pkg/mesh,pkg/wireguard: allow DNS name endpoints

This commit allows DNS names to be used when specifying the endpoint
for a node in the WireGuard mesh. This is useful in many scenarios, in
particular when operating an IoT device whose public IP is dynamic. This
change allows the administrator to use a dynamic DNS name in the node's
endpoint.

One of the side-effects of this change is that the WireGuard port can
now be specified individually for each node in the mesh, if the
administrator wishes to do so.

*Note*: this commit introduces a breaking change; the
`force-external-ip` node annotation has been removed; its functionality
has been ported over to the `force-endpoint` annotation. This annotation
is documented in the annotations.md file. The expected content of this
annotation is no longer a CIDR but rather a host:port. The host can be
either a DNS name or an IP.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

pkg/encapsulation/none.go

@@ -0,0 +1,63 @@
+// Copyright 2019 the Kilo authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package encapsulation
+
+import (
+	"net"
+
+	"github.com/squat/kilo/pkg/iptables"
+)
+
+type none Strategy
+
+// NewNone returns an new encapsulator that does not encapsulate.
+func NewNone(strategy Strategy) Encapsulator {
+	return none(strategy)
+}
+
+// CleanUp is a no-op.
+func (n none) CleanUp() error {
+	return nil
+}
+
+// Gw always returns nil.
+func (n none) Gw(_, _ net.IP, _ *net.IPNet) net.IP {
+	return nil
+}
+
+// Index always returns 0.
+func (n none) Index() int {
+	return 0
+}
+
+// Init is a no-op.
+func (n none) Init(base int) error {
+	return nil
+}
+
+// Rules always returns an empty list.
+func (n none) Rules(_ []*net.IPNet) []iptables.Rule {
+	return nil
+}
+
+// Set is a no-op.
+func (n none) Set(_ *net.IPNet) error {
+	return nil
+}
+
+// Strategy returns the configured strategy for encapsulation.
+func (n none) Strategy() Strategy {
+	return Strategy(n)
+}