Update minimum SSL version

Change default of ssl_min_protocol_version to TLSv1.2 (from TLSv1,
which means 1.0).  Older versions are still supported, just not by
default.

TLS 1.0 is widely deprecated, and TLS 1.1 only slightly less so.  All
OpenSSL versions that support TLS 1.1 also support TLS 1.2, so there
would be very little reason to, say, set the default to TLS 1.1
instead on grounds of better compatibility.

The test suite overrides this new setting, so it can still run with
older OpenSSL versions.

Discussion: https://www.postgresql.org/message-id/flat/b327f8df-da98-054d-0cc5-b76a857cfed9%402ndquadrant.com

doc/src/sgml/config.sgml

@@ -1365,10 +1365,8 @@ include_dir 'conf.d'
        </para>
 
        <para>
-        The default is <literal>TLSv1</literal>, mainly to support older
-        versions of the <productname>OpenSSL</productname> library.  You might
-        want to set this to a higher value if all software components can
-        support the newer protocol versions.
+        The default is <literal>TLSv1.2</literal>, which satisfies industry
+        best practices as of this writing.
        </para>
       </listitem>
      </varlistentry>

src/backend/utils/misc/guc.c

@@ -4573,7 +4573,7 @@ static struct config_enum ConfigureNamesEnum[] =
 			GUC_SUPERUSER_ONLY
 		},
 		&ssl_min_protocol_version,
-		PG_TLS1_VERSION,
+		PG_TLS1_2_VERSION,
 		ssl_protocol_versions_info + 1, /* don't allow PG_TLS_ANY */
 		NULL, NULL, NULL
 	},

src/backend/utils/misc/postgresql.conf.sample

@@ -105,7 +105,7 @@
 #ssl_ciphers = 'HIGH:MEDIUM:+3DES:!aNULL' # allowed SSL ciphers
 #ssl_prefer_server_ciphers = on
 #ssl_ecdh_curve = 'prime256v1'
-#ssl_min_protocol_version = 'TLSv1'
+#ssl_min_protocol_version = 'TLSv1.2'
 #ssl_max_protocol_version = ''
 #ssl_dh_params_file = ''
 #ssl_passphrase_command = ''

src/test/ssl/t/SSLServer.pm

@@ -132,6 +132,10 @@ sub configure_test_server_for_ssl
 	print $conf "listen_addresses='$serverhost'\n";
 	print $conf "log_statement=all\n";
 
+	# Accept even old TLS versions so that builds with older OpenSSL
+	# can run the test suite.
+	print $conf "ssl_min_protocol_version='TLSv1'\n";
+
 	# enable SSL and set up server key
 	print $conf "include 'sslconfig.conf'\n";