From 7413c833348f2e9be3dc2fc68b37171d9aa17b8e Mon Sep 17 00:00:00 2001
From: Shane Kerr <skerr@ns1.com>
Date: Thu, 26 May 2022 15:06:08 +0200
Subject: [PATCH] Disallow names that start with '.' in IsDomainName() (#1376)

* Disallow names that start with '.' in IsDomainName()

* Also update packDomain()
---
 defaults.go    | 5 +++++
 labels_test.go | 5 ++++-
 msg.go         | 5 +++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/defaults.go b/defaults.go
index d47b0b1f..f2cdbf43 100644
--- a/defaults.go
+++ b/defaults.go
@@ -218,6 +218,11 @@ func IsDomainName(s string) (labels int, ok bool) {
 
 			wasDot = false
 		case '.':
+			if i == 0 && len(s) > 1 {
+				// leading dots are not legal except for the root zone
+				return labels, false
+			}
+
 			if wasDot {
 				// two dots back to back is not legal
 				return labels, false
diff --git a/labels_test.go b/labels_test.go
index 3e672fec..dbe9a229 100644
--- a/labels_test.go
+++ b/labels_test.go
@@ -176,7 +176,10 @@ func TestIsDomainName(t *testing.T) {
 		lab int
 	}
 	names := map[string]*ret{
-		"..":                     {false, 1},
+		".":                      {true, 1},
+		"..":                     {false, 0},
+		"double-dot..test":       {false, 1},
+		".leading-dot.test":      {false, 0},
 		"@.":                     {true, 1},
 		"www.example.com":        {true, 3},
 		"www.e%ample.com":        {true, 3},
diff --git a/msg.go b/msg.go
index 60a2e8c3..89ebb64a 100644
--- a/msg.go
+++ b/msg.go
@@ -265,6 +265,11 @@ loop:
 
 			wasDot = false
 		case '.':
+			if i == 0 && len(s) > 1 {
+				// leading dots are not legal except for the root zone
+				return len(msg), ErrRdata
+			}
+
 			if wasDot {
 				// two dots back to back is not legal
 				return len(msg), ErrRdata