resolved: support for DNS-over-TLS

Add support for DNS-over-TLS using GnuTLS. To reduce latency also TLS False Start and TLS session resumption is supported.

meson.build

@@ -1137,6 +1137,18 @@ conf.set('DEFAULT_DNSSEC_MODE',
          'DNSSEC_' + default_dnssec.underscorify().to_upper())
 substs.set('DEFAULT_DNSSEC_MODE', default_dnssec)
 
+default_private_dns = get_option('default-private-dns')
+if fuzzer_build
+        default_private_dns = 'no'
+endif
+if default_private_dns != 'no' and conf.get('HAVE_GNUTLS') == 0
+        message('default-private-dns cannot be set to strict or opportunistic when gnutls is disabled. Setting default-private-dns to no.')
+        default_private_dns = 'no'
+endif
+conf.set('DEFAULT_PRIVATE_DNS_MODE',
+         'PRIVATE_DNS_' + default_private_dns.underscorify().to_upper())
+substs.set('DEFAULT_PRIVATE_DNS_MODE', default_private_dns)
+
 want_importd = get_option('importd')
 if want_importd != 'false'
         have = (conf.get('HAVE_LIBCURL') == 1 and
@@ -1585,6 +1597,7 @@ if conf.get('ENABLE_RESOLVE') == 1
                                 libbasic_gcrypt,
                                 libsystemd_resolve_core],
                    dependencies : [threads,
+                                   libgnutls,
                                    libgpg_error,
                                    libm,
                                    libidn],
@@ -2857,6 +2870,7 @@ status = [
         'symbolic gateway hostnames:        @0@'.format(', '.join(gateway_hostnames)),
 
         'default DNSSEC mode:               @0@'.format(default_dnssec),
+        'default private DNS mode:          @0@'.format(default_private_dns),
         'default cgroup hierarchy:          @0@'.format(default_hierarchy),
         'default KillUserProcesses setting: @0@'.format(kill_user_processes)]
 

meson_options.txt

@@ -193,6 +193,10 @@ option('default-dnssec', type : 'combo',
        description : 'default DNSSEC mode',
        choices : ['yes', 'allow-downgrade', 'no'],
        value : 'allow-downgrade')
+option('default-private-dns', type : 'combo',
+       description : 'default private DNS mode',
+       choices : ['opportunistic', 'no'],
+       value : 'no')
 option('dns-servers', type : 'string',
        description : 'space-separated list of default DNS servers',
        value : '8.8.8.8 8.8.4.4 2001:4860:4860::8888 2001:4860:4860::8844')

src/resolve/resolved-conf.c

@@ -396,6 +396,13 @@ int manager_parse_config_file(Manager *m) {
                 m->dnssec_mode = DNSSEC_NO;
         }
 #endif
+
+#if ! HAVE_GNUTLS
+        if (m->private_dns_mode != PRIVATE_DNS_NO) {
+                log_warning("Private DNS option cannot be set to opportunistic when systemd-resolved is built without gnutls support. Turning off private DNS support.");
+                m->private_dns_mode = PRIVATE_DNS_NO;
+        }
+#endif
         return 0;
 
 }

src/resolve/resolved-dns-scope.c

@@ -56,8 +56,12 @@ int dns_scope_new(Manager *m, DnsScope **ret, Link *l, DnsProtocol protocol, int
                         s->dnssec_mode = link_get_dnssec_mode(l);
                 else
                         s->dnssec_mode = manager_get_dnssec_mode(m);
-        } else
+
+                s->private_dns_mode = manager_get_private_dns_mode(m);
+        } else {
                 s->dnssec_mode = DNSSEC_NO;
+                s->private_dns_mode = PRIVATE_DNS_NO;
+        }
 
         LIST_PREPEND(scopes, m->dns_scopes, s);
 

src/resolve/resolved-dns-scope.h

@@ -35,6 +35,7 @@ struct DnsScope {
         DnsProtocol protocol;
         int family;
         DnssecMode dnssec_mode;
+        PrivateDnsMode private_dns_mode;
 
         Link *link;
 

src/resolve/resolved-dns-server.c

@@ -89,6 +89,11 @@ int dns_server_new(
 
         s->linked = true;
 
+#if HAVE_GNUTLS
+        /* Do not verify cerificate */
+        gnutls_certificate_allocate_credentials(&s->tls_cert_cred);
+#endif
+
         /* A new DNS server that isn't fallback is added and the one
          * we used so far was a fallback one? Then let's try to pick
          * the new one */
@@ -125,6 +130,14 @@ DnsServer* dns_server_unref(DnsServer *s)  {
 
         dns_stream_unref(s->stream);
 
