From f9f3b67f3a924af40a57b5f80935aa299c8f46ce Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 12 Mar 2021 08:39:43 -0800
Subject: [PATCH] wgengine{,tsdns}: rebind MagicDNS forwarders on link change

Fixes #1480

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 wgengine/tsdns/forwarder.go |  8 ++++++++
 wgengine/tsdns/tsdns.go     | 27 ++++++++++++++++++++++++++-
 wgengine/userspace.go       | 23 ++++++++++++-----------
 3 files changed, 46 insertions(+), 12 deletions(-)

diff --git a/wgengine/tsdns/forwarder.go b/wgengine/tsdns/forwarder.go
index 161379943..470748c4a 100644
--- a/wgengine/tsdns/forwarder.go
+++ b/wgengine/tsdns/forwarder.go
@@ -163,6 +163,14 @@ func (f *forwarder) Close() {
 	f.wg.Wait()
 }
 
+func (f *forwarder) rebindFromNetworkChange() {
+	for _, c := range f.conns {
+		c.mu.Lock()
+		c.reconnectLocked()
+		c.mu.Unlock()
+	}
+}
+
 func (f *forwarder) setUpstreams(upstreams []net.Addr) {
 	f.mu.Lock()
 	f.upstreams = upstreams
diff --git a/wgengine/tsdns/tsdns.go b/wgengine/tsdns/tsdns.go
index 23db7bba4..b68b8c04e 100644
--- a/wgengine/tsdns/tsdns.go
+++ b/wgengine/tsdns/tsdns.go
@@ -16,8 +16,10 @@ import (
 
 	dns "golang.org/x/net/dns/dnsmessage"
 	"inet.af/netaddr"
+	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/wgengine/monitor"
 )
 
 // maxResponseBytes is the maximum size of a response from a Resolver.
@@ -58,7 +60,9 @@ type Packet struct {
 // If it is asked to resolve a domain that is not of that form,
 // it delegates to upstream nameservers if any are set.
 type Resolver struct {
-	logf logger.Logf
+	logf         logger.Logf
+	linkMon      *monitor.Mon // or nil
+	unregLinkMon func()       // or nil
 	// forwarder forwards requests to upstream nameservers.
 	forwarder *forwarder
 
@@ -86,6 +90,10 @@ type ResolverConfig struct {
 	// Forward determines whether the resolver will forward packets to
 	// nameservers set with SetUpstreams if the domain name is not of a Tailscale node.
 	Forward bool
+	// LinkMonitor optionally provides a link monitor to use to rebind
+	// connections on link changes.
+	// If nil, rebinds are not performend.
+	LinkMonitor *monitor.Mon
 }
 
 // NewResolver constructs a resolver associated with the given root domain.
@@ -93,6 +101,7 @@ type ResolverConfig struct {
 func NewResolver(config ResolverConfig) *Resolver {
 	r := &Resolver{
 		logf:      logger.WithPrefix(config.Logf, "tsdns: "),
+		linkMon:   config.LinkMonitor,
 		queue:     make(chan Packet, queueSize),
 		responses: make(chan Packet),
 		errors:    make(chan error),
@@ -102,6 +111,9 @@ func NewResolver(config ResolverConfig) *Resolver {
 	if config.Forward {
 		r.forwarder = newForwarder(r.logf, r.responses)
 	}
+	if r.linkMon != nil {
+		r.unregLinkMon = r.linkMon.RegisterChangeCallback(r.onLinkMonitorChange)
+	}
 
 	return r
 }
@@ -130,6 +142,10 @@ func (r *Resolver) Close() {
 	}
 	close(r.closed)
 
+	if r.unregLinkMon != nil {
+		r.unregLinkMon()
+	}
+
 	if r.forwarder != nil {
 		r.forwarder.Close()
 	}
@@ -137,6 +153,15 @@ func (r *Resolver) Close() {
 	r.wg.Wait()
 }
 
+func (r *Resolver) onLinkMonitorChange(changed bool, state *interfaces.State) {
+	if !changed {
+		return
+	}
+	if r.forwarder != nil {
+		r.forwarder.rebindFromNetworkChange()
+	}
+}
+
 // SetMap sets the resolver's DNS map, taking ownership of it.
 func (r *Resolver) SetMap(m *Map) {
 	r.mu.Lock()
diff --git a/wgengine/userspace.go b/wgengine/userspace.go
index 8d73e9483..af8f823a2 100644
--- a/wgengine/userspace.go
+++ b/wgengine/userspace.go
@@ -219,18 +219,13 @@ func newUserspaceEngine(logf logger.Logf, rawTUNDev tun.Device, conf Config) (_
 	tsTUNDev := tstun.WrapTUN(logf, rawTUNDev)
 	closePool.add(tsTUNDev)
 
-	rconf := tsdns.ResolverConfig{
-		Logf:    logf,
-		Forward: true,
-	}
 	e := &userspaceEngine{
-		timeNow:  time.Now,
-		logf:     logf,
-		reqCh:    make(chan struct{}, 1),
-		waitCh:   make(chan struct{}),
-		tundev:   tsTUNDev,
-		resolver: tsdns.NewResolver(rconf),
-		pingers:  make(map[wgkey.Key]*pinger),
+		timeNow: time.Now,
+		logf:    logf,
+		reqCh:   make(chan struct{}, 1),
+		waitCh:  make(chan struct{}),
+		tundev:  tsTUNDev,
+		pingers: make(map[wgkey.Key]*pinger),
 	}
 	e.localAddrs.Store(map[netaddr.IP]bool{})
 
@@ -246,6 +241,12 @@ func newUserspaceEngine(logf logger.Logf, rawTUNDev tun.Device, conf Config) (_
 		e.linkMonOwned = true
 	}
 
+	e.resolver = tsdns.NewResolver(tsdns.ResolverConfig{
+		Logf:        logf,
+		Forward:     true,
+		LinkMonitor: e.linkMon,
+	})
+
 	logf("link state: %+v", e.linkMon.InterfaceState())
 
 	unregisterMonWatch := e.linkMon.RegisterChangeCallback(func(changed bool, st *interfaces.State) {