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505 HTTP Version Not Supported

505 HTTP Version Not Supported is a status that a server can emit if it doesn’t support the major HTTP version the client used to make the request.

To test this, I opened a telnet connection to a couple of major websites, and wrote the following:

GET / HTTP/4.0


A few sites returned a 400 Bad Request, but at least Cloudflair returned 505:

HTTP/1.1 505 HTTP Version Not Supported
Server: cloudflare
Date: Tue, 30 Jul 2019 15:23:35 GMT
Content-Type: text/html
Content-Length: 201
Connection: close
CF-RAY: -

<html>
<head><title>505 HTTP Version Not Supported</title></head>
<body bgcolor="white">
<center><h1>505 HTTP Version Not Supported</h1></center>
<hr><center>cloudflare</center>
</body>
</html>

Currently you’re unlikely to run into this error though, unless you have a buggy client.

We did get a new major HTTP version (2) and a new one is on the way (3), but the mechanism for switching to HTTP/2 or 3 is different, and will just be ignored if a HTTP/1.1 server didn’t support the new version.

References

HTTP series

This article is part of a series about the HTTP protocol. Read them all here:

Informational 1xx

Successful 2xx

Redirection 3xx

Client Error 4xx

Server Error 5xx

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