path: root/src/grp-resolve/systemd-resolved/resolved-dns-scope.c

/***
  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 <netinet/tcp.h>

#include "systemd-basic/af-list.h"
#include "systemd-basic/alloc-util.h"
#include "systemd-basic/fd-util.h"
#include "systemd-basic/hostname-util.h"
#include "systemd-basic/missing.h"
#include "systemd-basic/random-util.h"
#include "systemd-basic/socket-util.h"
#include "systemd-basic/strv.h"
#include "systemd-shared/dns-domain.h"

#include "resolved-dns-scope.h"
#include "resolved-llmnr.h"
#include "resolved-mdns.h"

#define MULTICAST_RATELIMIT_INTERVAL_USEC (1*USEC_PER_SEC)
#define MULTICAST_RATELIMIT_BURST 1000

/* After how much time to repeat LLMNR requests, see RFC 4795 Section 7 */
#define MULTICAST_RESEND_TIMEOUT_MIN_USEC (100 * USEC_PER_MSEC)
#define MULTICAST_RESEND_TIMEOUT_MAX_USEC (1 * USEC_PER_SEC)

int dns_scope_new(Manager *m, DnsScope **ret, Link *l, DnsProtocol protocol, int family) {
        DnsScope *s;

        assert(m);
        assert(ret);

        s = new0(DnsScope, 1);
        if (!s)
                return -ENOMEM;

        s->manager = m;
        s->link = l;
        s->protocol = protocol;
        s->family = family;
        s->resend_timeout = MULTICAST_RESEND_TIMEOUT_MIN_USEC;

        if (protocol == DNS_PROTOCOL_DNS) {
                /* Copy DNSSEC mode from the link if it is set there,
                 * otherwise take the manager's DNSSEC mode. Note that
                 * we copy this only at scope creation time, and do
                 * not update it from the on, even if the setting
                 * changes. */

                if (l)
                        s->dnssec_mode = link_get_dnssec_mode(l);
                else
                        s->dnssec_mode = manager_get_dnssec_mode(m);
        } else
                s->dnssec_mode = DNSSEC_NO;

        LIST_PREPEND(scopes, m->dns_scopes, s);

        dns_scope_llmnr_membership(s, true);
        dns_scope_mdns_membership(s, true);

        log_debug("New scope on link %s, protocol %s, family %s", l ? l->name : "*", dns_protocol_to_string(protocol), family == AF_UNSPEC ? "*" : af_to_name(family));

        /* Enforce ratelimiting for the multicast protocols */
        RATELIMIT_INIT(s->ratelimit, MULTICAST_RATELIMIT_INTERVAL_USEC, MULTICAST_RATELIMIT_BURST);

        *ret = s;
        return 0;
}

static void dns_scope_abort_transactions(DnsScope *s) {
        assert(s);

        while (s->transactions) {
                DnsTransaction *t = s->transactions;

                /* Abort the transaction, but make sure it is not
                 * freed while we still look at it */

                t->block_gc++;
                if (DNS_TRANSACTION_IS_LIVE(t->state))
                        dns_transaction_complete(t, DNS_TRANSACTION_ABORTED);
                t->block_gc--;

                dns_transaction_free(t);
        }
}

DnsScope* dns_scope_free(DnsScope *s) {
        DnsResourceRecord *rr;

        if (!s)
                return NULL;

        log_debug("Removing scope on link %s, protocol %s, family %s", s->link ? s->link->name : "*", dns_protocol_to_string(s->protocol), s->family == AF_UNSPEC ? "*" : af_to_name(s->family));

        dns_scope_llmnr_membership(s, false);
        dns_scope_mdns_membership(s, false);
        dns_scope_abort_transactions(s);

        while (s->query_candidates)
                dns_query_candidate_free(s->query_candidates);

        hashmap_free(s->transactions_by_key);

        while ((rr = ordered_hashmap_steal_first(s->conflict_queue)))
                dns_resource_record_unref(rr);

        ordered_hashmap_free(s->conflict_queue);
        sd_event_source_unref(s->conflict_event_source);

        dns_cache_flush(&s->cache);
        dns_zone_flush(&s->zone);

