505 HTTP Version Not Supported

Meaning & Description

Common Causes

• Using highly outdated tools, legacy hardware, or custom IoT devices that only speak HTTP/1.0.
• Hand-typing raw HTTP requests via telnet or netcat and making a typo in the protocol version.
• Misconfigured load balancers attempting to downgrade traffic to an unsupported version.

How to fix a 505 error

1. Inspect the raw request headers to see exactly which HTTP version is being sent (e.g., HTTP/1.0 vs HTTP/1.1).
2. If using cURL, use the `--http1.1` or `--http2` flags to explicitly define the protocol version.
3. Ensure your server configuration (Nginx, Apache) is set up to accept the protocol version the client is using.

Browser & SEO Behaviour

Code Examples

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain

HTTP/1.1 is required. HTTP/1.0 is no longer supported.

app.use((req, res, next) => {
  if (req.httpVersion === "1.0") {
    return res.status(505).send("HTTP/1.1 or higher is required.");
  }
  next();
});

package main

import "net/http"

func handler(w http.ResponseWriter, r *http.Request) {
	if r.ProtoMajor < 1 || (r.ProtoMajor == 1 && r.ProtoMinor == 0) {
		http.Error(w, "HTTP/1.1 or higher is required", http.StatusHTTPVersionNotSupported)
		return
	}
	w.WriteHeader(http.StatusOK)
}

func main() {
	http.HandleFunc("/", handler)
	http.ListenAndServe(":8080", nil)
}

# Forcing an ancient HTTP/1.0 request to a strict modern server
curl -0 https://api.example.com/

Raw HTTP Response Example

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: application/json

{
  "error": "HTTP Version Not Supported",
  "message": "This API requires HTTP/1.1 or HTTP/2."
}

Real-world Examples

Frequently Asked Questions

Developer Tips

• You generally do not need to implement this manually in application code; your web server (Nginx, Apache, IIS) handles HTTP protocol version validation natively.
• If you are building a custom raw TCP/HTTP server, ensure you gracefully handle legacy HTTP/1.0 requests or explicitly return a 505 with a clear explanation.

Interview Questions

These are questions you might face in a backend or API design interview that touch on HTTP 505.