Add a new "tls-mode=preferred" DSN parameter

Separating "preferred" into its own parameter instead of making it
a special value in the "tls=" parameter makes it possible to use
custom TLS config with this mode. This is useful when clients
don't need to authenticate servers using TLS but a server may
or may not need to authenticate the client using TLS.

Author: dweitzman

AUTHORS

@@ -25,6 +25,7 @@ Daniel Montoya <dsmontoyam at gmail.com>
 Daniel Nichter <nil at codenode.com>
 Daniël van Eeden <git at myname.nl>
 Dave Protasowski <dprotaso at gmail.com>
+David Weitzman <dweitzman at gmail.com>
 DisposaBoy <disposaboy at dby.me>
 Egor Smolyakov <egorsmkv at gmail.com>
 Evan Shaw <evan at vendhq.com>

README.md

@@ -335,6 +335,17 @@ Default:        false
 `tls=true` enables TLS / SSL encrypted connection to the server. Use `skip-verify` if you want to use a self-signed or invalid certificate (server side) or use `preferred` to use TLS only when advertised by the server. This is similar to `skip-verify`, but additionally allows a fallback to a connection which is not encrypted. Neither `skip-verify` nor `preferred` add any reliable security. You can use a custom TLS config after registering it with [`mysql.RegisterTLSConfig`](https://godoc.org/github.com/go-sql-driver/mysql#RegisterTLSConfig).
 
 
+##### `tls-mode`
+
+```
+Type:           string
+Valid Values:   preferred
+Default:        <none>
+```
+
+Use `tls-mode=preferred` to opt-in to TLS / SSL only with servers that support it. `preferred` does not authenticate the server but allows servers to optionally authenticate clients. The [`tls`](#tls) DSN parameter allows customizing the TLS config.
+
+
 ##### `writeTimeout`
 
 ```

dsn.go

@@ -46,6 +46,7 @@ type Config struct {
 	pubKey           *rsa.PublicKey    // Server public key
 	TLSConfig        string            // TLS configuration name
 	tls              *tls.Config       // TLS configuration
+	TLSOptional      bool              // Allows non-TLS for servers that don't have TLS capability
 	Timeout          time.Duration     // Dial timeout
 	ReadTimeout      time.Duration     // I/O read timeout
 	WriteTimeout     time.Duration     // I/O write timeout
@@ -287,6 +288,10 @@ func (cfg *Config) FormatDSN() string {
 		buf.WriteString(url.QueryEscape(cfg.TLSConfig))
 	}
 
+	if cfg.TLSOptional && cfg.TLSConfig != "preferred" {
+		buf.WriteString("&tls-mode=preferred")
+	}
+
 	if cfg.WriteTimeout > 0 {
 		if hasParam {
 			buf.WriteString("&writeTimeout=")
@@ -550,6 +555,18 @@ func parseDSNParams(cfg *Config, params string) (err error) {
 				return
 			}
 
+		// TLS enforcement settings
+		case "tls-mode":
+			switch value {
+			case "preferred":
+				cfg.TLSOptional = true
+				if cfg.tls == nil {
+					cfg.tls = &tls.Config{}
+				}
+			default:
+				return errors.New("invalid value / unknown tls-mode: " + value)
+			}
+
 		// TLS-Encryption
 		case "tls":
 			boolValue, isBool := readBool(value)
@@ -563,6 +580,9 @@ func parseDSNParams(cfg *Config, params string) (err error) {
 			} else if vl := strings.ToLower(value); vl == "skip-verify" || vl == "preferred" {
 				cfg.TLSConfig = vl
 				cfg.tls = &tls.Config{InsecureSkipVerify: true}
+				if vl == "preferred" {
+					cfg.TLSOptional = true
+				}
 			} else {
 				name, err := url.QueryUnescape(value)
 				if err != nil {

dsn_test.go

@@ -35,6 +35,12 @@ var testDSNs = []struct {
 }, {
 	"user:password@tcp(localhost:5555)/dbname?charset=utf8&tls=true",
 	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "true"},
+}, {
+	"user:password@tcp(localhost:5555)/dbname?charset=utf8&tls=preferred",
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "preferred", TLSOptional: true},
+}, {
+	"user:password@tcp(localhost:5555)/dbname?charset=utf8&tls=true&tls-mode=preferred",
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "true", TLSOptional: true},
 }, {
 	"user:password@tcp(localhost:5555)/dbname?charset=utf8mb4,utf8&tls=skip-verify",
 	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8mb4,utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "skip-verify"},

packets.go

@@ -194,7 +194,7 @@ func (mc *mysqlConn) readHandshakePacket() (data []byte, plugin string, err erro
 		return nil, "", ErrOldProtocol
 	}
 	if mc.flags&clientSSL == 0 && mc.cfg.tls != nil {
-		if mc.cfg.TLSConfig == "preferred" {
+		if mc.cfg.TLSOptional {
 			mc.cfg.tls = nil
 		} else {
 			return nil, "", ErrNoTLS