man: add basic documentation for resolved.conf's DNSSEC= switch

[thirdparty/systemd.git] / src / resolve / resolved-dns-transaction.c

/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/

/***
  This file is part of systemd.

  Copyright 2014 Lennart Poettering

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

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

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

#include <sd-messages.h>

#include "af-list.h"
#include "alloc-util.h"
#include "dns-domain.h"
#include "fd-util.h"
#include "random-util.h"
#include "resolved-dns-cache.h"
#include "resolved-dns-transaction.h"
#include "resolved-llmnr.h"
#include "string-table.h"

static void dns_transaction_reset_answer(DnsTransaction *t) {
        assert(t);

        t->received = dns_packet_unref(t->received);
        t->answer = dns_answer_unref(t->answer);
        t->answer_rcode = 0;
        t->answer_dnssec_result = _DNSSEC_RESULT_INVALID;
        t->answer_source = _DNS_TRANSACTION_SOURCE_INVALID;
        t->answer_authenticated = false;
}

static void dns_transaction_close_connection(DnsTransaction *t) {
        assert(t);

        t->stream = dns_stream_free(t->stream);
        t->dns_udp_event_source = sd_event_source_unref(t->dns_udp_event_source);
        t->dns_udp_fd = safe_close(t->dns_udp_fd);
}

static void dns_transaction_stop_timeout(DnsTransaction *t) {
        assert(t);

        t->timeout_event_source = sd_event_source_unref(t->timeout_event_source);
}

DnsTransaction* dns_transaction_free(DnsTransaction *t) {
        DnsQueryCandidate *c;
        DnsZoneItem *i;
        DnsTransaction *z;

        if (!t)
                return NULL;

        log_debug("Freeing transaction %" PRIu16 ".", t->id);

        dns_transaction_close_connection(t);
        dns_transaction_stop_timeout(t);

        dns_packet_unref(t->sent);
        dns_transaction_reset_answer(t);

        dns_server_unref(t->server);

        if (t->scope) {
                hashmap_remove_value(t->scope->transactions_by_key, t->key, t);
                LIST_REMOVE(transactions_by_scope, t->scope->transactions, t);

                if (t->id != 0)
                        hashmap_remove(t->scope->manager->dns_transactions, UINT_TO_PTR(t->id));
        }

        while ((c = set_steal_first(t->notify_query_candidates)))
                set_remove(c->transactions, t);
        set_free(t->notify_query_candidates);

        while ((i = set_steal_first(t->notify_zone_items)))
                i->probe_transaction = NULL;
        set_free(t->notify_zone_items);

        while ((z = set_steal_first(t->notify_transactions)))
                set_remove(z->dnssec_transactions, t);
        set_free(t->notify_transactions);

        while ((z = set_steal_first(t->dnssec_transactions))) {
                set_remove(z->notify_transactions, t);
                dns_transaction_gc(z);
        }
        set_free(t->dnssec_transactions);

        dns_answer_unref(t->validated_keys);
        dns_resource_key_unref(t->key);
        free(t->key_string);

        free(t);
        return NULL;
}

DEFINE_TRIVIAL_CLEANUP_FUNC(DnsTransaction*, dns_transaction_free);

bool dns_transaction_gc(DnsTransaction *t) {
        assert(t);

        if (t->block_gc > 0)
                return true;

        if (set_isempty(t->notify_query_candidates) &&
            set_isempty(t->notify_zone_items) &&
            set_isempty(t->notify_transactions)) {
                dns_transaction_free(t);
                return false;
        }

        return true;
}

int dns_transaction_new(DnsTransaction **ret, DnsScope *s, DnsResourceKey *key) {
        _cleanup_(dns_transaction_freep) DnsTransaction *t = NULL;
        int r;

        assert(ret);
        assert(s);
        assert(key);

        /* Don't allow looking up invalid or pseudo RRs */
        if (!dns_type_is_valid_query(key->type))
                return -EINVAL;

        /* We only support the IN class */
        if (key->class != DNS_CLASS_IN && key->class != DNS_CLASS_ANY)
                return -EOPNOTSUPP;

        r = hashmap_ensure_allocated(&s->manager->dns_transactions, NULL);
        if (r < 0)
                return r;

        r = hashmap_ensure_allocated(&s->transactions_by_key, &dns_resource_key_hash_ops);
        if (r < 0)
                return r;

        t = new0(DnsTransaction, 1);
        if (!t)
                return -ENOMEM;

        t->dns_udp_fd = -1;
        t->answer_source = _DNS_TRANSACTION_SOURCE_INVALID;
        t->answer_dnssec_result = _DNSSEC_RESULT_INVALID;
        t->key = dns_resource_key_ref(key);

        /* Find a fresh, unused transaction id */
        do
                random_bytes(&t->id, sizeof(t->id));
        while (t->id == 0 ||
               hashmap_get(s->manager->dns_transactions, UINT_TO_PTR(t->id)));

        r = hashmap_put(s->manager->dns_transactions, UINT_TO_PTR(t->id), t);
        if (r < 0) {
                t->id = 0;
                return r;
        }

        r = hashmap_replace(s->transactions_by_key, t->key, t);
        if (r < 0) {
                hashmap_remove(s->manager->dns_transactions, UINT_TO_PTR(t->id));
                return r;
        }

        LIST_PREPEND(transactions_by_scope, s->transactions, t);
        t->scope = s;

        s->manager->n_transactions_total ++;

        if (ret)
                *ret = t;

        t = NULL;

        return 0;
}

static void dns_transaction_tentative(DnsTransaction *t, DnsPacket *p) {
        _cleanup_free_ char *pretty = NULL;
        DnsZoneItem *z;

        assert(t);
        assert(p);

        if (manager_our_packet(t->scope->manager, p) != 0)
                return;

        in_addr_to_string(p->family, &p->sender, &pretty);

        log_debug("Transaction %" PRIu16 " for <%s> on scope %s on %s/%s got tentative packet from %s.",
                  t->id,
                  dns_transaction_key_string(t),
                  dns_protocol_to_string(t->scope->protocol),
                  t->scope->link ? t->scope->link->name : "*",
                  t->scope->family == AF_UNSPEC ? "*" : af_to_name(t->scope->family),
                  pretty);

        /* RFC 4795, Section 4.1 says that the peer with the
         * lexicographically smaller IP address loses */
        if (memcmp(&p->sender, &p->destination, FAMILY_ADDRESS_SIZE(p->family)) >= 0) {
                log_debug("Peer has lexicographically larger IP address and thus lost in the conflict.");
                return;
        }

        log_debug("We have the lexicographically larger IP address and thus lost in the conflict.");

        t->block_gc++;
        while ((z = set_first(t->notify_zone_items))) {
                /* First, make sure the zone item drops the reference
                 * to us */
                dns_zone_item_probe_stop(z);

                /* Secondly, report this as conflict, so that we might
                 * look for a different hostname */
                dns_zone_item_conflict(z);
        }
        t->block_gc--;

        dns_transaction_gc(t);
}

void dns_transaction_complete(DnsTransaction *t, DnsTransactionState state) {
        DnsQueryCandidate *c;
        DnsZoneItem *z;
        DnsTransaction *d;
        Iterator i;

        assert(t);
        assert(!DNS_TRANSACTION_IS_LIVE(state));

        if (state == DNS_TRANSACTION_DNSSEC_FAILED)
                log_struct(LOG_NOTICE,
                           LOG_MESSAGE_ID(SD_MESSAGE_DNSSEC_FAILURE),
                           LOG_MESSAGE("DNSSEC validation failed for question %s: %s", dns_transaction_key_string(t), dnssec_result_to_string(t->answer_dnssec_result)),
                           "DNS_TRANSACTION=%" PRIu16, t->id,
                           "DNS_QUESTION=%s", dns_transaction_key_string(t),
                           "DNSSEC_RESULT=%s", dnssec_result_to_string(t->answer_dnssec_result),
                           NULL);

        /* Note that this call might invalidate the query. Callers
         * should hence not attempt to access the query or transaction
         * after calling this function. */

        log_debug("Transaction %" PRIu16 " for <%s> on scope %s on %s/%s now complete with <%s> from %s (%s).",
                  t->id,
                  dns_transaction_key_string(t),
                  dns_protocol_to_string(t->scope->protocol),
                  t->scope->link ? t->scope->link->name : "*",
                  t->scope->family == AF_UNSPEC ? "*" : af_to_name(t->scope->family),
                  dns_transaction_state_to_string(state),
                  t->answer_source < 0 ? "none" : dns_transaction_source_to_string(t->answer_source),
                  t->answer_authenticated ? "authenticated" : "unsigned");

        t->state = state;

        dns_transaction_close_connection(t);
        dns_transaction_stop_timeout(t);

        /* Notify all queries that are interested, but make sure the
         * transaction isn't freed while we are still looking at it */
        t->block_gc++;

        SET_FOREACH(c, t->notify_query_candidates, i)
                dns_query_candidate_notify(c);
        SET_FOREACH(z, t->notify_zone_items, i)
                dns_zone_item_notify(z);

        if (!set_isempty(t->notify_transactions)) {
                DnsTransaction **nt;
                unsigned j, n = 0;

                /* We need to be careful when notifying other
                 * transactions, as that might destroy other
                 * transactions in our list. Hence, in order to be
                 * able to safely iterate through the list of
                 * transactions, take a GC lock on all of them
                 * first. Then, in a second loop, notify them, but
                 * first unlock that specific transaction. */

                nt = newa(DnsTransaction*, set_size(t->notify_transactions));
                SET_FOREACH(d, t->notify_transactions, i) {
                        nt[n++] = d;
                        d->block_gc++;
                }

                assert(n == set_size(t->notify_transactions));

                for (j = 0; j < n; j++) {
                        if (set_contains(t->notify_transactions, nt[j]))
                                dns_transaction_notify(nt[j], t);

