wgengine/router: dns: unify on *BSD, multimode on Linux, Magic DNS (#536)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns.go

@@ -0,0 +1,83 @@
+// 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 router
+
+import (
+	"time"
+
+	"inet.af/netaddr"
+)
+
+// DNSConfig is the subset of Config that contains DNS parameters.
+type DNSConfig struct {
+	// Nameservers are the IP addresses of the nameservers to use.
+	Nameservers []netaddr.IP
+	// Domains are the search domains to use.
+	Domains []string
+}
+
+// EquivalentTo determines whether its argument and receiver
+// represent equivalent DNS configurations (then DNS reconfig is a no-op).
+func (lhs DNSConfig) EquivalentTo(rhs DNSConfig) bool {
+	if len(lhs.Nameservers) != len(rhs.Nameservers) {
+		return false
+	}
+
+	if len(lhs.Domains) != len(rhs.Domains) {
+		return false
+	}
+
+	// With how we perform resolution order shouldn't matter,
+	// but it is unlikely that we will encounter different orders.
+	for i, server := range lhs.Nameservers {
+		if rhs.Nameservers[i] != server {
+			return false
+		}
+	}
+
+	for i, domain := range lhs.Domains {
+		if rhs.Domains[i] != domain {
+			return false
+		}
+	}
+
+	return true
+}
+
+// dnsReconfigTimeout is the timeout for DNS reconfiguration.
+//
+// This is useful because certain conditions can cause indefinite hangs
+// (such as improper dbus auth followed by contextless dbus.Object.Call).
+// Such operations should be wrapped in a timeout context.
+const dnsReconfigTimeout = time.Second
+
+// dnsMode determines how DNS settings are managed.
+type dnsMode uint8
+
+const (
+	// dnsDirect indicates that /etc/resolv.conf is edited directly.
+	dnsDirect dnsMode = iota
+	// dnsResolvconf indicates that a resolvconf binary is used.
+	dnsResolvconf
+	// dnsNetworkManager indicates that the NetworkManaer DBus API is used.
+	dnsNetworkManager
+	// dnsResolved indicates that the systemd-resolved DBus API is used.
+	dnsResolved
+)
+
+func (m dnsMode) String() string {
+	switch m {
+	case dnsDirect:
+		return "direct"
+	case dnsResolvconf:
+		return "resolvconf"
+	case dnsNetworkManager:
+		return "networkmanager"
+	case dnsResolved:
+		return "resolved"
+	default:
+		return "???"
+	}
+}