path: root/src/resolve/resolved-dns-stub.c

/* SPDX-License-Identifier: LGPL-2.1-or-later */

#include <net/if_arp.h>

#include "errno-util.h"
#include "fd-util.h"
#include "missing_network.h"
#include "missing_socket.h"
#include "resolved-dns-stub.h"
#include "socket-netlink.h"
#include "socket-util.h"
#include "string-table.h"

/* The MTU of the loopback device is 64K on Linux, advertise that as maximum datagram size, but subtract the Ethernet,
 * IP and UDP header sizes */
#define ADVERTISE_DATAGRAM_SIZE_MAX (65536U-14U-20U-8U)

/* On the extra stubs, use a more conservative choice */
#define ADVERTISE_EXTRA_DATAGRAM_SIZE_MAX DNS_PACKET_UNICAST_SIZE_LARGE_MAX

static int manager_dns_stub_fd_extra(Manager *m, DnsStubListenerExtra *l, int type);

static void dns_stub_listener_extra_hash_func(const DnsStubListenerExtra *a, struct siphash *state) {
        assert(a);

        siphash24_compress(&a->mode, sizeof(a->mode), state);
        siphash24_compress(&a->family, sizeof(a->family), state);
        siphash24_compress(&a->address, FAMILY_ADDRESS_SIZE(a->family), state);
        siphash24_compress(&a->port, sizeof(a->port), state);
}

static int dns_stub_listener_extra_compare_func(const DnsStubListenerExtra *a, const DnsStubListenerExtra *b) {
        int r;

        assert(a);
        assert(b);

        r = CMP(a->mode, b->mode);
        if (r != 0)
                return r;

        r = CMP(a->family, b->family);
        if (r != 0)
                return r;

        r = memcmp(&a->address, &b->address, FAMILY_ADDRESS_SIZE(a->family));
        if (r != 0)
                return r;

        return CMP(a->port, b->port);
}

DEFINE_HASH_OPS_WITH_KEY_DESTRUCTOR(
                dns_stub_listener_extra_hash_ops,
                DnsStubListenerExtra,
                dns_stub_listener_extra_hash_func,
                dns_stub_listener_extra_compare_func,
                dns_stub_listener_extra_free);

int dns_stub_listener_extra_new(
                Manager *m,
                DnsStubListenerExtra **ret) {

        DnsStubListenerExtra *l;

        l = new(DnsStubListenerExtra, 1);
        if (!l)
                return -ENOMEM;

        *l = (DnsStubListenerExtra) {
                .manager = m,
        };

        *ret = TAKE_PTR(l);
        return 0;
}

DnsStubListenerExtra *dns_stub_listener_extra_free(DnsStubListenerExtra *p) {
        if (!p)
                return NULL;

        p->udp_event_source = sd_event_source_unref(p->udp_event_source);
        p->tcp_event_source = sd_event_source_unref(p->tcp_event_source);

        return mfree(p);
}

static int dns_stub_make_reply_packet(
                DnsPacket **p,
                size_t max_size,
                DnsQuestion *q,
                DnsAnswer *answer,
                bool *ret_truncated) {

        bool truncated = false;
        DnsResourceRecord *rr;
        unsigned c = 0;
        int r;

        assert(p);

        /* Note that we don't bother with any additional RRs, as this is stub is for local lookups only, and hence
         * roundtrips aren't expensive. */

        if (!*p) {
                r = dns_packet_new(p, DNS_PROTOCOL_DNS, 0, max_size);
                if (r < 0)
                        return r;

                r = dns_packet_append_question(*p, q);
                if (r < 0)
                        return r;

                DNS_PACKET_HEADER(*p)->qdcount = htobe16(dns_question_size(q));
        }

        DNS_ANSWER_FOREACH(rr, answer) {

                r = dns_question_matches_rr(q, rr, NULL);
                if (r < 0)
                        return r;
                if (r > 0)
                        goto add;

                r = dns_question_matches_cname_or_dname(q, rr, NULL);
                if (r < 0)
                        return r;
                if (r > 0)
                        goto add;

                continue;
        add:
                r = dns_packet_append_rr(*p, rr, 0, NULL, NULL);
                if (r == -EMSGSIZE) {
                        truncated = true;
                        break;
                }
                if (r < 0)
                        return r;

                c++;
        }

        if (ret_truncated)
                *ret_truncated = truncated;
        else if (truncated)
                return -EMSGSIZE;

