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User Agent

Understand how to identify Prerender.io's crawlers and verify their authenticity to ensure proper handling of prerendered requests.

Overview

Prerender.io uses custom User-Agent strings to identify its crawlers when accessing your site. These crawlers serve pre-rendered HTML to search engine bots for better SEO performance on JavaScript-heavy websites. Recognizing and properly handling these User-Agents ensures your site responds correctly to Prerender requests. To prevent spoofing, additional verification methods like reverse DNS lookups are recommended.

How It Works

What is a User-Agent?

A User-Agent is a string that a browser or bot sends to a server to identify itself. It informs the server about the client making the request, including the browser type, operating system, and in Prerender’s case, the crawler identity.

Why It Matters

  • Ensures Prerender.io is properly served pre-rendered content.

  • Helps distinguish legitimate crawlers from spoofed bots.

  • Aids in SEO optimization by confirming that search engines are indexing the correct (rendered) content.

 

Prerender.io User-Agent Strings

Prerender (+https://github.com/prerender/prerender)
Crawler Full user agent string
Prerender.io desktop Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/W.X.Y.Z Safari/537.36 Prerender (+https://github.com/prerender/prerender)
Prerender.io mobile Mozilla/5.0 (Linux; Android 11; Pixel 5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/W.X.Y.Z Mobile Safari/537.36 Prerender (+https://github.com/prerender/prerender)
Prerender.io sitemap worker Prerender (+https://github.com/prerender/prerender)

Wherever you see the string Chrome/W.X.Y.Z in the user agent strings in the table, W.X.Y.Z is a placeholder that represents the version of the Chrome browser used by that user agent: for example, 117.0.5938.62-1.

These values can be spoofed. If you need to verify that the visitor is Prerender, you should use a reverse DNS lookup.

 

Prerender.io Crawler IP Address Ranges

See: IP Addresses

Steps to Verify a Prerender Request via IP

  1. Get the IP from your server logs for the request in question.

  2. Run a reverse DNS lookup:

    host 142.54.167.130

    Output:

    130.167.54.142.in-addr.arpa domain name pointer us-mci1-38.renders.prerender.io.
  3. Run a forward DNS lookup to confirm the pointer:
    host us-mci1-38.renders.prerender.io
    Output:
    us-mci1-38.renders.prerender.io has address 142.54.167.130
  4. If the IPs match in both directions, the request is valid and from Prerender.io.

 

Tips

  • Always perform reverse DNS lookup to prevent spoofed User-Agents from receiving prerendered content.

  • Do not rely solely on User-Agent strings for authentication or content switching.

  • Use Prerender’s official IP range list (available in the documentation) for whitelist filtering, especially for firewalls or CDN edge logic.

  • Different User-Agents for desktop and mobile — be sure to account for both in your logic if serving device-specific content.

  • Missed verification can lead to indexing issues, especially if bots get incorrect or unrendered content.

 

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