net/dns/resolver: teach the forwarder to do per-domain routing.

Given a DNS route map, the forwarder selects the right set of
upstreams for a given name.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/resolver/forwarder.go

@@ -17,10 +17,12 @@ import (
 	"sync"
 	"time"
 
+	dns "golang.org/x/net/dns/dnsmessage"
 	"inet.af/netaddr"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/netns"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/dnsname"
 )
 
 // headerBytes is the number of bytes in a DNS message header.
@@ -100,6 +102,11 @@ func getTxID(packet []byte) txid {
 	return (txid(hash) << 32) | txid(dnsid)
 }
 
+type route struct {
+	suffix    string
+	resolvers []netaddr.IPPort
+}
+
 // forwarder forwards DNS packets to a number of upstream nameservers.
 type forwarder struct {
 	logf logger.Logf
@@ -116,10 +123,9 @@ type forwarder struct {
 	conns []*fwdConn
 
 	mu sync.Mutex
-	// upstreams are the nameserver addresses that should be used for forwarding.
-	upstreams []net.Addr
-	// txMap maps DNS txids to active forwarding records.
-	txMap map[txid]forwardingRecord
+	// routes are per-suffix resolvers to use.
+	routes []route                   // most specific routes first
+	txMap  map[txid]forwardingRecord // txids to in-flight requests
 }
 
 func init() {
@@ -127,24 +133,22 @@ func init() {
 }
 
 func newForwarder(logf logger.Logf, responses chan packet) *forwarder {
-	return &forwarder{
+	ret := &forwarder{
 		logf:      logger.WithPrefix(logf, "forward: "),
 		responses: responses,
 		closed:    make(chan struct{}),
 		conns:     make([]*fwdConn, connCount),
 		txMap:     make(map[txid]forwardingRecord),
 	}
-}
 
-func (f *forwarder) Start() error {
-	f.wg.Add(connCount + 1)
-	for idx := range f.conns {
-		f.conns[idx] = newFwdConn(f.logf, idx)
-		go f.recv(f.conns[idx])
+	ret.wg.Add(connCount + 1)
+	for idx := range ret.conns {
+		ret.conns[idx] = newFwdConn(ret.logf, idx)
+		go ret.recv(ret.conns[idx])
 	}
-	go f.cleanMap()
+	go ret.cleanMap()
 
-	return nil
+	return ret
 }
 
 func (f *forwarder) Close() {
@@ -171,14 +175,15 @@ func (f *forwarder) rebindFromNetworkChange() {
 	}
 }
 
-func (f *forwarder) setUpstreams(upstreams []net.Addr) {
+func (f *forwarder) setRoutes(routes []route) {
+	fmt.Println(routes)
 	f.mu.Lock()
-	f.upstreams = upstreams
+	f.routes = routes
 	f.mu.Unlock()
 }
 
 // send sends packet to dst. It is best effort.
-func (f *forwarder) send(packet []byte, dst net.Addr) {
+func (f *forwarder) send(packet []byte, dst netaddr.IPPort) {
 	connIdx := rand.Intn(connCount)
 	conn := f.conns[connIdx]
 	conn.send(packet, dst)
@@ -256,24 +261,38 @@ func (f *forwarder) cleanMap() {
 
 // forward forwards the query to all upstream nameservers and returns the first response.
 func (f *forwarder) forward(query packet) error {
+	domain, err := nameFromQuery(query.bs)
+	if err != nil {
+		return err
+	}
+
 	txid := getTxID(query.bs)
 
 	f.mu.Lock()
+	routes := f.routes
+	f.mu.Unlock()
 
-	upstreams := f.upstreams
-	if len(upstreams) == 0 {
-		f.mu.Unlock()
+	var resolvers []netaddr.IPPort
+	for _, route := range routes {
+		if route.suffix != "." && !dnsname.HasSuffix(domain, route.suffix) {
+			continue
+		}
+		resolvers = route.resolvers
+		break
+	}
+	if len(resolvers) == 0 {
 		return errNoUpstreams
 	}
+
+	f.mu.Lock()
 	f.txMap[txid] = forwardingRecord{
 		src:       query.addr,
 		createdAt: time.Now(),
 	}
-
 	f.mu.Unlock()
 
-	for _, upstream := range upstreams {
-		f.send(query.bs, upstream)
+	for _, resolver := range resolvers {
+		f.send(query.bs, resolver)
 	}
 
 	return nil
@@ -309,7 +328,7 @@ func newFwdConn(logf logger.Logf, idx int) *fwdConn {
 
