fix subdomain extraction for rfc compliance

Author: gaby

ctx.go

@@ -1770,11 +1770,22 @@ func (c *DefaultCtx) Subdomains(offset ...int) []string {
 	// Normalize host according to RFC 3986
 	// Trim any trailing dot from a fully qualified domain name
 	host := strings.TrimSuffix(c.Hostname(), ".")
-	// Decode punycode labels to Unicode form
-	if u, err := idna.Lookup.ToUnicode(host); err == nil {
-		host = utils.ToLower(u)
-	} else {
-		host = utils.ToLower(host)
+	host = utils.ToLower(host)
+
+	// Decode punycode labels only when necessary
+	if strings.Contains(host, "xn--") {
+		if u, err := idna.Lookup.ToUnicode(host); err == nil {
+			host = utils.ToLower(u)
+		}
+	}
+
+	// Return nothing for IP addresses
+	ip := host
+	if strings.HasPrefix(ip, "[") && strings.HasSuffix(ip, "]") {
+		ip = strings.Trim(ip, "[]")
+	}
+	if utils.IsIPv4(ip) || utils.IsIPv6(ip) {
+		return []string{}
 	}
 
 	parts := strings.Split(host, ".")

ctx_test.go

@@ -3098,11 +3098,29 @@ func Test_Ctx_Subdomains(t *testing.T) {
 			want:   []string{"bücher"},
 		},
 		{
-			name:   "invalid domain keeps original lowercased",
+			name:   "punycode decode failure uses fallback",
+			host:   "xn--bcher--.example.com",
+			offset: nil,
+			want:   []string{"xn--bcher--"},
+		},
+		{
+			name:   "invalid host keeps original lowercased",
 			host:   "Foo Bar",
 			offset: []int{0},
 			want:   []string{"foo bar"},
 		},
+		{
+			name:   "IPv4 host returns empty",
+			host:   "192.168.0.1",
+			offset: nil,
+			want:   []string{},
+		},
+		{
+			name:   "IPv6 host returns empty",
+			host:   "[2001:db8::1]",
+			offset: nil,
+			want:   []string{},
+		},
 	}
 
 	for _, tc := range cases {