                        nt[j]->block_gc--;
                        dns_transaction_gc(nt[j]);
                }
        }

        t->block_gc--;
        dns_transaction_gc(t);
}

static int dns_transaction_pick_server(DnsTransaction *t) {
        DnsServer *server;

        assert(t);
        assert(t->scope->protocol == DNS_PROTOCOL_DNS);

        server = dns_scope_get_dns_server(t->scope);
        if (!server)
                return -ESRCH;

        t->current_features = dns_server_possible_feature_level(server);

        if (server == t->server)
                return 0;

        dns_server_unref(t->server);
        t->server = dns_server_ref(server);

        return 1;
}

static int on_stream_complete(DnsStream *s, int error) {
        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        DnsTransaction *t;

        assert(s);
        assert(s->transaction);

        /* Copy the data we care about out of the stream before we
         * destroy it. */
        t = s->transaction;
        p = dns_packet_ref(s->read_packet);

        t->stream = dns_stream_free(t->stream);

        if (IN_SET(error, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE)) {
                dns_transaction_complete(t, DNS_TRANSACTION_CONNECTION_FAILURE);
                return 0;
        }

        if (error != 0) {
                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                return 0;
        }

        if (dns_packet_validate_reply(p) <= 0) {
                log_debug("Invalid TCP reply packet.");
                dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                return 0;
        }

        dns_scope_check_conflicts(t->scope, p);

        t->block_gc++;
        dns_transaction_process_reply(t, p);
        t->block_gc--;

        /* If the response wasn't useful, then complete the transition
         * now. After all, we are the worst feature set now with TCP
         * sockets, and there's really no point in retrying. */
        if (t->state == DNS_TRANSACTION_PENDING)
                dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
        else
                dns_transaction_gc(t);

        return 0;
}

static int dns_transaction_open_tcp(DnsTransaction *t) {
        _cleanup_close_ int fd = -1;
        int r;

        assert(t);

        dns_transaction_close_connection(t);

        switch (t->scope->protocol) {

        case DNS_PROTOCOL_DNS:
                r = dns_transaction_pick_server(t);
                if (r < 0)
                        return r;

                r = dns_server_adjust_opt(t->server, t->sent, t->current_features);
                if (r < 0)
                        return r;

                fd = dns_scope_socket_tcp(t->scope, AF_UNSPEC, NULL, t->server, 53);
                break;

        case DNS_PROTOCOL_LLMNR:
                /* When we already received a reply to this (but it was truncated), send to its sender address */
                if (t->received)
                        fd = dns_scope_socket_tcp(t->scope, t->received->family, &t->received->sender, NULL, t->received->sender_port);
                else {
                        union in_addr_union address;
                        int family = AF_UNSPEC;

                        /* Otherwise, try to talk to the owner of a
                         * the IP address, in case this is a reverse
                         * PTR lookup */

                        r = dns_name_address(DNS_RESOURCE_KEY_NAME(t->key), &family, &address);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                return -EINVAL;
                        if (family != t->scope->family)
                                return -ESRCH;

                        fd = dns_scope_socket_tcp(t->scope, family, &address, NULL, LLMNR_PORT);
                }

                break;

        default:
                return -EAFNOSUPPORT;
        }

        if (fd < 0)
                return fd;

        r = dns_stream_new(t->scope->manager, &t->stream, t->scope->protocol, fd);
        if (r < 0)
                return r;
        fd = -1;

        r = dns_stream_write_packet(t->stream, t->sent);
        if (r < 0) {
                t->stream = dns_stream_free(t->stream);
                return r;
        }

        t->stream->complete = on_stream_complete;
        t->stream->transaction = t;

        /* The interface index is difficult to determine if we are
         * connecting to the local host, hence fill this in right away
         * instead of determining it from the socket */
        if (t->scope->link)
                t->stream->ifindex = t->scope->link->ifindex;

        dns_transaction_reset_answer(t);

        t->tried_stream = true;

        return 0;
}

static void dns_transaction_cache_answer(DnsTransaction *t) {
        assert(t);

        /* For mDNS we cache whenever we get the packet, rather than
         * in each transaction. */
        if (!IN_SET(t->scope->protocol, DNS_PROTOCOL_DNS, DNS_PROTOCOL_LLMNR))
                return;

        /* We never cache if this packet is from the local host, under
         * the assumption that a locally running DNS server would
         * cache this anyway, and probably knows better when to flush
         * the cache then we could. */
        if (!DNS_PACKET_SHALL_CACHE(t->received))
                return;

        dns_cache_put(&t->scope->cache,
                      t->key,
                      t->answer_rcode,
                      t->answer,
                      t->answer_authenticated,
                      0,
                      t->received->family,
                      &t->received->sender);
}

static bool dns_transaction_dnssec_is_live(DnsTransaction *t) {
        DnsTransaction *dt;
        Iterator i;

        assert(t);

        SET_FOREACH(dt, t->dnssec_transactions, i)
                if (DNS_TRANSACTION_IS_LIVE(dt->state))
                        return true;

        return false;
}

static void dns_transaction_process_dnssec(DnsTransaction *t) {
        int r;

        assert(t);

        /* Are there ongoing DNSSEC transactions? If so, let's wait for them. */
        if (dns_transaction_dnssec_is_live(t))
                return;

        /* All our auxiliary DNSSEC transactions are complete now. Try
         * to validate our RRset now. */
        r = dns_transaction_validate_dnssec(t);
        if (r < 0) {
                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                return;
        }

        if (t->answer_dnssec_result == DNSSEC_INCOMPATIBLE_SERVER &&
            t->scope->dnssec_mode == DNSSEC_YES) {
                /*  We are not in automatic downgrade mode, and the
                 *  server is bad, refuse operation. */
                dns_transaction_complete(t, DNS_TRANSACTION_DNSSEC_FAILED);
                return;
        }

        if (!IN_SET(t->answer_dnssec_result,
                    _DNSSEC_RESULT_INVALID,        /* No DNSSEC validation enabled */
                    DNSSEC_VALIDATED,              /* Answer is signed and validated successfully */
                    DNSSEC_UNSIGNED,               /* Answer is right-fully unsigned */
                    DNSSEC_INCOMPATIBLE_SERVER)) { /* Server does not do DNSSEC (Yay, we are downgrade attack vulnerable!) */
                dns_transaction_complete(t, DNS_TRANSACTION_DNSSEC_FAILED);
                return;
        }

        dns_transaction_cache_answer(t);

        if (t->answer_rcode == DNS_RCODE_SUCCESS)
                dns_transaction_complete(t, DNS_TRANSACTION_SUCCESS);
        else
                dns_transaction_complete(t, DNS_TRANSACTION_RCODE_FAILURE);
}

void dns_transaction_process_reply(DnsTransaction *t, DnsPacket *p) {
        usec_t ts;
        int r;

        assert(t);
        assert(p);
        assert(t->scope);
        assert(t->scope->manager);

        if (t->state != DNS_TRANSACTION_PENDING)
                return;

        /* Note that this call might invalidate the query. Callers
         * should hence not attempt to access the query or transaction
         * after calling this function. */

        log_debug("Processing incoming packet on transaction %" PRIu16".", t->id);

        switch (t->scope->protocol) {

        case DNS_PROTOCOL_LLMNR:
                assert(t->scope->link);

                /* For LLMNR we will not accept any packets from other
                 * interfaces */

                if (p->ifindex != t->scope->link->ifindex)
                        return;

                if (p->family != t->scope->family)
                        return;

                /* Tentative packets are not full responses but still
                 * useful for identifying uniqueness conflicts during
                 * probing. */
                if (DNS_PACKET_LLMNR_T(p)) {
                        dns_transaction_tentative(t, p);
                        return;
                }

                break;

        case DNS_PROTOCOL_MDNS:
                assert(t->scope->link);

                /* For mDNS we will not accept any packets from other interfaces */
                if (p->ifindex != t->scope->link->ifindex)
                        return;

                if (p->family != t->scope->family)
                        return;

                break;

        case DNS_PROTOCOL_DNS:
                /* Note that we do not need to verify the
                 * addresses/port numbers of incoming traffic, as we
                 * invoked connect() on our UDP socket in which case
                 * the kernel already does the needed verification for
                 * us. */
                break;

        default:
                assert_not_reached("Invalid DNS protocol.");
        }

        if (t->received != p) {
                dns_packet_unref(t->received);
                t->received = dns_packet_ref(p);
        }

        t->answer_source = DNS_TRANSACTION_NETWORK;

        if (p->ipproto == IPPROTO_TCP) {
                if (DNS_PACKET_TC(p)) {
                        /* Truncated via TCP? Somebody must be fucking with us */
                        dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                        return;
                }

                if (DNS_PACKET_ID(p) != t->id) {
                        /* Not the reply to our query? Somebody must be fucking with us */
                        dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                        return;
                }
        }

        assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);

        switch (t->scope->protocol) {

        case DNS_PROTOCOL_DNS:
                assert(t->server);

                if (IN_SET(DNS_PACKET_RCODE(p), DNS_RCODE_FORMERR, DNS_RCODE_SERVFAIL, DNS_RCODE_NOTIMP)) {

                        /* Request failed, immediately try again with reduced features */
                        log_debug("Server returned error: %s", dns_rcode_to_string(DNS_PACKET_RCODE(p)));

                        dns_server_packet_failed(t->server, t->current_features);

                        r = dns_transaction_go(t);
                        if (r < 0) {
                                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                                return;
                        }

                        return;
                } else
                        dns_server_packet_received(t->server, t->current_features, ts - t->start_usec, p->size);

                break;

        case DNS_PROTOCOL_LLMNR:
        case DNS_PROTOCOL_MDNS:
                dns_scope_packet_received(t->scope, ts - t->start_usec);
                break;

        default:
                assert_not_reached("Invalid DNS protocol.");
        }

        if (DNS_PACKET_TC(p)) {

                /* Truncated packets for mDNS are not allowed. Give up immediately. */
                if (t->scope->protocol == DNS_PROTOCOL_MDNS) {
                        dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                        return;
                }

                /* Response was truncated, let's try again with good old TCP */
                r = dns_transaction_open_tcp(t);
                if (r == -ESRCH) {
                        /* No servers found? Damn! */
                        dns_transaction_complete(t, DNS_TRANSACTION_NO_SERVERS);
                        return;
                }
                if (r < 0) {
                        /* On LLMNR, if we cannot connect to the host,
                         * we immediately give up */
                        if (t->scope->protocol == DNS_PROTOCOL_LLMNR) {
                                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                                return;
                        }

                        /* On DNS, couldn't send? Try immediately again, with a new server */
                        dns_scope_next_dns_server(t->scope);

                        r = dns_transaction_go(t);
                        if (r < 0) {
                                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                                return;
                        }
                }

                return;
        }

        /* Parse message, if it isn't parsed yet. */
        r = dns_packet_extract(p);
        if (r < 0) {
                dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                return;
        }

        if (IN_SET(t->scope->protocol, DNS_PROTOCOL_DNS, DNS_PROTOCOL_LLMNR)) {

                /* Only consider responses with equivalent query section to the request */
                r = dns_packet_is_reply_for(p, t->key);
                if (r < 0) {
                        dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                        return;
                }
                if (r == 0) {
                        dns_transaction_complete(t, DNS_TRANSACTION_INVALID_REPLY);
                        return;
                }

                /* Install the answer as answer to the transaction */
                dns_answer_unref(t->answer);
                t->answer = dns_answer_ref(p->answer);
                t->answer_rcode = DNS_PACKET_RCODE(p);
                t->answer_dnssec_result = _DNSSEC_RESULT_INVALID;
                t->answer_authenticated = false;

                /* Block GC while starting requests for additional DNSSEC RRs */
                t->block_gc++;
                r = dns_transaction_request_dnssec_keys(t);
                t->block_gc--;

                /* Maybe the transaction is ready for GC'ing now? If so, free it and return. */
                if (!dns_transaction_gc(t))
                        return;

                /* Requesting additional keys might have resulted in
                 * this transaction to fail, since the auxiliary
                 * request failed for some reason. If so, we are not
                 * in pending state anymore, and we should exit
                 * quickly. */
                if (t->state != DNS_TRANSACTION_PENDING)
                        return;
                if (r < 0) {
                        dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                        return;
                }
                if (r > 0) {
                        /* There are DNSSEC transactions pending now. Update the state accordingly. */
                        t->state = DNS_TRANSACTION_VALIDATING;
                        dns_transaction_close_connection(t);
                        dns_transaction_stop_timeout(t);
                        return;
                }
        }

        dns_transaction_process_dnssec(t);
}

static int on_dns_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        DnsTransaction *t = userdata;
        int r;

        assert(t);
        assert(t->scope);

        r = manager_recv(t->scope->manager, fd, DNS_PROTOCOL_DNS, &p);
        if (r <= 0)
                return r;

        if (dns_packet_validate_reply(p) > 0 &&
            DNS_PACKET_ID(p) == t->id)
                dns_transaction_process_reply(t, p);
        else
                log_debug("Invalid DNS UDP packet, ignoring.");

        return 0;
}

static int dns_transaction_emit_udp(DnsTransaction *t) {
        int r;

        assert(t);

        if (t->scope->protocol == DNS_PROTOCOL_DNS) {

                r = dns_transaction_pick_server(t);
                if (r < 0)
                        return r;

                if (t->current_features < DNS_SERVER_FEATURE_LEVEL_UDP)
                        return -EAGAIN;

                if (r > 0 || t->dns_udp_fd < 0) { /* Server changed, or no connection yet. */
                        int fd;

                        dns_transaction_close_connection(t);

                        fd = dns_scope_socket_udp(t->scope, t->server, 53);
                        if (fd < 0)
                                return fd;

                        r = sd_event_add_io(t->scope->manager->event, &t->dns_udp_event_source, fd, EPOLLIN, on_dns_packet, t);
                        if (r < 0) {
                                safe_close(fd);
                                return r;
                        }

                        t->dns_udp_fd = fd;
                }

                r = dns_server_adjust_opt(t->server, t->sent, t->current_features);
                if (r < 0)
                        return r;
        } else
                dns_transaction_close_connection(t);

        r = dns_scope_emit_udp(t->scope, t->dns_udp_fd, t->sent);
        if (r < 0)
                return r;

        dns_transaction_reset_answer(t);

        return 0;
}

static int on_transaction_timeout(sd_event_source *s, usec_t usec, void *userdata) {
        DnsTransaction *t = userdata;
        int r;

        assert(s);
        assert(t);

        if (!t->initial_jitter_scheduled || t->initial_jitter_elapsed) {
                /* Timeout reached? Increase the timeout for the server used */
                switch (t->scope->protocol) {

                case DNS_PROTOCOL_DNS:
                        assert(t->server);
                        dns_server_packet_lost(t->server, t->current_features, usec - t->start_usec);
                        break;

                case DNS_PROTOCOL_LLMNR:
                case DNS_PROTOCOL_MDNS:
                        dns_scope_packet_lost(t->scope, usec - t->start_usec);
                        break;

                default:
                        assert_not_reached("Invalid DNS protocol.");
                }

                if (t->initial_jitter_scheduled)
                        t->initial_jitter_elapsed = true;
        }

        log_debug("Timeout reached on transaction %" PRIu16 ".", t->id);

        /* ...and try again with a new server */
        dns_scope_next_dns_server(t->scope);

        r = dns_transaction_go(t);
        if (r < 0)
                dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);

        return 0;
}

static usec_t transaction_get_resend_timeout(DnsTransaction *t) {
        assert(t);
        assert(t->scope);

        switch (t->scope->protocol) {

        case DNS_PROTOCOL_DNS:
                assert(t->server);
                return t->server->resend_timeout;

        case DNS_PROTOCOL_MDNS:
                assert(t->n_attempts > 0);
                return (1 << (t->n_attempts - 1)) * USEC_PER_SEC;

        case DNS_PROTOCOL_LLMNR:
                return t->scope->resend_timeout;

        default:
                assert_not_reached("Invalid DNS protocol.");
        }
}

static int dns_transaction_prepare(DnsTransaction *t, usec_t ts) {
        int r;

        assert(t);

        dns_transaction_stop_timeout(t);

        if (t->n_attempts >= TRANSACTION_ATTEMPTS_MAX(t->scope->protocol)) {
                dns_transaction_complete(t, DNS_TRANSACTION_ATTEMPTS_MAX_REACHED);
                return 0;
        }

        if (t->scope->protocol == DNS_PROTOCOL_LLMNR && t->tried_stream) {
                /* If we already tried via a stream, then we don't
                 * retry on LLMNR. See RFC 4795, Section 2.7. */
                dns_transaction_complete(t, DNS_TRANSACTION_ATTEMPTS_MAX_REACHED);
                return 0;
        }

        t->n_attempts++;
        t->start_usec = ts;

        dns_transaction_reset_answer(t);

        /* Check the trust anchor. Do so only on classic DNS, since DNSSEC does not apply otherwise. */
        if (t->scope->protocol == DNS_PROTOCOL_DNS) {
                r = dns_trust_anchor_lookup_positive(&t->scope->manager->trust_anchor, t->key, &t->answer);
                if (r < 0)
                        return r;
                if (r > 0) {
                        t->answer_rcode = DNS_RCODE_SUCCESS;
                        t->answer_source = DNS_TRANSACTION_TRUST_ANCHOR;
                        t->answer_authenticated = true;
                        dns_transaction_complete(t, DNS_TRANSACTION_SUCCESS);
                        return 0;
                }

                if (dns_name_is_root(DNS_RESOURCE_KEY_NAME(t->key)) &&
                    t->key->type == DNS_TYPE_DS) {

                        /* Hmm, this is a request for the root DS? A
                         * DS RR doesn't exist in the root zone, and
                         * if our trust anchor didn't know it either,
                         * this means we cannot do any DNSSEC logic
                         * anymore. */

                        if (t->scope->dnssec_mode == DNSSEC_DOWNGRADE_OK) {
                                /* We are in downgrade mode. In this
                                 * case, synthesize an unsigned empty
                                 * response, so that the any lookup
                                 * depending on this one can continue
                                 * assuming there was no DS, and hence
                                 * the root zone was unsigned. */

                                t->answer_rcode = DNS_RCODE_SUCCESS;
                                t->answer_source = DNS_TRANSACTION_TRUST_ANCHOR;
                                t->answer_authenticated = false;
                                dns_transaction_complete(t, DNS_TRANSACTION_SUCCESS);
                        } else
                                /* If we are not in downgrade mode,
                                 * then fail the lookup, because we
                                 * cannot reasonably answer it. There
                                 * might be DS RRs, but we don't know
                                 * them, and the DNS server won't tell
                                 * them to us (and even if it would,
                                 * we couldn't validate it and trust
                                 * it). */
                                dns_transaction_complete(t, DNS_TRANSACTION_NO_TRUST_ANCHOR);