+#if HAVE_GNUTLS
+        if (s->tls_cert_cred)
+                gnutls_certificate_free_credentials(s->tls_cert_cred);
+
+        if (s->tls_session_data.data)
+                gnutls_free(s->tls_session_data.data);
+#endif
+
         free(s->server_string);
         return mfree(s);
 }
@@ -236,6 +249,7 @@ static void dns_server_reset_counters(DnsServer *s) {
 
         s->n_failed_udp = 0;
         s->n_failed_tcp = 0;
+        s->n_failed_tls = 0;
         s->packet_truncated = false;
         s->verified_usec = 0;
 
@@ -257,29 +271,32 @@ void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLeve
         if (protocol == IPPROTO_UDP) {
                 if (s->possible_feature_level == level)
                         s->n_failed_udp = 0;
-
-                /* If the RRSIG data is missing, then we can only validate EDNS0 at max */
-                if (s->packet_rrsig_missing && level >= DNS_SERVER_FEATURE_LEVEL_DO)
-                        level = DNS_SERVER_FEATURE_LEVEL_DO - 1;
-
-                /* If the OPT RR got lost, then we can only validate UDP at max */
-                if (s->packet_bad_opt && level >= DNS_SERVER_FEATURE_LEVEL_EDNS0)
-                        level = DNS_SERVER_FEATURE_LEVEL_EDNS0 - 1;
-
-                /* Even if we successfully receive a reply to a request announcing support for large packets,
-                   that does not mean we can necessarily receive large packets. */
-                if (level == DNS_SERVER_FEATURE_LEVEL_LARGE)
-                        level = DNS_SERVER_FEATURE_LEVEL_LARGE - 1;
-
         } else if (protocol == IPPROTO_TCP) {
+                if (DNS_SERVER_FEATURE_LEVEL_IS_TLS(level)) {
+                        if (s->possible_feature_level == level)
+                                s->n_failed_tls = 0;
+                } else {
+                        if (s->possible_feature_level == level)
+                                s->n_failed_tcp = 0;
 
-                if (s->possible_feature_level == level)
+                        /* Successful TCP connections are only useful to verify the TCP feature level. */
-                        s->n_failed_tcp = 0;
+                        level = DNS_SERVER_FEATURE_LEVEL_TCP;
-
+                }
-                /* Successful TCP connections are only useful to verify the TCP feature level. */
-                level = DNS_SERVER_FEATURE_LEVEL_TCP;
         }
 
+        /* If the RRSIG data is missing, then we can only validate EDNS0 at max */
+        if (s->packet_rrsig_missing && level >= DNS_SERVER_FEATURE_LEVEL_DO)
+                level = DNS_SERVER_FEATURE_LEVEL_IS_TLS(level) ? DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN : DNS_SERVER_FEATURE_LEVEL_EDNS0;
+
+        /* If the OPT RR got lost, then we can only validate UDP at max */
+        if (s->packet_bad_opt && level >= DNS_SERVER_FEATURE_LEVEL_EDNS0)
+                level = DNS_SERVER_FEATURE_LEVEL_EDNS0 - 1;
+
+        /* Even if we successfully receive a reply to a request announcing support for large packets,
+                that does not mean we can necessarily receive large packets. */
+        if (level == DNS_SERVER_FEATURE_LEVEL_LARGE)
+                level = DNS_SERVER_FEATURE_LEVEL_LARGE - 1;
+
         dns_server_verified(s, level);
 
         /* Remember the size of the largest UDP packet we received from a server,
@@ -304,8 +321,12 @@ void dns_server_packet_lost(DnsServer *s, int protocol, DnsServerFeatureLevel le
         if (s->possible_feature_level == level) {
                 if (protocol == IPPROTO_UDP)
                         s->n_failed_udp++;
-                else if (protocol == IPPROTO_TCP)
+                else if (protocol == IPPROTO_TCP) {
-                        s->n_failed_tcp++;
+                        if (DNS_SERVER_FEATURE_LEVEL_IS_TLS(level))
+                                s->n_failed_tls++;
+                        else
+                                s->n_failed_tcp++;
+                }
         }
 
         if (s->resend_timeout > usec)
@@ -333,7 +354,7 @@ void dns_server_packet_rrsig_missing(DnsServer *s, DnsServerFeatureLevel level)
 
         /* If the RRSIG RRs are missing, we have to downgrade what we previously verified */
         if (s->verified_feature_level >= DNS_SERVER_FEATURE_LEVEL_DO)
-                s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_DO-1;
+                s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_IS_TLS(level) ? DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN : DNS_SERVER_FEATURE_LEVEL_EDNS0;
 
         s->packet_rrsig_missing = true;
 }
@@ -395,9 +416,14 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) {
 
