ErrorFindly
5055xx Server ErrorHigh

HTTP 505 HTTP Version Not Supported

Defined in RFC 7231

What Does HTTP 505 Mean?

The server does not support the HTTP protocol version used in the request.

Common Causes

  1. 1Client using HTTP/2 with server that only supports HTTP/1.1
  2. 2Very old client using HTTP/0.9
  3. 3Misconfigured server rejecting valid protocol versions
  4. 4Proxy not supporting the protocol version

Impact

  • ⚠Cannot connect to the server
  • ⚠Need to use a different HTTP version

Developer Fix

For web developers and application engineers

  1. 1Check which HTTP versions the server supports
  2. 2Configure client to use a supported protocol version
  3. 3Add protocol version fallback logic

Server Admin Fix

For system administrators and DevOps engineers

  1. 1Update server software to support modern HTTP versions
  2. 2Enable HTTP/2 support if needed
  3. 3Check TLS configuration for HTTP/2 ALPN negotiation

Frequently Asked Questions

Is 505 common?
No, it is very rare. Most modern servers support HTTP/1.1 and HTTP/2. You might encounter it with very old or very restrictive server configurations.
How do I fix a 505 error?
As a client, try forcing HTTP/1.1. As a server admin, update your server software and enable support for the HTTP version your clients need.

Related HTTP Status Codes

426Upgrade Required

About the Author

Web Infrastructure Team

Verified against official RFC specifications and real-world server configurations. HTTP status code behavior confirmed across Apache, Nginx, and Cloudflare.