resolver: a root server's reply marked recursion available means interception (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. Such a
reply is now passed over, and when every root server refused or gave
one, the lookup fails with the existing "this network intercepts DNS
queries", whether or not that resolver had the name cached. Before, a
cached name was checked against the resolver's second-hand answer.

On such a network dnswatcher and its tests fail with that error, by
design; nothing asks again another way or works around it. A live test
passes public resolvers as the root servers and expects the error.

Model: opus-5-5

README.md

@@ -564,14 +564,13 @@ This approach ensures:
 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 stops the lookup at once with the error "this network intercepts DNS
-queries", and no other root server is asked. A lookup that every root server
-refuses 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.
+filter sends all DNS to one resolver. Such a reply is passed over for the next
+root server. When every root server refuses a query or gives such a reply, the
+lookup fails with the error "this network intercepts DNS queries". 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

TODO.md

@@ -19,10 +19,10 @@ 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 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).
+- 2026-10-16: a reply marked "recursion available" from a root server's address
+  is not used; when every root server refuses or gives one, the lookup fails
+  with "this network intercepts DNS queries", so on such a network 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

internal/resolver/errors.go

@@ -53,9 +53,16 @@ var (
 		"reply truncated and its retry over TCP failed",
 	)
 
-	// ErrIntercepted is returned when a root server's address gave a
-	// reply marked recursion available, which ends the lookup at once,
-	// or when every root server refused a query. Root servers do
+	// ErrRecursionAvailable is the reason a reply from a root server's
+	// address marked recursion available is not used. Root servers offer
+	// no recursion, so the reply came from something on the network
+	// answering in their place.
+	ErrRecursionAvailable = errors.New(
+		"reply marked recursion available, so not from a root server",
+	)
+
+	// ErrIntercepted is returned when every root server refused a
+	// query or gave a reply marked recursion available. 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.

internal/resolver/iterative.go

@@ -284,14 +284,11 @@ 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. When they are the root servers, the
-// first reply marked recursion available also 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.
+// order until one gives a reply it uses (see replyError). A server that
+// times out, refuses or gives a reply it does not use is passed over for
+// the next. When every server refused, the error says so. When they are
+// the root servers and each one refused or gave a reply marked recursion
+// available, the error is ErrIntercepted.
 func (r *Resolver) queryServers(
 	ctx context.Context,
 	servers []string,
@@ -302,6 +299,7 @@ func (r *Resolver) queryServers(
 	var lastErr error
 
 	refused := 0
+	recursionAvailable := 0
 
 	for _, ip := range shuffled(servers, rand.Shuffle) {
 		if checkCtx(ctx) != nil {
@@ -309,18 +307,8 @@ func (r *Resolver) queryServers(
 		}
 
 		resp, err := r.queryDNS(ctx, ip, name, qtype)
-		if err == 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,
-			)
+		if err == nil {
+			err = replyError(resp, ip, zone, name)
 		}
 
 		if err == nil {
@@ -331,6 +319,10 @@ func (r *Resolver) queryServers(
 			refused++
 		}
 
+		if errors.Is(err, ErrRecursionAvailable) {
+			recursionAvailable++
+		}
+
 		lastErr = err
 	}
 
@@ -341,6 +333,14 @@ func (r *Resolver) queryServers(
 		)
 	}
 
+	if refused+recursionAvailable == len(servers) && zone == "." {
+		return nil, fmt.Errorf(
+			"every root server refused a query for %s or gave a reply "+
+				"marked recursion available: %w",
+			name, ErrIntercepted,
+		)
+	}
+
 	if refused == len(servers) {
 		return nil, fmt.Errorf(
 			"every server of %s refused a query for %s: %w",
@@ -351,6 +351,27 @@ func (r *Resolver) queryServers(
 	return nil, fmt.Errorf("all servers failed: %w", lastErr)
 }
 
+// replyError returns why resp, the reply of the server at ip, one of the
+// servers of zone, to a query about name, is not used, or nil when it
+// is. A reply that is not usable is not used. Nor is a reply from a root
+// server marked recursion available: root servers offer no recursion,
+// so it came from something on the network answering in their place.
+func replyError(resp *dns.Msg, ip string, zone string, name string) error {
+	if zone == "." && resp.RecursionAvailable {
+		return fmt.Errorf(
+			"query %s @%s: %w", name, ip, ErrRecursionAvailable,
+		)
+	}
+
+	if !usableReply(resp, zone, name) {
+		return fmt.Errorf(
+			"query %s @%s: %w", name, ip, ErrUnusableReply,
+		)
+	}
+
+	return nil
+}
+
 // isErrorReply reports whether msg is an error reply: one with any code
 // but NOERROR and NXDOMAIN, such as SERVFAIL, NOTIMP or FORMERR. An error
 // reply says nothing about the name's records.

internal/resolver/resolver_test.go

@@ -853,12 +853,12 @@ func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 // TestQueryServers_RecursiveResolversAsRootServers passes public
 // recursive resolvers of four operators 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, 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. A resolver that refuses or gives no reply
-// is passed over for the next, so one of them answering is enough.
+// resolver would. Each refuses the query or gives a reply marked
+// recursion available, which no root server does: SERVFAIL, or an
+// answer from its cache, as 1.1.1.1 gives for com., which would be used
+// if a root server gave it. Either way the error is ErrIntercepted, and
+// since 1.1.1.1 and 8.8.8.8 refuse no such query, it says a reply was
+// marked recursion available.
 func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
 	t.Parallel()
 
@@ -877,11 +877,9 @@ func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
 			func(ctx context.Context) error {
 				_, err = r.QueryServers(ctx, resolvers, ".", name, dns.TypeNS)
 
-				// When no resolver gave a reply marked recursion
-				// available, each refused or gave no reply at all, so
-				// the attempt is tried again.
-				if err != nil &&
-					!strings.Contains(err.Error(), "marked recursion available") {
+				// When the error is another, one resolver may have given
+				// no reply at all, so the attempt is tried again.
+				if err != nil && !errors.Is(err, resolver.ErrIntercepted) {
 					return fmt.Errorf(
 						"%w: %w", livednstest.ErrNoAnswer, err,
 					)
@@ -892,18 +890,12 @@ func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
 		)
 
 		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())
+		require.EqualError(
+			t, err,
+			"every root server refused a query for "+name+" or gave a "+
+				"reply marked recursion available: "+
+				"this network intercepts DNS queries",
+		)
 	}
 }