         /* Determine the best feature level we care about. If DNSSEC mode is off there's no point in using anything
          * better than EDNS0, hence don't even try. */
-        best = dns_server_get_dnssec_mode(s) == DNSSEC_NO ?
+        if (dns_server_get_dnssec_mode(s) != DNSSEC_NO)
-                DNS_SERVER_FEATURE_LEVEL_EDNS0 :
+                best = dns_server_get_private_dns_mode(s) == PRIVATE_DNS_NO ?
-                DNS_SERVER_FEATURE_LEVEL_BEST;
+                        DNS_SERVER_FEATURE_LEVEL_LARGE :
+                        DNS_SERVER_FEATURE_LEVEL_TLS_DO;
+        else
+                best = dns_server_get_private_dns_mode(s) == PRIVATE_DNS_NO ?
+                        DNS_SERVER_FEATURE_LEVEL_EDNS0 :
+                        DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN;
 
         /* Clamp the feature level the highest level we care about. The DNSSEC mode might have changed since the last
          * time, hence let's downgrade if we are still at a higher level. */
@@ -431,7 +457,14 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) {
                          * work. Upgrade back to UDP again. */
                         log_debug("Reached maximum number of failed TCP connection attempts, trying UDP again...");
                         s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP;
+                } else if (s->n_failed_tls > 0 &&
+                           DNS_SERVER_FEATURE_LEVEL_IS_TLS(s->possible_feature_level)) {
 
+                        /* We tried to connect using DNS-over-TLS, and it didn't work. Downgrade to plaintext UDP
+                         * if we don't require DNS-over-TLS */
+
+                        log_debug("Server doesn't support seem to support DNS-over-TLS, downgrading protocol...");
+                        s->possible_feature_level--;
                 } else if (s->packet_bad_opt &&
                            s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_EDNS0) {
 
@@ -452,7 +485,7 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) {
                          * not. */
 
                         log_debug("Detected server responses lack RRSIG records, downgrading feature level...");
-                        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_EDNS0;
+                        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_IS_TLS(s->possible_feature_level) ? DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN : DNS_SERVER_FEATURE_LEVEL_EDNS0;
 
                 } else if (s->n_failed_udp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS &&
                            s->possible_feature_level >= (dns_server_get_dnssec_mode(s) == DNSSEC_YES ? DNS_SERVER_FEATURE_LEVEL_LARGE : DNS_SERVER_FEATURE_LEVEL_UDP)) {
@@ -795,6 +828,12 @@ DnssecMode dns_server_get_dnssec_mode(DnsServer *s) {
         return manager_get_dnssec_mode(s->manager);
 }
 
+PrivateDnsMode dns_server_get_private_dns_mode(DnsServer *s) {
+        assert(s);
+
+        return manager_get_private_dns_mode(s->manager);
+}
+
 void dns_server_flush_cache(DnsServer *s) {
         DnsServer *current;
         DnsScope *scope;
@@ -904,7 +943,9 @@ static const char* const dns_server_feature_level_table[_DNS_SERVER_FEATURE_LEVE
         [DNS_SERVER_FEATURE_LEVEL_TCP] = "TCP",
         [DNS_SERVER_FEATURE_LEVEL_UDP] = "UDP",
         [DNS_SERVER_FEATURE_LEVEL_EDNS0] = "UDP+EDNS0",
+        [DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN] = "TLS+EDNS0",
         [DNS_SERVER_FEATURE_LEVEL_DO] = "UDP+EDNS0+DO",
         [DNS_SERVER_FEATURE_LEVEL_LARGE] = "UDP+EDNS0+DO+LARGE",
+        [DNS_SERVER_FEATURE_LEVEL_TLS_DO] = "TLS+EDNS0+D0",
 };
 DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level, DnsServerFeatureLevel);

src/resolve/resolved-dns-server.h

@@ -9,6 +9,10 @@
 
 #include "in-addr-util.h"
 
+#if HAVE_GNUTLS
+#include <gnutls/gnutls.h>
+#endif
+
 typedef struct DnsServer DnsServer;
 
 typedef enum DnsServerType {
@@ -25,14 +29,17 @@ typedef enum DnsServerFeatureLevel {
         DNS_SERVER_FEATURE_LEVEL_TCP,
         DNS_SERVER_FEATURE_LEVEL_UDP,
         DNS_SERVER_FEATURE_LEVEL_EDNS0,
+        DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN,
         DNS_SERVER_FEATURE_LEVEL_DO,
         DNS_SERVER_FEATURE_LEVEL_LARGE,
+        DNS_SERVER_FEATURE_LEVEL_TLS_DO,
         _DNS_SERVER_FEATURE_LEVEL_MAX,
         _DNS_SERVER_FEATURE_LEVEL_INVALID = -1
 } DnsServerFeatureLevel;
 