        DNS_PACKET_HEADER(*p)->ancount = htobe16(be16toh(DNS_PACKET_HEADER(*p)->ancount) + c);

        return 0;
}

static int dns_stub_finish_reply_packet(
                DnsPacket *p,
                uint16_t id,
                int rcode,
                bool tc,        /* set the Truncated bit? */
                bool add_opt,   /* add an OPT RR to this packet? */
                bool edns0_do,  /* set the EDNS0 DNSSEC OK bit? */
                bool ad,        /* set the DNSSEC authenticated data bit? */
                uint16_t max_udp_size) { /* The maximum UDP datagram size to advertise to clients */

        int r;

        assert(p);

        if (add_opt) {
                r = dns_packet_append_opt(p, max_udp_size, edns0_do, /* include_rfc6975 = */ false, rcode, NULL);
                if (r == -EMSGSIZE) /* Hit the size limit? then indicate truncation */
                        tc = true;
                else if (r < 0)
                        return r;

        } else {
                /* If the client can't to EDNS0, don't do DO either */
                edns0_do = false;

                /* If the client didn't do EDNS, clamp the rcode to 4 bit */
                if (rcode > 0xF)
                        rcode = DNS_RCODE_SERVFAIL;
        }

        /* Don't set the AD bit unless DO is on, too */
        if (!edns0_do)
                ad = false;

        DNS_PACKET_HEADER(p)->id = id;

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

        return 0;
}

static int dns_stub_send(
                Manager *m,
                DnsStubListenerExtra *l,
                DnsStream *s,
                DnsPacket *p,
                DnsPacket *reply) {

        int r;

        assert(m);
        assert(p);
        assert(reply);

        if (s)
                r = dns_stream_write_packet(s, reply);
        else
                /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
                 * is because otherwise the kernel will choose it automatically based on the routing table and will
                 * thus pick 127.0.0.1 rather than 127.0.0.53. */
                r = manager_send(m,
                                 manager_dns_stub_fd_extra(m, l, SOCK_DGRAM),
                                 l ? p->ifindex : LOOPBACK_IFINDEX, /* force loopback iface if this is the main listener stub */
                                 p->family, &p->sender, p->sender_port, &p->destination,
                                 reply);
        if (r < 0)
                return log_debug_errno(r, "Failed to send reply packet: %m");

        return 0;
}

static int dns_stub_send_failure(
                Manager *m,
                DnsStubListenerExtra *l,
                DnsStream *s,
                DnsPacket *p,
                int rcode,
                bool authenticated) {

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

        assert(m);
        assert(p);

        r = dns_stub_make_reply_packet(&reply, DNS_PACKET_PAYLOAD_SIZE_MAX(p), p->question, NULL, NULL);
        if (r < 0)
                return log_debug_errno(r, "Failed to make failure packet: %m");

        r = dns_stub_finish_reply_packet(
                        reply,
                        DNS_PACKET_ID(p),
                        rcode,
                        /* truncated = */ false,
                        !!p->opt,
                        DNS_PACKET_DO(p),
                        authenticated,
                        l ? ADVERTISE_EXTRA_DATAGRAM_SIZE_MAX : ADVERTISE_DATAGRAM_SIZE_MAX);
        if (r < 0)
                return log_debug_errno(r, "Failed to build failure packet: %m");

        return dns_stub_send(m, l, s, p, reply);
}

static void dns_stub_query_complete(DnsQuery *q) {
        int r;

        assert(q);
        assert(q->request_dns_packet);

        switch (q->state) {

        case DNS_TRANSACTION_SUCCESS: {
                bool truncated;

                r = dns_stub_make_reply_packet(&q->reply_dns_packet, DNS_PACKET_PAYLOAD_SIZE_MAX(q->request_dns_packet), q->question_idna, q->answer, &truncated);
                if (r < 0) {
                        log_debug_errno(r, "Failed to build reply packet: %m");
                        break;
                }

