resolved: add strict mode for DNS-over-TLS

Add strict mode for DNS-over-TLS, which will require TLS support from the server. Closes #10755

5 files changed, 24 insertions, 3 deletions

diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index 79e388f8a2..7b2938fea3 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -394,7 +394,7 @@ int manager_parse_config_file(Manager *m) {
 
 #if ! ENABLE_DNS_OVER_TLS
         if (m->dns_over_tls_mode != DNS_OVER_TLS_NO) {
-                log_warning("DNS-over-TLS option cannot be set to opportunistic when systemd-resolved is built without DNS-over-TLS support. Turning off DNS-over-TLS support.");
+                log_warning("DNS-over-TLS option cannot be enabled or set to opportunistic when systemd-resolved is built without DNS-over-TLS support. Turning off DNS-over-TLS support.");
                 m->dns_over_tls_mode = DNS_OVER_TLS_NO;
         }
 #endif
diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c
index 0033de73b4..78e5953b30 100644
--- a/src/resolve/resolved-dns-server.c
+++ b/src/resolve/resolved-dns-server.c
@@ -419,7 +419,7 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) {
                         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)) {
+                           DNS_SERVER_FEATURE_LEVEL_IS_TLS(s->possible_feature_level) && dns_server_get_dns_over_tls_mode(s) != DNS_OVER_TLS_YES) {
 
                         /* 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 */
diff --git a/src/resolve/resolved-dnstls-gnutls.c b/src/resolve/resolved-dnstls-gnutls.c
index d824d6ca5a..6eef6117a3 100644
--- a/src/resolve/resolved-dnstls-gnutls.c
+++ b/src/resolve/resolved-dnstls-gnutls.c
@@ -54,6 +54,9 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
                 server->dnstls_data.session_data.size = 0;
         }
 
+        if (server->manager->dns_over_tls_mode == DNS_OVER_TLS_YES)
+                gnutls_session_set_verify_cert(gs, NULL, 0);
+
         gnutls_handshake_set_timeout(gs, GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);
 
         gnutls_transport_set_ptr2(gs, (gnutls_transport_ptr_t) (long) stream->fd, stream);
@@ -202,6 +205,10 @@ int dnstls_manager_init(Manager *manager) {
         if (r < 0)
                 return -ENOMEM;
 
+        r = gnutls_certificate_set_x509_system_trust(manager->dnstls_data.cert_cred);
+        if (r < 0)
+                log_warning("Failed to load system trust store: %s", gnutls_strerror(r));
+
         return 0;
 }
 
diff --git a/src/resolve/resolved-dnstls-openssl.c b/src/resolve/resolved-dnstls-openssl.c
index 22d579a7f7..85e202ff74 100644
--- a/src/resolve/resolved-dnstls-openssl.c
+++ b/src/resolve/resolved-dnstls-openssl.c
@@ -76,6 +76,17 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
         SSL_set_session(s, server->dnstls_data.session);
         SSL_set_bio(s, TAKE_PTR(rb), TAKE_PTR(wb));
 
+        if (server->manager->dns_over_tls_mode == DNS_OVER_TLS_YES) {
+                X509_VERIFY_PARAM *v;
+                const unsigned char *ip;
+
+                SSL_set_verify(s, SSL_VERIFY_PEER, NULL);
+                v = SSL_get0_param(s);
+                ip = server->family == AF_INET ? (const unsigned char*) &server->address.in.s_addr : server->address.in6.s6_addr;
+                if (!X509_VERIFY_PARAM_set1_ip(v, ip, FAMILY_ADDRESS_SIZE(server->family)))
+                        return -ECONNREFUSED;
+        }
+
         ERR_clear_error();
         stream->dnstls_data.handshake = SSL_do_handshake(s);
         if (stream->dnstls_data.handshake <= 0) {
@@ -357,6 +368,9 @@ int dnstls_manager_init(Manager *manager) {
 
         SSL_CTX_set_min_proto_version(manager->dnstls_data.ctx, TLS1_2_VERSION);
         SSL_CTX_set_options(manager->dnstls_data.ctx, SSL_OP_NO_COMPRESSION);
+        r = SSL_CTX_set_default_verify_paths(manager->dnstls_data.ctx);
+        if (r < 0)
+                log_warning("Failed to load system trust store: %s", ERR_error_string(ERR_get_error(), NULL));
 
         return 0;
 }
diff --git a/src/resolve/resolved-link.c b/src/resolve/resolved-link.c
index dd8b5a574b..96ebb4d23d 100644
--- a/src/resolve/resolved-link.c
+++ b/src/resolve/resolved-link.c
@@ -384,7 +384,7 @@ void link_set_dns_over_tls_mode(Link *l, DnsOverTlsMode mode) {
 
 #if ! ENABLE_DNS_OVER_TLS
         if (mode != DNS_OVER_TLS_NO)
-                log_warning("DNS-over-TLS option for the link cannot be set to opportunistic when systemd-resolved is built without DNS-over-TLS support. Turning off DNS-over-TLS support.");
+                log_warning("DNS-over-TLS option for the link cannot be enabled or set to opportunistic when systemd-resolved is built without DNS-over-TLS support. Turning off DNS-over-TLS support.");
         return;
 #endif