA and discard the result.???
+			 *
+			 * Continue to add the A now.
+			 * We'll make a note to not render it
+			 * if the recursion for the AAAA succeeds.
+			 */
+			result = ns_query_recurse(qctx->client,
+						  dns_rdatatype_aaaa,
+						  qctx->client->query.qname,
+						  NULL, NULL, qctx->resuming);
+			if (result == ISC_R_SUCCESS) {
+				client_state->flags |= FILTER_A_RECURSING;
+				qctx->client->query.attributes |=
+					NS_QUERYATTR_RECURSING;
+			}
+		}
+	} else if (qctx->qtype == dns_rdatatype_aaaa &&
+		   (client_state->flags & FILTER_A_RECURSING) != 0)
+	{
+		const section_filter_t filter_answer = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ANSWER,
+			.name = qctx->fname,
+			.type = dns_rdatatype_a,
+		};
+		process_section(&filter_answer);
+
+		client_state->flags &= ~FILTER_A_RECURSING;
+
+		result = ns_query_done(qctx);
+
+		*resp = result;
+
+		return (NS_HOOK_RETURN);
+	}
+
+	*resp = result;
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * When answering an ANY query, remove A if AAAA is present.
+ */
+static ns_hookresult_t
+filter_respond_any_found(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state != NULL && client_state->mode != NONE) {
+		/*
+		 * If we are authoritative, require an AAAA record to be
+		 * present before filtering out A records; otherwise,
+		 * just assume an AAAA record exists even if it was not in the
+		 * cache (and therefore is not in the response message),
+		 * thus proceeding with filtering out A records.
+		 */
+		const section_filter_t filter_answer = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ANSWER,
+			.name = qctx->tname,
+			.type = dns_rdatatype_a,
+			.only_if_aaaa_exists = qctx->authoritative,
+		};
+		process_section(&filter_answer);
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Hide A rrsets in the additional section if there is a matching AAAA, and
+ * hide NS in the authority section if A was filtered in the answer
+ * section.
+ */
+static ns_hookresult_t
+filter_query_done_send(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state != NULL && client_state->mode != NONE) {
+		const section_filter_t filter_additional = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ADDITIONAL,
+			.type = dns_rdatatype_a,
+			.only_if_aaaa_exists = true,
+		};
+		process_section(&filter_additional);
+
+		if ((client_state->flags & FILTER_A_FILTERED) != 0) {
+			const section_filter_t filter_authority = {
+				.qctx = qctx,
+				.mode = client_state->mode,
+				.section = DNS_SECTION_AUTHORITY,
+				.type = dns_rdatatype_ns,
+			};
+			process_section(&filter_authority);
+		}
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * If the client is being detached, then we can delete our persistent data
+ * from hash table and return it to the memory pool.
+ */
+static ns_hookresult_t
+filter_qctx_destroy(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+
+	*resp = ISC_R_UNSET;
+
+	if (!qctx->detach_client) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	client_state_destroy(qctx, inst);
+
+	return (NS_HOOK_CONTINUE);
+}
diff --git a/bin/plugins/filter-a.rst b/bin/plugins/filter-a.rst
new file mode 100644
index 0000000..16b3dee
--- /dev/null
+++ b/bin/plugins/filter-a.rst
@@ -0,0 +1,86 @@
+.. Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+..
+.. SPDX-License-Identifier: MPL-2.0
+..
+.. This Source Code Form is subject to the terms of the Mozilla Public
+.. License, v. 2.0.  If a copy of the MPL was not distributed with this
+.. file, you can obtain one at https://mozilla.org/MPL/2.0/.
+..
+.. See the COPYRIGHT file distributed with this work for additional
+.. information regarding copyright ownership.
+
+.. highlight: console
+
+.. iscman:: filter-a
+.. _man_filter-a:
+
+filter-a.so - filter A in DNS responses when AAAA is present
+---------------------------------------------------------------
+
+Synopsis
+~~~~~~~~
+
+:program:`plugin query` "filter-a.so" [{ parameters }];
+
+Description
+~~~~~~~~~~~
+
+:program:`filter-a.so` is a query plugin module for :iscman:`named`, enabling
+:iscman:`named` to omit some IPv4 addresses when responding to clients.
+
+For example:
+
+::
+
+   plugin query "filter-a.so" {
+           filter-a-on-v6 yes;
+           filter-a-on-v4 yes;
+           filter-a { 192.0.2.1; 2001:db8:2::1; };
+   };
+
+This module is intended to aid transition from IPv4 to IPv6 by
+withholding IPv4 addresses from DNS clients which are not connected to
+the IPv4 Internet, when the name being looked up has an IPv6 address
+available. Use of this module is not recommended unless absolutely
+necessary.
+
+Note: This mechanism can erroneously cause other servers not to give
+A records to their clients. If a recursing server with both IPv6 and
+IPv4 network connections queries an authoritative server using this
+mechanism via IPv6, it is denied A records even if its client is
+using IPv4.
