🤖 Overview

404checker is a legitimate web crawler operated by the company behind the SaaS tool 404Checker (404checker.com), designed to automatically scan websites for broken links (HTTP 404 and other client/server errors). Its primary purpose is to help webmasters and SEO professionals identify dead links, redirect loops, and other link-related issues that degrade user experience and search engine ranking. The tool is offered as a paid subscription service and is not associated with any malicious activity or threat actor campaigns.

🌐 Technical Behavior

According to the official documentation at 404checker.com, the crawler follows a breadth‑first crawl strategy, starting from a user‑provided sitemap URL or a manually entered seed URL. It sends HTTP GET requests sequentially at a moderate rate, typically not exceeding 10 requests per second per domain to avoid overloading servers. The crawler respects the robots.txt Disallow directives (see below) and uses standard HTTP/1.1 with persistent connections. IP addresses originate from a pool of AWS EC2 instances located primarily in the United States (us‑east‑1 and us‑west‑2) and, less frequently, from European data centers. The crawler does not execute JavaScript or render pages; it only parses HTML anchor tags (<a href>), CSS url() references, and <img src> to extract links.

📋 robots.txt Compliance

The 404checker crawler explicitly honors robots.txt directives. Its official site states: “We fully respect the rules set in your robots.txt file. If you disallow our user‑agent, we will not crawl your site.” The user‑agent string used in the crawl is 404checker (lowercase, no underscore). However, the operator also provides a secondary identifier Mozilla/5.0 (compatible; 404checker/2.0; +https://404checker.com/bot) to aid in server log identification. There is no evidence of deliberate violation of crawl rules; any reported incidents likely stem from misconfigured robots.txt or caching issues.

🔍 Detection Indicators

The primary User‑Agent string is 404checker or Mozilla/5.0 (compatible; 404checker/2.0; +https://404checker.com/bot). Behavioral indicators include a high ratio of HEAD requests (used to check link validity without downloading the full page) and a consistent request interval of approximately 0.5–1 second between successive requests. The crawler sends an Accept‑Encoding: gzip header and a standard Accept: text/html,application/xhtml+xml... header. No custom headers like X‑Powered‑By are typically present. Server logs show the referrer field often set to https://404checker.com/crawl/.

📊 Data Usage

The collected data is used exclusively to generate a detailed report for the paying subscriber: a list of broken links (HTTP 4xx and 5xx responses), redirect chains (HTTP 3xx), and pages that return a 200 status but contain no links (orphan pages). No data is used for AI training, search indexing, or aggregated analytics. The reports are stored temporarily on 404Checker’s servers and purged after 30 days, as per their privacy policy published on 404checker.com/privacy.

⚙️ Rate Limiting Policy

Although legitimate, the 404checker bot is often rate‑limited by server operators because its burst of sequential requests can mimic a low‑grade denial‑of‑service pattern, especially on shared hosting environments. A threshold of 50 requests per minute per IP is a reasonable policy to allow the bot to complete its scan without degrading site performance for other users, while still respecting its operational needs.

ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the bots listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.