                if (!truncated) {
                        r = dns_query_process_cname(q);
                        if (r == -ELOOP) {
                                (void) dns_stub_send_failure(q->manager, q->stub_listener_extra, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
                                break;
                        }
                        if (r < 0) {
                                log_debug_errno(r, "Failed to process CNAME: %m");
                                break;
                        }
                        if (r == DNS_QUERY_RESTARTED)
                                return;
                }

                r = dns_stub_finish_reply_packet(
                                q->reply_dns_packet,
                                DNS_PACKET_ID(q->request_dns_packet),
                                q->answer_rcode,
                                truncated,
                                !!q->request_dns_packet->opt,
                                DNS_PACKET_DO(q->request_dns_packet),
                                dns_query_fully_authenticated(q),
                                q->stub_listener_extra ? ADVERTISE_EXTRA_DATAGRAM_SIZE_MAX : ADVERTISE_DATAGRAM_SIZE_MAX);
                if (r < 0) {
                        log_debug_errno(r, "Failed to finish reply packet: %m");
                        break;
                }

                (void) dns_stub_send(q->manager, q->stub_listener_extra, q->request_dns_stream, q->request_dns_packet, q->reply_dns_packet);
                break;
        }

        case DNS_TRANSACTION_RCODE_FAILURE:
                (void) dns_stub_send_failure(q->manager, q->stub_listener_extra, q->request_dns_stream, q->request_dns_packet, q->answer_rcode, dns_query_fully_authenticated(q));
                break;

        case DNS_TRANSACTION_NOT_FOUND:
                (void) dns_stub_send_failure(q->manager, q->stub_listener_extra, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_NXDOMAIN, dns_query_fully_authenticated(q));
                break;

        case DNS_TRANSACTION_TIMEOUT:
        case DNS_TRANSACTION_ATTEMPTS_MAX_REACHED:
                /* Propagate a timeout as a no packet, i.e. that the client also gets a timeout */
                break;

        case DNS_TRANSACTION_NO_SERVERS:
        case DNS_TRANSACTION_INVALID_REPLY:
        case DNS_TRANSACTION_ERRNO:
        case DNS_TRANSACTION_ABORTED:
        case DNS_TRANSACTION_DNSSEC_FAILED:
        case DNS_TRANSACTION_NO_TRUST_ANCHOR:
        case DNS_TRANSACTION_RR_TYPE_UNSUPPORTED:
        case DNS_TRANSACTION_NETWORK_DOWN:
                (void) dns_stub_send_failure(q->manager, q->stub_listener_extra, q->request_dns_stream, q->request_dns_packet, DNS_RCODE_SERVFAIL, false);
                break;

        case DNS_TRANSACTION_NULL:
        case DNS_TRANSACTION_PENDING:
        case DNS_TRANSACTION_VALIDATING:
        default:
                assert_not_reached("Impossible state");
        }

        dns_query_free(q);
}

static int dns_stub_stream_complete(DnsStream *s, int error) {
        assert(s);

        log_debug_errno(error, "DNS TCP connection terminated, destroying queries: %m");

        for (;;) {
                DnsQuery *q;

                q = set_first(s->queries);
                if (!q)
                        break;

                dns_query_free(q);
        }

        /* This drops the implicit ref we keep around since it was allocated, as incoming stub connections
         * should be kept as long as the client wants to. */
        dns_stream_unref(s);
        return 0;
}

static void dns_stub_process_query(Manager *m, DnsStubListenerExtra *l, DnsStream *s, DnsPacket *p) {
        _cleanup_(dns_query_freep) DnsQuery *q = NULL;
        int r;

        assert(m);
        assert(p);
        assert(p->protocol == DNS_PROTOCOL_DNS);

        if (!l && /* l == NULL if this is the main stub */
            (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
             in_addr_is_localhost(p->family, &p->destination) <= 0)) {
                log_warning("Got packet on unexpected (i.e. non-localhost) IP range, ignoring.");
                return;
        }

        r = dns_packet_extract(p);
        if (r < 0) {
                log_debug_errno(r, "Failed to extract resources from incoming packet, ignoring packet: %m");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_FORMERR, false);
                return;
        }

        if (!DNS_PACKET_VERSION_SUPPORTED(p)) {
                log_debug("Got EDNS OPT field with unsupported version number.");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_BADVERS, false);
                return;
        }