+
+Options
+~~~~~~~
+
+``filter-a``
+   This option specifies a list of client addresses for which A filtering is to
+   be applied. The default is ``any``.
+
+``filter-a-on-v6``
+   If set to ``yes``, this option indicates that the DNS client is at an IPv6 address, in
+   ``filter-a``. If the response does not include DNSSEC
+   signatures, then all A records are deleted from the response. This
+   filtering applies to all responses, not only authoritative
+   ones.
+
+   If set to ``break-dnssec``, then A records are deleted even when
+   DNSSEC is enabled. As suggested by the name, this causes the response
+   to fail to verify, because the DNSSEC protocol is designed to detect
+   deletions.
+
+   This mechanism can erroneously cause other servers not to give A
+   records to their clients. If a recursing server with both IPv6 and IPv4
+   network connections queries an authoritative server using this
+   mechanism via IPv6, it is denied A records even if its client is
+   using IPv4.
+
+``filter-a-on-v4``
+   This option is identical to ``filter-a-on-v6``, except that it filters A responses
+   to queries from IPv4 clients instead of IPv6 clients. To filter all
+   responses, set both options to ``yes``.
+
+See Also
+~~~~~~~~
+
+BIND 9 Administrator Reference Manual.
diff --git a/bin/plugins/filter-aaaa.c b/bin/plugins/filter-aaaa.c
new file mode 100644
index 0000000..1ec1fd8
--- /dev/null
+++ b/bin/plugins/filter-aaaa.c
@@ -0,0 +1,881 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+/*! \file */
+
+/* aliases for the exported symbols */
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <string.h>
+
+#include <isc/buffer.h>
+#include <isc/hash.h>
+#include <isc/ht.h>
+#include <isc/log.h>
+#include <isc/mem.h>
+#include <isc/netaddr.h>
+#include <isc/result.h>
+#include <isc/types.h>
+#include <isc/util.h>
+
+#include <dns/acl.h>
+#include <dns/db.h>
+#include <dns/enumtype.h>
+#include <dns/log.h>
+#include <dns/message.h>
+#include <dns/rdataset.h>
+#include <dns/types.h>
+#include <dns/view.h>
+
+#include <isccfg/aclconf.h>
+#include <isccfg/cfg.h>
+#include <isccfg/grammar.h>
+
+#include <ns/client.h>
+#include <ns/hooks.h>
+#include <ns/log.h>
+#include <ns/query.h>
+#include <ns/types.h>
+
+#define CHECK(op)                              \
+	do {                                   \
+		result = (op);                 \
+		if (result != ISC_R_SUCCESS) { \
+			goto cleanup;          \
+		}                              \
+	} while (0)
+
+/*
+ * Possible values for the settings of filter-aaaa-on-v4 and
+ * filter-aaaa-on-v6: "no" is NONE, "yes" is FILTER, "break-dnssec"
+ * is BREAK_DNSSEC.
+ */
+typedef enum { NONE = 0, FILTER = 1, BREAK_DNSSEC = 2 } filter_aaaa_t;
+
+/*
+ * Persistent data for use by this module. This will be associated
+ * with client object address in the hash table, and will remain
+ * accessible until the client object is detached.
+ */
+typedef struct filter_data {
+	filter_aaaa_t mode;
+	uint32_t flags;
+} filter_data_t;
+
+typedef struct filter_instance {
+	ns_plugin_t *module;
+	isc_mem_t *mctx;
+
+	/*
+	 * Hash table associating a client object with its persistent data.
+	 */
+	isc_ht_t *ht;
+	isc_mutex_t hlock;
+
+	/*
+	 * Values configured when the module is loaded.
+	 */
+	filter_aaaa_t v4_aaaa;
+	filter_aaaa_t v6_aaaa;
+	dns_acl_t *aaaa_acl;
+} filter_instance_t;
+
+/*
+ * Per-client flags set by this module
+ */
+#define FILTER_AAAA_RECURSING 0x0001 /* Recursing for A */
+#define FILTER_AAAA_FILTERED  0x0002 /* AAAA was removed from answer */
+
+/*
+ * Client attribute tests.
+ */
+#define WANTDNSSEC(c)  (((c)->attributes & NS_CLIENTATTR_WANTDNSSEC) != 0)
+#define RECURSIONOK(c) (((c)->query.attributes & NS_QUERYATTR_RECURSIONOK) != 0)
+
+/*
+ * Forward declarations of functions referenced in install_hooks().
+ */
+static ns_hookresult_t
+filter_qctx_initialize(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_respond_begin(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_respond_any_found(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_prep_response_begin(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_query_done_send(void *arg, void *cbdata, isc_result_t *resp);
+static ns_hookresult_t
+filter_qctx_destroy(void *arg, void *cbdata, isc_result_t *resp);
+
+/*%
+ * Register the functions to be called at each hook point in 'hooktable', using
+ * memory context 'mctx' for allocating copies of stack-allocated structures
+ * passed to ns_hook_add().  Make sure 'inst' will be passed as the 'cbdata'
+ * argument to every callback.