                        return 0;
                }
        }

        /* Check the zone, but only if this transaction is not used
         * for probing or verifying a zone item. */
        if (set_isempty(t->notify_zone_items)) {

                r = dns_zone_lookup(&t->scope->zone, t->key, &t->answer, NULL, NULL);
                if (r < 0)
                        return r;
                if (r > 0) {
                        t->answer_rcode = DNS_RCODE_SUCCESS;
                        t->answer_source = DNS_TRANSACTION_ZONE;
                        t->answer_authenticated = true;
                        dns_transaction_complete(t, DNS_TRANSACTION_SUCCESS);
                        return 0;
                }
        }

        /* Check the cache, but only if this transaction is not used
         * for probing or verifying a zone item. */
        if (set_isempty(t->notify_zone_items)) {

                /* Before trying the cache, let's make sure we figured out a
                 * server to use. Should this cause a change of server this
                 * might flush the cache. */
                dns_scope_get_dns_server(t->scope);

                /* Let's then prune all outdated entries */
                dns_cache_prune(&t->scope->cache);

                r = dns_cache_lookup(&t->scope->cache, t->key, &t->answer_rcode, &t->answer, &t->answer_authenticated);
                if (r < 0)
                        return r;
                if (r > 0) {
                        t->answer_source = DNS_TRANSACTION_CACHE;
                        if (t->answer_rcode == DNS_RCODE_SUCCESS)
                                dns_transaction_complete(t, DNS_TRANSACTION_SUCCESS);
                        else
                                dns_transaction_complete(t, DNS_TRANSACTION_RCODE_FAILURE);
                        return 0;
                }
        }

        return 1;
}

static int dns_transaction_make_packet_mdns(DnsTransaction *t) {

        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        bool add_known_answers = false;
        DnsTransaction *other;
        unsigned qdcount;
        usec_t ts;
        int r;

        assert(t);
        assert(t->scope->protocol == DNS_PROTOCOL_MDNS);

        /* Discard any previously prepared packet, so we can start over and coalesce again */
        t->sent = dns_packet_unref(t->sent);

        r = dns_packet_new_query(&p, t->scope->protocol, 0, false);
        if (r < 0)
                return r;

        r = dns_packet_append_key(p, t->key, NULL);
        if (r < 0)
                return r;

        qdcount = 1;

        if (dns_key_is_shared(t->key))
                add_known_answers = true;

        /*
         * For mDNS, we want to coalesce as many open queries in pending transactions into one single
         * query packet on the wire as possible. To achieve that, we iterate through all pending transactions
         * in our current scope, and see whether their timing contraints allow them to be sent.
         */

        assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);

        LIST_FOREACH(transactions_by_scope, other, t->scope->transactions) {

                /* Skip ourselves */
                if (other == t)
                        continue;

                if (other->state != DNS_TRANSACTION_PENDING)
                        continue;

                if (other->next_attempt_after > ts)
                        continue;

                if (qdcount >= UINT16_MAX)
                        break;

                r = dns_packet_append_key(p, other->key, NULL);

                /*
                 * If we can't stuff more questions into the packet, just give up.
                 * One of the 'other' transactions will fire later and take care of the rest.
                 */
                if (r == -EMSGSIZE)
                        break;

                if (r < 0)
                        return r;

                r = dns_transaction_prepare(other, ts);
                if (r <= 0)
                        continue;

                ts += transaction_get_resend_timeout(other);

                r = sd_event_add_time(
                                other->scope->manager->event,
                                &other->timeout_event_source,
                                clock_boottime_or_monotonic(),
                                ts, 0,
                                on_transaction_timeout, other);
                if (r < 0)
                        return r;

                other->state = DNS_TRANSACTION_PENDING;
                other->next_attempt_after = ts;

                qdcount ++;

                if (dns_key_is_shared(other->key))
                        add_known_answers = true;
        }

        DNS_PACKET_HEADER(p)->qdcount = htobe16(qdcount);

        /* Append known answer section if we're asking for any shared record */
        if (add_known_answers) {
                r = dns_cache_export_shared_to_packet(&t->scope->cache, p);
                if (r < 0)
                        return r;
        }

        t->sent = p;
        p = NULL;

        return 0;
}

static int dns_transaction_make_packet(DnsTransaction *t) {
        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        int r;

        assert(t);

        if (t->scope->protocol == DNS_PROTOCOL_MDNS)
                return dns_transaction_make_packet_mdns(t);

        if (t->sent)
                return 0;

        r = dns_packet_new_query(&p, t->scope->protocol, 0, t->scope->dnssec_mode != DNSSEC_NO);
        if (r < 0)
                return r;

        r = dns_scope_good_key(t->scope, t->key);
        if (r < 0)
                return r;
        if (r == 0)
                return -EDOM;

        r = dns_packet_append_key(p, t->key, NULL);
        if (r < 0)
                return r;

        DNS_PACKET_HEADER(p)->qdcount = htobe16(1);
        DNS_PACKET_HEADER(p)->id = t->id;

        t->sent = p;
        p = NULL;

        return 0;
}

int dns_transaction_go(DnsTransaction *t) {
        usec_t ts;
        int r;

        assert(t);

        assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &ts) >= 0);

        r = dns_transaction_prepare(t, ts);
        if (r <= 0)
                return r;

        log_debug("Excercising transaction %" PRIu16 " for <%s> on scope %s on %s/%s.",
                  t->id,
                  dns_transaction_key_string(t),
                  dns_protocol_to_string(t->scope->protocol),
                  t->scope->link ? t->scope->link->name : "*",
                  t->scope->family == AF_UNSPEC ? "*" : af_to_name(t->scope->family));

        if (!t->initial_jitter_scheduled &&
            (t->scope->protocol == DNS_PROTOCOL_LLMNR ||
             t->scope->protocol == DNS_PROTOCOL_MDNS)) {
                usec_t jitter, accuracy;

                /* RFC 4795 Section 2.7 suggests all queries should be
                 * delayed by a random time from 0 to JITTER_INTERVAL. */

                t->initial_jitter_scheduled = true;

                random_bytes(&jitter, sizeof(jitter));

                switch (t->scope->protocol) {

                case DNS_PROTOCOL_LLMNR:
                        jitter %= LLMNR_JITTER_INTERVAL_USEC;
                        accuracy = LLMNR_JITTER_INTERVAL_USEC;
                        break;

                case DNS_PROTOCOL_MDNS:
                        jitter %= MDNS_JITTER_RANGE_USEC;
                        jitter += MDNS_JITTER_MIN_USEC;
                        accuracy = MDNS_JITTER_RANGE_USEC;
                        break;
                default:
                        assert_not_reached("bad protocol");
                }

                r = sd_event_add_time(
                                t->scope->manager->event,
                                &t->timeout_event_source,
                                clock_boottime_or_monotonic(),
                                ts + jitter, accuracy,
                                on_transaction_timeout, t);
                if (r < 0)
                        return r;

                t->n_attempts = 0;
                t->next_attempt_after = ts;
                t->state = DNS_TRANSACTION_PENDING;

                log_debug("Delaying %s transaction for " USEC_FMT "us.", dns_protocol_to_string(t->scope->protocol), jitter);
                return 0;
        }

        /* Otherwise, we need to ask the network */
        r = dns_transaction_make_packet(t);
        if (r == -EDOM) {
                /* Not the right request to make on this network?
                 * (i.e. an A request made on IPv6 or an AAAA request
                 * made on IPv4, on LLMNR or mDNS.) */
                dns_transaction_complete(t, DNS_TRANSACTION_NO_SERVERS);
                return 0;
        }
        if (r < 0)
                return r;

        if (t->scope->protocol == DNS_PROTOCOL_LLMNR &&
            (dns_name_endswith(DNS_RESOURCE_KEY_NAME(t->key), "in-addr.arpa") > 0 ||
             dns_name_endswith(DNS_RESOURCE_KEY_NAME(t->key), "ip6.arpa") > 0)) {

                /* RFC 4795, Section 2.4. says reverse lookups shall
                 * always be made via TCP on LLMNR */
                r = dns_transaction_open_tcp(t);
        } else {
                /* Try via UDP, and if that fails due to large size or lack of
                 * support try via TCP */
                r = dns_transaction_emit_udp(t);
                if (r == -EMSGSIZE || r == -EAGAIN)
                        r = dns_transaction_open_tcp(t);
        }

        if (r == -ESRCH) {
                /* No servers to send this to? */
                dns_transaction_complete(t, DNS_TRANSACTION_NO_SERVERS);
                return 0;
        } else if (r < 0) {
                if (t->scope->protocol != DNS_PROTOCOL_DNS) {
                        dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
                        return 0;
                }

                /* Couldn't send? Try immediately again, with a new server */
                dns_scope_next_dns_server(t->scope);

                return dns_transaction_go(t);
        }

        ts += transaction_get_resend_timeout(t);

        r = sd_event_add_time(
                        t->scope->manager->event,
                        &t->timeout_event_source,
                        clock_boottime_or_monotonic(),
                        ts, 0,
                        on_transaction_timeout, t);
        if (r < 0)
                return r;

        t->state = DNS_TRANSACTION_PENDING;
        t->next_attempt_after = ts;

        return 1;
}

static int dns_transaction_find_cyclic(DnsTransaction *t, DnsTransaction *aux) {
        DnsTransaction *n;
        Iterator i;
        int r;

        assert(t);
        assert(aux);

