migrate to golang.zx2c4.com/wireguard/wgctrl (#239)

* migrate to golang.zx2c4.com/wireguard/wgctrl

This commit introduces the usage of wgctrl.
It avoids the usage of exec calls of the wg command
and parsing the output of `wg show`.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* vendor wgctrl

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* apply suggestions from code review

Remove wireguard.Enpoint struct and use net.UDPAddr for the resolved
endpoint and addr string (dnsanme:port) if a DN was supplied.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/*: use wireguard.Enpoint

This commit introduces the wireguard.Enpoint struct.
It encapsulates a DN name with port and a net.UPDAddr.
The fields are private and only accessible over exported Methods
to avoid accidental modification.

Also iptables.GetProtocol is improved to avoid ipv4 rules being applied
by `ip6tables`.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/wireguard/conf_test.go: add tests for Endpoint

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* cmd/kg/main.go: validate port range

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* add suggestions from review

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* pkg/mesh/mesh.go: use Equal func

Implement an Equal func for Enpoint and use it instead of comparing
strings.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* cmd/kgctl/main.go: check port range

Signed-off-by: leonnicolas <leonloechner@gmx.de>

* vendor

Signed-off-by: leonnicolas <leonloechner@gmx.de>

vendor/github.com/mdlayher/netlink/netns_linux.go

@@ -0,0 +1,93 @@
+//+build linux
+
+package netlink
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// errNetNSDisabled is returned when network namespaces are unavailable on
+// a given system.
+var errNetNSDisabled = errors.New("netlink: network namespaces are not enabled on this system")
+
+// A netNS is a handle that can manipulate network namespaces.
+//
+// Operations performed on a netNS must use runtime.LockOSThread before
+// manipulating any network namespaces.
+type netNS struct {
+	// The handle to a network namespace.
+	f *os.File
+
+	// Indicates if network namespaces are disabled on this system, and thus
+	// operations should become a no-op or return errors.
+	disabled bool
+}
+
+// threadNetNS constructs a netNS using the network namespace of the calling
+// thread. If the namespace is not the default namespace, runtime.LockOSThread
+// should be invoked first.
+func threadNetNS() (*netNS, error) {
+	return fileNetNS(fmt.Sprintf("/proc/self/task/%d/ns/net", unix.Gettid()))
+}
+
+// fileNetNS opens file and creates a netNS. fileNetNS should only be called
+// directly in tests.
+func fileNetNS(file string) (*netNS, error) {
+	f, err := os.Open(file)
+	switch {
+	case err == nil:
+		return &netNS{f: f}, nil
+	case os.IsNotExist(err):
+		// Network namespaces are not enabled on this system. Use this signal
+		// to return errors elsewhere if the caller explicitly asks for a
+		// network namespace to be set.
+		return &netNS{disabled: true}, nil
+	default:
+		return nil, err
+	}
+}
+
+// Close releases the handle to a network namespace.
+func (n *netNS) Close() error {
+	return n.do(func() error {
+		return n.f.Close()
+	})
+}
+
+// FD returns a file descriptor which represents the network namespace.
+func (n *netNS) FD() int {
+	if n.disabled {
+		// No reasonable file descriptor value in this case, so specify a
+		// non-existent one.
+		return -1
+	}
+
+	return int(n.f.Fd())
+}
+
+// Restore restores the original network namespace for the calling thread.
+func (n *netNS) Restore() error {
+	return n.do(func() error {
+		return n.Set(n.FD())
+	})
+}
+
+// Set sets a new network namespace for the current thread using fd.
+func (n *netNS) Set(fd int) error {
+	return n.do(func() error {
+		return os.NewSyscallError("setns", unix.Setns(fd, unix.CLONE_NEWNET))
+	})
+}
+
+// do runs fn if network namespaces are enabled on this system.
+func (n *netNS) do(fn func() error) error {
+	if n.disabled {
+		return errNetNSDisabled
+	}
+
+	return fn()
+}