net/dnscache, net/tsdial: add DNS caching to tsdial UserDial

This is enough to handle the DNS queries as generated by Go's
net package (which our HTTP/SOCKS client uses), and the responses
generated by the ExitDNS DoH server.

This isn't yet suitable for putting on 100.100.100.100 where a number
of different DNS clients would hit it, as this doesn't yet do
EDNS0. It might work, but it's untested and likely incomplete.

Likewise, this doesn't handle anything about truncation, as the
exchanges are entirely in memory between Go or DoH. That would also
need to be handled later, if/when it's hooked up to 100.100.100.100.

Updates #3507

Change-Id: I1736b0ad31eea85ea853b310c52c5e6bf65c6e2a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tsdial/dohclient.go

@@ -13,15 +13,18 @@ import (
 	"net"
 	"net/http"
 	"time"
+
+	"tailscale.com/net/dnscache"
 )
 
 // dohConn is a net.PacketConn suitable for returning from
 // net.Dialer.Dial to send DNS queries over PeerAPI to exit nodes'
 // ExitDNS DoH proxy service.
 type dohConn struct {
-	ctx     context.Context
-	baseURL string
-	hc      *http.Client // if nil, default is used
+	ctx      context.Context
+	baseURL  string
+	hc       *http.Client // if nil, default is used
+	dnsCache *dnscache.MessageCache
 
 	rbuf bytes.Buffer
 }
@@ -52,6 +55,15 @@ func (c *dohConn) Read(p []byte) (n int, err error) {
 }
 
 func (c *dohConn) Write(packet []byte) (n int, err error) {
+	if c.dnsCache != nil {
+		err := c.dnsCache.ReplyFromCache(&c.rbuf, packet)
+		if err == nil {
+			// Cache hit.
+			// TODO(bradfitz): add clientmetric
+			return len(packet), nil
+		}
+		c.rbuf.Reset()
+	}
 	req, err := http.NewRequestWithContext(c.ctx, "POST", c.baseURL, bytes.NewReader(packet))
 	if err != nil {
 		return 0, err
@@ -77,6 +89,9 @@ func (c *dohConn) Write(packet []byte) (n int, err error) {
 	if err != nil {
 		return 0, err
 	}
+	if c.dnsCache != nil {
+		c.dnsCache.AddCacheEntry(packet, c.rbuf.Bytes())
+	}
 	return len(packet), nil
 }