        /* Try to find cyclic dependencies between transaction objects */

        if (t == aux)
                return 1;

        SET_FOREACH(n, aux->notify_transactions, i) {
                r = dns_transaction_find_cyclic(t, n);
                if (r != 0)
                        return r;
        }

        return r;
}

static int dns_transaction_add_dnssec_transaction(DnsTransaction *t, DnsResourceKey *key, DnsTransaction **ret) {
        DnsTransaction *aux;
        int r;

        assert(t);
        assert(ret);
        assert(key);

        aux = dns_scope_find_transaction(t->scope, key, true);
        if (!aux) {
                r = dns_transaction_new(&aux, t->scope, key);
                if (r < 0)
                        return r;
        } else {
                if (set_contains(t->dnssec_transactions, aux)) {
                        *ret = aux;
                        return 0;
                }

                r = dns_transaction_find_cyclic(t, aux);
                if (r < 0)
                        return r;
                if (r > 0) {
                        log_debug("Detected potential cyclic dependency, refusing to add transaction %" PRIu16 " (%s) as dependency for %" PRIu16 " (%s).",
                                  aux->id,
                                  strna(dns_transaction_key_string(aux)),
                                  t->id,
                                  strna(dns_transaction_key_string(t)));
                        return -ELOOP;
                }
        }

        r = set_ensure_allocated(&t->dnssec_transactions, NULL);
        if (r < 0)
                goto gc;

        r = set_ensure_allocated(&aux->notify_transactions, NULL);
        if (r < 0)
                goto gc;

        r = set_put(t->dnssec_transactions, aux);
        if (r < 0)
                goto gc;

        r = set_put(aux->notify_transactions, t);
        if (r < 0) {
                (void) set_remove(t->dnssec_transactions, aux);
                goto gc;
        }

        *ret = aux;
        return 1;

gc:
        dns_transaction_gc(aux);
        return r;
}

static int dns_transaction_request_dnssec_rr(DnsTransaction *t, DnsResourceKey *key) {
        _cleanup_(dns_answer_unrefp) DnsAnswer *a = NULL;
        DnsTransaction *aux;
        int r;

        assert(t);
        assert(key);

        /* Try to get the data from the trust anchor */
        r = dns_trust_anchor_lookup_positive(&t->scope->manager->trust_anchor, key, &a);
        if (r < 0)
                return r;
        if (r > 0) {
                r = dns_answer_extend(&t->validated_keys, a);
                if (r < 0)
                        return r;

                return 0;
        }

        /* This didn't work, ask for it via the network/cache then. */
        r = dns_transaction_add_dnssec_transaction(t, key, &aux);
        if (r == -ELOOP) /* This would result in a cyclic dependency */
                return 0;
        if (r < 0)
                return r;

        if (aux->state == DNS_TRANSACTION_NULL) {
                r = dns_transaction_go(aux);
                if (r < 0)
                        return r;
        }

        return 1;
}

static int dns_transaction_has_positive_answer(DnsTransaction *t, DnsAnswerFlags *flags) {
        int r;

        assert(t);

        /* Checks whether the answer is positive, i.e. either a direct
         * answer to the question, or a CNAME/DNAME for it */

        r = dns_answer_match_key(t->answer, t->key, flags);
        if (r != 0)
                return r;

        r = dns_answer_find_cname_or_dname(t->answer, t->key, NULL, flags);
        if (r != 0)
                return r;

        return false;
}

static int dns_transaction_has_unsigned_negative_answer(DnsTransaction *t) {
        int r;

        assert(t);

        /* Checks whether the answer is negative, and lacks NSEC/NSEC3
         * RRs to prove it */

        r = dns_transaction_has_positive_answer(t, NULL);
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        /* Is this key explicitly listed as a negative trust anchor?
         * If so, it's nothing we need to care about */
        r = dns_trust_anchor_lookup_negative(&t->scope->manager->trust_anchor, DNS_RESOURCE_KEY_NAME(t->key));
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        /* The answer does not contain any RRs that match to the
         * question. If so, let's see if there are any NSEC/NSEC3 RRs
         * included. If not, the answer is unsigned. */

        r = dns_answer_contains_nsec_or_nsec3(t->answer);
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        return true;
}

static int dns_transaction_is_primary_response(DnsTransaction *t, DnsResourceRecord *rr) {
        int r;

        assert(t);
        assert(rr);

        /* Check if the specified RR is the "primary" response,
         * i.e. either matches the question precisely or is a
         * CNAME/DNAME for it, or is any kind of NSEC/NSEC3 RR */

        r = dns_resource_key_match_rr(t->key, rr, NULL);
        if (r != 0)
                return r;

        r = dns_resource_key_match_cname_or_dname(t->key, rr->key, NULL);
        if (r != 0)
                return r;

        if (rr->key->type == DNS_TYPE_NSEC3) {
                const char *p;

                p = DNS_RESOURCE_KEY_NAME(rr->key);
                r = dns_name_parent(&p);
                if (r < 0)
                        return r;
                if (r > 0) {
                        r = dns_name_endswith(DNS_RESOURCE_KEY_NAME(t->key), p);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                return true;
                }
        }

        return rr->key->type == DNS_TYPE_NSEC;
}

int dns_transaction_request_dnssec_keys(DnsTransaction *t) {
        DnsResourceRecord *rr;

        int r;

        assert(t);

        /*
         * Retrieve all auxiliary RRs for the answer we got, so that
         * we can verify signatures or prove that RRs are rightfully
         * unsigned. Specifically:
         *
         * - For RRSIG we get the matching DNSKEY
         * - For DNSKEY we get the matching DS
         * - For unsigned SOA/NS we get the matching DS
         * - For unsigned CNAME/DNAME/DS we get the parent SOA RR
         * - For other unsigned RRs we get the matching SOA RR
         * - For SOA/NS/DS queries with no matching response RRs, and no NSEC/NSEC3, the parent's SOA RR
         * - For other queries with no matching response RRs, and no NSEC/NSEC3, the SOA RR
         */

        if (t->scope->dnssec_mode == DNSSEC_NO)
                return 0;

        if (t->current_features < DNS_SERVER_FEATURE_LEVEL_DO)
                return 0; /* Server doesn't do DNSSEC, there's no point in requesting any RRs then. */
        if (t->server && t->server->rrsig_missing)
                return 0; /* Server handles DNSSEC requests, but isn't augmenting responses with RRSIGs. No point in trying DNSSEC then. */

        DNS_ANSWER_FOREACH(rr, t->answer) {

                if (dns_type_is_pseudo(rr->key->type))
                        continue;

                /* If this RR is in the negative trust anchor, we don't need to validate it. */
                r = dns_trust_anchor_lookup_negative(&t->scope->manager->trust_anchor, DNS_RESOURCE_KEY_NAME(rr->key));
                if (r < 0)
                        return r;
                if (r > 0)
                        continue;

                switch (rr->key->type) {

                case DNS_TYPE_RRSIG: {
                        /* For each RRSIG we request the matching DNSKEY */
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *dnskey = NULL;

                        /* If this RRSIG is about a DNSKEY RR and the
                         * signer is the same as the owner, then we
                         * already have the DNSKEY, and we don't have
                         * to look for more. */
                        if (rr->rrsig.type_covered == DNS_TYPE_DNSKEY) {
                                r = dns_name_equal(rr->rrsig.signer, DNS_RESOURCE_KEY_NAME(rr->key));
                                if (r < 0)
                                        return r;
                                if (r > 0)
                                        continue;
                        }

                        /* If the signer is not a parent of our
                         * original query, then this is about an
                         * auxiliary RRset, but not anything we asked
                         * for. In this case we aren't interested,
                         * because we don't want to request additional
                         * RRs for stuff we didn't really ask for, and
                         * also to avoid request loops, where
                         * additional RRs from one transaction result
                         * in another transaction whose additonal RRs
                         * point back to the original transaction, and
                         * we deadlock. */
                        r = dns_name_endswith(DNS_RESOURCE_KEY_NAME(t->key), rr->rrsig.signer);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        dnskey = dns_resource_key_new(rr->key->class, DNS_TYPE_DNSKEY, rr->rrsig.signer);
                        if (!dnskey)
                                return -ENOMEM;

                        log_debug("Requesting DNSKEY to validate transaction %" PRIu16" (%s, RRSIG with key tag: %" PRIu16 ").", t->id, DNS_RESOURCE_KEY_NAME(rr->key), rr->rrsig.key_tag);
                        r = dns_transaction_request_dnssec_rr(t, dnskey);
                        if (r < 0)
                                return r;
                        break;
                }

                case DNS_TYPE_DNSKEY: {
                        /* For each DNSKEY we request the matching DS */
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *ds = NULL;

                        /* If the DNSKEY we are looking at is not for
                         * zone we are interested in, nor any of its
                         * parents, we aren't interested, and don't
                         * request it. After all, we don't want to end
                         * up in request loops, and want to keep
                         * additional traffic down. */

                        r = dns_name_endswith(DNS_RESOURCE_KEY_NAME(t->key), DNS_RESOURCE_KEY_NAME(rr->key));
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        ds = dns_resource_key_new(rr->key->class, DNS_TYPE_DS, DNS_RESOURCE_KEY_NAME(rr->key));
                        if (!ds)
                                return -ENOMEM;

                        log_debug("Requesting DS to validate transaction %" PRIu16" (%s, DNSKEY with key tag: %" PRIu16 ").", t->id, DNS_RESOURCE_KEY_NAME(rr->key), dnssec_keytag(rr, false));
                        r = dns_transaction_request_dnssec_rr(t, ds);
                        if (r < 0)
                                return r;

