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/configuration/settings/settings.go

@@ -26,6 +26,7 @@ type Settings struct {
 	Updater       Updater
 	Version       Version
 	VPN           VPN
+	IPv6          IPv6
 	Pprof         pprof.Settings
 	BoringPoll    BoringPoll
 }
@@ -53,6 +54,7 @@ func (s *Settings) Validate(filterChoicesGetter FilterChoicesGetter, ipv6Support
 		"system":          s.System.validate,
 		"updater":         s.Updater.Validate,
 		"version":         s.Version.validate,
+		"ipv6":            s.IPv6.validate,
 		// Pprof validation done in pprof constructor
 		"VPN": func() error {
 			return s.VPN.Validate(filterChoicesGetter, ipv6Supported, warner)
@@ -87,6 +89,7 @@ func (s *Settings) copy() (copied Settings) {
 		VPN:           s.VPN.Copy(),
 		Pprof:         s.Pprof.Copy(),
 		BoringPoll:    s.BoringPoll.Copy(),
+		IPv6:          s.IPv6.copy(),
 	}
 }
 
@@ -109,6 +112,7 @@ func (s *Settings) OverrideWith(other Settings,
 	patchedSettings.VPN.OverrideWith(other.VPN)
 	patchedSettings.Pprof.OverrideWith(other.Pprof)
 	patchedSettings.BoringPoll.overrideWith(other.BoringPoll)
+	patchedSettings.IPv6.overrideWith(other.IPv6)
 	err = patchedSettings.Validate(filterChoicesGetter, ipv6Supported, warner)
 	if err != nil {
 		return err
@@ -124,6 +128,8 @@ func (s *Settings) SetDefaults() {
 	s.Firewall.setDefaults(s.Log.Level)
 	s.Health.SetDefaults()
 	s.HTTPProxy.setDefaults()
+	s.Log.setDefaults()
+	s.IPv6.setDefaults()
 	s.PublicIP.setDefaults()
 	s.Shadowsocks.setDefaults()
 	s.Storage.setDefaults()
@@ -146,6 +152,7 @@ func (s Settings) toLinesNode() (node *gotree.Node) {
 	node.AppendNode(s.DNS.toLinesNode())
 	node.AppendNode(s.Firewall.toLinesNode())
 	node.AppendNode(s.Log.toLinesNode())
+	node.AppendNode(s.IPv6.toLinesNode())
 	node.AppendNode(s.Health.toLinesNode())
 	node.AppendNode(s.Shadowsocks.toLinesNode())
 	node.AppendNode(s.HTTPProxy.toLinesNode())
@@ -211,6 +218,7 @@ func (s *Settings) Read(r *reader.Reader, warner Warner) (err error) {
 		"updater":     s.Updater.read,
 		"version":     s.Version.read,
 		"VPN":         s.VPN.read,
+		"IPv6":        s.IPv6.read,
 		"profiling":   s.Pprof.Read,
 		"boring poll": s.BoringPoll.read,
 	}