        LIST_REMOVE(scopes, s->manager->dns_scopes, s);
        return mfree(s);
}

DnsServer *dns_scope_get_dns_server(DnsScope *s) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_DNS)
                return NULL;

        if (s->link)
                return link_get_dns_server(s->link);
        else
                return manager_get_dns_server(s->manager);
}

void dns_scope_next_dns_server(DnsScope *s) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_DNS)
                return;

        if (s->link)
                link_next_dns_server(s->link);
        else
                manager_next_dns_server(s->manager);
}

void dns_scope_packet_received(DnsScope *s, usec_t rtt) {
        assert(s);

        if (rtt <= s->max_rtt)
                return;

        s->max_rtt = rtt;
        s->resend_timeout = MIN(MAX(MULTICAST_RESEND_TIMEOUT_MIN_USEC, s->max_rtt * 2), MULTICAST_RESEND_TIMEOUT_MAX_USEC);
}

void dns_scope_packet_lost(DnsScope *s, usec_t usec) {
        assert(s);

        if (s->resend_timeout <= usec)
                s->resend_timeout = MIN(s->resend_timeout * 2, MULTICAST_RESEND_TIMEOUT_MAX_USEC);
}

static int dns_scope_emit_one(DnsScope *s, int fd, DnsPacket *p) {
        union in_addr_union addr;
        int ifindex = 0, r;
        int family;
        uint32_t mtu;

        assert(s);
        assert(p);
        assert(p->protocol == s->protocol);

        if (s->link) {
                mtu = s->link->mtu;
                ifindex = s->link->ifindex;
        } else
                mtu = manager_find_mtu(s->manager);

        switch (s->protocol) {

        case DNS_PROTOCOL_DNS:
                assert(fd >= 0);

                if (DNS_PACKET_QDCOUNT(p) > 1)
                        return -EOPNOTSUPP;

                if (p->size > DNS_PACKET_UNICAST_SIZE_MAX)
                        return -EMSGSIZE;

                if (p->size + UDP_PACKET_HEADER_SIZE > mtu)
                        return -EMSGSIZE;

                r = manager_write(s->manager, fd, p);
                if (r < 0)
                        return r;

                break;

        case DNS_PROTOCOL_LLMNR:
                assert(fd < 0);

                if (DNS_PACKET_QDCOUNT(p) > 1)
                        return -EOPNOTSUPP;

                if (!ratelimit_test(&s->ratelimit))
                        return -EBUSY;

                family = s->family;

                if (family == AF_INET) {
                        addr.in = LLMNR_MULTICAST_IPV4_ADDRESS;
                        fd = manager_llmnr_ipv4_udp_fd(s->manager);
                } else if (family == AF_INET6) {
                        addr.in6 = LLMNR_MULTICAST_IPV6_ADDRESS;
                        fd = manager_llmnr_ipv6_udp_fd(s->manager);
                } else
                        return -EAFNOSUPPORT;
                if (fd < 0)
                        return fd;

                r = manager_send(s->manager, fd, ifindex, family, &addr, LLMNR_PORT, NULL, p);
                if (r < 0)
                        return r;

                break;

        case DNS_PROTOCOL_MDNS:
                assert(fd < 0);

                if (!ratelimit_test(&s->ratelimit))
                        return -EBUSY;

                family = s->family;

                if (family == AF_INET) {
                        addr.in = MDNS_MULTICAST_IPV4_ADDRESS;
                        fd = manager_mdns_ipv4_fd(s->manager);
                } else if (family == AF_INET6) {
                        addr.in6 = MDNS_MULTICAST_IPV6_ADDRESS;
                        fd = manager_mdns_ipv6_fd(s->manager);
                } else
                        return -EAFNOSUPPORT;
                if (fd < 0)
                        return fd;

                r = manager_send(s->manager, fd, ifindex, family, &addr, MDNS_PORT, NULL, p);
                if (r < 0)
                        return r;

                break;

        default:
                return -EAFNOSUPPORT;
        }

        return 1;
}

int dns_scope_emit_udp(DnsScope *s, int fd, DnsPacket *p) {
        int r;

