resolver: a reply marked recursion available from a root server's address stops the lookup (closes #300)

dnswatcher never asks for recursion and a root server never offers it,
so a reply marked recursion available from a root server's address
came from a resolver on the network answering in its place. The first
such reply, a refusal included, now stops the lookup at once with the
existing "this network intercepts DNS queries", naming the address,
and no other root server is asked, whether or not that resolver had
the name cached. queryDNS now returns a refused reply along with
ErrRefused, so the mark on a refusal can be seen. A refusal without
the mark is passed over and counted as before.

On such a network dnswatcher and its tests fail with that error, by
design. A live test passes public resolvers as the root servers and
expects it.

Model: opus-5-5

README.md

@@ -561,6 +561,18 @@ This approach ensures:
 - Ability to detect split-horizon or inconsistent responses across authoritative
   servers.
 
+A root server never offers recursion, so a reply marked "recursion available"
+from a root server's address came from a recursive resolver on the network
+answering in its place, as on a network where a router rule, a VPN or a DNS
+filter sends all DNS to one resolver. The first such reply from a root server's
+address, a refusal included, stops the lookup at once with the error "this
+network intercepts DNS queries", naming that address, and no other root server
+is asked. A lookup that every root server refuses without that mark fails with
+the same error. Every lookup starts at the root servers, so on such a network
+every lookup fails with that error, whether or not that resolver has the name
+cached, and dnswatcher's tests fail there too. That is by design: nothing asks
+again another way or otherwise works around the interception.
+
 A watched name's records are stored as its nameservers return them, CNAME
 included. When they return a CNAME and no address, the chain of every CNAME
 target they gave is followed (with a depth limit to prevent loops) to the A and

TODO.md

@@ -19,6 +19,14 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-16: the first reply marked "recursion available" from a root server's
+  address, a refusal included, stops the lookup with "this network intercepts
+  DNS queries", so on a network that sends DNS to a resolver dnswatcher and its
+  tests fail, by design (closes #300).
+- 2026-10-15: a query its caller's context ends before it is sent records no
+  reason about the nameserver, and a try the context's deadline cuts short is a
+  timeout, so a nameserver whose only failures were timeouts is `timeout`, not a
+  network error or `ok` (closes #298).
 - 2026-10-15: the resolver no longer says its query timeout grows on each try;
   every try waits the same 2 seconds, and the unused `timeoutMultiplier` is gone
   (closes #296).

internal/resolver/errors.go

@@ -53,9 +53,13 @@ var (
 		"reply truncated and its retry over TCP failed",
 	)
 
-	// ErrIntercepted is returned when every root server refused a
-	// query. Root servers refuse no query, so the refusals came from
-	// something on the network answering in their place.
+	// ErrIntercepted is returned when a root server's address gave a
+	// reply marked recursion available, a refusal included, which ends
+	// the lookup at once, or when every root server refused a query.
+	// Root servers do neither, so those replies came from something on
+	// the network answering in their place. On such a network
+	// dnswatcher fails with it, by design: nothing works around the
+	// interception.
 	ErrIntercepted = errors.New("this network intercepts DNS queries")
 
 	// ErrCNAMEDepthExceeded is returned when a CNAME chain

internal/resolver/iterative.go

@@ -62,7 +62,10 @@ func checkCtx(ctx context.Context) error {
 }
 
 // tryExchange sends msg to addr over UDP, up to maxRetries times, until
-// a reply comes. Every try waits the same queryTimeoutDuration.
+// a reply comes. Every try waits the same queryTimeoutDuration, or less
+// when ctx's deadline comes first. When ctx has ended before a further
+// try, it returns the error of the try already made, such as a timeout,
+// or ErrContextCanceled when none was made.
 func (r *Resolver) tryExchange(
 	ctx context.Context,
 	msg *dns.Msg,
@@ -74,7 +77,11 @@ func (r *Resolver) tryExchange(
 
 	for range maxRetries {
 		if checkCtx(ctx) != nil {
-			return nil, ErrContextCanceled
+			if err == nil {
+				return nil, ErrContextCanceled
+			}
+
+			return nil, err
 		}
 
 		resp, _, err = r.client.ExchangeContext(ctx, msg, addr)
@@ -108,7 +115,8 @@ func (r *Resolver) retryTCP(
 }
 
 // queryDNS sends a DNS query to a specific server IP, never asking it
-// for recursion. A reply of REFUSED is returned as ErrRefused.
+// for recursion. A reply of REFUSED is returned together with
+// ErrRefused, so that the caller can still read the reply's flags.
 func (r *Resolver) queryDNS(
 	ctx context.Context,
 	serverIP string,
@@ -132,7 +140,7 @@ func (r *Resolver) queryDNS(
 	}
 
 	if resp.Rcode == dns.RcodeRefused {
-		return nil, fmt.Errorf(
+		return resp, fmt.Errorf(
 			"query %s @%s: %w", name, serverIP, ErrRefused,
 		)
 	}
@@ -277,10 +285,15 @@ func shuffled(
 }
 
