HTTP Version Not Supported
The server does not support the HTTP protocol version that was used in the request.
What it means
The HTTP 505 HTTP Version Not Supported status code indicates that the web server is unable or unwilling to process the request using the major HTTP version provided by the client (such as a request explicitly forcing `HTTP/3` on an old server that only understands `HTTP/1.1`).
How it works
- 1A specialized or highly experimental client targets a legacy server.
- 2The client sends a request explicitly specifying a version header protocol like `HTTP/3.0`.
- 3The legacy server reads the header, determines it cannot parse or support it safely, and throws a 505.
Why you're seeing this
- • A client or script is intentionally forcing an un-supported or malformed version string in the request line.
How to fix it
- • Ensure your server software (Apache, Nginx, IIS) is upgraded to its latest stable release to automatically gain multi-version support.
- • Fix the client-side tool or HTTP library to fall back gracefully to widely supported versions (`HTTP/1.1` or `HTTP/2`).
Framework Implementations
Node.js (Native HTTP)
const http = require('http');
const server = http.createServer((req, res) => {
if (req.httpVersionMajor > 2) {
res.writeHead(505, { 'Content-Type': 'text/plain' });
res.end('HTTP Version Not Supported');
} else {
res.writeHead(200); res.end('OK');
}
});Real-Life Scenarios
Rarely manually triggered. Use this inside internal embedded network software or legacy servers when a client sends a payload formatting standard you cannot parse.
Legacy Embedded Devices
An old manufacturing IoT gateway server running software from 2005 receives an explicit `HTTP/2` multiplexed request from a modern client and rejects it with a 505.
HTTP Transaction
1. Client Request
GET / HTTP/3.0 Host: legacy.example.com
2. Server Response
HTTP/1.1 505 HTTP Version Not Supported Content-Type: text/plain This server only supports HTTP/1.1 and HTTP/2 connections.
SEO Implications
No impact under standard circumstances. Major search bots dynamically adjust their connection capabilities down to matching server baselines.