tlsutil: Fix check TLS configuration

.changelog/17481.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+tlsutil: Default setting of ServerName field in outgoing TLS configuration for checks now handled by crypto/tls.
+```

tlsutil/config.go

@@ -857,10 +857,23 @@ func (c *Configurator) IncomingHTTPSConfig() *tls.Config {
 	return config
 }
 
-// OutgoingTLSConfigForCheck generates a *tls.Config for outgoing TLS connections
-// for checks. This function is separated because there is an extra flag to
-// consider for checks. EnableAgentTLSForChecks and InsecureSkipVerify has to
-// be checked for checks.
+// OutgoingTLSConfigForCheck creates a client *tls.Config for executing checks.
+// It is RECOMMENDED that the serverName be left unspecified. The crypto/tls
+// client will deduce the ServerName (for SNI) from the check address unless
+// it's an IP (RFC 6066, Section 3). However, there are two instances where
+// supplying a serverName is useful:
+//
+//  1. When the check address is an IP, a serverName can be supplied for SNI.
+//     Note: setting serverName will also override the hostname used to verify
+//     the certificate presented by the server being checked.
+//
+//  2. When the hostname in the check address won't be present in the SAN
+//     (Subject Alternative Name) field of the certificate presented by the
+//     server being checked. Note: setting serverName will also override the
+//     ServerName used for SNI.
+//
+// Setting skipVerify will disable verification of the server's certificate
+// chain and hostname, which is generally not suitable for production use.
 func (c *Configurator) OutgoingTLSConfigForCheck(skipVerify bool, serverName string) *tls.Config {
 	c.log("OutgoingTLSConfigForCheck")
 
@@ -875,13 +888,9 @@ func (c *Configurator) OutgoingTLSConfigForCheck(skipVerify bool, serverName str
 		}
 	}
 
-	if serverName == "" {
-		serverName = c.serverNameOrNodeName()
-	}
 	config := c.internalRPCTLSConfig(false)
 	config.InsecureSkipVerify = skipVerify
 	config.ServerName = serverName
-
 	return config
 }
 

tlsutil/config_test.go

@@ -1376,7 +1376,7 @@ func TestConfigurator_OutgoingTLSConfigForCheck(t *testing.T) {
 			},
 		},
 		{
-			name: "agent tls, default server name",
+			name: "agent tls, default consul server name, no override",
 			conf: func() (*Configurator, error) {
 				return NewConfigurator(Config{
 					InternalRPC: ProtocolConfig{
@@ -1389,11 +1389,11 @@ func TestConfigurator_OutgoingTLSConfigForCheck(t *testing.T) {
 			},
 			expected: &tls.Config{
 				MinVersion: tls.VersionTLS12,
-				ServerName: "servername",
+				ServerName: "",
 			},
 		},
 		{
-			name: "agent tls, skip verify, node name for server name",
+			name: "agent tls, skip verify, consul node name for server name, no override",
 			conf: func() (*Configurator, error) {
 				return NewConfigurator(Config{
 					InternalRPC: ProtocolConfig{
@@ -1407,7 +1407,7 @@ func TestConfigurator_OutgoingTLSConfigForCheck(t *testing.T) {
 			expected: &tls.Config{
 				InsecureSkipVerify: true,
 				MinVersion:         tls.VersionTLS12,
-				ServerName:         "nodename",
+				ServerName:         "",
 			},
 		},
 		{

website/content/docs/services/configuration/checks-configuration-reference.mdx

@@ -35,8 +35,8 @@ Specify health check options in the `check` block. To register two or more heath
 | `h2ping` | String value that specifies the HTTP2 endpoint, including port number, to send HTTP2 requests to. | <li>H2ping</li> |        
 | `h2ping_use_tls` | Boolean value that enables TLS for H2ping checks when set to `true`. | <li>H2ping</li> |
 | `http` | String value that specifies an HTTP endpoint to send requests to. | <li>HTTP</li> |
-| `tls_server_name` | String value that specifies the name of the TLS server that issues certificates. Defaults to the SNI determined by the address specified in the `http` field. Set the `tls_skip_verify` to `false` to disable this field. | <li>HTTP</li> |
-| `tls_skip_verify` | Boolean value that disbles TLS for HTTP checks when set to `true`. Default is `false`. | <li>HTTP</li> |
+| `tls_server_name` | String value that specifies the server name used to verify the hostname on the returned certificates unless `tls_skip_verify` is given. Also included in the client's handshake to support SNI. It is recommended that this field be left unspecified. The TLS client will deduce the server name for SNI from the check address unless it's an IP ([RFC 6066, Section 3](https://tools.ietf.org/html/rfc6066#section-3)). There are two common circumstances where supplying a `tls_server_name` can be beneficial: <li>When the check address is an IP, `tls_server_name` can be specified for SNI. Note: setting `tls_server_name` will also override the hostname used to verify the certificate presented by the server being checked.</li><li>When the hostname in the check address won't be present in the SAN (Subject Alternative Name) field of the certificate presented by the server being checked. Note: setting `tls_server_name` will also override the hostname used for SNI.</li> | <li>HTTP </li> <li>H2Ping </li> <li>gRPC </li> |
+| `tls_skip_verify` | Boolean value that determines if the check verifies the chain and hostname of the certificate that the server presents. Set to `true` to disable verification. We recommend setting to `false` for production use. Default is `false`. | <li>HTTP </li> <li>H2Ping </li> <li>gRPC </li> |
 | `method` | String value that specifies the request method to send during HTTP checks. Default is `GET`. | <li>HTTP</li> |
 | `header` | Object that specifies header fields to send in HTTP check requests. Each header specified in `header` object contains a list of string values. | <li>HTTP</li> |
 | `body` | String value that contains JSON attributes to send in HTTP check requests. You must escap the quotation marks around the keys and values for each attribute. | <li>HTTP</li> |