From d49c86087ef1d6582f3c401cb2d4c1db5bad0fa1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Christoffer=20Fjellstr=C3=B6m?= <c.fjellstrom@gmail.com>
Date: Tue, 12 Mar 2019 17:31:33 +0100
Subject: [PATCH] Add checks on data length for A and AAAA records (#919)

* Add checks on data length for A and AAAA records

Fixes panic when parsing A or AAAA records with no data

* Add tests Field() on empty A/AAAA data

* Refactor format test

* Add return value check on format test
---
 format.go      |  6 ++++++
 format_test.go | 16 ++++++++++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 format_test.go

diff --git a/format.go b/format.go
index 86057f99..0ec79f2f 100644
--- a/format.go
+++ b/format.go
@@ -31,6 +31,9 @@ func Field(r RR, i int) string {
 		switch reflect.ValueOf(r).Elem().Type().Field(i).Tag {
 		case `dns:"a"`:
 			// TODO(miek): Hmm store this as 16 bytes
+			if d.Len() < net.IPv4len {
+				return ""
+			}
 			if d.Len() < net.IPv6len {
 				return net.IPv4(byte(d.Index(0).Uint()),
 					byte(d.Index(1).Uint()),
@@ -42,6 +45,9 @@ func Field(r RR, i int) string {
 				byte(d.Index(14).Uint()),
 				byte(d.Index(15).Uint())).String()
 		case `dns:"aaaa"`:
+			if d.Len() < net.IPv6len {
+				return ""
+			}
 			return net.IP{
 				byte(d.Index(0).Uint()),
 				byte(d.Index(1).Uint()),
diff --git a/format_test.go b/format_test.go
new file mode 100644
index 00000000..d0dce4ff
--- /dev/null
+++ b/format_test.go
@@ -0,0 +1,16 @@
+package dns
+
+import (
+	"testing"
+)
+
+func TestFieldEmptyAOrAAAAData(t *testing.T) {
+	res := Field(new(A), 1)
+	if res != "" {
+		t.Errorf("expected empty string but got %v", res)
+	}
+	res = Field(new(AAAA), 1)
+	if res != "" {
+		t.Errorf("expected empty string but got %v", res)
+	}
+}