all: imp code

main.go

@@ -67,7 +67,8 @@ type Options struct {
 	EDNSAddr string `yaml:"edns-addr" long:"edns-addr" description:"Send EDNS Client Address"`
 
 	// UpstreamMode determines the logic through which upstreams will be used.
-	UpstreamMode string `yaml:"upstream-mode" long:"upstream-mode" description:"" optional:"yes" optional-value:"load_balance"`
+	// If not specified the [proxy.UpstreamModeLoadBalance] is used.
+	UpstreamMode string `yaml:"upstream-mode" long:"upstream-mode" description:"Upstreams logic mode" optional:"yes" optional-value:"load_balance"`
 
 	// ListenAddrs is the list of server's listen addresses.
 	ListenAddrs []string `yaml:"listen-addrs" short:"l" long:"listen" description:"Listening addresses"`

proxy/config.go

@@ -106,6 +106,7 @@ type Config struct {
 	HTTPSServerName string
 
 	// UpstreamMode determines the logic through which upstreams will be used.
+	// If not specified the [proxy.UpstreamModeLoadBalance] is used.
 	UpstreamMode UpstreamMode
 
 	// UDPListenAddr is the set of UDP addresses to listen for plain

proxy/proxy.go

@@ -262,12 +262,9 @@ func New(c *Config) (p *Proxy, err error) {
 		p.requestsSema = syncutil.EmptySemaphore{}
 	}
 
-	p.logger.Info("upstream mode is set", "mode", p.UpstreamMode)
-	if p.UpstreamMode == UpstreamModeFastestAddr {
-		p.fastestAddr = fastip.New(&fastip.Config{
-			Logger:          p.Logger,
-			PingWaitTimeout: p.FastestPingTimeout,
-		})
+	err = p.setupUpstreamMode()
+	if err != nil {
+		return nil, fmt.Errorf("setting up upstream mode: %w", err)
 	}
 
 	err = p.setupDNS64()
@@ -281,6 +278,28 @@ func New(c *Config) (p *Proxy, err error) {
 	return p, nil
 }
 
+// setupUpstreamMode validates and sets up upstream mode for proxy.
+func (p *Proxy) setupUpstreamMode() (err error) {
+	switch p.UpstreamMode {
+	case "":
+		p.UpstreamMode = UpstreamModeLoadBalance
+	case UpstreamModeFastestAddr, UpstreamModeLoadBalance, UpstreamModeParallel:
+		// Go on.
+	default:
+		return fmt.Errorf("bad upstream mode: %q", p.UpstreamMode)
+	}
+
+	p.logger.Info("upstream mode is set", "mode", p.UpstreamMode)
+	if p.UpstreamMode == UpstreamModeFastestAddr {
+		p.fastestAddr = fastip.New(&fastip.Config{
+			Logger:          p.Logger,
+			PingWaitTimeout: p.FastestPingTimeout,
+		})
+	}
+
+	return nil
+}
+
 // validateBasicAuth validates the basic-auth mode settings if p.Config.Userinfo
 // is set.
 func (p *Proxy) validateBasicAuth() (err error) {