+ */
+static void
+install_hooks(ns_hooktable_t *hooktable, isc_mem_t *mctx,
+	      filter_instance_t *inst) {
+	const ns_hook_t filter_init = {
+		.action = filter_qctx_initialize,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_respbegin = {
+		.action = filter_respond_begin,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_respanyfound = {
+		.action = filter_respond_any_found,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_prepresp = {
+		.action = filter_prep_response_begin,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_donesend = {
+		.action = filter_query_done_send,
+		.action_data = inst,
+	};
+
+	const ns_hook_t filter_destroy = {
+		.action = filter_qctx_destroy,
+		.action_data = inst,
+	};
+
+	ns_hook_add(hooktable, mctx, NS_QUERY_QCTX_INITIALIZED, &filter_init);
+	ns_hook_add(hooktable, mctx, NS_QUERY_RESPOND_BEGIN, &filter_respbegin);
+	ns_hook_add(hooktable, mctx, NS_QUERY_RESPOND_ANY_FOUND,
+		    &filter_respanyfound);
+	ns_hook_add(hooktable, mctx, NS_QUERY_PREP_RESPONSE_BEGIN,
+		    &filter_prepresp);
+	ns_hook_add(hooktable, mctx, NS_QUERY_DONE_SEND, &filter_donesend);
+	ns_hook_add(hooktable, mctx, NS_QUERY_QCTX_DESTROYED, &filter_destroy);
+}
+
+/**
+** Support for parsing of parameters and configuration of the module.
+**/
+
+/*
+ * Support for parsing of parameters.
+ */
+static const char *filter_aaaa_enums[] = { "break-dnssec", NULL };
+
+static isc_result_t
+parse_filter_aaaa(cfg_parser_t *pctx, const cfg_type_t *type, cfg_obj_t **ret) {
+	return (cfg_parse_enum_or_other(pctx, type, &cfg_type_boolean, ret));
+}
+
+static void
+doc_filter_aaaa(cfg_printer_t *pctx, const cfg_type_t *type) {
+	cfg_doc_enum_or_other(pctx, type, &cfg_type_boolean);
+}
+
+static cfg_type_t cfg_type_filter_aaaa = {
+	"filter_aaaa",	 parse_filter_aaaa, cfg_print_ustring,
+	doc_filter_aaaa, &cfg_rep_string,   filter_aaaa_enums,
+};
+
+static cfg_clausedef_t param_clauses[] = {
+	{ "filter-aaaa", &cfg_type_bracketed_aml, 0 },
+	{ "filter-aaaa-on-v4", &cfg_type_filter_aaaa, 0 },
+	{ "filter-aaaa-on-v6", &cfg_type_filter_aaaa, 0 },
+};
+
+static cfg_clausedef_t *param_clausesets[] = { param_clauses, NULL };
+
+static cfg_type_t cfg_type_parameters = {
+	"filter-aaaa-params", cfg_parse_mapbody, cfg_print_mapbody,
+	cfg_doc_mapbody,      &cfg_rep_map,	 param_clausesets
+};
+
+static isc_result_t
+parse_filter_aaaa_on(const cfg_obj_t *param_obj, const char *param_name,
+		     filter_aaaa_t *dstp) {
+	const cfg_obj_t *obj = NULL;
+	isc_result_t result;
+
+	result = cfg_map_get(param_obj, param_name, &obj);
+	if (result != ISC_R_SUCCESS) {
+		return (ISC_R_SUCCESS);
+	}
+
+	if (cfg_obj_isboolean(obj)) {
+		if (cfg_obj_asboolean(obj)) {
+			*dstp = FILTER;
+		} else {
+			*dstp = NONE;
+		}
+	} else if (strcasecmp(cfg_obj_asstring(obj), "break-dnssec") == 0) {
+		*dstp = BREAK_DNSSEC;
+	} else {
+		result = ISC_R_UNEXPECTED;
+	}
+
+	return (result);
+}
+
+static isc_result_t
+check_syntax(cfg_obj_t *fmap, const void *cfg, isc_mem_t *mctx, isc_log_t *lctx,
+	     void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	const cfg_obj_t *aclobj = NULL;
+	dns_acl_t *acl = NULL;
+	filter_aaaa_t f4 = NONE, f6 = NONE;
+
+	cfg_map_get(fmap, "filter-aaaa", &aclobj);
+	if (aclobj == NULL) {
+		return (result);
+	}
+
