ipn/ipnlocal, net/tsdial: make SOCKS/HTTP dials use ExitDNS

And simplify, unexport some tsdial/netstack stuff in the the process.

Fixes #3475

Change-Id: I186a5a5cbd8958e25c075b4676f7f6e70f3ff76e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tsdial/dohclient.go

@@ -0,0 +1,86 @@
+// 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 tsdial
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"time"
+)
+
+// 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
+
+	rbuf bytes.Buffer
+}
+
+var (
+	_ net.Conn       = (*dohConn)(nil)
+	_ net.PacketConn = (*dohConn)(nil) // be a PacketConn to change net.Resolver semantics
+)
+
+func (*dohConn) Close() error                       { return nil }
+func (*dohConn) LocalAddr() net.Addr                { return todoAddr{} }
+func (*dohConn) RemoteAddr() net.Addr               { return todoAddr{} }
+func (*dohConn) SetDeadline(t time.Time) error      { return nil }
+func (*dohConn) SetReadDeadline(t time.Time) error  { return nil }
+func (*dohConn) SetWriteDeadline(t time.Time) error { return nil }
+
+func (c *dohConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
+	return c.Write(p)
+}
+
+func (c *dohConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
+	n, err = c.Read(p)
+	return n, todoAddr{}, err
+}
+
+func (c *dohConn) Read(p []byte) (n int, err error) {
+	return c.rbuf.Read(p)
+}
+
+func (c *dohConn) Write(packet []byte) (n int, err error) {
+	req, err := http.NewRequestWithContext(c.ctx, "POST", c.baseURL, bytes.NewReader(packet))
+	if err != nil {
+		return 0, err
+	}
+	const dohType = "application/dns-message"
+	req.Header.Set("Content-Type", dohType)
+	hc := c.hc
+	if hc == nil {
+		hc = http.DefaultClient
+	}
+	hres, err := hc.Do(req)
+	if err != nil {
+		return 0, err
+	}
+	defer hres.Body.Close()
+	if hres.StatusCode != 200 {
+		return 0, errors.New(hres.Status)
+	}
+	if ct := hres.Header.Get("Content-Type"); ct != dohType {
+		return 0, fmt.Errorf("unexpected response Content-Type %q", ct)
+	}
+	_, err = io.Copy(&c.rbuf, hres.Body)
+	if err != nil {
+		return 0, err
+	}
+	return len(packet), nil
+}
+
+type todoAddr struct{}
+
+func (todoAddr) Network() string { return "unused" }
+func (todoAddr) String() string  { return "unused-todoAddr" }