net/dns: add an OSConfigurator interface.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/osconfig.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2021 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 dns
+
+// DNSRoutingMode describes the type of per-domain DNS routing that
+// the OS is capable of.
+type RoutingMode int
+
+const (
+	// RoutingModeNone means the OS only supports setting a single
+	// primary set of DNS resolvers.
+	RoutingModeNone RoutingMode = iota
+	// RoutingModeSingle means the OS supports a set of
+	// primary resolvers, as well as one set of additional per-suffix
+	// resolvers per network interface.
+	RoutingModeSingle
+	// RoutingModeMulti means the OS supports a set of primary
+	// resolvers, as well as an arbitrary overlay of DNS routes.
+	RoutingModeMulti
+)
+
+// An OSConfigurator applies DNS settings to the operating system.
+type OSConfigurator interface {
+	// Set updates the OS's DNS configuration to match cfg.
+	// If cfg nil or the zero value, all Tailscale-related DNS
+	// configuration is removed.
+	// Set must not be called after Close.
+	Set(cfg *OSConfig) error
+	// DNSRoutingMode reports the DNS routing capabilities of this OS
+	// configurator.
+	RoutingMode() RoutingMode
+	// Close removes Tailscale-related DNS configuration from the OS.
+	Close() error
+}