fix: fall back to ipv4 when ipv6 is unreachable

pkg/net/proxytransport.go

@@ -1,8 +1,11 @@
 package netutil
 
 import (
+	"context"
+	"errors"
 	"net"
 	"net/http"
+	"syscall"
 	"time"
 )
 
@@ -16,10 +19,28 @@ func newTransport(dialTimeout, tlsTimeout, headerTimeout time.Duration) *http.Tr
 		TLSHandshakeTimeout:   tlsTimeout,
 		ResponseHeaderTimeout: headerTimeout,
 		ExpectContinueTimeout: 1 * time.Second,
-		DialContext:           dialer.DialContext,
+		DialContext:           dialWithIPv4Fallback(dialer),
 	}
 }
 
+func dialWithIPv4Fallback(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error) {
+	return func(ctx context.Context, network string, address string) (net.Conn, error) {
+		conn, err := dialer.DialContext(ctx, network, address)
+		if err == nil {
+			return conn, nil
+		}
+		if network != "tcp" || !isUnreachableNetwork(err) {
+			return nil, err
+		}
+
+		return dialer.DialContext(ctx, "tcp4", address)
+	}
+}
+
+func isUnreachableNetwork(err error) bool {
+	return errors.Is(err, syscall.ENETUNREACH) || errors.Is(err, syscall.EHOSTUNREACH)
+}
+
 func NewClient() *http.Client {
 	return &http.Client{
 		Transport: newTransport(10*time.Second, 10*time.Second, 30*time.Second),