        if (dns_type_is_obsolete(p->question->keys[0]->type)) {
                log_debug("Got message with obsolete key type, refusing.");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_NOTIMP, false);
                return;
        }

        if (dns_type_is_zone_transer(p->question->keys[0]->type)) {
                log_debug("Got request for zone transfer, refusing.");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_NOTIMP, false);
                return;
        }

        if (!DNS_PACKET_RD(p))  {
                /* If the "rd" bit is off (i.e. recursion was not requested), then refuse operation */
                log_debug("Got request with recursion disabled, refusing.");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_REFUSED, false);
                return;
        }

        if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
                log_debug("Got request with DNSSEC CD bit set, refusing.");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_NOTIMP, false);
                return;
        }

        r = dns_query_new(m, &q, p->question, p->question, 0, SD_RESOLVED_PROTOCOLS_ALL|SD_RESOLVED_NO_SEARCH);
        if (r < 0) {
                log_error_errno(r, "Failed to generate query object: %m");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_SERVFAIL, false);
                return;
        }

        /* Request that the TTL is corrected by the cached time for this lookup, so that we return vaguely useful TTLs */
        q->clamp_ttl = true;

        q->request_dns_packet = dns_packet_ref(p);
        q->request_dns_stream = dns_stream_ref(s); /* make sure the stream stays around until we can send a reply through it */
        q->stub_listener_extra = l;
        q->complete = dns_stub_query_complete;

        if (s) {
                /* Remember which queries belong to this stream, so that we can cancel them when the stream
                 * is disconnected early */

                r = set_ensure_put(&s->queries, NULL, q);
                if (r < 0) {
                        log_oom();
                        return;
                }
                assert(r > 0);
        }

        r = dns_query_go(q);
        if (r < 0) {
                log_error_errno(r, "Failed to start query: %m");
                dns_stub_send_failure(m, l, s, p, DNS_RCODE_SERVFAIL, false);
                return;
        }

        log_debug("Processing query...");
        TAKE_PTR(q);
}

static int on_dns_stub_packet_internal(sd_event_source *s, int fd, uint32_t revents, Manager *m, DnsStubListenerExtra *l) {
        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;
        int r;

        r = manager_recv(m, fd, DNS_PROTOCOL_DNS, &p);
        if (r <= 0)
                return r;

        if (dns_packet_validate_query(p) > 0) {
                log_debug("Got DNS stub UDP query packet for id %u", DNS_PACKET_ID(p));

                dns_stub_process_query(m, l, NULL, p);
        } else
                log_debug("Invalid DNS stub UDP packet, ignoring.");

        return 0;
}

static int on_dns_stub_packet(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
        return on_dns_stub_packet_internal(s, fd, revents, userdata, NULL);
}

static int on_dns_stub_packet_extra(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
        DnsStubListenerExtra *l = userdata;

        assert(l);

        return on_dns_stub_packet_internal(s, fd, revents, l->manager, l);
}

static int on_dns_stub_stream_packet(DnsStream *s) {
        _cleanup_(dns_packet_unrefp) DnsPacket *p = NULL;

        assert(s);

        p = dns_stream_take_read_packet(s);
        assert(p);

        if (dns_packet_validate_query(p) > 0) {
                log_debug("Got DNS stub TCP query packet for id %u", DNS_PACKET_ID(p));

                dns_stub_process_query(s->manager, s->stub_listener_extra, s, p);
        } else
                log_debug("Invalid DNS stub TCP packet, ignoring.");

        return 0;
}

static int on_dns_stub_stream_internal(sd_event_source *s, int fd, uint32_t revents, Manager *m, DnsStubListenerExtra *l) {
        DnsStream *stream;
        int cfd, r;

        cfd = accept4(fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
        if (cfd < 0) {
                if (ERRNO_IS_ACCEPT_AGAIN(errno))
                        return 0;

                return -errno;
        }

        r = dns_stream_new(m, &stream, DNS_STREAM_STUB, DNS_PROTOCOL_DNS, cfd, NULL);
        if (r < 0) {
                safe_close(cfd);
                return r;
        }

        stream->stub_listener_extra = l;
        stream->on_packet = on_dns_stub_stream_packet;
        stream->complete = dns_stub_stream_complete;