 // queryServers asks servers, the servers of zone, about name in a random
-// order until one gives a usable reply. A server that times out, refuses
-// or gives a reply that is not usable is passed over for the next. When
-// every server refused, the error says so, and when they are the root
-// servers it is ErrIntercepted.
+// order until one gives a usable reply. When they are the root servers,
+// the first reply marked recursion available, a refusal included, ends
+// the lookup with ErrIntercepted, naming the address it came from, and
+// no other root server is asked: root servers offer no recursion, so
+// that reply came from something on the network answering in their
+// place. Otherwise a server that times out, refuses or gives a reply
+// that is not usable is passed over for the next. When every server
+// refused, the error says so, and when they are the root servers it is
+// ErrIntercepted.
 func (r *Resolver) queryServers(
 	ctx context.Context,
 	servers []string,
@@ -297,7 +310,17 @@ func (r *Resolver) queryServers(
 			return nil, ErrContextCanceled
 		}
 
+		// queryDNS returns a refusal together with its reply, so a
+		// refusal's mark is checked here too.
 		resp, err := r.queryDNS(ctx, ip, name, qtype)
+		if resp != nil && zone == "." && resp.RecursionAvailable {
+			return nil, fmt.Errorf(
+				"query %s @%s: reply marked recursion available, "+
+					"so not from a root server: %w",
+				name, ip, ErrIntercepted,
+			)
+		}
+
 		if err == nil && !usableReply(resp, zone, name) {
 			err = fmt.Errorf(
 				"query %s @%s: %w", name, ip, ErrUnusableReply,
@@ -863,6 +886,10 @@ func (r *Resolver) querySingleType(
 			if err != nil {
 				err = fmt.Errorf("query %s @%s: %w", hostname, nsIP, err)
 			}
+		case errors.Is(err, ErrContextCanceled):
+			// The caller's context ended before the query was sent.
+			// That says nothing about the nameserver, so no reason is
+			// recorded.
 		case isTimeout(err):
 			state.gotTimeout = true
 		case errors.Is(err, ErrRefused):

internal/resolver/iterative_internal_test.go

@@ -1,6 +1,8 @@
 package resolver
 
 import (
+	"context"
+	"log/slog"
 	"strconv"
 	"syscall"
 	"testing"
@@ -132,3 +134,33 @@ func TestReadReply(t *testing.T) {
 		})
 	}
 }
+
+// TestQuerySingleType_ContextEnded asks a nameserver about one record
+// type with a context that has already ended, so no query is sent. A
+// query the context ended before it was sent says nothing about the
+// nameserver and records no reason: a nameserver whose earlier query
+// timed out is still reported as timed out, not as a network error.
+func TestQuerySingleType_ContextEnded(t *testing.T) {
+	t.Parallel()
+
+	r := NewFromLogger(slog.New(slog.DiscardHandler))
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	resp := &NameserverResponse{Records: map[string][]string{}, Status: StatusOK}
+
+	// The query for an earlier record type timed out.
+	state := queryState{gotTimeout: true}
+
+	_, err := r.querySingleType(
+		ctx, []string{"192.0.2.1"}, "example.com.", dns.TypeA, resp, &state,
+	)
+	require.ErrorIs(t, err, ErrContextCanceled)
+	assert.Equal(t, queryState{gotTimeout: true}, state)
+
+	classifyResponse(resp, state)
+
+	assert.Equal(t, StatusTimeout, resp.Status)
+	assert.Equal(t, "all queries timed out", resp.Error)
+}

internal/resolver/resolver_test.go

@@ -850,6 +850,74 @@ func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 	require.NotContains(t, err.Error(), "refused a query for")
 }
 
