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Implement DNS64 synthesis.
When unwind(8) learns new autoconf resolvers (from dhcp or router
advertisements) it checks if a DNS64 is present in this network
location and tries to recover the IPv6 prefix used according to
RFC7050.
The learned autoconf resolvers are then prevented from upgrading to
the validating state since DNS64 breaks DNSSEC.
unwind(8) can now perform its own synthesis. If a query for a AAAA
record results in no answer we re-send the query for A and if that
leads to an answer we synthesize an AAAA answer using the learned
prefixes.

Testing & OK kn

/*	$OpenBSD: dns64_synth.c,v 1.1 2021/01/24 18:29:15 florian Exp $	*/

/*
 * dns64/dns64.h - DNS64 module
 *
 * Copyright (c) 2009, Viagénie. All rights reserved.
 *
 * This software is open source.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation
 * and/or other materials provided with the distribution.
 *
 * Neither the name of the NLNET LABS nor the names of its contributors may
 * be used to endorse or promote products derived from this software without
 * specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Copyright (c) 2021 Florian Obser <florian@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>
#include <sys/socket.h>

#include <netinet/in.h>
#include <net/route.h>

#include <string.h>

#include "libunbound/config.h"
#include "libunbound/util/regional.h"
#include "libunbound/util/data/msgencode.h"
#include "libunbound/util/data/msgparse.h"
#include "libunbound/util/data/msgreply.h"

#include "log.h"
#include "unwind.h"
#include "frontend.h"
#include "dns64_synth.h"

extern struct dns64_prefix	*dns64_prefixes;
extern int			 dns64_prefix_count;

static void
synthesize_aaaa(const struct in6_addr *in6, int prefixlen, const uint8_t *a,
    size_t a_len, uint8_t *aaaa, size_t aaaa_len)
{
	size_t i, pos;
	memcpy(aaaa, in6->s6_addr, sizeof(in6->s6_addr));
	for (i = 0, pos = prefixlen / 8; i < a_len && pos < aaaa_len;
	    i++, pos++) {
		if (pos == 8)
			aaaa[pos++] = 0;
		aaaa[pos] = a[i];
	}
}

/*
 * Copied from unbound/dns64/dns64.c and slightly extended to work with more
 * than one IPv6 prefix.
 */

void
dns64_synth_aaaa_data(const struct ub_packed_rrset_key* fk, const struct
    packed_rrset_data* fd, struct ub_packed_rrset_key *dk, struct
    packed_rrset_data **dd_out, struct regional *region)
{
	struct packed_rrset_data	*dd;
	size_t				 i, pos;
	int				 j;

	/*
	 * Create synthesized AAAA RR set data. We need to allocated extra
	 * memory for the RRs themselves. Each RR has a length, TTL, pointer to
	 * wireformat data, 2 bytes of data length, and 16 bytes of IPv6
	 * address.
	 */
	if(fd->count > RR_COUNT_MAX) {
		*dd_out = NULL;
		return; /* integer overflow protection in alloc */
	}
	if (!(dd = *dd_out = regional_alloc(region, sizeof(struct
	    packed_rrset_data) + fd->count * dns64_prefix_count *
	    (sizeof(size_t) + sizeof(time_t) + sizeof(uint8_t*) + 2 + 16)))) {
		log_warnx("out of memory");
		return;
	}

	/* Copy attributes from A RR set. */
	dd->ttl = fd->ttl;
	dd->count = fd->count * dns64_prefix_count;
	dd->rrsig_count = 0;
	dd->trust = fd->trust;
	dd->security = fd->security;

	/* Synthesize AAAA records. Adjust pointers in structure. */
	dd->rr_len = (size_t*)((uint8_t*)dd + sizeof(struct packed_rrset_data));
	dd->rr_data = (uint8_t**)&dd->rr_len[dd->count];
	dd->rr_ttl = (time_t*)&dd->rr_data[dd->count];
	for(i = 0, pos = 0; i < fd->count; ++i) {
		if (fd->rr_len[i] != 6 || fd->rr_data[i][0] != 0 ||
		    fd->rr_data[i][1] != 4) {
			*dd_out = NULL;
			return;
		}
		for (j = 0; j < dns64_prefix_count; j++, pos++) {
			dd->rr_len[pos] = 18;
			dd->rr_ttl[pos] = fd->rr_ttl[i];
			dd->rr_data[pos] = (uint8_t*)&dd->rr_ttl[dd->count] +
			    18 * pos;
			dd->rr_data[pos][0] = 0;
			dd->rr_data[pos][1] = 16;
			synthesize_aaaa(&dns64_prefixes[j].in6,
			    dns64_prefixes[j].prefixlen, &fd->rr_data[i][2],
			    fd->rr_len[i]-2, &dd->rr_data[pos][2],
			    dd->rr_len[pos]-2);
		}
	}

	/*
	 * Create synthesized AAAA RR set key. This is mostly just bookkeeping,
	 * nothing interesting here.
	 */
	if(!dk) {
		log_warnx("no key");
		*dd_out = NULL;
		return;
	}

	dk->rk.dname = (uint8_t*)regional_alloc_init(region, fk->rk.dname,
	    fk->rk.dname_len);

	if(!dk->rk.dname) {
		log_warnx("out of memory");
		*dd_out = NULL;
		return;
	}

	dk->rk.type = htons(LDNS_RR_TYPE_AAAA);
	memset(&dk->entry, 0, sizeof(dk->entry));
	dk->entry.key = dk;
	dk->entry.hash = rrset_key_hash(&dk->rk);
	dk->entry.data = dd;
}