 // send sends packet to dst using c's connection.
 // It is best effort. It is UDP, after all. Failures are logged.
-func (c *fwdConn) send(packet []byte, dst net.Addr) {
+func (c *fwdConn) send(packet []byte, dst netaddr.IPPort) {
 	var b *backoff.Backoff // lazily initialized, since it is not needed in the common case
 	backOff := func(err error) {
 		if b == nil {
@@ -335,8 +354,9 @@ func (c *fwdConn) send(packet []byte, dst net.Addr) {
 		}
 		c.mu.Unlock()
 
+		a := dst.UDPAddr()
 		c.wg.Add(1)
-		_, err := conn.WriteTo(packet, dst)
+		_, err := conn.WriteTo(packet, a)
 		c.wg.Done()
 		if err == nil {
 			// Success
@@ -469,3 +489,24 @@ func (c *fwdConn) close() {
 	// Unblock any remaining readers.
 	c.change.Broadcast()
 }
+
+// nameFromQuery extracts the normalized query name from bs.
+func nameFromQuery(bs []byte) (string, error) {
+	var parser dns.Parser
+
+	hdr, err := parser.Start(bs)
+	if err != nil {
+		return "", err
+	}
+	if hdr.Response {
+		return "", errNotQuery
+	}
+
+	q, err := parser.Question()
+	if err != nil {
+		return "", err
+	}
+
+	n := q.Name.Data[:q.Name.Length]
+	return rawNameToLower(n), nil
+}

net/dns/resolver/tsdns.go

@@ -10,7 +10,6 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
-	"net"
 	"sort"
 	"strings"
 	"sync"
@@ -68,11 +67,6 @@ type Config struct {
 	LocalDomains []string
 }
 
-type route struct {
-	suffix    string
-	resolvers []netaddr.IPPort
-}
-
 // Resolver is a DNS resolver for nodes on the Tailscale network,
 // associating them with domain names of the form <mynode>.<mydomain>.<root>.
 // If it is asked to resolve a domain that is not of that form,
@@ -100,7 +94,6 @@ type Resolver struct {
 	localDomains []string
 	hostToIP     map[string][]netaddr.IP
 	ipToHost     map[netaddr.IP]string
-	routes       []route // most specific routes first
 }
 
 // New returns a new resolver.
@@ -121,10 +114,6 @@ func New(logf logger.Logf, linkMon *monitor.Mon) (*Resolver, error) {
 		r.unregLinkMon = r.linkMon.RegisterChangeCallback(r.onLinkMonitorChange)
 	}
 
-	if err := r.forwarder.Start(); err != nil {
-		return nil, err
-	}
-
 	r.wg.Add(1)
 	go r.poll()
 
@@ -138,7 +127,6 @@ func isFQDN(s string) bool {
 func (r *Resolver) SetConfig(cfg Config) error {
 	routes := make([]route, 0, len(cfg.Routes))
 	reverse := make(map[netaddr.IP]string, len(cfg.Hosts))
-	var defaultUpstream []net.Addr
 
 	for host, ips := range cfg.Hosts {
 		if !isFQDN(host) {
@@ -162,32 +150,19 @@ func (r *Resolver) SetConfig(cfg Config) error {
 			suffix:    suffix,
 			resolvers: ips,
 		})
-		if suffix == "." {
-			// TODO: this is a temporary hack to forward upstream
-			// resolvers to the forwarder, which doesn't yet
-			// understand per-domain resolvers. Effectively, SetConfig
-			// currently ignores all routes except for ".", which it
-			// sets as the only resolver.
-			for _, ip := range ips {
-				up := ip.UDPAddr()
-				defaultUpstream = append(defaultUpstream, up)
-			}
-		}
 	}
 	// Sort from longest prefix to shortest.
 	sort.Slice(routes, func(i, j int) bool {
-		return strings.Count(routes[i].suffix, ".") > strings.Count(routes[j].suffix, ".")
+		return dnsname.NumLabels(routes[i].suffix) > dnsname.NumLabels(routes[j].suffix)
 	})
 
-	r.forwarder.setUpstreams(defaultUpstream)
+	r.forwarder.setRoutes(routes)
 
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	r.localDomains = cfg.LocalDomains
 	r.hostToIP = cfg.Hosts
 	r.ipToHost = reverse
-	r.routes = routes
-
 	return nil
 }
 
@@ -386,6 +361,8 @@ type response struct {
 }
 
 // parseQuery parses the query in given packet into a response struct.
+// if the parse is successful, resp.Name contains the normalized name being queried.
+// TODO: stuffing the query name in resp.Name temporarily is a hack. Clean it up.
 func parseQuery(query []byte, resp *response) error {
 	var parser dns.Parser
 	var err error

net/dns/resolver/tsdns_server_test.go

@@ -5,7 +5,7 @@
 package resolver
 
 import (
-	"log"
+	"fmt"
 	"testing"
 
 	"github.com/miekg/dns"
@@ -16,8 +16,6 @@ import (
 // that depends on github.com/miekg/dns
 // from the rest, which only depends on dnsmessage.
 