 #define DNS_SERVER_FEATURE_LEVEL_WORST 0
 #define DNS_SERVER_FEATURE_LEVEL_BEST (_DNS_SERVER_FEATURE_LEVEL_MAX - 1)
+#define DNS_SERVER_FEATURE_LEVEL_IS_TLS(x) IN_SET(x, DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN, DNS_SERVER_FEATURE_LEVEL_TLS_DO)
 
 const char* dns_server_feature_level_to_string(int i) _const_;
 int dns_server_feature_level_from_string(const char *s) _pure_;
@@ -55,6 +62,11 @@ struct DnsServer {
         char *server_string;
         DnsStream *stream;
 
+#if HAVE_GNUTLS
+        gnutls_certificate_credentials_t tls_cert_cred;
+        gnutls_datum_t tls_session_data;
+#endif
+
         usec_t resend_timeout;
         usec_t max_rtt;
 
@@ -65,6 +77,7 @@ struct DnsServer {
 
         unsigned n_failed_udp;
         unsigned n_failed_tcp;
+        unsigned n_failed_tls;
 
         bool packet_truncated:1;
         bool packet_bad_opt:1;
@@ -134,6 +147,7 @@ void manager_next_dns_server(Manager *m);
 bool dns_server_address_valid(int family, const union in_addr_union *sa);
 
 DnssecMode dns_server_get_dnssec_mode(DnsServer *s);
+PrivateDnsMode dns_server_get_private_dns_mode(DnsServer *s);
 
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsServer*, dns_server_unref);
 

src/resolve/resolved-dns-stream.c

@@ -16,6 +16,8 @@
 #define DNS_STREAM_TIMEOUT_USEC (10 * USEC_PER_SEC)
 #define DNS_STREAMS_MAX 128
 
+#define WRITE_TLS_DATA 1
+
 static void dns_stream_stop(DnsStream *s) {
         assert(s);
 
@@ -47,7 +49,19 @@ static int dns_stream_update_io(DnsStream *s) {
 static int dns_stream_complete(DnsStream *s, int error) {
         assert(s);
 
-        dns_stream_stop(s);
+#if HAVE_GNUTLS
+        if (s->tls_session && IN_SET(error, ETIMEDOUT, 0)) {
+                int r;
+
+                r = gnutls_bye(s->tls_session, GNUTLS_SHUT_RDWR);
+                if (r == GNUTLS_E_AGAIN && !s->tls_bye) {
+                        dns_stream_ref(s); /* keep reference for closing TLS session */
+                        s->tls_bye = true;
+                } else
+                        dns_stream_stop(s);
+        } else
+#endif
+                dns_stream_stop(s);
 
         if (s->complete)
                 s->complete(s, error);
@@ -182,12 +196,39 @@ static int dns_stream_identify(DnsStream *s) {
         return 0;
 }
 
-static ssize_t dns_stream_writev(DnsStream *s, const struct iovec *iov, size_t iovcnt) {
+static ssize_t dns_stream_writev(DnsStream *s, const struct iovec *iov, size_t iovcnt, int flags) {
         ssize_t r;
 
         assert(s);
         assert(iov);
 
+#if HAVE_GNUTLS
+        if (s->tls_session && !(flags & WRITE_TLS_DATA)) {
+                ssize_t ss;
+                size_t i;
+
+                r = 0;
+                for (i = 0; i < iovcnt; i++) {
+                        ss = gnutls_record_send(s->tls_session, iov[i].iov_base, iov[i].iov_len);
+                        if (ss < 0) {
+                                switch(ss) {
+
+                                case GNUTLS_E_INTERRUPTED:
+                                        return -EINTR;
+                                case GNUTLS_E_AGAIN:
+                                        return -EAGAIN;
+                                default:
+                                        log_debug("Failed to invoke gnutls_record_send: %s", gnutls_strerror(ss));
+                                        return -EIO;
+                                }
+                        }
+
+                        r += ss;
+                        if (ss != (ssize_t) iov[i].iov_len)
+                                continue;
+                }
+        } else
+#endif
         if (s->tfo_salen > 0) {
                 struct msghdr hdr = {
                         .msg_iov = (struct iovec*) iov,
@@ -215,6 +256,54 @@ static ssize_t dns_stream_writev(DnsStream *s, const struct iovec *iov, size_t i
         return r;
 }
 
