core: fix crash with no plugins (#4184)

* core: fix crash with no plugins

A Corefile that defines a zone without plugins crashes coredns with the
stack trace below. Change this to return a refused.

~~~ corefile
 example.org {
    whoami
    log
    cache
    debug
}

example.net {
}
~~~

Asking for anyhing in example.net does this. Add test that tests this.

~~~
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xa5e6a4]

goroutine 55 [running]:
github.com/coredns/coredns/core/dnsserver.(*Server).ServeDNS(0xc000438f60, 0x2059420, 0xc0005a4030, 0x206c0e0, 0xc000522140, 0xc0005ae000)
	/home/miek/src/github.com/coredns/coredns/core/dnsserver/server.go:247 +0x884
github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket.func1(0x206dd00, 0xc00009e180, 0xc0005ae000)
	/home/miek/src/github.com/coredns/coredns/core/dnsserver/server.go:126 +0xaf
github.com/miekg/dns.HandlerFunc.ServeDNS(0xc000529270, 0x206dd00, 0xc00009e180, 0xc0005ae000)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:37 +0x44
github.com/miekg/dns.(*Server).serveDNS(0xc000286c60, 0xc000282400, 0x34, 0x200, 0xc00009e180)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:609 +0x2f7
github.com/miekg/dns.(*Server).serveUDPPacket(0xc000286c60, 0xc0003b03b4, 0xc000282400, 0x34, 0x200, 0xc00000e320, 0xc000522080)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:549 +0xb2
created by github.com/miekg/dns.(*Server).serveUDP
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:479 +0x292
~~~

Also fix single typo in chaos_test.go

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix naming

Signed-off-by: Miek Gieben <miek@miek.nl>

3 files changed, 33 insertions, 1 deletions

diff --git a/core/dnsserver/server.go b/core/dnsserver/server.go
index 52a2d6efc..eb23346e0 100644
--- a/core/dnsserver/server.go
+++ b/core/dnsserver/server.go
@@ -240,6 +240,10 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 
 	for {
 		if h, ok := s.zones[q[off:]]; ok {
+			if h.pluginChain == nil { // zone defined, but has not got any plugins
+				errorAndMetricsFunc(s.Addr, w, r, dns.RcodeRefused)
+				return
+			}
 			if r.Question[0].Qtype != dns.TypeDS {
 				if h.FilterFunc == nil {
 					rcode, _ := h.pluginChain.ServeDNS(ctx, w, r)
diff --git a/test/chaos_test.go b/test/chaos_test.go
index 5178e5fc3..eb83a2759 100644
--- a/test/chaos_test.go
+++ b/test/chaos_test.go
@@ -32,6 +32,6 @@ func TestChaos(t *testing.T) {
 	chTxt := resp.Answer[0].(*dns.TXT).Txt[0]
 	version := caddy.AppName + "-" + caddy.AppVersion
 	if chTxt != version {
-		t.Fatalf("Expected version to bo %s, got %s", version, chTxt)
+		t.Fatalf("Expected version to be %s, got %s", version, chTxt)
 	}
 }
diff --git a/test/no_plugins_test.go b/test/no_plugins_test.go
new file mode 100644
index 000000000..51d546db3
--- /dev/null
+++ b/test/no_plugins_test.go
@@ -0,0 +1,28 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestNoPlugins(t *testing.T) {
+	corefile := `example.org:0 {
+	}`
+
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	resp, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %v", err)
+	}
+	if resp.Rcode != dns.RcodeRefused {
+		t.Fatalf("Expected rcode to be %d, got %d", dns.RcodeRefused, resp.Rcode)
+	}
+}