        assert(s);
        assert(p);
        assert(p->protocol == s->protocol);
        assert((s->protocol == DNS_PROTOCOL_DNS) == (fd >= 0));

        do {
                /* If there are multiple linked packets, set the TC bit in all but the last of them */
                if (p->more) {
                        assert(p->protocol == DNS_PROTOCOL_MDNS);
                        dns_packet_set_flags(p, true, true);
                }

                r = dns_scope_emit_one(s, fd, p);
                if (r < 0)
                        return r;

                p = p->more;
        } while (p);

        return 0;
}

static int dns_scope_socket(
                DnsScope *s,
                int type,
                int family,
                const union in_addr_union *address,
                DnsServer *server,
                uint16_t port) {

        _cleanup_close_ int fd = -1;
        union sockaddr_union sa = {};
        socklen_t salen;
        static const int one = 1;
        int ret, r, ifindex;

        assert(s);

        if (server) {
                assert(family == AF_UNSPEC);
                assert(!address);

                ifindex = dns_server_ifindex(server);

                sa.sa.sa_family = server->family;
                if (server->family == AF_INET) {
                        sa.in.sin_port = htobe16(port);
                        sa.in.sin_addr = server->address.in;
                        salen = sizeof(sa.in);
                } else if (server->family == AF_INET6) {
                        sa.in6.sin6_port = htobe16(port);
                        sa.in6.sin6_addr = server->address.in6;
                        sa.in6.sin6_scope_id = ifindex;
                        salen = sizeof(sa.in6);
                } else
                        return -EAFNOSUPPORT;
        } else {
                assert(family != AF_UNSPEC);
                assert(address);

                sa.sa.sa_family = family;
                ifindex = s->link ? s->link->ifindex : 0;

                if (family == AF_INET) {
                        sa.in.sin_port = htobe16(port);
                        sa.in.sin_addr = address->in;
                        salen = sizeof(sa.in);
                } else if (family == AF_INET6) {
                        sa.in6.sin6_port = htobe16(port);
                        sa.in6.sin6_addr = address->in6;
                        sa.in6.sin6_scope_id = ifindex;
                        salen = sizeof(sa.in6);
                } else
                        return -EAFNOSUPPORT;
        }

        fd = socket(sa.sa.sa_family, type|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
        if (fd < 0)
                return -errno;

        if (type == SOCK_STREAM) {
                r = setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));
                if (r < 0)
                        return -errno;
        }

        if (s->link) {
                be32_t ifindex_be = htobe32(ifindex);

                if (sa.sa.sa_family == AF_INET) {
                        r = setsockopt(fd, IPPROTO_IP, IP_UNICAST_IF, &ifindex_be, sizeof(ifindex_be));
                        if (r < 0)
                                return -errno;
                } else if (sa.sa.sa_family == AF_INET6) {
                        r = setsockopt(fd, IPPROTO_IPV6, IPV6_UNICAST_IF, &ifindex_be, sizeof(ifindex_be));
                        if (r < 0)
                                return -errno;
                }
        }

        if (s->protocol == DNS_PROTOCOL_LLMNR) {
                /* RFC 4795, section 2.5 requires the TTL to be set to 1 */

                if (sa.sa.sa_family == AF_INET) {
                        r = setsockopt(fd, IPPROTO_IP, IP_TTL, &one, sizeof(one));
                        if (r < 0)
                                return -errno;
                } else if (sa.sa.sa_family == AF_INET6) {
                        r = setsockopt(fd, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &one, sizeof(one));
                        if (r < 0)
                                return -errno;
                }
        }

        r = connect(fd, &sa.sa, salen);
        if (r < 0 && errno != EINPROGRESS)
                return -errno;

        ret = fd;
        fd = -1;

        return ret;
}

int dns_scope_socket_udp(DnsScope *s, DnsServer *server, uint16_t port) {
        return dns_scope_socket(s, SOCK_DGRAM, AF_UNSPEC, NULL, server, port);
}

int dns_scope_socket_tcp(DnsScope *s, int family, const union in_addr_union *address, DnsServer *server, uint16_t port) {
        return dns_scope_socket(s, SOCK_STREAM, family, address, server, port);
}

DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, const char *domain) {
        DnsSearchDomain *d;
        DnsServer *dns_server;

        assert(s);
        assert(domain);

        /* Checks if the specified domain is something to look up on
         * this scope. Note that this accepts non-qualified hostnames,
         * i.e. those without any search path prefixed yet. */

        if (ifindex != 0 && (!s->link || s->link->ifindex != ifindex))
                return DNS_SCOPE_NO;

        if ((SD_RESOLVED_FLAGS_MAKE(s->protocol, s->family, 0) & flags) == 0)
                return DNS_SCOPE_NO;

        /* Never resolve any loopback hostname or IP address via DNS,
         * LLMNR or mDNS. Instead, always rely on synthesized RRs for
         * these. */
        if (is_localhost(domain) ||
            dns_name_endswith(domain, "127.in-addr.arpa") > 0 ||
            dns_name_equal(domain, "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa") > 0)
                return DNS_SCOPE_NO;

        /* Never respond to some of the domains listed in RFC6303 */
        if (dns_name_endswith(domain, "0.in-addr.arpa") > 0 ||
            dns_name_equal(domain, "255.255.255.255.in-addr.arpa") > 0 ||
            dns_name_equal(domain, "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa") > 0)
                return DNS_SCOPE_NO;

        /* Never respond to some of the domains listed in RFC6761 */
        if (dns_name_endswith(domain, "invalid") > 0)
                return DNS_SCOPE_NO;

        /* Always honour search domains for routing queries. Note that
         * we return DNS_SCOPE_YES here, rather than just
         * DNS_SCOPE_MAYBE, which means wildcard scopes won't be
         * considered anymore. */
        LIST_FOREACH(domains, d, dns_scope_get_search_domains(s))
                if (dns_name_endswith(domain, d->name) > 0)
                        return DNS_SCOPE_YES;

        /* If the DNS server has route-only domains, don't send other requests
         * to it. This would be a privacy violation, will most probably fail
         * anyway, and adds unnecessary load. */
        dns_server = dns_scope_get_dns_server(s);
        if (dns_server && dns_server_limited_domains(dns_server))
                return DNS_SCOPE_NO;

        switch (s->protocol) {

        case DNS_PROTOCOL_DNS:

                /* Exclude link-local IP ranges */
                if (dns_name_endswith(domain, "254.169.in-addr.arpa") == 0 &&
                    dns_name_endswith(domain, "8.e.f.ip6.arpa") == 0 &&
                    dns_name_endswith(domain, "9.e.f.ip6.arpa") == 0 &&
                    dns_name_endswith(domain, "a.e.f.ip6.arpa") == 0 &&
                    dns_name_endswith(domain, "b.e.f.ip6.arpa") == 0 &&
                    /* If networks use .local in their private setups, they are supposed to also add .local to their search
                     * domains, which we already checked above. Otherwise, we consider .local specific to mDNS and won't
                     * send such queries ordinary DNS servers. */
                    dns_name_endswith(domain, "local") == 0)
                        return DNS_SCOPE_MAYBE;

                return DNS_SCOPE_NO;

        case DNS_PROTOCOL_MDNS:
                if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
                    (s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0) ||
                    (dns_name_endswith(domain, "local") > 0 && /* only resolve names ending in .local via mDNS */
                     dns_name_equal(domain, "local") == 0 &&   /* but not the single-label "local" name itself */
                     manager_is_own_hostname(s->manager, domain) <= 0)) /* never resolve the local hostname via mDNS */
                        return DNS_SCOPE_MAYBE;

                return DNS_SCOPE_NO;

        case DNS_PROTOCOL_LLMNR:
                if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
                    (s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0) ||
                    (dns_name_is_single_label(domain) && /* only resolve single label names via LLMNR */
                     !is_gateway_hostname(domain) && /* don't resolve "gateway" with LLMNR, let nss-myhostname handle this */
                     manager_is_own_hostname(s->manager, domain) <= 0))  /* never resolve the local hostname via LLMNR */
                        return DNS_SCOPE_MAYBE;

                return DNS_SCOPE_NO;

        default:
                assert_not_reached("Unknown scope protocol");
        }
}

bool dns_scope_good_key(DnsScope *s, const DnsResourceKey *key) {
        int key_family;

        assert(s);
        assert(key);