+	CHECK(cfg_acl_fromconfig(aclobj, (const cfg_obj_t *)cfg, lctx,
+				 (cfg_aclconfctx_t *)actx, mctx, 0, &acl));
+
+	CHECK(parse_filter_aaaa_on(fmap, "filter-aaaa-on-v4", &f4));
+	CHECK(parse_filter_aaaa_on(fmap, "filter-aaaa-on-v6", &f6));
+
+	if ((f4 != NONE || f6 != NONE) && dns_acl_isnone(acl)) {
+		cfg_obj_log(aclobj, lctx, ISC_LOG_WARNING,
+			    "\"filter-aaaa\" is 'none;' but "
+			    "either filter-aaaa-on-v4 or filter-aaaa-on-v6 "
+			    "is enabled");
+		result = ISC_R_FAILURE;
+	} else if (f4 == NONE && f6 == NONE && !dns_acl_isnone(acl)) {
+		cfg_obj_log(aclobj, lctx, ISC_LOG_WARNING,
+			    "\"filter-aaaa\" is set but "
+			    "neither filter-aaaa-on-v4 or filter-aaaa-on-v6 "
+			    "is enabled");
+		result = ISC_R_FAILURE;
+	}
+
+cleanup:
+	if (acl != NULL) {
+		dns_acl_detach(&acl);
+	}
+
+	return (result);
+}
+
+static isc_result_t
+parse_parameters(filter_instance_t *inst, const char *parameters,
+		 const void *cfg, const char *cfg_file, unsigned long cfg_line,
+		 isc_mem_t *mctx, isc_log_t *lctx, void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	cfg_parser_t *parser = NULL;
+	cfg_obj_t *param_obj = NULL;
+	const cfg_obj_t *obj = NULL;
+	isc_buffer_t b;
+
+	CHECK(cfg_parser_create(mctx, lctx, &parser));
+
+	isc_buffer_constinit(&b, parameters, strlen(parameters));
+	isc_buffer_add(&b, strlen(parameters));
+	CHECK(cfg_parse_buffer(parser, &b, cfg_file, cfg_line,
+			       &cfg_type_parameters, 0, &param_obj));
+
+	CHECK(check_syntax(param_obj, cfg, mctx, lctx, actx));
+
+	CHECK(parse_filter_aaaa_on(param_obj, "filter-aaaa-on-v4",
+				   &inst->v4_aaaa));
+	CHECK(parse_filter_aaaa_on(param_obj, "filter-aaaa-on-v6",
+				   &inst->v6_aaaa));
+
+	result = cfg_map_get(param_obj, "filter-aaaa", &obj);
+	if (result == ISC_R_SUCCESS) {
+		CHECK(cfg_acl_fromconfig(obj, (const cfg_obj_t *)cfg, lctx,
+					 (cfg_aclconfctx_t *)actx, mctx, 0,
+					 &inst->aaaa_acl));
+	} else {
+		CHECK(dns_acl_any(mctx, &inst->aaaa_acl));
+	}
+
+cleanup:
+	if (param_obj != NULL) {
+		cfg_obj_destroy(parser, &param_obj);
+	}
+	if (parser != NULL) {
+		cfg_parser_destroy(&parser);
+	}
+	return (result);
+}
+
+/**
+** Mandatory plugin API functions:
+**
+** - plugin_destroy
+** - plugin_register
+** - plugin_version
+** - plugin_check
+**/
+
+/*
+ * Called by ns_plugin_register() to initialize the plugin and
+ * register hook functions into the view hook table.
+ */
+isc_result_t
+plugin_register(const char *parameters, const void *cfg, const char *cfg_file,
+		unsigned long cfg_line, isc_mem_t *mctx, isc_log_t *lctx,
+		void *actx, ns_hooktable_t *hooktable, void **instp) {
+	filter_instance_t *inst = NULL;
+	isc_result_t result = ISC_R_SUCCESS;
+
+	isc_log_write(lctx, NS_LOGCATEGORY_GENERAL, NS_LOGMODULE_HOOKS,
+		      ISC_LOG_INFO,
+		      "registering 'filter-aaaa' "
+		      "module from %s:%lu, %s parameters",
+		      cfg_file, cfg_line, parameters != NULL ? "with" : "no");
+
+	inst = isc_mem_get(mctx, sizeof(*inst));
+	memset(inst, 0, sizeof(*inst));
+	isc_mem_attach(mctx, &inst->mctx);
+
+	if (parameters != NULL) {
+		CHECK(parse_parameters(inst, parameters, cfg, cfg_file,
+				       cfg_line, mctx, lctx, actx));
+	}
+
+	isc_ht_init(&inst->ht, mctx, 16, ISC_HT_CASE_SENSITIVE);
+	isc_mutex_init(&inst->hlock);
+
+	/*
+	 * Set hook points in the view's hooktable.
