monitor: refactor for architecture-specific connection implementations

Signed-off-by: wardn <wardn@users.noreply.github.com>

wgengine/monitor/monitor_linux.go

@@ -0,0 +1,60 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package monitor
+
+import (
+	"fmt"
+
+	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
+)
+
+const (
+	RTMGRP_IPV4_IFADDR = 0x10
+	RTMGRP_IPV4_ROUTE  = 0x40
+)
+
+// nlConn wraps a *netlink.Conn and returns a monitor.Message
+// instead of a netlink.Message. Currently, messages are discarded,
+// but down the line, when messages trigger different logic depending
+// on the type of event, this provides the capability of handling
+// each architecture-specific message in a generic fashion.
+type nlConn struct {
+	conn *netlink.Conn
+}
+
+func NewConn() (Conn, error) {
+	conn, err := netlink.Dial(unix.NETLINK_ROUTE, &netlink.Config{
+		// IPv4 address and route changes. Routes get us most of the
+		// events of interest, but we need address as well to cover
+		// things like DHCP deciding to give us a new address upon
+		// renewal - routing wouldn't change, but all reachability
+		// would.
+		//
+		// Why magic numbers? These aren't exposed in x/sys/unix
+		// yet. The values come from rtnetlink.h, RTMGRP_IPV4_IFADDR
+		// and RTMGRP_IPV4_ROUTE.
+		Groups: RTMGRP_IPV4_IFADDR | RTMGRP_IPV4_ROUTE,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("dialing netlink socket: %v", err)
+	}
+	return &nlConn{conn}, nil
+}
+
+func (c *nlConn) Close() error {
+	return c.conn.Close()
+}
+
+func (c *nlConn) Receive() (Message, error) {
+	// currently ignoring the message
+	_, err := c.conn.Receive()
+	if err != nil {
+		return nil, err
+	}
+	// TODO(]|[): this is where the NetLink-specific message would
+	// get converted into a "standard" event message and returned.
+	return nil, nil
+}