        /* Check if it makes sense to resolve the specified key on
         * this scope. Note that this call assumes as fully qualified
         * name, i.e. the search suffixes already appended. */

        if (key->class != DNS_CLASS_IN)
                return false;

        if (s->protocol == DNS_PROTOCOL_DNS) {

                /* On classic DNS, looking up non-address RRs is always
                 * fine. (Specifically, we want to permit looking up
                 * DNSKEY and DS records on the root and top-level
                 * domains.) */
                if (!dns_resource_key_is_address(key))
                        return true;

                /* However, we refuse to look up A and AAAA RRs on the
                 * root and single-label domains, under the assumption
                 * that those should be resolved via LLMNR or search
                 * path only, and should not be leaked onto the
                 * internet. */
                return !(dns_name_is_single_label(dns_resource_key_name(key)) ||
                         dns_name_is_root(dns_resource_key_name(key)));
        }

        /* On mDNS and LLMNR, send A and AAAA queries only on the
         * respective scopes */

        key_family = dns_type_to_af(key->type);
        if (key_family < 0)
                return true;

        return key_family == s->family;
}

static int dns_scope_multicast_membership(DnsScope *s, bool b, struct in_addr in, struct in6_addr in6) {
        int fd;

        assert(s);
        assert(s->link);

        if (s->family == AF_INET) {
                struct ip_mreqn mreqn = {
                        .imr_multiaddr = in,
                        .imr_ifindex = s->link->ifindex,
                };

                fd = manager_llmnr_ipv4_udp_fd(s->manager);
                if (fd < 0)
                        return fd;

                /* Always first try to drop membership before we add
                 * one. This is necessary on some devices, such as
                 * veth. */
                if (b)
                        (void) setsockopt(fd, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreqn, sizeof(mreqn));

                if (setsockopt(fd, IPPROTO_IP, b ? IP_ADD_MEMBERSHIP : IP_DROP_MEMBERSHIP, &mreqn, sizeof(mreqn)) < 0)
                        return -errno;

        } else if (s->family == AF_INET6) {
                struct ipv6_mreq mreq = {
                        .ipv6mr_multiaddr = in6,
                        .ipv6mr_interface = s->link->ifindex,
                };

                fd = manager_llmnr_ipv6_udp_fd(s->manager);
                if (fd < 0)
                        return fd;

                if (b)
                        (void) setsockopt(fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP, &mreq, sizeof(mreq));

                if (setsockopt(fd, IPPROTO_IPV6, b ? IPV6_ADD_MEMBERSHIP : IPV6_DROP_MEMBERSHIP, &mreq, sizeof(mreq)) < 0)
                        return -errno;
        } else
                return -EAFNOSUPPORT;

        return 0;
}

int dns_scope_llmnr_membership(DnsScope *s, bool b) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_LLMNR)
                return 0;

        return dns_scope_multicast_membership(s, b, LLMNR_MULTICAST_IPV4_ADDRESS, LLMNR_MULTICAST_IPV6_ADDRESS);
}

int dns_scope_mdns_membership(DnsScope *s, bool b) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_MDNS)
                return 0;

        return dns_scope_multicast_membership(s, b, MDNS_MULTICAST_IPV4_ADDRESS, MDNS_MULTICAST_IPV6_ADDRESS);
}

static int dns_scope_make_reply_packet(
                DnsScope *s,
                uint16_t id,
                int rcode,
                DnsQuestion *q,
                DnsAnswer *answer,
                DnsAnswer *soa,
                bool tentative,
                DnsPacket **ret) {

        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        int r;

        assert(s);
        assert(ret);

        if (dns_question_isempty(q) &&
            dns_answer_isempty(answer) &&
            dns_answer_isempty(soa))
                return -EINVAL;

        r = dns_packet_new(&p, s->protocol, 0);
        if (r < 0)
                return r;

