From 70edcab16b7d7a6900d1a2499ff12e07f039a057 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <stapelberg@google.com>
Date: Sat, 7 Sep 2019 19:27:12 +0200
Subject: [PATCH] dns: return NXDOMAIN for DHCP leases once they expire

---
 internal/dns/dns.go      |  9 +++++++--
 internal/dns/dns_test.go | 22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/internal/dns/dns.go b/internal/dns/dns.go
index 12eac48..b48fbac 100644
--- a/internal/dns/dns.go
+++ b/internal/dns/dns.go
@@ -448,7 +448,12 @@ func (s *Server) resolveSubname(hostname string, q dns.Question) (dns.RR, error)
 
 		if lower := strings.ToLower(q.Name); lower == hostname+"." ||
 			lower == hostname+"."+s.domain+"." {
-			host, _ := s.hostByName(hostname)
+			host, ok := s.hostByName(hostname)
+			if !ok {
+				// The corresponding DHCP lease might have expired, but this
+				// handler is still installed on the mux.
+				return nil, nil // NXDOMAIN
+			}
 			if q.Qtype == dns.TypeA {
 				return dns.NewRR(q.Name + " 3600 IN A " + host)
 			}
@@ -482,7 +487,7 @@ func (s *Server) subnameHandler(hostname string) func(w dns.ResponseWriter, r *d
 				w.WriteMsg(m)
 				return
 			}
-			log.Fatal(err)
+			log.Fatalf("question %#v: %v", r.Question[0], err)
 		}
 		if rr != nil {
 			m := new(dns.Msg)
diff --git a/internal/dns/dns_test.go b/internal/dns/dns_test.go
index 4153b19..c278cab 100644
--- a/internal/dns/dns_test.go
+++ b/internal/dns/dns_test.go
@@ -187,6 +187,11 @@ func TestDHCP(t *testing.T) {
 			Addr:     net.IP{192, 168, 42, 150},
 			Expiry:   expired,
 		},
+		{
+			Hostname: "aged",
+			Addr:     net.IP{192, 168, 42, 42},
+			Expiry:   time.Now().Add(1 * time.Minute),
+		},
 	})
 
 	t.Run("testtarget.lan. (expired)", func(t *testing.T) {
@@ -203,6 +208,23 @@ func TestDHCP(t *testing.T) {
 			t.Fatalf("unexpected rcode: got %v, want %v", got, want)
 		}
 	})
+
+	s.SetLeases([]dhcp4d.Lease{
+		{
+			Hostname: "aged",
+			Addr:     net.IP{192, 168, 42, 42},
+			Expiry:   expired,
+		},
+	})
+
+	t.Run("aged.lan. (expired)", func(t *testing.T) {
+		m := new(dns.Msg)
+		m.SetQuestion("aged.lan.", dns.TypeA)
+		s.Mux.ServeDNS(r, m)
+		if got, want := r.response.Rcode, dns.RcodeNameError; got != want {
+			t.Fatalf("unexpected rcode: got %v, want %v", got, want)
+		}
+	})
 }
 
 func TestHostname(t *testing.T) {