                        break;
                }

                case DNS_TYPE_SOA:
                case DNS_TYPE_NS: {
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *ds = NULL;

                        /* For an unsigned SOA or NS, try to acquire
                         * the matching DS RR, as we are at a zone cut
                         * then, and whether a DS exists tells us
                         * whether the zone is signed. Do so only if
                         * this RR matches our original question,
                         * however. */

                        r = dns_resource_key_match_rr(t->key, rr, NULL);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        r = dnssec_has_rrsig(t->answer, rr->key);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                continue;

                        ds = dns_resource_key_new(rr->key->class, DNS_TYPE_DS, DNS_RESOURCE_KEY_NAME(rr->key));
                        if (!ds)
                                return -ENOMEM;

                        log_debug("Requesting DS to validate transaction %" PRIu16 " (%s, unsigned SOA/NS RRset).", t->id, DNS_RESOURCE_KEY_NAME(rr->key));
                        r = dns_transaction_request_dnssec_rr(t, ds);
                        if (r < 0)
                                return r;

                        break;
                }

                case DNS_TYPE_DS:
                case DNS_TYPE_CNAME:
                case DNS_TYPE_DNAME: {
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *soa = NULL;
                        const char *name;

                        /* CNAMEs and DNAMEs cannot be located at a
                         * zone apex, hence ask for the parent SOA for
                         * unsigned CNAME/DNAME RRs, maybe that's the
                         * apex. But do all that only if this is
                         * actually a response to our original
                         * question.
                         *
                         * Similar for DS RRs, which are signed when
                         * the parent SOA is signed. */

                        r = dns_transaction_is_primary_response(t, rr);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        r = dnssec_has_rrsig(t->answer, rr->key);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                continue;

                        name = DNS_RESOURCE_KEY_NAME(rr->key);
                        r = dns_name_parent(&name);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        soa = dns_resource_key_new(rr->key->class, DNS_TYPE_SOA, name);
                        if (!soa)
                                return -ENOMEM;

                        log_debug("Requesting parent SOA to validate transaction %" PRIu16 " (%s, unsigned CNAME/DNAME/DS RRset).", t->id, DNS_RESOURCE_KEY_NAME(rr->key));
                        r = dns_transaction_request_dnssec_rr(t, soa);
                        if (r < 0)
                                return r;

                        break;
                }

                default: {
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *soa = NULL;

                        /* For other unsigned RRsets (including
                         * NSEC/NSEC3!), look for proof the zone is
                         * unsigned, by requesting the SOA RR of the
                         * zone. However, do so only if they are
                         * directly relevant to our original
                         * question. */

                        r = dns_transaction_is_primary_response(t, rr);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        r = dnssec_has_rrsig(t->answer, rr->key);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                continue;

                        soa = dns_resource_key_new(rr->key->class, DNS_TYPE_SOA, DNS_RESOURCE_KEY_NAME(rr->key));
                        if (!soa)
                                return -ENOMEM;

                        log_debug("Requesting SOA to validate transaction %" PRIu16 " (%s, unsigned non-SOA/NS RRset <%s>).", t->id, DNS_RESOURCE_KEY_NAME(rr->key), dns_resource_record_to_string(rr));
                        r = dns_transaction_request_dnssec_rr(t, soa);
                        if (r < 0)
                                return r;
                        break;
                }}
        }

        /* Above, we requested everything necessary to validate what
         * we got. Now, let's request what we need to validate what we
         * didn't get... */

        r = dns_transaction_has_unsigned_negative_answer(t);
        if (r < 0)
                return r;
        if (r > 0) {
                const char *name;

                name = DNS_RESOURCE_KEY_NAME(t->key);

                /* If this was a SOA or NS request, then this
                 * indicates that we are not at a zone apex, hence ask
                 * the parent name instead. If this was a DS request,
                 * then it's signed when the parent zone is signed,
                 * hence ask the parent in that case, too. */

                if (IN_SET(t->key->type, DNS_TYPE_SOA, DNS_TYPE_NS, DNS_TYPE_DS)) {
                        r = dns_name_parent(&name);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                log_debug("Requesting parent SOA to validate transaction %" PRIu16 " (%s, unsigned empty SOA/NS/DS response).", t->id, DNS_RESOURCE_KEY_NAME(t->key));
                        else
                                name = NULL;
                } else
                        log_debug("Requesting SOA to validate transaction %" PRIu16 " (%s, unsigned empty non-SOA/NS/DS response).", t->id, DNS_RESOURCE_KEY_NAME(t->key));

                if (name) {
                        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *soa = NULL;

                        soa = dns_resource_key_new(t->key->class, DNS_TYPE_SOA, name);
                        if (!soa)
                                return -ENOMEM;

                        r = dns_transaction_request_dnssec_rr(t, soa);
                        if (r < 0)
                                return r;
                }
        }

        return dns_transaction_dnssec_is_live(t);
}

void dns_transaction_notify(DnsTransaction *t, DnsTransaction *source) {
        int r;

        assert(t);
        assert(source);

        if (!IN_SET(t->state, DNS_TRANSACTION_PENDING, DNS_TRANSACTION_VALIDATING))
                return;

        /* Invoked whenever any of our auxiliary DNSSEC transactions
           completed its work. We copy any RRs from that transaction
           over into our list of validated keys -- but only if the
           answer is authenticated.

           Note that we fail our transaction if the auxiliary
           transaction failed, except on NXDOMAIN. This is because
           some broken DNS servers (Akamai...) will return NXDOMAIN
           for empty non-terminals. */

        switch (source->state) {

        case DNS_TRANSACTION_DNSSEC_FAILED:

                log_debug("Auxiliary DNSSEC RR query failed validation: %s", dnssec_result_to_string(source->answer_dnssec_result));
                t->answer_dnssec_result = source->answer_dnssec_result; /* Copy error code over */
                dns_transaction_complete(t, DNS_TRANSACTION_DNSSEC_FAILED);
                break;

        case DNS_TRANSACTION_RCODE_FAILURE:

                if (source->answer_rcode != DNS_RCODE_NXDOMAIN) {
                        log_debug("Auxiliary DNSSEC RR query failed with rcode=%i.", source->answer_rcode);
                        goto fail;
                }

                /* fall-through: NXDOMAIN is good enough for us */

        case DNS_TRANSACTION_SUCCESS:
                if (source->answer_authenticated) {
                        r = dns_answer_extend(&t->validated_keys, source->answer);
                        if (r < 0) {
                                log_error_errno(r, "Failed to merge validated DNSSEC key data: %m");
                                goto fail;
                        }
                }

                /* If the state is still PENDING, we are still in the loop
                 * that adds further DNSSEC transactions, hence don't check if
                 * we are ready yet. If the state is VALIDATING however, we
                 * should check if we are complete now. */
                if (t->state == DNS_TRANSACTION_VALIDATING)
                        dns_transaction_process_dnssec(t);
                break;

        default:
                log_debug("Auxiliary DNSSEC RR query failed with %s", dns_transaction_state_to_string(source->state));
                goto fail;
        }

        return;

fail:
        t->answer_dnssec_result = DNSSEC_FAILED_AUXILIARY;
        dns_transaction_complete(t, DNS_TRANSACTION_DNSSEC_FAILED);
}

static int dns_transaction_validate_dnskey_by_ds(DnsTransaction *t) {
        DnsResourceRecord *rr;
        int ifindex, r;

        assert(t);

        /* Add all DNSKEY RRs from the answer that are validated by DS
         * RRs from the list of validated keys to the list of
         * validated keys. */

        DNS_ANSWER_FOREACH_IFINDEX(rr, ifindex, t->answer) {

                r = dnssec_verify_dnskey_search(rr, t->validated_keys);
                if (r < 0)
                        return r;
                if (r == 0)
                        continue;

                /* If so, the DNSKEY is validated too. */
                r = dns_answer_add_extend(&t->validated_keys, rr, ifindex, DNS_ANSWER_AUTHENTICATED);
                if (r < 0)
                        return r;
        }

        return 0;
}

static int dns_transaction_requires_rrsig(DnsTransaction *t, DnsResourceRecord *rr) {
        int r;

        assert(t);
        assert(rr);

        /* Checks if the RR we are looking for must be signed with an
         * RRSIG. This is used for positive responses. */

        if (t->scope->dnssec_mode == DNSSEC_NO)
                return false;

        if (dns_type_is_pseudo(rr->key->type))
                return -EINVAL;

        r = dns_trust_anchor_lookup_negative(&t->scope->manager->trust_anchor, DNS_RESOURCE_KEY_NAME(rr->key));
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        switch (rr->key->type) {

        case DNS_TYPE_RRSIG:
                /* RRSIGs are the signatures themselves, they need no signing. */
                return false;

        case DNS_TYPE_SOA:
        case DNS_TYPE_NS: {
                DnsTransaction *dt;
                Iterator i;

                /* For SOA or NS RRs we look for a matching DS transaction */

                SET_FOREACH(dt, t->dnssec_transactions, i) {

                        if (dt->key->class != rr->key->class)
                                continue;
                        if (dt->key->type != DNS_TYPE_DS)
                                continue;

                        r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), DNS_RESOURCE_KEY_NAME(rr->key));
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        /* We found a DS transactions for the SOA/NS
                         * RRs we are looking at. If it discovered signed DS
                         * RRs, then we need to be signed, too. */

                        if (!dt->answer_authenticated)
                                return false;

