feat(netlink): detect ipv6 support level

- 'supported' if one ipv6 route is found that is not loopback and not a default route
- 'internet' if one default ipv6 route is found

internal/netlink/ipv6.go

@@ -4,34 +4,56 @@ import (
 	"fmt"
 )
 
-func (n *NetLink) IsIPv6Supported() (supported bool, err error) {
+type IPv6SupportLevel uint8
+
+const (
+	IPv6Unsupported = iota
+	// IPv6Supported indicates the host supports IPv6 but has no access to the
+	// Internet via IPv6. It is true if one IPv6 route is found and no default
+	// IPv6 route is found.
+	IPv6Supported
+	// IPv6Internet indicates the host has access to the Internet via IPv6,
+	// which is detected when a default IPv6 route is found.
+	IPv6Internet
+)
+
+func (i IPv6SupportLevel) IsSupported() bool {
+	return i == IPv6Supported || i == IPv6Internet
+}
+
+func (n *NetLink) FindIPv6SupportLevel() (level IPv6SupportLevel, err error) {
 	routes, err := n.RouteList(FamilyV6)
 	if err != nil {
-		return false, fmt.Errorf("listing IPv6 routes: %w", err)
+		return IPv6Unsupported, fmt.Errorf("listing IPv6 routes: %w", err)
 	}
 
 	// Check each route for IPv6 due to Podman bug listing IPv4 routes
 	// as IPv6 routes at container start, see:
 	// https://github.com/qdm12/gluetun/issues/1241#issuecomment-1333405949
+	level = IPv6Unsupported
 	for _, route := range routes {
 		link, err := n.LinkByIndex(route.LinkIndex)
 		if err != nil {
-			return false, fmt.Errorf("finding link corresponding to route: %w", err)
+			return IPv6Unsupported, fmt.Errorf("finding link corresponding to route: %w", err)
 		}
 
-		sourceIsIPv6 := route.Src.IsValid() && route.Src.Is6()
+		sourceIsIPv4 := route.Src.IsValid() && route.Src.Is4()
+		destinationIsIPv4 := route.Dst.IsValid() && route.Dst.Addr().Is4()
 		destinationIsIPv6 := route.Dst.IsValid() && route.Dst.Addr().Is6()
 		switch {
-		case !sourceIsIPv6 && !destinationIsIPv6,
+		case sourceIsIPv4 && destinationIsIPv4,
 			destinationIsIPv6 && route.Dst.Addr().IsLoopback():
-			continue
+		case route.Dst.Addr().IsUnspecified(): // default ipv6 route
+			n.debugLogger.Debugf("IPv6 internet access is enabled on link %s", link.Name)
+			return IPv6Internet, nil
+		default: // non-default ipv6 route found
+			n.debugLogger.Debugf("IPv6 is supported by link %s", link.Name)
+			level = IPv6Supported
 		}
-
-		n.debugLogger.Debugf("IPv6 is supported by link %s", link.Name)
-		return true, nil
 	}
 
-	n.debugLogger.Debugf("IPv6 is not supported after searching %d routes",
-		len(routes))
-	return false, nil
+	if level == IPv6Unsupported {
+		n.debugLogger.Debugf("no IPv6 route found in %d routes", len(routes))
+	}
+	return level, nil
 }