        DNS_PACKET_HEADER(p)->id = id;
        DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
                                                              1 /* qr */,
                                                              0 /* opcode */,
                                                              0 /* c */,
                                                              0 /* tc */,
                                                              tentative,
                                                              0 /* (ra) */,
                                                              0 /* (ad) */,
                                                              0 /* (cd) */,
                                                              rcode));

        r = dns_packet_append_question(p, q);
        if (r < 0)
                return r;
        DNS_PACKET_HEADER(p)->qdcount = htobe16(dns_question_size(q));

        r = dns_packet_append_answer(p, answer);
        if (r < 0)
                return r;
        DNS_PACKET_HEADER(p)->ancount = htobe16(dns_answer_size(answer));

        r = dns_packet_append_answer(p, soa);
        if (r < 0)
                return r;
        DNS_PACKET_HEADER(p)->arcount = htobe16(dns_answer_size(soa));

        *ret = p;
        p = NULL;

        return 0;
}

static void dns_scope_verify_conflicts(DnsScope *s, DnsPacket *p) {
        DnsResourceRecord *rr;
        DnsResourceKey *key;

        assert(s);
        assert(p);

        DNS_QUESTION_FOREACH(key, p->question)
                dns_zone_verify_conflicts(&s->zone, key);

        DNS_ANSWER_FOREACH(rr, p->answer)
                dns_zone_verify_conflicts(&s->zone, rr->key);
}

void dns_scope_process_query(DnsScope *s, DnsStream *stream, DnsPacket *p) {
        _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL, *soa = NULL;
        _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
        DnsResourceKey *key = NULL;
        bool tentative = false;
        int r;

        assert(s);
        assert(p);

        if (p->protocol != DNS_PROTOCOL_LLMNR)
                return;

        if (p->ipproto == IPPROTO_UDP) {
                /* Don't accept UDP queries directed to anything but
                 * the LLMNR multicast addresses. See RFC 4795,
                 * section 2.5. */

                if (p->family == AF_INET && !in_addr_equal(AF_INET, &p->destination, (union in_addr_union*) &LLMNR_MULTICAST_IPV4_ADDRESS))
                        return;

                if (p->family == AF_INET6 && !in_addr_equal(AF_INET6, &p->destination, (union in_addr_union*) &LLMNR_MULTICAST_IPV6_ADDRESS))
                        return;
        }

        r = dns_packet_extract(p);
        if (r < 0) {
                log_debug_errno(r, "Failed to extract resource records from incoming packet: %m");
                return;
        }

        if (DNS_PACKET_LLMNR_C(p)) {
                /* Somebody notified us about a possible conflict */
                dns_scope_verify_conflicts(s, p);
                return;
        }

        assert(dns_question_size(p->question) == 1);
        key = p->question->keys[0];

        r = dns_zone_lookup(&s->zone, key, 0, &answer, &soa, &tentative);
        if (r < 0) {
                log_debug_errno(r, "Failed to lookup key: %m");
                return;
        }
        if (r == 0)
                return;

        if (answer)
                dns_answer_order_by_scope(answer, in_addr_is_link_local(p->family, &p->sender) > 0);

        r = dns_scope_make_reply_packet(s, DNS_PACKET_ID(p), DNS_RCODE_SUCCESS, p->question, answer, soa, tentative, &reply);
        if (r < 0) {
                log_debug_errno(r, "Failed to build reply packet: %m");
                return;
        }

        if (stream) {
                r = dns_stream_write_packet(stream, reply);
                if (r < 0) {
                        log_debug_errno(r, "Failed to enqueue reply packet: %m");
                        return;
                }

                /* Let's take an extra reference on this stream, so that it stays around after returning. The reference
                 * will be dangling until the stream is disconnected, and the default completion handler of the stream
                 * will then unref the stream and destroy it */
                if (DNS_STREAM_QUEUED(stream))
                        dns_stream_ref(stream);
        } else {
                int fd;

                if (!ratelimit_test(&s->ratelimit))
                        return;

                if (p->family == AF_INET)
                        fd = manager_llmnr_ipv4_udp_fd(s->manager);
                else if (p->family == AF_INET6)
                        fd = manager_llmnr_ipv6_udp_fd(s->manager);
                else {
                        log_debug("Unknown protocol");
                        return;
                }
                if (fd < 0) {
                        log_debug_errno(fd, "Failed to get reply socket: %m");
                        return;
                }