+// TestQueryServers_RecursiveResolversAsRootServers passes public
+// recursive resolvers to QueryServers as the servers of the root zone,
+// as a network that sends every DNS query to a resolver would. Their
+// replies are marked recursion available, which no root server's is:
+// SERVFAIL, REFUSED, or an answer from a resolver's cache, as 1.1.1.1
+// gives for com., which would be used if a root server gave it. The
+// first such reply ends the lookup with ErrIntercepted, naming the
+// resolver that gave it. In the first group the replies differ; every
+// resolver in the second refuses a query that does not ask for
+// recursion, as 9.9.9.9 does, so there a refusal ends the lookup
+// instead of being passed over. Each group has resolvers of four
+// operators, and a resolver that gives no reply is passed over for the
+// next, so one of them answering is enough.
+func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+	groups := [][]string{
+		{"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222"},
+		{"64.6.64.6", "185.222.222.222", "4.2.2.1", "94.140.14.14"},
+	}
+
+	for _, resolvers := range groups {
+		for _, name := range []string{"com.", "desec.io."} {
+			var err error
+
+			livednstest.Retry(
+				t,
+				"QueryServers("+strings.Join(resolvers, ", ")+
+					" as root servers, "+name+")",
+				func(ctx context.Context) error {
+					_, err = r.QueryServers(
+						ctx, resolvers, ".", name, dns.TypeNS,
+					)
+
+					// When no resolver gave a reply marked recursion
+					// available, none may have replied at all, so the
+					// attempt is tried again.
+					if err != nil && !strings.Contains(
+						err.Error(), "marked recursion available",
+					) {
+						return fmt.Errorf(
+							"%w: %w", livednstest.ErrNoAnswer, err,
+						)
+					}
+
+					return nil
+				},
+			)
+
+			require.ErrorIs(t, err, resolver.ErrIntercepted)
+
+			// The error names the resolver whose reply ended the
+			// lookup.
+			messages := make([]string, 0, len(resolvers))
+			for _, ip := range resolvers {
+				messages = append(messages,
+					"query "+name+" @"+ip+": reply marked recursion "+
+						"available, so not from a root server: "+
+						"this network intercepts DNS queries",
+				)
+			}
+
+			require.Contains(t, messages, err.Error())
+		}
+	}
+}
+
 func TestQueryNameserver_RecordsSorted(t *testing.T) {
 	t.Parallel()
 
@@ -1380,10 +1448,10 @@ func TestQueryNameserverIP_Timeout(t *testing.T) {
 	r := newTestResolver(t)
 
 	// Nothing answers at 192.0.2.1, a documentation address. The
-	// resolver tries each query twice, and the first try gives up
-	// after two seconds. A deadline that ends during the first try
-	// makes the status vary from run to run between error and
-	// timeout, so the deadline must outlast the first try.
+	// resolver tries each query twice, and each try gives up after two
+	// seconds. This deadline ends during the second try of the first
+	// record type; TestQueryNameserverIP_DeadlineDuringFirstTry covers
+	// one that ends during the first.
 	ctx, cancel := context.WithTimeout(
 		context.Background(), 3*time.Second,
 	)
@@ -1399,6 +1467,62 @@ func TestQueryNameserverIP_Timeout(t *testing.T) {
 	assert.NotEmpty(t, resp.Error)
 }
 
+// TestQueryNameserverIP_DeadlineDuringFirstTry gives a query to
+// 192.0.2.1, where nothing answers, a deadline that ends during its
+// first try. That try ends at the deadline as a timeout, so the
+// nameserver is reported as timed out, not as answering.
+func TestQueryNameserverIP_DeadlineDuringFirstTry(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	// Well under the two seconds one try waits.
+	ctx, cancel := context.WithTimeout(
+		context.Background(), 500*time.Millisecond,
+	)
+	t.Cleanup(cancel)
+
+	resp, err := r.QueryNameserverIP(
+		ctx, "unreachable.test.", "192.0.2.1",
+		"example.com",
+	)
+	require.NoError(t, err)
+
+	assert.Equal(t, resolver.StatusTimeout, resp.Status)
+	assert.NotEmpty(t, resp.Error)
+}
+
+// TestQueryNameserverIP_CancelDuringFirstTry cancels the context while
+// the first try of a query to 192.0.2.1, where nothing answers, waits
+// for a reply. That try's timeout is all that is learned about the
+// nameserver, so it is reported as timed out, not as answering.
+func TestQueryNameserverIP_CancelDuringFirstTry(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	// The first try starts at once and waits two seconds for a reply;
+	// the cancel comes a second in. The DNS client cuts a try short at
+	// the context's deadline but not at a cancel, so the try still
+	// waits its full two seconds and ends as a timeout a second after
+	// the cancel. By then the context has certainly ended, so no second
+	// try is made. With a deadline instead, the try ends at the moment
+	// the context does, and whether a second try is made depends on
+	// which the resolver sees first.
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	time.AfterFunc(time.Second, cancel)
+
+	resp, err := r.QueryNameserverIP(
+		ctx, "unreachable.test.", "192.0.2.1",
+		"example.com",
+	)
+	require.NoError(t, err)
+
+	assert.Equal(t, resolver.StatusTimeout, resp.Status)
+	assert.NotEmpty(t, resp.Error)
+}
+
 // TestQueryNameserverIP_CancelledLogsNothing cancels the context while
 // a query to 192.0.2.1, where nothing answers, is waiting for a reply,
 // as shutdown does. The query was cut short, not failed, so nothing is