fix(isFQDN): enforce 253-char domain length unless ignored

src/lib/isFQDN.js

@@ -24,6 +24,11 @@ export default function isFQDN(str, options) {
     str = str.substring(2);
   }
 
+  /* Presentation-form FQDN max length is 253 (RFC 1035 / RFC 3696). */
+  if (!options.ignore_max_length && str.length > 253) {
+    return false;
+  }
+
   const parts = str.split('.');
   const tld = parts[parts.length - 1];
 

test/validators.test.js

@@ -1332,6 +1332,33 @@ describe('Validators', () => {
       ],
     });
   });
+  it('should reject FQDNs longer than 253 characters unless ignore_max_length is set', () => {
+    const label = 'a'.repeat(63);
+    // Each label stays within the 63-char limit; only the overall length differs.
+    const atLimit = [label, label, label, 'a'.repeat(57), 'com'].join('.'); // 253
+    const tooLongByOne = [label, label, label, 'a'.repeat(58), 'com'].join('.'); // 254
+    const tooLong = [label, label, label, label, 'comm'].join('.'); // 260
+    test({
+      validator: 'isFQDN',
+      valid: [
+        atLimit,
+      ],
+      invalid: [
+        tooLongByOne,
+        tooLong,
+      ],
+    });
+    test({
+      validator: 'isFQDN',
+      args: [
+        { ignore_max_length: true },
+      ],
+      valid: [
+        tooLongByOne,
+        tooLong,
+      ],
+    });
+  });
   it('should validate FQDN with trailing dot option', () => {
     test({
       validator: 'isFQDN',

test/validators/isFQDN.test.js

@@ -23,4 +23,26 @@ describe('isFQDN', () => {
       ],
     });
   });
+
+  it('should enforce the 253-character domain length limit', () => {
+    const label = 'a'.repeat(63);
+    const atLimit = [label, label, label, 'a'.repeat(57), 'com'].join('.');
+    const tooLong = [label, label, label, label, 'comm'].join('.');
+    test({
+      validator: 'isFQDN',
+      valid: [
+        atLimit,
+      ],
+      invalid: [
+        tooLong,
+      ],
+    });
+    test({
+      validator: 'isFQDN',
+      args: [{ ignore_max_length: true }],
+      valid: [
+        tooLong,
+      ],
+    });
+  });
 });