net/dnscache, control/controlclient: use DNS cache when dialing control

Cache DNS results of earlier login.tailscale.com control dials, and use
them for future dials if DNS is slow or broken.

Fixes various issues with trickier setups with the domain's DNS server
behind a subnet router.

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

control/controlclient/direct.go

@@ -36,6 +36,7 @@ import (
 	"golang.org/x/oauth2"
 	"inet.af/netaddr"
 	"tailscale.com/log/logheap"
+	"tailscale.com/net/dnscache"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tshttpproxy"
@@ -172,11 +173,15 @@ func NewDirect(opts Options) (*Direct, error) {
 
 	httpc := opts.HTTPTestClient
 	if httpc == nil {
+		dnsCache := &dnscache.Resolver{
+			Forward:     dnscache.Get().Forward, // use default cache's forwarder
+			UseLastGood: true,
+		}
 		dialer := netns.NewDialer()
 		tr := http.DefaultTransport.(*http.Transport).Clone()
 		tr.Proxy = tshttpproxy.ProxyFromEnvironment
 		tshttpproxy.SetTransportGetProxyConnectHeader(tr)
-		tr.DialContext = dialer.DialContext
+		tr.DialContext = dnscache.Dialer(dialer.DialContext, dnsCache)
 		tr.ForceAttemptHTTP2 = true
 		tr.TLSClientConfig = tlsdial.Config(serverURL.Host, tr.TLSClientConfig)
 		httpc = &http.Client{Transport: tr}