+static ssize_t dns_stream_read(DnsStream *s, void *buf, size_t count) {
+        ssize_t ss;
+
+#if HAVE_GNUTLS
+        if (s->tls_session) {
+                ss = gnutls_record_recv(s->tls_session, buf, count);
+                if (ss < 0) {
+                        switch(ss) {
+
+                        case GNUTLS_E_INTERRUPTED:
+                                return -EINTR;
+                        case GNUTLS_E_AGAIN:
+                                return -EAGAIN;
+                        default:
+                                log_debug("Failed to invoke gnutls_record_send: %s", gnutls_strerror(ss));
+                                return -EIO;
+                        }
+                } else if (s->on_connection) {
+                        int r;
+
+                        r = s->on_connection(s);
+                        s->on_connection = NULL; /* only call once */
+                        if (r < 0)
+                                return r;
+                }
+        } else
+#endif
+                ss = read(s->fd, buf, count);
+
+        return ss;
+}
+
+#if HAVE_GNUTLS
+static ssize_t dns_stream_tls_writev(gnutls_transport_ptr_t p, const giovec_t * iov, int iovcnt) {
+        int r;
+
+        assert(p);
+
+        r = dns_stream_writev((DnsStream*) p, (struct iovec*) iov, iovcnt, WRITE_TLS_DATA);
+        if (r < 0) {
+                errno = -r;
+                return -1;
+        }
+
+        return r;
+}
+#endif
+
 static int on_stream_timeout(sd_event_source *es, usec_t usec, void *userdata) {
         DnsStream *s = userdata;
 
@@ -229,6 +318,40 @@ static int on_stream_io(sd_event_source *es, int fd, uint32_t revents, void *use
 
         assert(s);
 
+#if HAVE_GNUTLS
+        if (s->tls_bye) {
+                assert(s->tls_session);
+
+                r = gnutls_bye(s->tls_session, GNUTLS_SHUT_RDWR);
+                if (r != GNUTLS_E_AGAIN) {
+                        s->tls_bye = false;
+                        dns_stream_unref(s);
+                }
+
+                return 0;
+        }
+
+        if (s->tls_handshake < 0) {
+                assert(s->tls_session);
+
+                s->tls_handshake = gnutls_handshake(s->tls_session);
+                if (s->tls_handshake >= 0) {
+                        if (s->on_connection && !(gnutls_session_get_flags(s->tls_session) & GNUTLS_SFLAGS_FALSE_START)) {
+                                r = s->on_connection(s);
+                                s->on_connection = NULL; /* only call once */
+                                if (r < 0)
+                                        return r;
+                        }
+                } else {
+                        if (gnutls_error_is_fatal(s->tls_handshake))
+                                return dns_stream_complete(s, ECONNREFUSED);
+                        else
+                                return 0;
+                }
+
+        }
+#endif
+
         /* only identify after connecting */
         if (s->tfo_salen == 0) {
                 r = dns_stream_identify(s);
@@ -250,7 +373,7 @@ static int on_stream_io(sd_event_source *es, int fd, uint32_t revents, void *use
 
                 IOVEC_INCREMENT(iov, 2, s->n_written);
 
-                ss = dns_stream_writev(s, iov, 2);
+                ss = dns_stream_writev(s, iov, 2, 0);
                 if (ss < 0) {
                         if (!IN_SET(errno, EINTR, EAGAIN))
                                 return dns_stream_complete(s, errno);
@@ -272,7 +395,7 @@ static int on_stream_io(sd_event_source *es, int fd, uint32_t revents, void *use
                 if (s->n_read < sizeof(s->read_size)) {
                         ssize_t ss;
 
-                        ss = read(fd, (uint8_t*) &s->read_size + s->n_read, sizeof(s->read_size) - s->n_read);
+                        ss = dns_stream_read(s, (uint8_t*) &s->read_size + s->n_read, sizeof(s->read_size) - s->n_read);
                         if (ss < 0) {
                                 if (!IN_SET(errno, EINTR, EAGAIN))
                                         return dns_stream_complete(s, errno);
@@ -320,7 +443,7 @@ static int on_stream_io(sd_event_source *es, int fd, uint32_t revents, void *use
                                         }
                                 }
 
-                                ss = read(fd,
+                                ss = dns_stream_read(s,
                                           (uint8_t*) DNS_PACKET_DATA(s->read_packet) + s->n_read - sizeof(s->read_size),
                                           sizeof(s->read_size) + be16toh(s->read_size) - s->n_read);
                                 if (ss < 0) {
@@ -380,6 +503,11 @@ DnsStream *dns_stream_unref(DnsStream *s) {
                 s->manager->n_dns_streams--;
         }
 
+#if HAVE_GNUTLS
+        if (s->tls_session)
+                gnutls_deinit(s->tls_session);
+#endif
+
         ORDERED_SET_FOREACH(p, s->write_queue, i)
                 dns_packet_unref(ordered_set_remove(s->write_queue, p));
 
@@ -456,6 +584,21 @@ int dns_stream_new(Manager *m, DnsStream **ret, DnsProtocol protocol, int fd, co
         return 0;
 }
 
