manifests,pkg/encapsulation: Flannel compatibility

This commit adds basic support to run in compatibility mode with
Flannel. This allows clusters running Flannel as their principal
networking solution to leverage some advances Kilo features. In certain
Flannel setups, the clusters can even leverage muti-cloud. For this, the
cluster needs to either run in a full mesh, or Flannel needs to use the
API server's external IP address.

pkg/encapsulation/ipip.go

@@ -22,45 +22,13 @@ import (
 	"github.com/squat/kilo/pkg/iptables"
 )
 
-// Strategy identifies which packets within a location should
-// be encapsulated.
-type Strategy string
-
-const (
-	// Never indicates that no packets within a location
-	// should be encapsulated.
-	Never Strategy = "never"
-	// CrossSubnet indicates that only packets that
-	// traverse subnets within a location should be encapsulated.
-	CrossSubnet Strategy = "crosssubnet"
-	// Always indicates that all packets within a location
-	// should be encapsulated.
-	Always Strategy = "always"
-)
-
-// Interface can configure
-// the encapsulation interface, init itself,
-// get the encapsulation interface index,
-// set the interface IP address,
-// return the required IPTables rules,
-// return the encapsulation strategy,
-// and clean up any changes applied to the backend.
-type Interface interface {
-	CleanUp() error
-	Index() int
-	Init(int) error
-	Rules([]*net.IPNet) []iptables.Rule
-	Set(*net.IPNet) error
-	Strategy() Strategy
-}
-
 type ipip struct {
 	iface    int
 	strategy Strategy
 }
 
-// NewIPIP returns an encapsulation that uses IPIP.
-func NewIPIP(strategy Strategy) Interface {
+// NewIPIP returns an encapsulator that uses IPIP.
+func NewIPIP(strategy Strategy) Encapsulator {
 	return &ipip{strategy: strategy}
 }
 
@@ -72,6 +40,11 @@ func (i *ipip) CleanUp() error {
 	return iproute.RemoveInterface(i.iface)
 }
 
+// Gw returns the correct gateway IP associated with the given node.
+func (i *ipip) Gw(_, internal net.IP, _ *net.IPNet) net.IP {
+	return internal
+}
+
 // Index returns the index of the IPIP interface.
 func (i *ipip) Index() int {
 	return i.iface