-var dnsHandleFunc = dns.HandleFunc
-
 // resolveToIP returns a handler function which responds
 // to queries of type A it receives with an A record containing ipv4,
 // to queries of type AAAA with an AAAA record containing ipv6,
@@ -68,28 +66,38 @@ func resolveToIP(ipv4, ipv6 netaddr.IP, ns string) dns.HandlerFunc {
 	}
 }
 
-func resolveToNXDOMAIN(w dns.ResponseWriter, req *dns.Msg) {
+var resolveToNXDOMAIN = dns.HandlerFunc(func(w dns.ResponseWriter, req *dns.Msg) {
 	m := new(dns.Msg)
 	m.SetRcode(req, dns.RcodeNameError)
 	w.WriteMsg(m)
-}
-
-func serveDNS(tb testing.TB, addr string) (*dns.Server, chan error) {
-	server := &dns.Server{Addr: addr, Net: "udp"}
+})
 
+func serveDNS(tb testing.TB, addr string, records ...interface{}) *dns.Server {
+	if len(records)%2 != 0 {
+		panic("must have an even number of record values")
+	}
+	mux := dns.NewServeMux()
+	for i := 0; i < len(records); i += 2 {
+		name := records[i].(string)
+		handler := records[i+1].(dns.Handler)
+		mux.Handle(name, handler)
+	}
 	waitch := make(chan struct{})
-	server.NotifyStartedFunc = func() { close(waitch) }
+	server := &dns.Server{
+		Addr:              addr,
+		Net:               "udp",
+		Handler:           mux,
+		NotifyStartedFunc: func() { close(waitch) },
+		ReusePort:         true,
+	}
 
-	errch := make(chan error, 1)
 	go func() {
 		err := server.ListenAndServe()
 		if err != nil {
-			log.Printf("ListenAndServe(%q): %v", addr, err)
+			panic(fmt.Sprintf("ListenAndServe(%q): %v", addr, err))
 		}
-		errch <- err
-		close(errch)
 	}()
 
 	<-waitch
-	return server, errch
+	return server
 }

net/dns/resolver/tsdns_test.go

@@ -15,13 +15,8 @@ import (
 	"tailscale.com/tstest"
 )
 
-var testipv4 = netaddr.IPv4(1, 2, 3, 4)
-var testipv6 = netaddr.IPv6Raw([16]byte{
-	0x00, 0x01, 0x02, 0x03,
-	0x04, 0x05, 0x06, 0x07,
-	0x08, 0x09, 0x0a, 0x0b,
-	0x0c, 0x0d, 0x0e, 0x0f,
-})
+var testipv4 = netaddr.MustParseIP("1.2.3.4")
+var testipv6 = netaddr.MustParseIP("0001:0203:0405:0607:0809:0a0b:0c0d:0e0f")
 
 var dnsCfg = Config{
 	Hosts: map[string][]netaddr.IP{
@@ -283,32 +278,14 @@ func TestDelegate(t *testing.T) {
 		t.Skip("skipping test that requires localhost IPv6")
 	}
 
-	dnsHandleFunc("test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
-	dnsHandleFunc("nxdomain.site.", resolveToNXDOMAIN)
-
-	v4server, v4errch := serveDNS(t, "127.0.0.1:0")
-	v6server, v6errch := serveDNS(t, "[::1]:0")
-
-	defer func() {
-		if err := <-v4errch; err != nil {
-			t.Errorf("v4 server error: %v", err)
-		}
-		if err := <-v6errch; err != nil {
-			t.Errorf("v6 server error: %v", err)
-		}
-	}()
-	if v4server != nil {
-		defer v4server.Shutdown()
-	}
-	if v6server != nil {
-		defer v6server.Shutdown()
-	}
-
-	if v4server == nil || v6server == nil {
-		// There is an error in at least one of the channels
-		// and we cannot proceed; return to see it.
-		return
-	}
+	v4server := serveDNS(t, "127.0.0.1:0",
+		"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."),
+		"nxdomain.site.", resolveToNXDOMAIN)
+	defer v4server.Shutdown()
+	v6server := serveDNS(t, "[::1]:0",
+		"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."),
+		"nxdomain.site.", resolveToNXDOMAIN)
+	defer v6server.Shutdown()
 
 	r, err := New(t.Logf, nil)
 	if err != nil {
@@ -377,19 +354,75 @@ func TestDelegate(t *testing.T) {
 	}
 }
 
-func TestDelegateCollision(t *testing.T) {
-	dnsHandleFunc("test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
+func TestDelegateSplitRoute(t *testing.T) {
+	test4 := netaddr.MustParseIP("2.3.4.5")
+	test6 := netaddr.MustParseIP("ff::1")
 