+#if HAVE_GNUTLS
+int dns_stream_connect_tls(DnsStream *s, gnutls_session_t tls_session) {
+        gnutls_transport_set_ptr2(tls_session, (gnutls_transport_ptr_t) (long) s->fd, s);
+        gnutls_transport_set_vec_push_function(tls_session, &dns_stream_tls_writev);
+
+        s->encrypted = true;
+        s->tls_session = tls_session;
+        s->tls_handshake = gnutls_handshake(tls_session);
+        if (s->tls_handshake < 0 && gnutls_error_is_fatal(s->tls_handshake))
+                return -ECONNREFUSED;
+
+        return 0;
+}
+#endif
+
 int dns_stream_write_packet(DnsStream *s, DnsPacket *p) {
         int r;
 

src/resolve/resolved-dns-stream.h

@@ -15,6 +15,10 @@ typedef struct DnsStream DnsStream;
 #include "resolved-dns-transaction.h"
 #include "resolved-manager.h"
 
+#if HAVE_GNUTLS
+#include <gnutls/gnutls.h>
+#endif
+
 /* Streams are used by three subsystems:
  *
  *   1. The normal transaction logic when doing a DNS or LLMNR lookup via TCP
@@ -41,6 +45,12 @@ struct DnsStream {
         union sockaddr_union tfo_address;
         socklen_t tfo_salen;
 
+#if HAVE_GNUTLS
+        gnutls_session_t tls_session;
+        int tls_handshake;
+        bool tls_bye;
+#endif
+
         sd_event_source *io_event_source;
         sd_event_source *timeout_event_source;
 
@@ -49,6 +59,7 @@ struct DnsStream {
         size_t n_written, n_read;
         OrderedSet *write_queue;
 
+        int (*on_connection)(DnsStream *s);
         int (*on_packet)(DnsStream *s);
         int (*complete)(DnsStream *s, int error);
 
@@ -56,10 +67,16 @@ struct DnsStream {
         DnsServer *server;                       /* when used by the transaction logic */
         DnsQuery *query;             /* when used by the DNS stub logic */
 
+        /* used when DNS-over-TLS is enabled */
+        bool encrypted:1;
+
         LIST_FIELDS(DnsStream, streams);
 };
 
 int dns_stream_new(Manager *m, DnsStream **s, DnsProtocol protocol, int fd, const union sockaddr_union *tfo_address);
+#if HAVE_GNUTLS
+int dns_stream_connect_tls(DnsStream *s, gnutls_session_t tls_session);
+#endif
 DnsStream *dns_stream_unref(DnsStream *s);
 DnsStream *dns_stream_ref(DnsStream *s);
 

src/resolve/resolved-dns-transaction.c

@@ -18,6 +18,10 @@
 #include "resolved-llmnr.h"
 #include "string-table.h"
 
+#if HAVE_GNUTLS
+#include <gnutls/socket.h>
+#endif
+
 #define TRANSACTIONS_MAX 4096
 #define TRANSACTION_TCP_TIMEOUT_USEC (10U*USEC_PER_SEC)
 
@@ -499,6 +503,20 @@ static int dns_transaction_on_stream_packet(DnsTransaction *t, DnsPacket *p) {
         return 0;
 }
 
+static int on_stream_connection(DnsStream *s) {
+#if HAVE_GNUTLS
+        /* Store TLS Ticket for faster succesive TLS handshakes */
+        if (s->tls_session && s->server) {
+                if (s->server->tls_session_data.data)
+                        gnutls_free(s->server->tls_session_data.data);
+
+                gnutls_session_get_data2(s->tls_session, &s->server->tls_session_data);
+        }
+#endif
+
+        return 0;
+}
+
 static int on_stream_complete(DnsStream *s, int error) {
         DnsTransaction *t, *n;
         int r = 0;
@@ -559,6 +577,9 @@ static int dns_transaction_emit_tcp(DnsTransaction *t) {
         _cleanup_(dns_stream_unrefp) DnsStream *s = NULL;
         union sockaddr_union sa;
         int r;
+#if HAVE_GNUTLS
+        gnutls_session_t gs;
+#endif
 
         assert(t);
 
@@ -578,10 +599,10 @@ static int dns_transaction_emit_tcp(DnsTransaction *t) {
                 if (r < 0)
                         return r;
 
-                if (t->server->stream)
+                if (t->server->stream && (DNS_SERVER_FEATURE_LEVEL_IS_TLS(t->current_feature_level) == t->server->stream->encrypted))
                         s = dns_stream_ref(t->server->stream);
                 else
-                        fd = dns_scope_socket_tcp(t->scope, AF_UNSPEC, NULL, t->server, 53, &sa);
+                        fd = dns_scope_socket_tcp(t->scope, AF_UNSPEC, NULL, t->server, DNS_SERVER_FEATURE_LEVEL_IS_TLS(t->current_feature_level) ? 853 : 53, &sa);
 
                 break;
 
@@ -630,6 +651,33 @@ static int dns_transaction_emit_tcp(DnsTransaction *t) {
                         s->server = dns_server_ref(t->server);
                 }
 