+	 */
+	install_hooks(hooktable, mctx, inst);
+
+	*instp = inst;
+
+cleanup:
+	if (result != ISC_R_SUCCESS) {
+		plugin_destroy((void **)&inst);
+	}
+
+	return (result);
+}
+
+isc_result_t
+plugin_check(const char *parameters, const void *cfg, const char *cfg_file,
+	     unsigned long cfg_line, isc_mem_t *mctx, isc_log_t *lctx,
+	     void *actx) {
+	isc_result_t result = ISC_R_SUCCESS;
+	cfg_parser_t *parser = NULL;
+	cfg_obj_t *param_obj = NULL;
+	isc_buffer_t b;
+
+	CHECK(cfg_parser_create(mctx, lctx, &parser));
+
+	isc_buffer_constinit(&b, parameters, strlen(parameters));
+	isc_buffer_add(&b, strlen(parameters));
+	CHECK(cfg_parse_buffer(parser, &b, cfg_file, cfg_line,
+			       &cfg_type_parameters, 0, &param_obj));
+
+	CHECK(check_syntax(param_obj, cfg, mctx, lctx, actx));
+
+cleanup:
+	if (param_obj != NULL) {
+		cfg_obj_destroy(parser, &param_obj);
+	}
+	if (parser != NULL) {
+		cfg_parser_destroy(&parser);
+	}
+	return (result);
+}
+
+/*
+ * Called by ns_plugins_free(); frees memory allocated by
+ * the module when it was registered.
+ */
+void
+plugin_destroy(void **instp) {
+	filter_instance_t *inst = (filter_instance_t *)*instp;
+
+	if (inst->ht != NULL) {
+		isc_ht_destroy(&inst->ht);
+		isc_mutex_destroy(&inst->hlock);
+	}
+	if (inst->aaaa_acl != NULL) {
+		dns_acl_detach(&inst->aaaa_acl);
+	}
+
+	isc_mem_putanddetach(&inst->mctx, inst, sizeof(*inst));
+	*instp = NULL;
+
+	return;
+}
+
+/*
+ * Returns plugin API version for compatibility checks.
+ */
+int
+plugin_version(void) {
+	return (NS_PLUGIN_VERSION);
+}
+
+/**
+** "filter-aaaa" feature implementation begins here.
+**/
+
+/*%
+ * Structure describing the filtering to be applied by process_section().
+ */
+typedef struct section_filter {
+	query_ctx_t *qctx;
+	filter_aaaa_t mode;
+	dns_section_t section;
+	const dns_name_t *name;
+	dns_rdatatype_t type;
+	bool only_if_a_exists;
+} section_filter_t;
+
+/*
+ * Check whether this is an IPv4 client.
+ */
+static bool
+is_v4_client(ns_client_t *client) {
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET) {
+		return (true);
+	}
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET6 &&
+	    IN6_IS_ADDR_V4MAPPED(&client->peeraddr.type.sin6.sin6_addr))
+	{
+		return (true);
+	}
+	return (false);
+}
+
+/*
+ * Check whether this is an IPv6 client.
+ */
+static bool
+is_v6_client(ns_client_t *client) {
+	if (isc_sockaddr_pf(&client->peeraddr) == AF_INET6 &&
+	    !IN6_IS_ADDR_V4MAPPED(&client->peeraddr.type.sin6.sin6_addr))
+	{
+		return (true);
+	}
+	return (false);
+}
+
+static filter_data_t *
+client_state_get(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state = NULL;
+	isc_result_t result;
+
+	LOCK(&inst->hlock);
+	result = isc_ht_find(inst->ht, (const unsigned char *)&qctx->client,
+			     sizeof(qctx->client), (void **)&client_state);
+	UNLOCK(&inst->hlock);
+
+	return (result == ISC_R_SUCCESS ? client_state : NULL);
+}
+
+static void
+client_state_create(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state;
+	isc_result_t result;
+
+	client_state = isc_mem_get(inst->mctx, sizeof(*client_state));
+
+	client_state->mode = NONE;
+	client_state->flags = 0;
+
+	LOCK(&inst->hlock);
+	result = isc_ht_add(inst->ht, (const unsigned char *)&qctx->client,
+			    sizeof(qctx->client), client_state);
+	UNLOCK(&inst->hlock);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+}
+
+static void
+client_state_destroy(const query_ctx_t *qctx, filter_instance_t *inst) {
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result;
+
+	if (client_state == NULL) {
+		return;
+	}
+
+	LOCK(&inst->hlock);
+	result = isc_ht_delete(inst->ht, (const unsigned char *)&qctx->client,
+			       sizeof(qctx->client));
+	UNLOCK(&inst->hlock);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+
+	isc_mem_put(inst->mctx, client_state, sizeof(*client_state));
+}
+
+/*%
+ * Mark 'rdataset' and 'sigrdataset' as rendered, gracefully handling NULL
+ * pointers and non-associated rdatasets.
+ */
+static void
+mark_as_rendered(dns_rdataset_t *rdataset, dns_rdataset_t *sigrdataset) {
+	if (rdataset != NULL && dns_rdataset_isassociated(rdataset)) {
+		rdataset->attributes |= DNS_RDATASETATTR_RENDERED;
+	}
+	if (sigrdataset != NULL && dns_rdataset_isassociated(sigrdataset)) {
+		sigrdataset->attributes |= DNS_RDATASETATTR_RENDERED;
+	}
+}
+
+/*%
+ * Check whether an RRset of given 'type' is present at given 'name'.  If
+ * it is found and either it is not signed or the combination of query
+ * flags and configured processing 'mode' allows it, mark the RRset and its
+ * associated signatures as already rendered to prevent them from appearing
+ * in the response message stored in 'qctx'.  If 'only_if_a_exists' is
+ * true, an RRset of type A must also exist at 'name' in order for the
+ * above processing to happen.
