dhcp4d: add header describing whether lease is active

Should be interpreted by the DHCP server so that other programs don’t need to
hard-code the lease time.

cmd/dhcp4d/dhcp4d.go

@@ -283,6 +283,7 @@ func newSrv(permDir string) (*srv, error) {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
+		w.Header().Set("X-Lease-Active", fmt.Sprint(lease.Expiry.After(time.Now().Add(handler.LeasePeriod/2))))
 		if _, err := io.Copy(w, bytes.NewReader(b)); err != nil {
 			log.Printf("/lease/%s: %v", hostname, err)
 		}

cmd/dhcp4d/dhcp4d_test.go

@@ -92,6 +92,24 @@ func TestLeaseHandler(t *testing.T) {
 	if diff := cmp.Diff(lease, got); diff != "" {
 		t.Fatalf("/lease/midna: unexpected lease: diff (-want +got):\n%s", diff)
 	}
+
+	if got, want := resp.Header.Get("X-Lease-Active"), "true"; got != want {
+		t.Fatalf("Unexpected X-Lease-Active header: got %q, want %s", got, want)
+	}
+	lease.Expiry = time.Now().Add(-1 * time.Minute)
+	srv.leases([]*dhcp4d.Lease{&lease}, &lease)
+	resp, err = http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := resp.StatusCode, http.StatusOK; got != want {
+		b, _ := ioutil.ReadAll(resp.Body)
+		t.Fatalf("unexpected HTTP response code: got %v (%s), want %v", resp.Status, strings.TrimSpace(string(b)), want)
+	}
+	if got, want := resp.Header.Get("X-Lease-Active"), "false"; got != want {
+		t.Fatalf("Unexpected X-Lease-Active header: got %q, want %s", got, want)
+	}
+
 	canc()
 	if err := eg.Wait(); err != nil && err != context.Canceled {
 		t.Fatal(err)

internal/dhcp4d/dhcp4d.go

@@ -47,7 +47,7 @@ type Handler struct {
 	serverIP    net.IP
 	start       net.IP // first IP address to hand out
 	leaseRange  int    // number of IP addresses to hand out
-	leasePeriod time.Duration
+	LeasePeriod time.Duration
 	options     dhcp4.Options
 	leasesHW    map[string]int // points into leasesIP
 	leasesIP    map[int]*Lease
@@ -89,7 +89,7 @@ func NewHandler(dir string, iface *net.Interface, ifaceName string, conn net.Pac
 		serverIP:    serverIP,
 		start:       start,
 		leaseRange:  230,
-		leasePeriod: 20 * time.Minute,
+		LeasePeriod: 20 * time.Minute,
 		options: dhcp4.Options{
 			dhcp4.OptionSubnetMask:       []byte{255, 255, 255, 0},
 			dhcp4.OptionRouter:           []byte(serverIP),
@@ -251,7 +251,7 @@ func (h *Handler) serveDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 			dhcp4.Offer,
 			h.serverIP,
 			dhcp4.IPAdd(h.start, free),
-			h.leasePeriod,
+			h.LeasePeriod,
 			h.options.SelectOrderOrAll(options[dhcp4.OptionParameterRequestList]))
 
 	case dhcp4.Request:
@@ -267,7 +267,7 @@ func (h *Handler) serveDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 			Num:          leaseNum,
 			Addr:         make([]byte, 4),
 			HardwareAddr: p.CHAddr().String(),
-			Expiry:       h.timeNow().Add(h.leasePeriod),
+			Expiry:       h.timeNow().Add(h.LeasePeriod),
 			Hostname:     string(options[dhcp4.OptionHostName]),
 		}
 		copy(lease.Addr, reqIP.To4())
@@ -296,7 +296,7 @@ func (h *Handler) serveDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 			}
 			h.Leases(leases, lease)
 		}
-		return dhcp4.ReplyPacket(p, dhcp4.ACK, h.serverIP, reqIP, h.leasePeriod,
+		return dhcp4.ReplyPacket(p, dhcp4.ACK, h.serverIP, reqIP, h.LeasePeriod,
 			h.options.SelectOrderOrAll(options[dhcp4.OptionParameterRequestList]))
 	}
 	return nil