+#if HAVE_GNUTLS
+                if (DNS_SERVER_FEATURE_LEVEL_IS_TLS(t->current_feature_level)) {
+                        r = gnutls_init(&gs, GNUTLS_CLIENT | GNUTLS_ENABLE_FALSE_START | GNUTLS_NONBLOCK);
+                        if (r < 0)
+                                return r;
+
+                        /* As DNS-over-TLS is a recent protocol, older TLS versions can be disabled */
+                        r = gnutls_priority_set_direct(gs, "NORMAL:-VERS-ALL:+VERS-TLS1.2", NULL);
+                        if (r < 0)
+                                return r;
+
+                        r = gnutls_credentials_set(gs, GNUTLS_CRD_CERTIFICATE, t->server->tls_cert_cred);
+                        if (r < 0)
+                                return r;
+
+                        if (t->server && t->server->tls_session_data.size > 0)
+                                gnutls_session_set_data(gs, t->server->tls_session_data.data, t->server->tls_session_data.size);
+
+                        gnutls_handshake_set_timeout(gs, GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);
+
+                        r = dns_stream_connect_tls(s, gs);
+                        if (r < 0)
+                                return r;
+                }
+#endif
+
+                s->on_connection = on_stream_connection;
                 s->complete = on_stream_complete;
                 s->on_packet = dns_stream_on_packet;
 
@@ -993,7 +1041,17 @@ void dns_transaction_process_reply(DnsTransaction *t, DnsPacket *p) {
                         }
 
                         /* Reduce this feature level by one and try again. */
-                        t->clamp_feature_level = t->current_feature_level - 1;
+                        switch (t->current_feature_level) {
+                        case DNS_SERVER_FEATURE_LEVEL_TLS_DO:
+                                t->clamp_feature_level = DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN;
+                                break;
+                        case DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN + 1:
+                                /* Skip plain TLS when TLS is not supported */
+                                t->clamp_feature_level = DNS_SERVER_FEATURE_LEVEL_TLS_PLAIN - 1;
+                                break;
+                        default:
+                                t->clamp_feature_level = t->current_feature_level - 1;
+                        }
 
                         log_debug("Server returned error %s, retrying transaction with reduced feature level %s.",
                                   dns_rcode_to_string(DNS_PACKET_RCODE(p)),
@@ -1209,7 +1267,7 @@ static int dns_transaction_emit_udp(DnsTransaction *t) {
                 if (r < 0)
                         return r;
 
-                if (t->current_feature_level < DNS_SERVER_FEATURE_LEVEL_UDP)
+                if (t->current_feature_level < DNS_SERVER_FEATURE_LEVEL_UDP || DNS_SERVER_FEATURE_LEVEL_IS_TLS(t->current_feature_level))
                         return -EAGAIN; /* Sorry, can't do UDP, try TCP! */
 
                 if (!dns_server_dnssec_supported(t->server) && dns_type_is_dnssec(t->key->type))
@@ -1699,7 +1757,7 @@ int dns_transaction_go(DnsTransaction *t) {
                 if (r == -EMSGSIZE)
                         log_debug("Sending query via TCP since it is too large.");
                 else if (r == -EAGAIN)
-                        log_debug("Sending query via TCP since server doesn't support UDP.");
+                        log_debug("Sending query via TCP since UDP isn't supported.");
                 if (IN_SET(r, -EMSGSIZE, -EAGAIN))
                         r = dns_transaction_emit_tcp(t);
         }

src/resolve/resolved-gperf.gperf

@@ -23,5 +23,6 @@ Resolve.Domains,         config_parse_search_domains,         0,
 Resolve.LLMNR,           config_parse_resolve_support,        0,                   offsetof(Manager, llmnr_support)
 Resolve.MulticastDNS,    config_parse_resolve_support,        0,                   offsetof(Manager, mdns_support)
 Resolve.DNSSEC,          config_parse_dnssec_mode,            0,                   offsetof(Manager, dnssec_mode)
+Resolve.PrivateDNS,      config_parse_private_dns_mode,       0,                   offsetof(Manager, private_dns_mode)
 Resolve.Cache,           config_parse_bool,                   0,                   offsetof(Manager, enable_cache)
 Resolve.DNSStubListener, config_parse_dns_stub_listener_mode, 0,                   offsetof(Manager, dns_stub_listener_mode)

src/resolve/resolved-manager.c

@@ -580,6 +580,7 @@ int manager_new(Manager **ret) {
         m->llmnr_support = RESOLVE_SUPPORT_YES;
         m->mdns_support = RESOLVE_SUPPORT_YES;
         m->dnssec_mode = DEFAULT_DNSSEC_MODE;
+        m->private_dns_mode = DEFAULT_PRIVATE_DNS_MODE;
         m->enable_cache = true;
         m->dns_stub_listener_mode = DNS_STUB_LISTENER_UDP;
         m->read_resolv_conf = true;
@@ -1384,6 +1385,15 @@ bool manager_dnssec_supported(Manager *m) {
         return true;
 }
 
