validate all addresses resolved from hostname

Signed-off-by: Artemiy Ryabinkov <getlag@ya.ru>

config.go

@@ -37,6 +37,7 @@ type Config struct {
 	Password      string
 	TLSConfig     *tls.Config // nil disables TLS
 	DialFunc      DialFunc    // e.g. net.Dialer.DialContext
+	LookupFunc    LookupFunc  // e.g. net.Resolver.LookupHost
 	BuildFrontend BuildFrontendFunc
 	RuntimeParams map[string]string // Run-time parameters to set on connection as session default values (e.g. search_path or application_name)
 
@@ -77,7 +78,7 @@ func NetworkAddress(host string, port uint16) (network, address string) {
 		address = filepath.Join(host, ".s.PGSQL.") + strconv.FormatInt(int64(port), 10)
 	} else {
 		network = "tcp"
-		address = fmt.Sprintf("%s:%d", host, port)
+		address = net.JoinHostPort(host, strconv.Itoa(int(port)))
 	}
 	return network, address
 }
@@ -190,6 +191,8 @@ func ParseConfig(connString string) (*Config, error) {
 		config.DialFunc = defaultDialer.DialContext
 	}
 
+	config.LookupFunc = makeDefaultResolver().LookupHost
+
 	notRuntimeParams := map[string]struct{}{
 		"host":                 struct{}{},
 		"port":                 struct{}{},
@@ -495,6 +498,10 @@ func makeDefaultDialer() *net.Dialer {
 	return &net.Dialer{KeepAlive: 5 * time.Minute}
 }
 
+func makeDefaultResolver() *net.Resolver {
+	return net.DefaultResolver
+}
+
 func makeDefaultBuildFrontendFunc(minBufferLen int) BuildFrontendFunc {
 	return func(r io.Reader, w io.Writer) Frontend {
 		cr, err := chunkreader.NewConfig(r, chunkreader.Config{MinBufLen: minBufferLen})

pgconn.go

@@ -43,6 +43,9 @@ type Notification struct {
 // DialFunc is a function that can be used to connect to a PostgreSQL server.
 type DialFunc func(ctx context.Context, network, addr string) (net.Conn, error)
 
+// LookupFunc is a function that can be used to lookup IPs addrs from host.
+type LookupFunc func(ctx context.Context, host string) (addrs []string, err error)
+
 // BuildFrontendFunc is a function that can be used to create Frontend implementation for connection.
 type BuildFrontendFunc func(r io.Reader, w io.Writer) Frontend
 
@@ -123,6 +126,15 @@ func ConnectConfig(ctx context.Context, config *Config) (pgConn *PgConn, err err
 	}
 	fallbackConfigs = append(fallbackConfigs, config.Fallbacks...)
 
+	fallbackConfigs, err = expandWithIPs(ctx, config.LookupFunc, fallbackConfigs)
+	if err != nil {
+		return nil, &connectError{config: config, msg: "hostname resolving error", err: err}
+	}
+
+	if len(fallbackConfigs) == 0 {
+		return nil, &connectError{config: config, msg: "hostname resolving error", err: errors.New("ip addr wasn't found")}
+	}
+
 	for _, fc := range fallbackConfigs {
 		pgConn, err = connect(ctx, config, fc)
 		if err == nil {
@@ -147,6 +159,27 @@ func ConnectConfig(ctx context.Context, config *Config) (pgConn *PgConn, err err
 	return pgConn, nil
 }
 
+func expandWithIPs(ctx context.Context, lookupFn LookupFunc, fallbacks []*FallbackConfig) ([]*FallbackConfig, error) {
+	var configs []*FallbackConfig
+
+	for _, fb := range fallbacks {
+		ips, err := lookupFn(ctx, fb.Host)
+		if err != nil {
+			return nil, err
+		}
+
+		for _, ip := range ips {
+			configs = append(configs, &FallbackConfig{
+				Host:      ip,
+				Port:      fb.Port,
+				TLSConfig: fb.TLSConfig,
+			})
+		}
+	}
+
+	return configs, nil
+}
+
 func connect(ctx context.Context, config *Config, fallbackConfig *FallbackConfig) (*PgConn, error) {
 	pgConn := new(PgConn)
 	pgConn.config = config