resolver: a nameserver is asked at each of its addresses in turn (closes #285)

A nameserver's queries went only to the first IPv4 address its name
resolves to, so one unreachable address made it fail whenever that
address came first in the answer. Each record type is now asked at the
addresses one at a time until one gives a usable reply, and the next
type is asked first at the address that gave it. The README's DNS
Resolution Strategy says which addresses are asked.

Model: opus-5-5

internal/resolver/iterative.go

@@ -708,8 +708,9 @@ func (r *Resolver) QueryNameserver(
 	return r.queryNameserver(ctx, nsHostname, hostname, recordTypes())
 }
 
-// queryNameserver queries a specific nameserver for the record types
-// in qtypes and builds a NameserverResponse.
+// queryNameserver queries a specific nameserver, at the IPv4 addresses
+// its name resolves to, for the record types in qtypes and builds a
+// NameserverResponse.
 func (r *Resolver) queryNameserver(
 	ctx context.Context,
 	nsHostname string,
@@ -727,7 +728,7 @@ func (r *Resolver) queryNameserver(
 
 	hostname = dns.Fqdn(hostname)
 
-	return r.queryTypes(ctx, nsHostname, nsIPs[0], hostname, qtypes)
+	return r.queryTypes(ctx, nsHostname, nsIPs, hostname, qtypes)
 }
 
 // QueryNameserverIP queries a nameserver by its IP address directly,
@@ -744,13 +745,18 @@ func (r *Resolver) QueryNameserverIP(
 
 	hostname = dns.Fqdn(hostname)
 
-	return r.queryTypes(ctx, nsHostname, nsIP, hostname, recordTypes())
+	return r.queryTypes(
+		ctx, nsHostname, []string{nsIP}, hostname, recordTypes(),
+	)
 }
 
+// queryTypes asks the nameserver nsHostname, at its addresses nsIPs,
+// about hostname's records of each type in qtypes and builds a
+// NameserverResponse.
 func (r *Resolver) queryTypes(
 	ctx context.Context,
 	nsHostname string,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtypes []uint16,
 ) (*NameserverResponse, error) {
@@ -760,7 +766,7 @@ func (r *Resolver) queryTypes(
 		Status:     StatusOK,
 	}
 
-	state := r.queryEachType(ctx, nsIP, hostname, qtypes, resp)
+	state := r.queryEachType(ctx, nsIPs, hostname, qtypes, resp)
 	classifyResponse(resp, state)
 
 	return resp, nil
@@ -778,28 +784,37 @@ type queryState struct {
 	answered      bool
 }
 
-// queryEachType asks the nameserver at nsIP about hostname once for each
-// record type in qtypes, and lists in resp.FailedTypes the types whose
-// query got no usable reply, logging each with the reason unless ctx was
-// cancelled: shutdown cancels it, and a query it cut short did not fail.
+// queryEachType asks the nameserver at its addresses nsIPs about hostname
+// once for each record type in qtypes, and lists in resp.FailedTypes the
+// types whose query got no usable reply at any of them, logging each with
+// the reason unless ctx was cancelled: shutdown cancels it, and a query
+// it cut short did not fail.
 func (r *Resolver) queryEachType(
 	ctx context.Context,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtypes []uint16,
 	resp *NameserverResponse,
 ) queryState {
 	var state queryState
 
+	// Reordered below; the caller's list is left as it was.
+	nsIPs = slices.Clone(nsIPs)
+
 	for _, qtype := range qtypes {
 		if checkCtx(ctx) != nil {
 			break
 		}
 
-		err := r.querySingleType(ctx, nsIP, hostname, qtype, resp, &state)
+		answeredAt, err := r.querySingleType(
+			ctx, nsIPs, hostname, qtype, resp, &state,
+		)
 		if err == nil {
 			state.answered = true
 
+			// The next type is asked first at the address that answered.
+			nsIPs[0], nsIPs[answeredAt] = nsIPs[answeredAt], nsIPs[0]
+
 			continue
 		}
 
@@ -833,20 +848,29 @@ func (r *Resolver) queryEachType(
 	return state
 }
 
-// querySingleType asks the nameserver at nsIP about hostname's records
-// of type qtype. It returns nil when the nameserver answered: with
-// records, with none, or with NXDOMAIN; otherwise it returns why not.
+// querySingleType asks the nameserver about hostname's records of type
+// qtype at its addresses nsIPs, one at a time, until one answers: with
+// records, with none, or with NXDOMAIN. It returns the index in nsIPs of
+// the address that answered; when none did, it returns why the last one
+// did not.
 func (r *Resolver) querySingleType(
 	ctx context.Context,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtype uint16,
 	resp *NameserverResponse,
 	state *queryState,
-) error {
-	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
-	if err != nil {
+) (int, error) {
+	var err error
+
+	for i, nsIP := range nsIPs {
+		var msg *dns.Msg
+
+		msg, err = r.queryDNS(ctx, nsIP, hostname, qtype)
+
 		switch {
+		case err == nil:
+			err = readReply(msg, resp, state)
 		case isTimeout(err):
 			state.gotTimeout = true
 		case errors.Is(err, ErrRefused):
@@ -855,10 +879,12 @@ func (r *Resolver) querySingleType(
 			state.netErr = err
 		}
 
-		return err
+		if err == nil {
+			return i, nil
+		}
 	}
 
-	return readReply(msg, resp, state)
+	return 0, err
 }
 
 // readReply adds to resp the records in msg, a nameserver's reply to a