feat(netlink): detect ipv6 support level (#2523)

- add option `IPV6_CHECK_ADDRESSESES=[2001:4860:4860::8888]:53,[2606:4700:4700::1111]:53`
- gluetun needs access to the addresses above through the host firewall, to test ipv6 support before setting up the vpn

internal/cli/noopfirewall.go

@@ -0,0 +1,14 @@
+package cli
+
+import (
+	"context"
+	"net/netip"
+)
+
+type noopFirewall struct{}
+
+func (f *noopFirewall) AcceptOutput(_ context.Context, _, _ string, _ netip.Addr,
+	_ uint16, _ bool,
+) (err error) {
+	return nil
+}

internal/cli/openvpnconfig.go

@@ -11,6 +11,7 @@ import (
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/netlink"
 	"github.com/qdm12/gluetun/internal/openvpn/extract"
 	"github.com/qdm12/gluetun/internal/provider"
 	"github.com/qdm12/gluetun/internal/storage"
@@ -40,7 +41,9 @@ type IPFetcher interface {
 }
 
 type IPv6Checker interface {
-	IsIPv6Supported() (supported bool, err error)
+	FindIPv6SupportLevel(ctx context.Context,
+		checkAddresses []netip.AddrPort, firewall netlink.Firewall,
+	) (level netlink.IPv6SupportLevel, err error)
 }
 
 func (c *CLI) OpenvpnConfig(logger OpenvpnConfigLogger, reader *reader.Reader,
@@ -58,12 +61,14 @@ func (c *CLI) OpenvpnConfig(logger OpenvpnConfigLogger, reader *reader.Reader,
 	}
 	allSettings.SetDefaults()
 
-	ipv6Supported, err := ipv6Checker.IsIPv6Supported()
+	ipv6SupportLevel, err := ipv6Checker.FindIPv6SupportLevel(context.Background(),
+		allSettings.IPv6.CheckAddresses, &noopFirewall{})
 	if err != nil {
 		return fmt.Errorf("checking for IPv6 support: %w", err)
 	}
 
-	if err = allSettings.Validate(storage, ipv6Supported, logger); err != nil {
+	err = allSettings.Validate(storage, ipv6SupportLevel.IsSupported(), logger)
+	if err != nil {
 		return fmt.Errorf("validating settings: %w", err)
 	}
 
@@ -79,13 +84,13 @@ func (c *CLI) OpenvpnConfig(logger OpenvpnConfigLogger, reader *reader.Reader,
 		unzipper, parallelResolver, ipFetcher, openvpnFileExtractor, allSettings.Updater)
 	providerConf := providers.Get(allSettings.VPN.Provider.Name)
 	connection, err := providerConf.GetConnection(
-		allSettings.VPN.Provider.ServerSelection, ipv6Supported)
+		allSettings.VPN.Provider.ServerSelection, ipv6SupportLevel == netlink.IPv6Internet)
 	if err != nil {
 		return err
 	}
 
 	lines := providerConf.OpenVPNConfig(connection,
-		allSettings.VPN.OpenVPN, ipv6Supported)
+		allSettings.VPN.OpenVPN, ipv6SupportLevel.IsSupported())
 
 	fmt.Println(strings.Join(lines, "\n"))
 	return nil