resolved: support TLS 1.3 when using GnuTLS for DNS-over-TLS

src/resolve/resolved-dnstls-gnutls.c

@@ -9,6 +9,11 @@
 #include "resolved-dns-stream.h"
 #include "resolved-dnstls.h"
 
+#if GNUTLS_VERSION_NUMBER >= 0x030600
+#define PRIORTY_STRING "NORMAL:-VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3"
+#else
+#define PRIORTY_STRING "NORMAL:-VERS-ALL:+VERS-TLS1.2"
+#endif
 DEFINE_TRIVIAL_CLEANUP_FUNC(gnutls_session_t, gnutls_deinit);
 
 static ssize_t dnstls_stream_writev(gnutls_transport_ptr_t p, const giovec_t *iov, int iovcnt) {
@@ -37,7 +42,7 @@ int dnstls_stream_connect_tls(DnsStream *stream, DnsServer *server) {
                 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);
+        r = gnutls_priority_set_direct(gs, PRIORTY_STRING, NULL);
         if (r < 0)
                 return r;