ipn/ipnlocal: start adding DoH DNS server to peerapi when exit node

Updates #1713

Change-Id: I8d9c488f779e7acc811a9bc18166a2726198a429
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/userspace.go

@@ -147,6 +147,20 @@ func (e *userspaceEngine) GetInternals() (_ *tstun.Wrapper, _ *magicsock.Conn, o
 	return e.tundev, e.magicConn, true
 }
 
+// ResolvingEngine is implemented by Engines that have DNS resolvers.
+type ResolvingEngine interface {
+	GetResolver() (_ *resolver.Resolver, ok bool)
+}
+
+var (
+	_ ResolvingEngine = (*userspaceEngine)(nil)
+	_ ResolvingEngine = (*watchdogEngine)(nil)
+)
+
+func (e *userspaceEngine) GetResolver() (r *resolver.Resolver, ok bool) {
+	return e.dns.Resolver(), true
+}
+
 // BIRDClient handles communication with the BIRD Internet Routing Daemon.
 type BIRDClient interface {
 	EnableProtocol(proto string) error

wgengine/watchdog.go

@@ -15,6 +15,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/tstun"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -139,6 +140,12 @@ func (e *watchdogEngine) GetInternals() (tw *tstun.Wrapper, c *magicsock.Conn, o
 	}
 	return
 }
+func (e *watchdogEngine) GetResolver() (r *resolver.Resolver, ok bool) {
+	if re, ok := e.wrap.(ResolvingEngine); ok {
+		return re.GetResolver()
+	}
+	return nil, false
+}
 func (e *watchdogEngine) Wait() {
 	e.wrap.Wait()
 }