                        return dns_answer_match_key(dt->answer, dt->key, NULL);
                }

                /* We found nothing that proves this is safe to leave
                 * this unauthenticated, hence ask inist on
                 * authentication. */
                return true;
        }

        case DNS_TYPE_DS:
        case DNS_TYPE_CNAME:
        case DNS_TYPE_DNAME: {
                const char *parent = NULL;
                DnsTransaction *dt;
                Iterator i;

                /*
                 * CNAME/DNAME RRs cannot be located at a zone apex, hence look directly for the parent SOA.
                 *
                 * DS RRs are signed if the parent is signed, hence also look at the parent SOA
                 */

                SET_FOREACH(dt, t->dnssec_transactions, i) {

                        if (dt->key->class != rr->key->class)
                                continue;
                        if (dt->key->type != DNS_TYPE_SOA)
                                continue;

                        if (!parent) {
                                parent = DNS_RESOURCE_KEY_NAME(rr->key);
                                r = dns_name_parent(&parent);
                                if (r < 0)
                                        return r;
                                if (r == 0) {
                                        if (rr->key->type == DNS_TYPE_DS)
                                                return true;

                                        /* A CNAME/DNAME without a parent? That's sooo weird. */
                                        log_debug("Transaction %" PRIu16 " claims CNAME/DNAME at root. Refusing.", t->id);
                                        return -EBADMSG;
                                }
                        }

                        r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), parent);
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        return t->answer_authenticated;
                }

                return true;
        }

        default: {
                DnsTransaction *dt;
                Iterator i;

                /* Any other kind of RR (including DNSKEY/NSEC/NSEC3). Let's see if our SOA lookup was authenticated */

                SET_FOREACH(dt, t->dnssec_transactions, i) {

                        if (dt->key->class != rr->key->class)
                                continue;
                        if (dt->key->type != DNS_TYPE_SOA)
                                continue;

                        r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), DNS_RESOURCE_KEY_NAME(rr->key));
                        if (r < 0)
                                return r;
                        if (r == 0)
                                continue;

                        /* We found the transaction that was supposed to find
                         * the SOA RR for us. It was successful, but found no
                         * RR for us. This means we are not at a zone cut. In
                         * this case, we require authentication if the SOA
                         * lookup was authenticated too. */
                        return t->answer_authenticated;
                }

                return true;
        }}
}

static int dns_transaction_requires_nsec(DnsTransaction *t) {
        DnsTransaction *dt;
        const char *name;
        Iterator i;
        int r;

        assert(t);

        /* Checks if we need to insist on NSEC/NSEC3 RRs for proving
         * this negative reply */

        if (t->scope->dnssec_mode == DNSSEC_NO)
                return false;

        if (dns_type_is_pseudo(t->key->type))
                return -EINVAL;

        r = dns_trust_anchor_lookup_negative(&t->scope->manager->trust_anchor, DNS_RESOURCE_KEY_NAME(t->key));
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        name = DNS_RESOURCE_KEY_NAME(t->key);

        if (IN_SET(t->key->type, DNS_TYPE_SOA, DNS_TYPE_NS, DNS_TYPE_DS)) {

                /* We got a negative reply for this SOA/NS lookup? If
                 * so, then we are not at a zone apex, and thus should
                 * look at the result of the parent SOA lookup.
                 *
                 * We got a negative reply for this DS lookup? DS RRs
                 * are signed when their parent zone is signed, hence
                 * also check the parent SOA in this case. */

                r = dns_name_parent(&name);
                if (r < 0)
                        return r;
                if (r == 0)
                        return true;
        }

        /* For all other RRs we check the SOA on the same level to see
         * if it's signed. */

        SET_FOREACH(dt, t->dnssec_transactions, i) {

                if (dt->key->class != t->key->class)
                        continue;
                if (dt->key->type != DNS_TYPE_SOA)
                        continue;

                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), name);
                if (r < 0)
                        return r;
                if (r == 0)
                        continue;

                return dt->answer_authenticated;
        }

        /* If in doubt, require NSEC/NSEC3 */
        return true;
}

static int dns_transaction_dnskey_authenticated(DnsTransaction *t, DnsResourceRecord *rr) {
        DnsResourceRecord *rrsig;
        bool found = false;
        int r;

        /* Checks whether any of the DNSKEYs used for the RRSIGs for
         * the specified RRset is authenticated (i.e. has a matching
         * DS RR). */

        r = dns_trust_anchor_lookup_negative(&t->scope->manager->trust_anchor, DNS_RESOURCE_KEY_NAME(rr->key));
        if (r < 0)
                return r;
        if (r > 0)
                return false;

        DNS_ANSWER_FOREACH(rrsig, t->answer) {
                DnsTransaction *dt;
                Iterator i;

                r = dnssec_key_match_rrsig(rr->key, rrsig);
                if (r < 0)
                        return r;
                if (r == 0)
                        continue;

                SET_FOREACH(dt, t->dnssec_transactions, i) {

                        if (dt->key->class != rr->key->class)
                                continue;

                        if (dt->key->type == DNS_TYPE_DNSKEY) {

                                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), rrsig->rrsig.signer);
                                if (r < 0)
                                        return r;
                                if (r == 0)
                                        continue;

                                /* OK, we found an auxiliary DNSKEY
                                 * lookup. If that lookup is
                                 * authenticated, report this. */

                                if (dt->answer_authenticated)
                                        return true;

                                found = true;

                        } else if (dt->key->type == DNS_TYPE_DS) {

                                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(dt->key), rrsig->rrsig.signer);
                                if (r < 0)
                                        return r;
                                if (r == 0)
                                        continue;

                                /* OK, we found an auxiliary DS
                                 * lookup. If that lookup is
                                 * authenticated and non-zero, we
                                 * won! */

                                if (!dt->answer_authenticated)
                                        return false;

                                return dns_answer_match_key(dt->answer, dt->key, NULL);
                        }
                }
        }

        return found ? false : -ENXIO;
}

static int dns_transaction_known_signed(DnsTransaction *t, DnsResourceRecord *rr) {
        assert(t);
        assert(rr);

        /* We know that the root domain is signed, hence if it appears
         * not to be signed, there's a problem with the DNS server */

        return rr->key->class == DNS_CLASS_IN &&
                dns_name_is_root(DNS_RESOURCE_KEY_NAME(rr->key));
}

int dns_transaction_validate_dnssec(DnsTransaction *t) {
        _cleanup_(dns_answer_unrefp) DnsAnswer *validated = NULL;
        bool dnskeys_finalized = false;
        DnsResourceRecord *rr;
        DnsAnswerFlags flags;
        int r;

        assert(t);

        /* We have now collected all DS and DNSKEY RRs in
         * t->validated_keys, let's see which RRs we can now
         * authenticate with that. */

        if (t->scope->dnssec_mode == DNSSEC_NO)
                return 0;

        /* Already validated */
        if (t->answer_dnssec_result != _DNSSEC_RESULT_INVALID)
                return 0;

        /* Our own stuff needs no validation */
        if (IN_SET(t->answer_source, DNS_TRANSACTION_ZONE, DNS_TRANSACTION_TRUST_ANCHOR)) {
                t->answer_dnssec_result = DNSSEC_VALIDATED;
                t->answer_authenticated = true;
                return 0;
        }

        /* Cached stuff is not affected by validation. */
        if (t->answer_source != DNS_TRANSACTION_NETWORK)
                return 0;

        if (t->current_features < DNS_SERVER_FEATURE_LEVEL_DO ||
            (t->server && t->server->rrsig_missing)) {
                /* The server does not support DNSSEC, or doesn't augment responses with RRSIGs. */
                t->answer_dnssec_result = DNSSEC_INCOMPATIBLE_SERVER;
                return 0;
        }

        log_debug("Validating response from transaction %" PRIu16 " (%s).", t->id, dns_transaction_key_string(t));

        /* First see if there are DNSKEYs we already known a validated DS for. */
        r = dns_transaction_validate_dnskey_by_ds(t);
        if (r < 0)
                return r;

        for (;;) {
                bool changed = false;

                DNS_ANSWER_FOREACH(rr, t->answer) {
                        DnssecResult result;

                        if (rr->key->type == DNS_TYPE_RRSIG)
                                continue;

                        r = dnssec_verify_rrset_search(t->answer, rr->key, t->validated_keys, USEC_INFINITY, &result);
                        if (r < 0)
                                return r;

                        log_debug("Looking at %s: %s", strna(dns_resource_record_to_string(rr)), dnssec_result_to_string(result));

                        if (result == DNSSEC_VALIDATED) {

                                if (rr->key->type == DNS_TYPE_DNSKEY) {
                                        /* If we just validated a
                                         * DNSKEY RRset, then let's
                                         * add these keys to the set
                                         * of validated keys for this
                                         * transaction. */

                                        r = dns_answer_copy_by_key(&t->validated_keys, t->answer, rr->key, DNS_ANSWER_AUTHENTICATED);
                                        if (r < 0)
                                                return r;

                                        /* Maybe warn the user that we
                                         * encountered a revoked
                                         * DNSKEY for a key from our
                                         * trust anchor */
                                        r = dns_trust_anchor_check_revoked(&t->scope->manager->trust_anchor, t->answer, rr->key);
                                        if (r < 0)
                                                return r;
                                }

                                /* Add the validated RRset to the new
                                 * list of validated RRsets, and
                                 * remove it from the unvalidated
                                 * RRsets. We mark the RRset as
                                 * authenticated and cacheable. */
                                r = dns_answer_move_by_key(&validated, &t->answer, rr->key, DNS_ANSWER_AUTHENTICATED|DNS_ANSWER_CACHEABLE);
                                if (r < 0)
                                        return r;

                                t->scope->manager->n_dnssec_secure++;