+ */
+static bool
+process_name(query_ctx_t *qctx, filter_aaaa_t mode, const dns_name_t *name,
+	     dns_rdatatype_t type, bool only_if_a_exists) {
+	dns_rdataset_t *rdataset = NULL, *sigrdataset = NULL;
+	isc_result_t result;
+	bool modified = false;
+
+	if (only_if_a_exists) {
+		CHECK(dns_message_findtype(name, dns_rdatatype_a, 0, NULL));
+	}
+
+	(void)dns_message_findtype(name, type, 0, &rdataset);
+	(void)dns_message_findtype(name, dns_rdatatype_rrsig, type,
+				   &sigrdataset);
+
+	if (rdataset != NULL &&
+	    (sigrdataset == NULL || !WANTDNSSEC(qctx->client) ||
+	     mode == BREAK_DNSSEC))
+	{
+		/*
+		 * An RRset of given 'type' was found at 'name' and at least
+		 * one of the following is true:
+		 *
+		 *   - the RRset is not signed,
+		 *   - the client did not set the DO bit in its request,
+		 *   - configuration allows us to tamper with signed responses.
+		 *
+		 * This means it is okay to filter out this RRset and its
+		 * signatures, if any, from the response.
+		 */
+		mark_as_rendered(rdataset, sigrdataset);
+		modified = true;
+	}
+
+cleanup:
+	return (modified);
+}
+
+/*%
+ * Apply the requested section filter, i.e. prevent (when possible, as
+ * determined by process_name()) RRsets of given 'type' from being rendered
+ * in the given 'section' of the response message stored in 'qctx'.  Clear
+ * the AD bit if the answer and/or authority section was modified.  If
+ * 'name' is NULL, all names in the given 'section' are processed;
+ * otherwise, only 'name' is.  'only_if_a_exists' is passed through to
+ * process_name().
+ */
+static void
+process_section(const section_filter_t *filter) {
+	query_ctx_t *qctx = filter->qctx;
+	filter_aaaa_t mode = filter->mode;
+	dns_section_t section = filter->section;
+	const dns_name_t *name = filter->name;
+	dns_rdatatype_t type = filter->type;
+	bool only_if_a_exists = filter->only_if_a_exists;
+
+	dns_message_t *message = qctx->client->message;
+	isc_result_t result;
+
+	for (result = dns_message_firstname(message, section);
+	     result == ISC_R_SUCCESS;
+	     result = dns_message_nextname(message, section))
+	{
+		dns_name_t *cur = NULL;
+		dns_message_currentname(message, section, &cur);
+		if (name != NULL && !dns_name_equal(name, cur)) {
+			/*
+			 * We only want to process 'name' and this is not it.
+			 */
+			continue;
+		}
+
+		if (!process_name(qctx, mode, cur, type, only_if_a_exists)) {
+			/*
+			 * Response was not modified, do not touch the AD bit.
+			 */
+			continue;
+		}
+
+		if (section == DNS_SECTION_ANSWER ||
+		    section == DNS_SECTION_AUTHORITY)
+		{
+			message->flags &= ~DNS_MESSAGEFLAG_AD;
+		}
+	}
+}
+
+/*
+ * Initialize filter state, fetching it from a memory pool and storing it
+ * in a hash table keyed according to the client object; this enables us to
+ * retrieve persistent data related to a client query for as long as the
+ * object persists.
+ */
+static ns_hookresult_t
+filter_qctx_initialize(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state;
+
+	*resp = ISC_R_UNSET;
+
+	client_state = client_state_get(qctx, inst);
+	if (client_state == NULL) {
+		client_state_create(qctx, inst);
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Determine whether this client should have AAAA filtered or not, based on
+ * the client address family and the settings of filter-aaaa-on-v4 and
+ * filter-aaaa-on-v6.
+ */
+static ns_hookresult_t
+filter_prep_response_begin(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result;
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state == NULL) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (inst->v4_aaaa != NONE || inst->v6_aaaa != NONE) {
+		result = ns_client_checkaclsilent(qctx->client, NULL,
+						  inst->aaaa_acl, true);
+		if (result == ISC_R_SUCCESS && inst->v4_aaaa != NONE &&
+		    is_v4_client(qctx->client))
+		{
+			client_state->mode = inst->v4_aaaa;
+		} else if (result == ISC_R_SUCCESS && inst->v6_aaaa != NONE &&
+			   is_v6_client(qctx->client))
+		{
+			client_state->mode = inst->v6_aaaa;
+		}
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Hide AAAA rrsets if there is a matching A. Trigger recursion if
+ * necessary to find out whether an A exists.