                /* Note that we always immediately reply to all LLMNR
                 * requests, and do not wait any time, since we
                 * verified uniqueness for all records. Also see RFC
                 * 4795, Section 2.7 */

                r = manager_send(s->manager, fd, p->ifindex, p->family, &p->sender, p->sender_port, NULL, reply);
                if (r < 0) {
                        log_debug_errno(r, "Failed to send reply packet: %m");
                        return;
                }
        }
}

DnsTransaction *dns_scope_find_transaction(DnsScope *scope, DnsResourceKey *key, bool cache_ok) {
        DnsTransaction *t;

        assert(scope);
        assert(key);

        /* Try to find an ongoing transaction that is a equal to the
         * specified question */
        t = hashmap_get(scope->transactions_by_key, key);
        if (!t)
                return NULL;

        /* Refuse reusing transactions that completed based on cached
         * data instead of a real packet, if that's requested. */
        if (!cache_ok &&
            IN_SET(t->state, DNS_TRANSACTION_SUCCESS, DNS_TRANSACTION_RCODE_FAILURE) &&
            t->answer_source != DNS_TRANSACTION_NETWORK)
                return NULL;

        return t;
}

static int dns_scope_make_conflict_packet(
                DnsScope *s,
                DnsResourceRecord *rr,
                DnsPacket **ret) {

        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        int r;

        assert(s);
        assert(rr);
        assert(ret);

        r = dns_packet_new(&p, s->protocol, 0);
        if (r < 0)
                return r;

        DNS_PACKET_HEADER(p)->flags = htobe16(DNS_PACKET_MAKE_FLAGS(
                                                              0 /* qr */,
                                                              0 /* opcode */,
                                                              1 /* conflict */,
                                                              0 /* tc */,
                                                              0 /* t */,
                                                              0 /* (ra) */,
                                                              0 /* (ad) */,
                                                              0 /* (cd) */,
                                                              0));

        /* For mDNS, the transaction ID should always be 0 */
        if (s->protocol != DNS_PROTOCOL_MDNS)
                random_bytes(&DNS_PACKET_HEADER(p)->id, sizeof(uint16_t));

        DNS_PACKET_HEADER(p)->qdcount = htobe16(1);
        DNS_PACKET_HEADER(p)->arcount = htobe16(1);

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

        r = dns_packet_append_rr(p, rr, NULL, NULL);
        if (r < 0)
                return r;

        *ret = p;
        p = NULL;

        return 0;
}

static int on_conflict_dispatch(sd_event_source *es, usec_t usec, void *userdata) {
        DnsScope *scope = userdata;
        int r;

        assert(es);
        assert(scope);

        scope->conflict_event_source = sd_event_source_unref(scope->conflict_event_source);

        for (;;) {
                _cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
                _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;

                rr = ordered_hashmap_steal_first(scope->conflict_queue);
                if (!rr)
                        break;

                r = dns_scope_make_conflict_packet(scope, rr, &p);
                if (r < 0) {
                        log_error_errno(r, "Failed to make conflict packet: %m");
                        return 0;
                }

                r = dns_scope_emit_udp(scope, -1, p);
                if (r < 0)
                        log_debug_errno(r, "Failed to send conflict packet: %m");
        }

        return 0;
}

int dns_scope_notify_conflict(DnsScope *scope, DnsResourceRecord *rr) {
        usec_t jitter;
        int r;

        assert(scope);
        assert(rr);

        /* We don't send these queries immediately. Instead, we queue
         * them, and send them after some jitter delay. */
        r = ordered_hashmap_ensure_allocated(&scope->conflict_queue, &dns_resource_key_hash_ops);
        if (r < 0) {
                log_oom();
                return r;
        }

