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/ipv6.go

@@ -0,0 +1,58 @@
+package settings
+
+import (
+	"net/netip"
+
+	"github.com/qdm12/gosettings"
+	"github.com/qdm12/gosettings/reader"
+	"github.com/qdm12/gotree"
+)
+
+// IPv6 contains settings regarding IPv6 configuration.
+type IPv6 struct {
+	// CheckAddresses are the TCP ip:port addresses to dial to check if
+	// IPv6 is supported, in case a default IPv6 route is found.
+	// It defaults to google and cloudflare IPv6 anycast addresses
+	// [2001:4860:4860::8888]:53,[2606:4700:4700::1111]:53
+	CheckAddresses []netip.AddrPort
+}
+
+func (i IPv6) validate() (err error) {
+	return nil
+}
+
+func (i *IPv6) copy() (copied IPv6) {
+	return IPv6{
+		CheckAddresses: gosettings.CopySlice(i.CheckAddresses),
+	}
+}
+
+func (i *IPv6) overrideWith(other IPv6) {
+	i.CheckAddresses = gosettings.OverrideWithSlice(i.CheckAddresses, other.CheckAddresses)
+}
+
+func (i *IPv6) setDefaults() {
+	defaultCheckAddresses := []netip.AddrPort{
+		netip.MustParseAddrPort("[2001:4860:4860::8888]:53"),
+		netip.MustParseAddrPort("[2606:4700:4700::1111]:53"),
+	}
+	i.CheckAddresses = gosettings.DefaultSlice(i.CheckAddresses, defaultCheckAddresses)
+}
+
+func (i IPv6) String() string {
+	return i.toLinesNode().String()
+}
+
+func (i IPv6) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("IPv6 settings:")
+	addrsNode := node.Appendf("Check addresses:")
+	for _, addr := range i.CheckAddresses {
+		addrsNode.Append(addr.String())
+	}
+	return node
+}
+
+func (i *IPv6) read(r *reader.Reader) (err error) {
+	i.CheckAddresses, err = r.CSVNetipAddrPorts("IPV6_CHECK_ADDRESSES")
+	return err
+}

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,
 	}

internal/configuration/settings/settings_test.go

@@ -67,6 +67,10 @@ func Test_Settings_String(t *testing.T) {
 |       └── Log level: INFO
 ├── Log settings:
 |   └── Log level: INFO
+├── IPv6 settings:
+|   └── Check addresses:
+|       ├── [2001:4860:4860::8888]:53
+|       └── [2606:4700:4700::1111]:53
 ├── Health settings:
 |   ├── Server listening address: 127.0.0.1:9999
 |   ├── Target addresses: