net/dnsfallback: cache most recent DERP map on disk (#5545)

This is especially helpful as we launch newer DERPs over time, and older
clients have progressively out-of-date static DERP maps baked in. After
this, as long as the client has successfully connected once, it'll cache
the most recent DERP map it knows about.

Resolves an in-code comment from @bradfitz

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

ipn/ipnlocal/local.go

@@ -34,6 +34,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/ipn/policy"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/tsaddr"
@@ -731,6 +732,9 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 		b.e.SetNetworkMap(st.NetMap)
 		b.e.SetDERPMap(st.NetMap.DERPMap)
 
+		// Update our cached DERP map
+		dnsfallback.UpdateCache(st.NetMap.DERPMap)
+
 		b.send(ipn.Notify{NetMap: st.NetMap})
 	}
 	if st.URL != "" {

ipn/ipnserver/server.go

@@ -37,6 +37,7 @@ import (
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/localapi"
 	"tailscale.com/logtail/backoff"
+	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/netstat"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/tsdial"
@@ -786,6 +787,8 @@ func New(logf logger.Logf, logid string, store ipn.StateStore, eng wgengine.Engi
 			b.SetTailnetKeyAuthority(authority, storage)
 			logf("tka initialized at head %x", authority.Head())
 		}
+
+		dnsfallback.SetCachePath(filepath.Join(root, "derpmap.cached.json"))
 	} else {
 		logf("network-lock unavailable; no state directory")
 	}

net/dnsfallback/dnsfallback.go

@@ -20,8 +20,12 @@ import (
 	"net/http"
 	"net/netip"
 	"net/url"
+	"os"
+	"reflect"
+	"sync/atomic"
 	"time"
 
+	"tailscale.com/atomicfile"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tshttpproxy"
@@ -39,6 +43,7 @@ func Lookup(ctx context.Context, host string) ([]netip.Addr, error) {
 	}
 
 	dm := getDERPMap()
+
 	var cands4, cands6 []nameIP
 	for _, dr := range dm.Regions {
 		for _, n := range dr.Nodes {
@@ -128,12 +133,45 @@ type dnsMap map[string][]netip.Addr
 // getDERPMap returns some DERP map. The DERP servers also run a fallback
 // DNS server.
 func getDERPMap() *tailcfg.DERPMap {
-	// TODO(bradfitz): try to read the last known DERP map from disk,
-	// at say /var/lib/tailscale/derpmap.txt and write it when it changes,
-	// and read it here.
-	// But ultimately the fallback will be to use a copy baked into the binary,
-	// which is this part:
+	dm := getStaticDERPMap()
+
+	// Merge in any DERP servers from the cached map that aren't in the
+	// static map; this ensures that we're getting new region(s) while not
+	// overriding the built-in fallbacks if things go horribly wrong and we
+	// get a bad DERP map.
+	//
+	// TODO(andrew): should we expect OmitDefaultRegions here? We're not
+	// forwarding traffic, just resolving DNS, so maybe we can ignore that
+	// value anyway?
+	cached := cachedDERPMap.Load()
+	if cached == nil {
+		return dm
+	}
+
+	for id, region := range cached.Regions {
+		dr, ok := dm.Regions[id]
+		if !ok {
+			dm.Regions[id] = region
+			continue
+		}
 
+		// Add any nodes that we don't already have.
+		seen := make(map[string]bool)
+		for _, n := range dr.Nodes {
+			seen[n.HostName] = true
+		}
+		for _, n := range region.Nodes {
+			if !seen[n.HostName] {
+				dr.Nodes = append(dr.Nodes, n)
+			}
+		}
+	}
+
+	return dm
+}
+
+// getStaticDERPMap returns the DERP map that was compiled into this binary.
+func getStaticDERPMap() *tailcfg.DERPMap {
 	dm := new(tailcfg.DERPMap)
 	if err := json.Unmarshal(staticDERPMapJSON, dm); err != nil {
 		panic(err)
@@ -143,3 +181,67 @@ func getDERPMap() *tailcfg.DERPMap {
 
 //go:embed dns-fallback-servers.json
 var staticDERPMapJSON []byte
+
+// cachedDERPMap is the path to a cached DERP map that we loaded from our on-disk cache.
+var cachedDERPMap atomic.Pointer[tailcfg.DERPMap]
+
+// cachePath is the path to the DERP map cache file, set by SetCachePath via
+// ipnserver.New() if we have a state directory.
+var cachePath string
+
+// UpdateCache stores the DERP map cache back to disk.
+//
+// The caller must not mutate 'c' after calling this function.
+func UpdateCache(c *tailcfg.DERPMap) {
+	// Don't do anything if nothing changed.
+	curr := cachedDERPMap.Load()
+	if reflect.DeepEqual(curr, c) {
+		return
+	}
+
+	d, err := json.Marshal(c)
+	if err != nil {
+		log.Printf("[v1] dnsfallback: UpdateCache error marshaling: %v", err)
+		return
+	}
+
+	// Only store after we're confident this is at least valid JSON
+	cachedDERPMap.Store(c)
+
+	// Don't try writing if we don't have a cache path set; this can happen
+	// when we don't have a state path (e.g. /var/lib/tailscale) configured.
+	if cachePath != "" {
+		err = atomicfile.WriteFile(cachePath, d, 0600)
+		if err != nil {
+			log.Printf("[v1] dnsfallback: UpdateCache error writing: %v", err)
+			return
+		}
+	}
+	log.Printf("[v2] dnsfallback: UpdateCache succeeded")
+}
+
+// SetCachePath sets the path to the on-disk DERP map cache that we store and
+// update. Additionally, if a file at this path exists, we load it and merge it
+// with the DERP map baked into the binary.
+//
+// This function should be called before any calls to UpdateCache, as it is not
+// concurrency-safe.
+func SetCachePath(path string) {
+	cachePath = path
+
+	f, err := os.Open(path)
+	if err != nil {
+		log.Printf("[v1] dnsfallback: SetCachePath error reading %q: %v", path, err)
+		return
+	}
+	defer f.Close()
+
+	dm := new(tailcfg.DERPMap)
+	if err := json.NewDecoder(f).Decode(dm); err != nil {
+		log.Printf("[v1] dnsfallback: SetCachePath error decoding %q: %v", path, err)
+		return
+	}
+
+	cachedDERPMap.Store(dm)
+	log.Printf("[v2] dnsfallback: SetCachePath loaded cached DERP map")
+}

net/dnsfallback/dnsfallback_test.go

@@ -4,7 +4,15 @@
 
 package dnsfallback
 
-import "testing"
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+
+	"tailscale.com/tailcfg"
+)
 
 func TestGetDERPMap(t *testing.T) {
 	dm := getDERPMap()
@@ -15,3 +23,151 @@ func TestGetDERPMap(t *testing.T) {
 		t.Fatal("no regions")
 	}
 }
+
+func TestCache(t *testing.T) {
+	cacheFile := filepath.Join(t.TempDir(), "cache.json")
+
+	// Write initial cache value
+	initialCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "99a",
+					RegionID: 99,
+					HostName: "derp99a.tailscale.com",
+					IPv4:     "1.2.3.4",
+				}},
+			},
+
+			// Intentionally attempt to "overwrite" something
+			1: {
+				RegionID:   1,
+				RegionCode: "r1",
+				RegionName: "r1",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "1c",
+					RegionID: 1,
+					HostName: "derp1c.tailscale.com",
+					IPv4:     "127.0.0.1",
+					IPv6:     "::1",
+				}},
+			},
+		},
+	}
+	d, err := json.Marshal(initialCache)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(cacheFile, d, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Clear any existing cached DERP map(s)
+	cachedDERPMap.Store(nil)
+
+	// Load the cache
+	SetCachePath(cacheFile)
+	if cm := cachedDERPMap.Load(); !reflect.DeepEqual(initialCache, cm) {
+		t.Fatalf("cached map was %+v; want %+v", cm, initialCache)
+	}
+
+	// Verify that our DERP map is merged with the cache.
+	dm := getDERPMap()
+	region, ok := dm.Regions[99]
+	if !ok {
+		t.Fatal("expected region 99")
+	}
+	if !reflect.DeepEqual(region, initialCache.Regions[99]) {
+		t.Fatalf("region 99: got %+v; want %+v", region, initialCache.Regions[99])
+	}
+
+	// Verify that our cache can't override a statically-baked-in DERP server.
+	n0 := dm.Regions[1].Nodes[0]
+	if n0.IPv4 == "127.0.0.1" || n0.IPv6 == "::1" {
+		t.Errorf("got %+v; expected no overwrite for node", n0)
+	}
+
+	// Also, make sure that the static DERP map still has the same first
+	// node as when this test was last written/updated; this ensures that
+	// we don't accidentally start allowing overwrites due to some of the
+	// test's assumptions changing out from underneath us as we update the
+	// JSON file of fallback servers.
+	if getStaticDERPMap().Regions[1].Nodes[0].HostName != "derp1c.tailscale.com" {
+		t.Errorf("DERP server has a different name; please update this test")
+	}
+}
+
+func TestCacheUnchanged(t *testing.T) {
+	cacheFile := filepath.Join(t.TempDir(), "cache.json")
+
+	// Write initial cache value
+	initialCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "99a",
+					RegionID: 99,
+					HostName: "derp99a.tailscale.com",
+					IPv4:     "1.2.3.4",
+				}},
+			},
+		},
+	}
+	d, err := json.Marshal(initialCache)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(cacheFile, d, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Clear any existing cached DERP map(s)
+	cachedDERPMap.Store(nil)
+
+	// Load the cache
+	SetCachePath(cacheFile)
+	if cm := cachedDERPMap.Load(); !reflect.DeepEqual(initialCache, cm) {
+		t.Fatalf("cached map was %+v; want %+v", cm, initialCache)
+	}
+
+	// Remove the cache file on-disk, then re-set to the current value. If
+	// our equality comparison is working, we won't rewrite the file
+	// on-disk since the cached value won't have changed.
+	if err := os.Remove(cacheFile); err != nil {
+		t.Fatal(err)
+	}
+
+	UpdateCache(initialCache)
+	if _, err := os.Stat(cacheFile); !os.IsNotExist(err) {
+		t.Fatalf("got err=%v; expected to not find cache file", err)
+	}
+
+	// Now, update the cache with something slightly different and verify
+	// that we did re-write the file on-disk.
+	updatedCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes:      []*tailcfg.DERPNode{ /* set below */ },
+			},
+		},
+	}
+	clonedNode := *initialCache.Regions[99].Nodes[0]
+	clonedNode.IPv4 = "1.2.3.5"
+	updatedCache.Regions[99].Nodes = append(updatedCache.Regions[99].Nodes, &clonedNode)
+
+	UpdateCache(updatedCache)
+	if st, err := os.Stat(cacheFile); err != nil {
+		t.Fatalf("could not stat cache file; err=%v", err)
+	} else if !st.Mode().IsRegular() || st.Size() == 0 {
+		t.Fatalf("didn't find non-empty regular file; mode=%v size=%d", st.Mode(), st.Size())
+	}
+}