resolver: query a hostname at its own zone's servers (closes #189)

A hostname's nameservers came from its last two labels, so a name under
co.uk was asked at the co.uk servers and a name in a delegated subdomain
at the parent's servers; both only refer onward. The hostname now goes
through FindAuthoritativeNameservers, which follows delegations for the
name and walks up its labels until it finds the zone it is in.

followDelegation now stops at an authoritative reply: that server holds
the zone, so its reply is not a referral. Without this a CNAME answered
with the zone's NS records in the authority section, as Route 53 does,
was followed as a referral until the delegation limit.

Model: opus-5-5

internal/resolver/resolver_test.go

@@ -79,9 +79,10 @@ func TestFindAuthoritativeNameservers_Subdomain(
 	t.Parallel()
 
 	r := newTestResolver(t)
-	nameservers := liveFindAuthoritative(t, r, "www.google.com")
+	fromHost := liveFindAuthoritative(t, r, "www.google.com")
+	fromZone := liveFindAuthoritative(t, r, "google.com")
 
-	assert.NotEmpty(t, nameservers)
+	assert.Equal(t, fromZone, fromHost)
 }
 
 func TestFindAuthoritativeNameservers_ReturnsSorted(
@@ -350,37 +351,57 @@ func TestQueryAllNameservers_ReturnsAllNS(t *testing.T) {
 func TestQueryAllNameservers_AllReturnOK(t *testing.T) {
 	t.Parallel()
 
-	r := newTestResolver(t)
-	results := liveQueryAllNameservers(t, r, "google.com")
+	// The last two names are in zones other than their last two
+	// labels: google.co.uk, under the two-label suffix co.uk, and
+	// compute-1.amazonaws.com, where EC2 host names are, which
+	// amazonaws.com delegates to other servers. Servers above a
+	// name's zone only refer onward, which gives nodata, so ok shows
+	// the name was asked at its own zone's servers.
+	hostnames := []string{
+		"google.com",
+		"www.google.co.uk",
+		"ec2-3-80-0-1.compute-1.amazonaws.com",
+	}
 
-	// A quorum, not unanimity: one authoritative server being
-	// slow or rate-limiting us is a property of the live
-	// internet, not a resolver defect.
-	assert.GreaterOrEqual(
-		t,
-		countStatus(results, resolver.StatusOK),
-		liveQuorum(len(results)),
-		"a quorum of nameservers should answer OK: %s",
-		describeStatuses(results),
-	)
+	for _, hostname := range hostnames {
+		t.Run(hostname, func(t *testing.T) {
+			t.Parallel()
 
-	// Quorum tolerates SILENCE only. Every individual result must
-	// be either the expected answer or a non-answer: ok, timeout
-	// or error, and nothing else. Stated as a closed allowlist so
-	// that a wrong answer no one thought to ban — nxdomain and
-	// nodata today, any status added later — fails here rather
-	// than sliding through under the quorum.
-	assert.Empty(
-		t,
-		unsanctionedStatuses(
-			results,
-			resolver.StatusOK,
-			resolver.StatusTimeout,
-			resolver.StatusError,
-		),
-		"every nameserver must answer OK or not answer at all: %s",
-		describeStatuses(results),
-	)
+			r := newTestResolver(t)
+			results := liveQueryAllNameservers(t, r, hostname)
+
+			// A quorum, not unanimity: one authoritative server
+			// being slow or rate-limiting us is a property of the
+			// live internet, not a resolver defect.
+			assert.GreaterOrEqual(
+				t,
+				countStatus(results, resolver.StatusOK),
+				liveQuorum(len(results)),
+				"a quorum of nameservers should answer OK: %s",
+				describeStatuses(results),
+			)
+
+			// Quorum tolerates SILENCE only. Every individual
+			// result must be either the expected answer or a
+			// non-answer: ok, timeout or error, and nothing else.
+			// Stated as a closed allowlist so that a wrong answer
+			// no one thought to ban — nxdomain and nodata today,
+			// any status added later — fails here rather than
+			// sliding through under the quorum.
+			assert.Empty(
+				t,
+				unsanctionedStatuses(
+					results,
+					resolver.StatusOK,
+					resolver.StatusTimeout,
+					resolver.StatusError,
+				),
+				"every nameserver must answer OK or not answer "+
+					"at all: %s",
+				describeStatuses(results),
+			)
+		})
+	}
 }
 
 func TestQueryAllNameservers_NXDomainFromAllNS(