-	server, errch := serveDNS(t, "127.0.0.1:0")
-	defer func() {
-		if err := <-errch; err != nil {
-			t.Errorf("server error: %v", err)
-		}
-	}()
+	server1 := serveDNS(t, "127.0.0.1:0",
+		"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
+	defer server1.Shutdown()
+	server2 := serveDNS(t, "127.0.0.1:0",
+		"test.other.", resolveToIP(test4, test6, "dns.other."))
+	defer server2.Shutdown()
+
+	r, err := New(t.Logf, nil)
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	defer r.Close()
 
-	if server == nil {
-		return
+	cfg := dnsCfg
+	cfg.Routes = map[string][]netaddr.IPPort{
+		".":      {netaddr.MustParseIPPort(server1.PacketConn.LocalAddr().String())},
+		"other.": {netaddr.MustParseIPPort(server2.PacketConn.LocalAddr().String())},
+	}
+	r.SetConfig(cfg)
+
+	tests := []struct {
+		title    string
+		query    []byte
+		response dnsResponse
+	}{
+		{
+			"general",
+			dnspacket("test.site.", dns.TypeA),
+			dnsResponse{ip: testipv4, rcode: dns.RCodeSuccess},
+		},
+		{
+			"override",
+			dnspacket("test.other.", dns.TypeA),
+			dnsResponse{ip: test4, rcode: dns.RCodeSuccess},
+		},
 	}
+
+	for _, tt := range tests {
+		t.Run(tt.title, func(t *testing.T) {
+			payload, err := syncRespond(r, tt.query)
+			if err != nil {
+				t.Errorf("err = %v; want nil", err)
+				return
+			}
+			response, err := unpackResponse(payload)
+			if err != nil {
+				t.Errorf("extract: err = %v; want nil (in %x)", err, payload)
+				return
+			}
+			if response.rcode != tt.response.rcode {
+				t.Errorf("rcode = %v; want %v", response.rcode, tt.response.rcode)
+			}
+			if response.ip != tt.response.ip {
+				t.Errorf("ip = %v; want %v", response.ip, tt.response.ip)
+			}
+			if response.name != tt.response.name {
+				t.Errorf("name = %v; want %v", response.name, tt.response.name)
+			}
+		})
+	}
+}
+
+func TestDelegateCollision(t *testing.T) {
+	server := serveDNS(t, "127.0.0.1:0",
+		"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
 	defer server.Shutdown()
 
 	r, err := New(t.Logf, nil)
@@ -628,8 +661,8 @@ func TestFull(t *testing.T) {
 		{"ipv6", dnspacket("test2.ipn.dev.", dns.TypeAAAA), ipv6Response},
 		{"no-ipv6", dnspacket("test1.ipn.dev.", dns.TypeAAAA), emptyResponse},
 		{"upper", dnspacket("TEST1.IPN.DEV.", dns.TypeA), ipv4UppercaseResponse},
-		{"ptr", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR), ptrResponse},
-		{"ptr", dnspacket("f.0.e.0.d.0.c.0.b.0.a.0.9.0.8.0.7.0.6.0.5.0.4.0.3.0.2.0.1.0.0.0.ip6.arpa.",
+		{"ptr4", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR), ptrResponse},
+		{"ptr6", dnspacket("f.0.e.0.d.0.c.0.b.0.a.0.9.0.8.0.7.0.6.0.5.0.4.0.3.0.2.0.1.0.0.0.ip6.arpa.",
 			dns.TypePTR), ptrResponse6},
 		{"nxdomain", dnspacket("test3.ipn.dev.", dns.TypeA), nxdomainResponse},
 	}
@@ -702,18 +735,8 @@ func TestTrimRDNSBonjourPrefix(t *testing.T) {
 }
 
 func BenchmarkFull(b *testing.B) {
-	dnsHandleFunc("test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
-
-	server, errch := serveDNS(b, "127.0.0.1:0")
-	defer func() {
-		if err := <-errch; err != nil {
-			b.Errorf("server error: %v", err)
-		}
-	}()
-
-	if server == nil {
-		return
-	}
+	server := serveDNS(b, "127.0.0.1:0",
+		"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."))
 	defer server.Shutdown()
 
 	r, err := New(b.Logf, nil)

util/dnsname/dnsname.go

@@ -124,3 +124,12 @@ func SanitizeHostname(hostname string) string {
 	hostname = TrimCommonSuffixes(hostname)
 	return SanitizeLabel(hostname)
 }
+
+// NumLabels returns the number of DNS labels in hostname.
+// If hostname is empty or the top-level name ".", returns 0.
+func NumLabels(hostname string) int {
+	if hostname == "" || hostname == "." {
+		return 0
+	}
+	return strings.Count(hostname, ".")
+}