        /* We let the reference to the stream dangle here, it will be dropped later by the complete callback. */

        return 0;
}

static int on_dns_stub_stream(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
        return on_dns_stub_stream_internal(s, fd, revents, userdata, NULL);
}

static int on_dns_stub_stream_extra(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
        DnsStubListenerExtra *l = userdata;

        assert(l);
        return on_dns_stub_stream_internal(s, fd, revents, l->manager, l);
}

static int set_dns_stub_common_socket_options(int fd, int family) {
        int r;

        assert(fd >= 0);
        assert(IN_SET(family, AF_INET, AF_INET6));

        r = setsockopt_int(fd, SOL_SOCKET, SO_REUSEADDR, true);
        if (r < 0)
                return r;

        r = socket_set_recvpktinfo(fd, family, true);
        if (r < 0)
                return r;

        r = socket_set_recvttl(fd, family, true);
        if (r < 0)
                return r;

        return 0;
}

static int manager_dns_stub_fd(Manager *m, int type) {
        union sockaddr_union sa = {
                .in.sin_family = AF_INET,
                .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
                .in.sin_port = htobe16(53),
        };
        _cleanup_close_ int fd = -1;
        int r;

        assert(IN_SET(type, SOCK_DGRAM, SOCK_STREAM));

        sd_event_source **event_source = type == SOCK_DGRAM ? &m->dns_stub_udp_event_source : &m->dns_stub_tcp_event_source;
        if (*event_source)
                return sd_event_source_get_io_fd(*event_source);

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

        r = set_dns_stub_common_socket_options(fd, AF_INET);
        if (r < 0)
                return r;

        /* Make sure no traffic from outside the local host can leak to onto this socket */
        r = socket_bind_to_ifindex(fd, LOOPBACK_IFINDEX);
        if (r < 0)
                return r;

        r = setsockopt_int(fd, IPPROTO_IP, IP_TTL, 1);
        if (r < 0)
                return r;

        if (bind(fd, &sa.sa, sizeof(sa.in)) < 0)
                return -errno;

        if (type == SOCK_STREAM &&
            listen(fd, SOMAXCONN) < 0)
                return -errno;

        r = sd_event_add_io(m->event, event_source, fd, EPOLLIN,
                            type == SOCK_DGRAM ? on_dns_stub_packet : on_dns_stub_stream,
                            m);
        if (r < 0)
                return r;

        r = sd_event_source_set_io_fd_own(*event_source, true);
        if (r < 0)
                return r;

        (void) sd_event_source_set_description(*event_source,
                                               type == SOCK_DGRAM ? "dns-stub-udp" : "dns-stub-tcp");

        return TAKE_FD(fd);
}

static int manager_dns_stub_fd_extra(Manager *m, DnsStubListenerExtra *l, int type) {
        _cleanup_free_ char *pretty = NULL;
        _cleanup_close_ int fd = -1;
        union sockaddr_union sa;
        int r;

        assert(m);
        assert(IN_SET(type, SOCK_DGRAM, SOCK_STREAM));

        if (!l)
                return manager_dns_stub_fd(m, type);

        sd_event_source **event_source = type == SOCK_DGRAM ? &l->udp_event_source : &l->tcp_event_source;
        if (*event_source)
                return sd_event_source_get_io_fd(*event_source);

        if (l->family == AF_INET)
                sa = (union sockaddr_union) {
                        .in.sin_family = l->family,
                        .in.sin_port = htobe16(l->port != 0 ? l->port : 53U),
                        .in.sin_addr = l->address.in,
                };
        else
                sa = (union sockaddr_union) {
                        .in6.sin6_family = l->family,
                        .in6.sin6_port = htobe16(l->port != 0 ? l->port : 53U),
                        .in6.sin6_addr = l->address.in6,
                };

        fd = socket(l->family, type | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
        if (fd < 0) {
                r = -errno;
                goto fail;
        }

        r = set_dns_stub_common_socket_options(fd, l->family);
        if (r < 0)
                goto fail;