                                /* Exit the loop, we dropped something from the answer, start from the beginning */
                                changed = true;
                                break;

                        } else if (dnskeys_finalized) {

                                /* If we haven't read all DNSKEYs yet
                                 * a negative result of the validation
                                 * is irrelevant, as there might be
                                 * more DNSKEYs coming. */

                                if (result == DNSSEC_NO_SIGNATURE) {
                                        r = dns_transaction_requires_rrsig(t, rr);
                                        if (r < 0)
                                                return r;
                                        if (r == 0) {
                                                /* Data does not require signing. In that case, just copy it over,
                                                 * but remember that this is by no means authenticated.*/
                                                r = dns_answer_move_by_key(&validated, &t->answer, rr->key, 0);
                                                if (r < 0)
                                                        return r;

                                                t->scope->manager->n_dnssec_insecure++;

                                                changed = true;
                                                break;
                                        }

                                        r = dns_transaction_known_signed(t, rr);
                                        if (r < 0)
                                                return r;
                                        if (r > 0) {
                                                /* This is an RR we know has to be signed. If it isn't this means
                                                 * the server is not attaching RRSIGs, hence complain. */

                                                dns_server_packet_rrsig_missing(t->server);

                                                if (t->scope->dnssec_mode == DNSSEC_DOWNGRADE_OK) {

                                                        /* Downgrading is OK? If so, just consider the information unsigned */

                                                        r = dns_answer_move_by_key(&validated, &t->answer, rr->key, 0);
                                                        if (r < 0)
                                                                return r;

                                                        t->scope->manager->n_dnssec_insecure++;
                                                        changed = true;
                                                        break;
                                                }

                                                /* Otherwise, fail */
                                                t->answer_dnssec_result = DNSSEC_INCOMPATIBLE_SERVER;
                                                return 0;
                                        }
                                }

                                if (IN_SET(result,
                                           DNSSEC_MISSING_KEY,
                                           DNSSEC_SIGNATURE_EXPIRED,
                                           DNSSEC_UNSUPPORTED_ALGORITHM)) {

                                        r = dns_transaction_dnskey_authenticated(t, rr);
                                        if (r < 0 && r != -ENXIO)
                                                return r;
                                        if (r == 0) {
                                                /* The DNSKEY transaction was not authenticated, this means there's
                                                 * no DS for this, which means it's OK if no keys are found for this signature. */

                                                r = dns_answer_move_by_key(&validated, &t->answer, rr->key, 0);
                                                if (r < 0)
                                                        return r;

                                                t->scope->manager->n_dnssec_insecure++;

                                                changed = true;
                                                break;
                                        }
                                }

                                if (IN_SET(result,
                                           DNSSEC_INVALID,
                                           DNSSEC_SIGNATURE_EXPIRED,
                                           DNSSEC_NO_SIGNATURE,
                                           DNSSEC_UNSUPPORTED_ALGORITHM))
                                        t->scope->manager->n_dnssec_bogus++;
                                else
                                        t->scope->manager->n_dnssec_indeterminate++;

                                r = dns_transaction_is_primary_response(t, rr);
                                if (r < 0)
                                        return r;
                                if (r > 0) {
                                        /* This is a primary response
                                         * to our question, and it
                                         * failed validation. That's
                                         * fatal. */
                                        t->answer_dnssec_result = result;
                                        return 0;
                                }

                                /* This is just some auxiliary
                                 * data. Just remove the RRset and
                                 * continue. */
                                r = dns_answer_remove_by_key(&t->answer, rr->key);
                                if (r < 0)
                                        return r;

                                /* Exit the loop, we dropped something from the answer, start from the beginning */
                                changed = true;
                                break;
                        }
                }

                if (changed)
                        continue;

                if (!dnskeys_finalized) {
                        /* OK, now we know we have added all DNSKEYs
                         * we possibly could to our validated
                         * list. Now run the whole thing once more,
                         * and strip everything we still cannot
                         * validate.
                         */
                        dnskeys_finalized = true;
                        continue;
                }

                /* We're done */
                break;
        }

        dns_answer_unref(t->answer);
        t->answer = validated;
        validated = NULL;

        /* At this point the answer only contains validated
         * RRsets. Now, let's see if it actually answers the question
         * we asked. If so, great! If it doesn't, then see if
         * NSEC/NSEC3 can prove this. */
        r = dns_transaction_has_positive_answer(t, &flags);
        if (r > 0) {
                /* Yes, it answers the question! */

                if (flags & DNS_ANSWER_AUTHENTICATED) {
                        /* The answer is fully authenticated, yay. */
                        t->answer_dnssec_result = DNSSEC_VALIDATED;
                        t->answer_rcode = DNS_RCODE_SUCCESS;
                        t->answer_authenticated = true;
                } else {
                        /* The answer is not fully authenticated. */
                        t->answer_dnssec_result = DNSSEC_UNSIGNED;
                        t->answer_authenticated = false;
                }

        } else if (r == 0) {
                DnssecNsecResult nr;
                bool authenticated = false;

                /* Bummer! Let's check NSEC/NSEC3 */
                r = dnssec_test_nsec(t->answer, t->key, &nr, &authenticated);
                if (r < 0)
                        return r;

                switch (nr) {

                case DNSSEC_NSEC_NXDOMAIN:
                        /* NSEC proves the domain doesn't exist. Very good. */
                        log_debug("Proved NXDOMAIN via NSEC/NSEC3 for transaction %u (%s)", t->id, dns_transaction_key_string(t));
                        t->answer_dnssec_result = DNSSEC_VALIDATED;
                        t->answer_rcode = DNS_RCODE_NXDOMAIN;
                        t->answer_authenticated = authenticated;
                        break;

                case DNSSEC_NSEC_NODATA:
                        /* NSEC proves that there's no data here, very good. */
                        log_debug("Proved NODATA via NSEC/NSEC3 for transaction %u (%s)", t->id, dns_transaction_key_string(t));
                        t->answer_dnssec_result = DNSSEC_VALIDATED;
                        t->answer_rcode = DNS_RCODE_SUCCESS;
                        t->answer_authenticated = authenticated;
                        break;

                case DNSSEC_NSEC_OPTOUT:
                        /* NSEC3 says the data might not be signed */
                        log_debug("Data is NSEC3 opt-out via NSEC/NSEC3 for transaction %u (%s)", t->id, dns_transaction_key_string(t));
                        t->answer_dnssec_result = DNSSEC_UNSIGNED;
                        t->answer_authenticated = false;
                        break;

                case DNSSEC_NSEC_NO_RR:
                        /* No NSEC data? Bummer! */

                        r = dns_transaction_requires_nsec(t);
                        if (r < 0)
                                return r;
                        if (r > 0)
                                t->answer_dnssec_result = DNSSEC_NO_SIGNATURE;
                        else {
                                t->answer_dnssec_result = DNSSEC_UNSIGNED;
                                t->answer_authenticated = false;
                        }

                        break;

                case DNSSEC_NSEC_UNSUPPORTED_ALGORITHM:
                        /* We don't know the NSEC3 algorithm used? */
                        t->answer_dnssec_result = DNSSEC_UNSUPPORTED_ALGORITHM;
                        break;

                case DNSSEC_NSEC_FOUND:
                case DNSSEC_NSEC_CNAME:
                        /* NSEC says it needs to be there, but we couldn't find it? Bummer! */
                        t->answer_dnssec_result = DNSSEC_NSEC_MISMATCH;
                        break;

                default:
                        assert_not_reached("Unexpected NSEC result.");
                }
        }

        return 1;
}

const char *dns_transaction_key_string(DnsTransaction *t) {
        assert(t);

        if (!t->key_string) {
                if (dns_resource_key_to_string(t->key, &t->key_string) < 0)
                        return "n/a";
        }

        return strstrip(t->key_string);
}

static const char* const dns_transaction_state_table[_DNS_TRANSACTION_STATE_MAX] = {
        [DNS_TRANSACTION_NULL] = "null",
        [DNS_TRANSACTION_PENDING] = "pending",
        [DNS_TRANSACTION_VALIDATING] = "validating",
        [DNS_TRANSACTION_RCODE_FAILURE] = "rcode-failure",
        [DNS_TRANSACTION_SUCCESS] = "success",
        [DNS_TRANSACTION_NO_SERVERS] = "no-servers",
        [DNS_TRANSACTION_TIMEOUT] = "timeout",
        [DNS_TRANSACTION_ATTEMPTS_MAX_REACHED] = "attempts-max-reached",
        [DNS_TRANSACTION_INVALID_REPLY] = "invalid-reply",
        [DNS_TRANSACTION_RESOURCES] = "resources",
        [DNS_TRANSACTION_CONNECTION_FAILURE] = "connection-failure",
        [DNS_TRANSACTION_ABORTED] = "aborted",
        [DNS_TRANSACTION_DNSSEC_FAILED] = "dnssec-failed",
        [DNS_TRANSACTION_NO_TRUST_ANCHOR] = "no-trust-anchor",
};
DEFINE_STRING_TABLE_LOOKUP(dns_transaction_state, DnsTransactionState);

static const char* const dns_transaction_source_table[_DNS_TRANSACTION_SOURCE_MAX] = {
        [DNS_TRANSACTION_NETWORK] = "network",
        [DNS_TRANSACTION_CACHE] = "cache",
        [DNS_TRANSACTION_ZONE] = "zone",
        [DNS_TRANSACTION_TRUST_ANCHOR] = "trust-anchor",
};
DEFINE_STRING_TABLE_LOOKUP(dns_transaction_source, DnsTransactionSource);