W3C-checklink
W3C •
Since 1998
Quick Actions
Official Docs
What is W3C-checklink?
W3C Link Checker is a tool provided by the W3C that crawls websites to find broken links, redirects, and other link-related issues. The service helps maintain website quality by identifying problematic links that could harm user experience and SEO. It can check both internal and external links, providing detailed reports about link status, redirect chains, and anchor text issues. The tool is particularly useful for maintaining large websites where manual link checking would be impractical.
User Agent String
W3C-checklink/5.0.0
How to Control W3C-checklink
Block Completely
To prevent W3C-checklink from accessing your entire website, add this to your robots.txt file:
# Block W3C-checklink
User-agent: W3C-checklink
Disallow: /
Block Specific Directories
To restrict access to certain parts of your site while allowing others:
User-agent: W3C-checklink
Disallow: /admin/
Disallow: /private/
Disallow: /wp-admin/
Allow: /public/
Set Crawl Delay
To slow down the crawl rate (note: not all bots respect this directive):
User-agent: W3C-checklink
Crawl-delay: 10
How to Verify W3C-checklink
Verification Method:
W3C-checklink user agent
W3C-checklink user agent
Learn more in the official documentation.
Detection Patterns
Multiple ways to detect W3C-checklink in your application:
Basic Pattern
/W3C\-checklink/iStrict Pattern
/^W3C\-checklink/5\.0\.0$/Flexible Pattern
/W3C\-checklink[\s\/]?[\d\.]*?/iVendor Match
/.*W3C.*W3C\-checklink/iImplementation Examples
// PHP Detection for W3C-checklink
function detect_w3c_checklink() {
$user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
$pattern = '/W3C\\-checklink/i';
if (preg_match($pattern, $user_agent)) {
// Log the detection
error_log('W3C-checklink detected from IP: ' . $_SERVER['REMOTE_ADDR']);
// Set cache headers
header('Cache-Control: public, max-age=3600');
header('X-Robots-Tag: noarchive');
// Optional: Serve cached version
if (file_exists('cache/' . md5($_SERVER['REQUEST_URI']) . '.html')) {
readfile('cache/' . md5($_SERVER['REQUEST_URI']) . '.html');
exit;
}
return true;
}
return false;
}
# Python/Flask Detection for W3C-checklink
import re
from flask import request, make_responsedef detect_w3c_checklink():
user_agent = request.headers.get('User-Agent', '')
pattern = r'W3C-checklink'
if re.search(pattern, user_agent, re.IGNORECASE):
# Create response with caching
response = make_response()
response.headers['Cache-Control'] = 'public, max-age=3600'
response.headers['X-Robots-Tag'] = 'noarchive'
return True
return False# Django Middleware
class W3CchecklinkMiddleware:
def __init__(self, get_response):
self.get_response = get_response
def __call__(self, request):
if self.detect_bot(request):
# Handle bot traffic
pass
return self.get_response(request)
// JavaScript/Node.js Detection for W3C-checklink
const express = require('express');
const app = express();// Middleware to detect W3C-checklink
function detectW3Cchecklink(req, res, next) {
const userAgent = req.headers['user-agent'] || '';
const pattern = /W3C-checklink/i;
if (pattern.test(userAgent)) {
// Log bot detection
console.log('W3C-checklink detected from IP:', req.ip);
// Set cache headers
res.set({
'Cache-Control': 'public, max-age=3600',
'X-Robots-Tag': 'noarchive'
});
// Mark request as bot
req.isBot = true;
req.botName = 'W3C-checklink';
}
next();
}app.use(detectW3Cchecklink);
# Apache .htaccess rules for W3C-checklink# Block completely
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} W3C\-checklink [NC]
RewriteRule .* - [F,L]# Or redirect to a static version
RewriteCond %{HTTP_USER_AGENT} W3C\-checklink [NC]
RewriteCond %{REQUEST_URI} !^/static/
RewriteRule ^(.*)$ /static/$1 [L]# Or set environment variable for PHP
SetEnvIfNoCase User-Agent "W3C\-checklink" is_bot=1# Add cache headers for this bot
<If "%{HTTP_USER_AGENT} =~ /W3C\-checklink/i">
Header set Cache-Control "public, max-age=3600"
Header set X-Robots-Tag "noarchive"
</If>
# Nginx configuration for W3C-checklink# Map user agent to variable
map $http_user_agent $is_w3c_checklink {
default 0;
~*W3C\-checklink 1;
}server {
# Block the bot completely
if ($is_w3c_checklink) {
return 403;
}
# Or serve cached content
location / {
if ($is_w3c_checklink) {
root /var/www/cached;
try_files $uri $uri.html $uri/index.html @backend;
}
try_files $uri @backend;
}
# Add headers for bot requests
location @backend {
if ($is_w3c_checklink) {
add_header Cache-Control "public, max-age=3600";
add_header X-Robots-Tag "noarchive";
}
proxy_pass http://backend;
}
}
Should You Block This Bot?
Recommendations based on your website type:
| Site Type | Recommendation | Reasoning |
|---|---|---|
| E-commerce | Optional | Evaluate based on bandwidth usage vs. benefits |
| Blog/News | Allow | Increases content reach and discoverability |
| SaaS Application | Block | No benefit for application interfaces; preserve resources |
| Documentation | Selective | Allow for public docs, block for internal docs |
| Corporate Site | Limit | Allow for public pages, block sensitive areas like intranets |
Advanced robots.txt Configurations
E-commerce Site Configuration
User-agent: W3C-checklink
Crawl-delay: 5
Disallow: /cart/
Disallow: /checkout/
Disallow: /my-account/
Disallow: /api/
Disallow: /*?sort=
Disallow: /*?filter=
Disallow: /*&page=
Allow: /products/
Allow: /categories/
Sitemap: https://example.com/sitemap.xml
Publishing/Blog Configuration
User-agent: W3C-checklink
Crawl-delay: 10
Disallow: /wp-admin/
Disallow: /drafts/
Disallow: /preview/
Disallow: /*?replytocom=
Allow: /
SaaS/Application Configuration
User-agent: W3C-checklink
Disallow: /app/
Disallow: /api/
Disallow: /dashboard/
Disallow: /settings/
Allow: /
Allow: /pricing/
Allow: /features/
Allow: /docs/
Quick Reference
User Agent Match
W3C-checklinkRobots.txt Name
W3C-checklinkCategory
otherRespects robots.txt
Yes
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