        /* We only place one RR per key in the conflict
         * messages, not all of them. That should be enough to
         * indicate where there might be a conflict */
        r = ordered_hashmap_put(scope->conflict_queue, rr->key, rr);
        if (r == -EEXIST || r == 0)
                return 0;
        if (r < 0)
                return log_debug_errno(r, "Failed to queue conflicting RR: %m");

        dns_resource_record_ref(rr);

        if (scope->conflict_event_source)
                return 0;

        random_bytes(&jitter, sizeof(jitter));
        jitter %= LLMNR_JITTER_INTERVAL_USEC;

        r = sd_event_add_time(scope->manager->event,
                              &scope->conflict_event_source,
                              clock_boottime_or_monotonic(),
                              now(clock_boottime_or_monotonic()) + jitter,
                              LLMNR_JITTER_INTERVAL_USEC,
                              on_conflict_dispatch, scope);
        if (r < 0)
                return log_debug_errno(r, "Failed to add conflict dispatch event: %m");

        (void) sd_event_source_set_description(scope->conflict_event_source, "scope-conflict");

        return 0;
}

void dns_scope_check_conflicts(DnsScope *scope, DnsPacket *p) {
        unsigned i;
        int r;

        assert(scope);
        assert(p);

        if (p->protocol != DNS_PROTOCOL_LLMNR)
                return;

        if (DNS_PACKET_RRCOUNT(p) <= 0)
                return;

        if (DNS_PACKET_LLMNR_C(p) != 0)
                return;

        if (DNS_PACKET_LLMNR_T(p) != 0)
                return;

        if (manager_our_packet(scope->manager, p))
                return;

        r = dns_packet_extract(p);
        if (r < 0) {
                log_debug_errno(r, "Failed to extract packet: %m");
                return;
        }

        log_debug("Checking for conflicts...");

        for (i = 0; i < p->answer->n_rrs; i++) {

                /* Check for conflicts against the local zone. If we
                 * found one, we won't check any further */
                r = dns_zone_check_conflicts(&scope->zone, p->answer->items[i].rr);
                if (r != 0)
                        continue;

                /* Check for conflicts against the local cache. If so,
                 * send out an advisory query, to inform everybody */
                r = dns_cache_check_conflicts(&scope->cache, p->answer->items[i].rr, p->family, &p->sender);
                if (r <= 0)
                        continue;

                dns_scope_notify_conflict(scope, p->answer->items[i].rr);
        }
}

void dns_scope_dump(DnsScope *s, FILE *f) {
        assert(s);

        if (!f)
                f = stdout;

        fputs("[Scope protocol=", f);
        fputs(dns_protocol_to_string(s->protocol), f);

        if (s->link) {
                fputs(" interface=", f);
                fputs(s->link->name, f);
        }

        if (s->family != AF_UNSPEC) {
                fputs(" family=", f);
                fputs(af_to_name(s->family), f);
        }

        fputs("]\n", f);

        if (!dns_zone_is_empty(&s->zone)) {
                fputs("ZONE:\n", f);
                dns_zone_dump(&s->zone, f);
        }

        if (!dns_cache_is_empty(&s->cache)) {
                fputs("CACHE:\n", f);
                dns_cache_dump(&s->cache, f);
        }
}

DnsSearchDomain *dns_scope_get_search_domains(DnsScope *s) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_DNS)
                return NULL;

        if (s->link)
                return s->link->search_domains;

        return s->manager->search_domains;
}

bool dns_scope_name_needs_search_domain(DnsScope *s, const char *name) {
        assert(s);

        if (s->protocol != DNS_PROTOCOL_DNS)
                return false;

        return dns_name_is_single_label(name);
}

bool dns_scope_network_good(DnsScope *s) {
        /* Checks whether the network is in good state for lookups on this scope. For mDNS/LLMNR/Classic DNS scopes
         * bound to links this is easy, as they don't even exist if the link isn't in a suitable state. For the global
         * DNS scope we check whether there are any links that are up and have an address. */

        if (s->link)
                return true;

        return manager_routable(s->manager, AF_UNSPEC);
}

int dns_scope_ifindex(DnsScope *s) {
        assert(s);

        if (s->link)
                return s->link->ifindex;

        return 0;
}