netns,wgengine: add OpenBSD support to netns via an rtable

When an exit node has been set and a new default route is added,
create a new rtable in the default rdomain and add the current
default route via its physical interface.  When control() is
requesting a connection not go through the exit-node default route,
we can use the SO_RTABLE socket option to force it through the new
rtable we created.

Updates #17321

Signed-off-by: joshua stein <jcs@jcs.org>

net/netmon/interfaces_bsdroute.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// FreeBSD and OpenBSD routing table functions.
+
+//go:build freebsd || openbsd
+
+package netmon
+
+import (
+	"syscall"
+
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+)
+
+// fetchRoutingTable calls route.FetchRIB, fetching NET_RT_DUMP.
+func fetchRoutingTable() (rib []byte, err error) {
+	return route.FetchRIB(syscall.AF_UNSPEC, unix.NET_RT_DUMP, 0)
+}
+
+func parseRoutingTable(rib []byte) ([]route.Message, error) {
+	return route.ParseRIB(syscall.NET_RT_IFLIST, rib)
+}
+
+func getDelegatedInterface(ifIndex int) (int, error) {
+	return 0, nil
+}