resolver: ask a referral's nameservers that come without addresses (closes #221)

Looking up a nameserver's own address followed only the addresses a
referral gave, so a nameserver whose zone is delegated without them,
such as a.ntpns.org of pool.ntp.org, never resolved. The walk to a
name's nameservers looked addresses up only when a referral gave none.
Both now ask the nameservers whose addresses the referral gives first
and, if none of them gives a usable reply, look up and ask the others;
with no addresses given, all are looked up, as before. maxLookupDepth
stops lookups three deep, so delegations that point at each other
still end. g.ntpns.org's address needs all three when anyns.pch.net
gives the referral to g.ntpns.org without addresses.

Model: opus-5-5

internal/resolver/export_test.go

@@ -55,12 +55,31 @@ func (r *Resolver) QueryEachNS(
 	return r.queryEachNS(ctx, nameservers, hostname, recordTypes())
 }
 
-// ResolveNSIPs exports resolveNSIPs for testing.
+// ResolveNSIPs exports resolveNSIPs for testing, looking each name up
+// as a lookup that no other lookup started.
 func (r *Resolver) ResolveNSIPs(
 	ctx context.Context,
 	nsNames []string,
 ) []string {
-	return r.resolveNSIPs(ctx, nsNames)
+	return r.resolveNSIPs(ctx, nsNames, 1)
+}
+
+// MaxLookupDepth exports maxLookupDepth for testing.
+const MaxLookupDepth = maxLookupDepth
+
+// QueryNameservers exports queryNameservers for testing.
+func (r *Resolver) QueryNameservers(
+	ctx context.Context,
+	given []string,
+	withoutAddresses []string,
+	zone string,
+	name string,
+	qtype uint16,
+	depth int,
+) (*dns.Msg, error) {
+	return r.queryNameservers(
+		ctx, given, withoutAddresses, zone, name, qtype, depth,
+	)
 }
 
 // RootServerList exports rootServerList for testing.