        /* Do not set IP_TTL for extra DNS stub listeners, as the address may not be local and in that case
         * people may want ttl > 1. */

        r = socket_set_freebind(fd, l->family, true);
        if (r < 0)
                goto fail;

        if (bind(fd, &sa.sa, SOCKADDR_LEN(sa)) < 0) {
                r = -errno;
                goto fail;
        }

        if (type == SOCK_STREAM &&
            listen(fd, SOMAXCONN) < 0) {
                r = -errno;
                goto fail;
        }

        r = sd_event_add_io(m->event, event_source, fd, EPOLLIN,
                            type == SOCK_DGRAM ? on_dns_stub_packet_extra : on_dns_stub_stream_extra,
                            l);
        if (r < 0)
                goto fail;

        r = sd_event_source_set_io_fd_own(*event_source, true);
        if (r < 0)
                goto fail;

        (void) sd_event_source_set_description(*event_source,
                                               type == SOCK_DGRAM ? "dns-stub-udp-extra" : "dns-stub-tcp-extra");

        if (DEBUG_LOGGING) {
                (void) in_addr_port_to_string(l->family, &l->address, l->port, &pretty);
                log_debug("Listening on %s socket %s.",
                          type == SOCK_DGRAM ? "UDP" : "TCP",
                          strnull(pretty));
        }

        return TAKE_FD(fd);

fail:
        assert(r < 0);
        (void) in_addr_port_to_string(l->family, &l->address, l->port, &pretty);
        return log_warning_errno(r,
                                 r == -EADDRINUSE ? "Another process is already listening on %s socket %s: %m" :
                                                    "Failed to listen on %s socket %s: %m",
                                 type == SOCK_DGRAM ? "UDP" : "TCP",
                                 strnull(pretty));
}

int manager_dns_stub_start(Manager *m) {
        const char *t = "UDP";
        int r = 0;

        assert(m);

        if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_NO)
                log_debug("Not creating stub listener.");
        else
                log_debug("Creating stub listener using %s.",
                          m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP ? "UDP" :
                          m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP ? "TCP" :
                          "UDP/TCP");

        if (FLAGS_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_UDP))
                r = manager_dns_stub_fd(m, SOCK_DGRAM);

        if (r >= 0 &&
            FLAGS_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_TCP)) {
                t = "TCP";
                r = manager_dns_stub_fd(m, SOCK_STREAM);
        }

        if (IN_SET(r, -EADDRINUSE, -EPERM)) {
                log_warning_errno(r,
                                  r == -EADDRINUSE ? "Another process is already listening on %s socket 127.0.0.53:53.\n"
                                                     "Turning off local DNS stub support." :
                                                     "Failed to listen on %s socket 127.0.0.53:53: %m.\n"
                                                     "Turning off local DNS stub support.",
                                  t);
                manager_dns_stub_stop(m);
        } else if (r < 0)
                return log_error_errno(r, "Failed to listen on %s socket 127.0.0.53:53: %m", t);

        if (!ordered_set_isempty(m->dns_extra_stub_listeners)) {
                DnsStubListenerExtra *l;

                log_debug("Creating extra stub listeners.");

                ORDERED_SET_FOREACH(l, m->dns_extra_stub_listeners) {
                        if (FLAGS_SET(l->mode, DNS_STUB_LISTENER_UDP))
                                (void) manager_dns_stub_fd_extra(m, l, SOCK_DGRAM);
                        if (FLAGS_SET(l->mode, DNS_STUB_LISTENER_TCP))
                                (void) manager_dns_stub_fd_extra(m, l, SOCK_STREAM);
                }
        }

        return 0;
}

void manager_dns_stub_stop(Manager *m) {
        assert(m);

        m->dns_stub_udp_event_source = sd_event_source_unref(m->dns_stub_udp_event_source);
        m->dns_stub_tcp_event_source = sd_event_source_unref(m->dns_stub_tcp_event_source);
}

static const char* const dns_stub_listener_mode_table[_DNS_STUB_LISTENER_MODE_MAX] = {
        [DNS_STUB_LISTENER_NO] = "no",
        [DNS_STUB_LISTENER_UDP] = "udp",
        [DNS_STUB_LISTENER_TCP] = "tcp",
        [DNS_STUB_LISTENER_YES] = "yes",
};
DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(dns_stub_listener_mode, DnsStubListenerMode, DNS_STUB_LISTENER_YES);