logpolicy, ipn/ipnserver: connect to logtail via tailscaled when needed

This is for use by the Windows GUI client to log via when an
exit node is in use, so the logs don't go out via the exit node and
instead go directly, like tailscaled's. The dialer tried to do that
in the unprivileged GUI by binding to a specific interface, but the
"Internet Kill Switch" installed by tailscaled for exit nodes
precludes that from working and instead the GUI fails to dial out.
So, go through tailscaled (with a CONNECT request) instead.

Fixes tailscale/corp#3169

Change-Id: I17a8efdc1d4b8fed53a29d1c19995592b651b215
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/proxyconnect.go

@@ -0,0 +1,74 @@
+// 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 ipnserver
+
+import (
+	"bufio"
+	"context"
+	"io"
+	"net"
+	"net/http"
+	"time"
+
+	"tailscale.com/logpolicy"
+	"tailscale.com/types/logger"
+)
+
+// handleProxyConnectConn handles a CONNECT request to
+// log.tailscale.io (or whatever the configured log server is). This
+// is intended for use by the Windows GUI client to log via when an
+// exit node is in use, so the logs don't go out via the exit node and
+// instead go directly, like tailscaled's. The dialer tried to do that
+// in the unprivileged GUI by binding to a specific interface, but the
+// "Internet Kill Switch" installed by tailscaled for exit nodes
+// precludes that from working and instead the GUI fails to dial out.
+// So, go through tailscaled (with a CONNECT request) instead.
+func (s *Server) handleProxyConnectConn(ctx context.Context, br *bufio.Reader, c net.Conn, logf logger.Logf) {
+	defer c.Close()
+
+	c.SetReadDeadline(time.Now().Add(5 * time.Second)) // should be long enough to send the HTTP headers
+	req, err := http.ReadRequest(br)
+	if err != nil {
+		logf("ReadRequest: %v", err)
+		return
+	}
+	c.SetReadDeadline(time.Time{})
+
+	if req.Method != "CONNECT" {
+		logf("ReadRequest: unexpected method %q, not CONNECT", req.Method)
+		return
+	}
+
+	hostPort := req.RequestURI
+	logHost := logpolicy.LogHost()
+	allowed := net.JoinHostPort(logHost, "443")
+	if hostPort != allowed {
+		logf("invalid CONNECT target %q; want %q", hostPort, allowed)
+		io.WriteString(c, "HTTP/1.1 403 Forbidden\r\n\r\nBad CONNECT target.\n")
+		return
+	}
+
+	tr := logpolicy.NewLogtailTransport(logHost)
+	back, err := tr.DialContext(ctx, "tcp", hostPort)
+	if err != nil {
+		logf("error CONNECT dialing %v: %v", hostPort, err)
+		io.WriteString(c, "HTTP/1.1 502 Fail\r\n\r\nConnect failure.\n")
+		return
+	}
+	defer back.Close()
+
+	io.WriteString(c, "HTTP/1.1 200 OK\r\n\r\n")
+
+	errc := make(chan error, 2)
+	go func() {
+		_, err := io.Copy(c, back)
+		errc <- err
+	}()
+	go func() {
+		_, err := io.Copy(back, br)
+		errc <- err
+	}()
+	<-errc
+}

ipn/ipnserver/server.go

@@ -238,12 +238,28 @@ func bufferHasHTTPRequest(br *bufio.Reader) bool {
 		mem.Contains(mem.B(peek), mem.S(" HTTP/"))
 }
 
+// bufferIsConnect reports whether br looks like it's likely an HTTP
+// CONNECT request.
+//
+// Invariant: br has already had at least 4 bytes Peek'ed.
+func bufferIsConnect(br *bufio.Reader) bool {
+	peek, _ := br.Peek(br.Buffered())
+	return mem.HasPrefix(mem.B(peek), mem.S("CONN"))
+}
+
 func (s *Server) serveConn(ctx context.Context, c net.Conn, logf logger.Logf) {
 	// First see if it's an HTTP request.
 	br := bufio.NewReader(c)
 	c.SetReadDeadline(time.Now().Add(time.Second))
 	br.Peek(4)
 	c.SetReadDeadline(time.Time{})
+
+	// Handle logtail CONNECT requests early. (See docs on handleProxyConnectConn)
+	if bufferIsConnect(br) {
+		s.handleProxyConnectConn(ctx, br, c, logf)
+		return
+	}
+
 	isHTTPReq := bufferHasHTTPRequest(br)
 
 	ci, err := s.addConn(c, isHTTPReq)