feat(vpn): auto detection of IPv6 support

- `OPENVPN_IPV6` removed
- Affects OpenVPN
- Use the same mechanism for OpenVPN and Wireguard
- Check only once at program start since this is unlikely to change at runtime
- Log if IPv6 is supported
- Remove `IPv6` boolean from settings structs
- Move IPv6 detection as a method on NetLinker

internal/wireguard/settings.go

@@ -30,6 +30,9 @@ type Settings struct {
 	// RulePriority is the priority for the rule created with the
 	// FirewallMark.
 	RulePriority int
+	// IPv6 can bet set to true if IPv6 should be handled.
+	// It defaults to false if left unset.
+	IPv6 *bool
 }
 
 func (s *Settings) SetDefaults() {
@@ -47,6 +50,11 @@ func (s *Settings) SetDefaults() {
 		const defaultFirewallMark = 51820
 		s.FirewallMark = defaultFirewallMark
 	}
+
+	if s.IPv6 == nil {
+		ipv6 := false // this should be injected from host
+		s.IPv6 = &ipv6
+	}
 }
 
 var (
@@ -187,6 +195,12 @@ func (s Settings) ToLines(settings ToLinesSettings) (lines []string) {
 	}
 	lines = append(lines, fieldPrefix+"Endpoint: "+endpointStr)
 
+	ipv6Status := "disabled"
+	if *s.IPv6 {
+		ipv6Status = "enabled"
+	}
+	lines = append(lines, fieldPrefix+"IPv6: "+ipv6Status)
+
 	if s.FirewallMark != 0 {
 		lines = append(lines, fieldPrefix+"Firewall mark: "+fmt.Sprint(s.FirewallMark))
 	}