+ *
+ * (This version is for processing answers to explicit AAAA queries; ANY
+ * queries are handled in filter_respond_any_found().)
+ */
+static ns_hookresult_t
+filter_respond_begin(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+	isc_result_t result = ISC_R_UNSET;
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state == NULL) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (client_state->mode != BREAK_DNSSEC &&
+	    (client_state->mode != FILTER ||
+	     (WANTDNSSEC(qctx->client) && qctx->sigrdataset != NULL &&
+	      dns_rdataset_isassociated(qctx->sigrdataset))))
+	{
+		return (NS_HOOK_CONTINUE);
+	}
+
+	if (qctx->qtype == dns_rdatatype_aaaa) {
+		dns_rdataset_t *trdataset;
+		trdataset = ns_client_newrdataset(qctx->client);
+		result = dns_db_findrdataset(
+			qctx->db, qctx->node, qctx->version, dns_rdatatype_a, 0,
+			qctx->client->now, trdataset, NULL);
+		if (dns_rdataset_isassociated(trdataset)) {
+			dns_rdataset_disassociate(trdataset);
+		}
+		ns_client_putrdataset(qctx->client, &trdataset);
+
+		/*
+		 * We found an AAAA. If we also found an A, then the AAAA
+		 * must not be rendered.
+		 *
+		 * If the A is not in our cache, then any result other than
+		 * DNS_R_DELEGATION or ISC_R_NOTFOUND means there is no A,
+		 * and so AAAAs are okay.
+		 *
+		 * We assume there is no A if we can't recurse for this
+		 * client. That might be the wrong answer, but what else
+		 * can we do?  Besides, the fact that we have the AAAA and
+		 * are using this mechanism in the first place suggests
+		 * that we care more about As than AAAAs, and would have
+		 * cached an A if it existed.
+		 */
+		if (result == ISC_R_SUCCESS) {
+			mark_as_rendered(qctx->rdataset, qctx->sigrdataset);
+			qctx->client->message->flags &= ~DNS_MESSAGEFLAG_AD;
+			client_state->flags |= FILTER_AAAA_FILTERED;
+		} else if (!qctx->authoritative && RECURSIONOK(qctx->client) &&
+			   (result == DNS_R_DELEGATION ||
+			    result == ISC_R_NOTFOUND))
+		{
+			/*
+			 * This is an ugly kludge to recurse
+			 * for the A and discard the result.
+			 *
+			 * Continue to add the AAAA now.
+			 * We'll make a note to not render it
+			 * if the recursion for the A succeeds.
+			 */
+			result = ns_query_recurse(qctx->client, dns_rdatatype_a,
+						  qctx->client->query.qname,
+						  NULL, NULL, qctx->resuming);
+			if (result == ISC_R_SUCCESS) {
+				client_state->flags |= FILTER_AAAA_RECURSING;
+				qctx->client->query.attributes |=
+					NS_QUERYATTR_RECURSING;
+			}
+		}
+	} else if (qctx->qtype == dns_rdatatype_a &&
+		   (client_state->flags & FILTER_AAAA_RECURSING) != 0)
+	{
+		const section_filter_t filter_answer = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ANSWER,
+			.name = qctx->fname,
+			.type = dns_rdatatype_aaaa,
+		};
+		process_section(&filter_answer);
+
+		client_state->flags &= ~FILTER_AAAA_RECURSING;
+
+		result = ns_query_done(qctx);
+
+		*resp = result;
+
+		return (NS_HOOK_RETURN);
+	}
+
+	*resp = result;
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * When answering an ANY query, remove AAAA if A is present.
+ */
+static ns_hookresult_t
+filter_respond_any_found(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state != NULL && client_state->mode != NONE) {
+		/*
+		 * If we are authoritative, require an A record to be
+		 * present before filtering out AAAA records; otherwise,
+		 * just assume an A record exists even if it was not in the
+		 * cache (and therefore is not in the response message),
+		 * thus proceeding with filtering out AAAA records.
+		 */
+		const section_filter_t filter_answer = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ANSWER,
+			.name = qctx->tname,
+			.type = dns_rdatatype_aaaa,
+			.only_if_a_exists = qctx->authoritative,
+		};
+		process_section(&filter_answer);
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * Hide AAAA rrsets in the additional section if there is a matching A, and
+ * hide NS in the authority section if AAAA was filtered in the answer
+ * section.
+ */
+static ns_hookresult_t
+filter_query_done_send(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+	filter_data_t *client_state = client_state_get(qctx, inst);
+
+	*resp = ISC_R_UNSET;
+
+	if (client_state != NULL && client_state->mode != NONE) {
+		const section_filter_t filter_additional = {
+			.qctx = qctx,
+			.mode = client_state->mode,
+			.section = DNS_SECTION_ADDITIONAL,
+			.type = dns_rdatatype_aaaa,
+			.only_if_a_exists = true,
+		};
+		process_section(&filter_additional);
+
+		if ((client_state->flags & FILTER_AAAA_FILTERED) != 0) {
+			const section_filter_t filter_authority = {
+				.qctx = qctx,
+				.mode = client_state->mode,
+				.section = DNS_SECTION_AUTHORITY,
+				.type = dns_rdatatype_ns,
+			};
+			process_section(&filter_authority);
+		}
+	}
+
+	return (NS_HOOK_CONTINUE);
+}
+
+/*
+ * If the client is being detached, then we can delete our persistent data
+ * from hash table and return it to the memory pool.
+ */
+static ns_hookresult_t
+filter_qctx_destroy(void *arg, void *cbdata, isc_result_t *resp) {
+	query_ctx_t *qctx = (query_ctx_t *)arg;
+	filter_instance_t *inst = (filter_instance_t *)cbdata;
+
+	*resp = ISC_R_UNSET;
+
+	if (!qctx->detach_client) {
+		return (NS_HOOK_CONTINUE);
+	}
+
+	client_state_destroy(qctx, inst);
+
+	return (NS_HOOK_CONTINUE);
+}
diff --git a/bin/plugins/filter-aaaa.rst b/bin/plugins/filter-aaaa.rst
new file mode 100644
index 0000000..8cd7556
--- /dev/null
+++ b/bin/plugins/filter-aaaa.rst
@@ -0,0 +1,90 @@
+.. Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+..
+.. SPDX-License-Identifier: MPL-2.0
+..
+.. This Source Code Form is subject to the terms of the Mozilla Public
+.. License, v. 2.0.  If a copy of the MPL was not distributed with this
+.. file, you can obtain one at https://mozilla.org/MPL/2.0/.
+..
+.. See the COPYRIGHT file distributed with this work for additional
+.. information regarding copyright ownership.
+
+.. highlight: console
+
+.. iscman:: filter-aaaa
+.. _man_filter-aaaa:
+
+filter-aaaa.so - filter AAAA in DNS responses when A is present
+---------------------------------------------------------------
+
+Synopsis
+~~~~~~~~
+
+:program:`plugin query` "filter-aaaa.so" [{ parameters }];
+
+Description
+~~~~~~~~~~~
+
+:program:`filter-aaaa.so` is a query plugin module for :iscman:`named`, enabling
+:iscman:`named` to omit some IPv6 addresses when responding to clients.
+
+Until BIND 9.12, this feature was implemented natively in :iscman:`named` and
+enabled with the ``filter-aaaa`` ACL and the ``filter-aaaa-on-v4`` and
+``filter-aaaa-on-v6`` options. These options are now deprecated in
+:iscman:`named.conf` but can be passed as parameters to the
+``filter-aaaa.so`` plugin, for example:
+
+::
+
+   plugin query "filter-aaaa.so" {
+           filter-aaaa-on-v4 yes;
+           filter-aaaa-on-v6 yes;
+           filter-aaaa { 192.0.2.1; 2001:db8:2::1; };
+   };
+
+This module is intended to aid transition from IPv4 to IPv6 by
+withholding IPv6 addresses from DNS clients which are not connected to
+the IPv6 Internet, when the name being looked up has an IPv4 address
+available. Use of this module is not recommended unless absolutely
+necessary.
+
+Note: This mechanism can erroneously cause other servers not to give
+AAAA records to their clients. If a recursing server with both IPv6 and
+IPv4 network connections queries an authoritative server using this
+mechanism via IPv4, it is denied AAAA records even if its client is
+using IPv6.
+
+Options
+~~~~~~~
+
+``filter-aaaa``
+   This option specifies a list of client addresses for which AAAA filtering is to
+   be applied. The default is ``any``.
+
+``filter-aaaa-on-v4``
+   If set to ``yes``, this option indicates that the DNS client is at an IPv4 address, in
+   ``filter-aaaa``. If the response does not include DNSSEC
+   signatures, then all AAAA records are deleted from the response. This
+   filtering applies to all responses, not only authoritative
+   ones.
+
+   If set to ``break-dnssec``, then AAAA records are deleted even when
+   DNSSEC is enabled. As suggested by the name, this causes the response
+   to fail to verify, because the DNSSEC protocol is designed to detect
+   deletions.
+
+   This mechanism can erroneously cause other servers not to give AAAA
+   records to their clients. If a recursing server with both IPv6 and IPv4
+   network connections queries an authoritative server using this
+   mechanism via IPv4, it is denied AAAA records even if its client is
+   using IPv6.
+
+``filter-aaaa-on-v6``
+   This option is identical to ``filter-aaaa-on-v4``, except that it filters AAAA responses
+   to queries from IPv6 clients instead of IPv4 clients. To filter all
+   responses, set both options to ``yes``.
+
+See Also
+~~~~~~~~
+
+BIND 9 Administrator Reference Manual.