+PrivateDnsMode manager_get_private_dns_mode(Manager *m) {
+        assert(m);
+
+        if (m->private_dns_mode != _PRIVATE_DNS_MODE_INVALID)
+                return m->private_dns_mode;
+
+        return _PRIVATE_DNS_MODE_INVALID;
+}
+
 void manager_dnssec_verdict(Manager *m, DnssecVerdict verdict, const DnsResourceKey *key) {
 
         assert(verdict >= 0);

src/resolve/resolved-manager.h

@@ -35,6 +35,7 @@ struct Manager {
         ResolveSupport llmnr_support;
         ResolveSupport mdns_support;
         DnssecMode dnssec_mode;
+        PrivateDnsMode private_dns_mode;
         bool enable_cache;
         DnsStubListenerMode dns_stub_listener_mode;
 
@@ -172,6 +173,8 @@ int manager_compile_search_domains(Manager *m, OrderedSet **domains, int filter_
 DnssecMode manager_get_dnssec_mode(Manager *m);
 bool manager_dnssec_supported(Manager *m);
 
+PrivateDnsMode manager_get_private_dns_mode(Manager *m);
+
 void manager_dnssec_verdict(Manager *m, DnssecVerdict verdict, const DnsResourceKey *key);
 
 bool manager_routable(Manager *m, int family);

src/resolve/resolved.conf.in

@@ -18,5 +18,6 @@
 #LLMNR=yes
 #MulticastDNS=yes
 #DNSSEC=@DEFAULT_DNSSEC_MODE@
+#PrivateDNS=@DEFAULT_PRIVATE_DNS@
 #Cache=yes
 #DNSStubListener=udp

src/shared/resolve-util.c

@@ -11,6 +11,7 @@
 
 DEFINE_CONFIG_PARSE_ENUM(config_parse_resolve_support, resolve_support, ResolveSupport, "Failed to parse resolve support setting");
 DEFINE_CONFIG_PARSE_ENUM(config_parse_dnssec_mode, dnssec_mode, DnssecMode, "Failed to parse DNSSEC mode setting");
+DEFINE_CONFIG_PARSE_ENUM(config_parse_private_dns_mode, private_dns_mode, PrivateDnsMode, "Failed to parse private DNS mode setting");
 
 static const char* const resolve_support_table[_RESOLVE_SUPPORT_MAX] = {
         [RESOLVE_SUPPORT_NO] = "no",
@@ -25,3 +26,9 @@ static const char* const dnssec_mode_table[_DNSSEC_MODE_MAX] = {
         [DNSSEC_YES] = "yes",
 };
 DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(dnssec_mode, DnssecMode, DNSSEC_YES);
+
+static const char* const private_dns_mode_table[_PRIVATE_DNS_MODE_MAX] = {
+        [PRIVATE_DNS_NO] = "no",
+        [PRIVATE_DNS_OPPORTUNISTIC] = "opportunistic",
+};
+DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(private_dns_mode, PrivateDnsMode, PRIVATE_DNS_OPPORTUNISTIC);

src/shared/resolve-util.h

@@ -12,6 +12,7 @@
 
 typedef enum ResolveSupport ResolveSupport;
 typedef enum DnssecMode DnssecMode;
+typedef enum PrivateDnsMode PrivateDnsMode;
 
 enum ResolveSupport {
         RESOLVE_SUPPORT_NO,
@@ -39,11 +40,27 @@ enum DnssecMode {
         _DNSSEC_MODE_INVALID = -1
 };
 
+enum PrivateDnsMode {
+        /* No connection is made for DNS-over-TLS */
+        PRIVATE_DNS_NO,
+
+        /* Try to connect using DNS-over-TLS, but if connection fails,
+         * fallback to using an unencrypted connection */
+        PRIVATE_DNS_OPPORTUNISTIC,
+
+        _PRIVATE_DNS_MODE_MAX,
+        _PRIVATE_DNS_MODE_INVALID = -1
+};
+
 CONFIG_PARSER_PROTOTYPE(config_parse_resolve_support);
 CONFIG_PARSER_PROTOTYPE(config_parse_dnssec_mode);
+CONFIG_PARSER_PROTOTYPE(config_parse_private_dns_mode);
 
 const char* resolve_support_to_string(ResolveSupport p) _const_;
 ResolveSupport resolve_support_from_string(const char *s) _pure_;
 
 const char* dnssec_mode_to_string(DnssecMode p) _const_;
 DnssecMode dnssec_mode_from_string(const char *s) _pure_;
+
+const char* private_dns_mode_to_string(PrivateDnsMode p) _const_;
+PrivateDnsMode private_dns_mode_from_string(const char *s) _pure_;