Error Code 520: Meaning, Causes & How to Fix It (2026)

Cloudflare Error 520 — "Web Server Is Returning an Unknown Error"

Error 520 is one of the most frustrating server errors to troubleshoot because Cloudflare's own message — "Web Server Is Returning an Unknown Error" — tells you almost nothing about what went wrong. That vagueness is intentional: 520 is a catch-all for every situation where Cloudflare got a response from your origin server that it could not interpret as a valid HTTP reply. The origin was reachable. The connection opened. But what came back was empty, garbled, or cut off mid-stream.

This guide covers the exact architecture behind the 520, the six documented root causes, how to use the CF-RAY ID to pinpoint the problem, and a complete fix checklist organised by audience — end users, server administrators, and developers running scraping pipelines.

What Error 520 Means: The Architecture Behind It

To understand 520, you need to understand where Cloudflare sits in the request path. Cloudflare operates as a reverse proxy between the internet and your origin web server. Every visitor request passes through Cloudflare's edge network before reaching your server.

The critical distinction: a 520 means Cloudflare did reach your origin server and the TCP connection was established. The failure happens at the HTTP response layer — the origin either sent nothing back, sent something with no valid HTTP status code, sent malformed headers, or closed the connection before completing the response.

As Cloudflare's official documentation states: "This error occurs when the origin server returns an empty, unknown, or unexpected response to Cloudflare." The error page shown to visitors includes a CF-RAY ID — a unique request identifier that links to the specific event in Cloudflare's logs and your origin server's access log.

520 vs. Other Cloudflare 5xx Errors

Error 520 is part of a family of Cloudflare-specific codes (52x range) that go beyond standard HTTP. Knowing the differences prevents misdiagnosis:

Error code definitions: Cloudflare official documentation; Contabo Cloudflare 520 guide (March 2026).

8 Root Causes of Error 520

3 Real-World Scenarios

How to Fix Error 520: Complete Checklist

Work through the section that matches your role and access level.

First: Use the CF-RAY ID to Narrow the Cause

Every 520 error page includes a CF-RAY ID at the bottom (format: 7d1234567890abcd-LAX). This ID maps to a specific request in Cloudflare's logs and your origin server's access log. Before any other troubleshooting, note this ID — it dramatically narrows the time window and request to inspect. In Cloudflare Dashboard: Analytics → Origin Analytics → filter by edgeResponseStatus = 520 and match to originResponseStatus. If the origin returned 200 but Cloudflare served 520, the response content was malformed.

For End Users

For Server Administrators

keepalive_timeout 320s;
keepalive_requests 10000;

For Developers & Web Scrapers

import time
import requests

def fetch_with_retry(url, proxies, max_retries=4):
    for attempt in range(max_retries):
        try:
            resp = requests.get(url, proxies=proxies, timeout=20)
            if resp.status_code == 520:
                wait = 2 ** attempt
                print(f"520 error — retrying in {wait}s")
                time.sleep(wait)
                continue
            return resp
        except requests.RequestException as e:
            time.sleep(2 ** attempt)
    return None
  

Error 520 and Web Scraping: What It Means for Your Pipeline

For data collection teams, a 520 from a Cloudflare-protected target is important to distinguish from other block responses:

• A 403 Forbidden means Cloudflare's WAF has actively blocked the request — the origin was never reached. Changing IPs or headers addresses this.
• A 520 means the origin was reached but returned a bad response. This could be an anti-scraping measure (the server detects bot traffic and intentionally returns an empty response), but is more commonly a genuine server-side instability triggered by the volume or pattern of requests.

The practical response differs accordingly. For genuine 520 errors from resource exhaustion, exponential backoff retry logic (Fix 10 above) resolves most cases. For scraping patterns that trigger intentional 520 responses, residential IP rotation through a clean IP pool reduces the scraping footprint that triggered the server-side detection in the first place.

Nstproxy's residential proxy network provides the clean, ethically sourced IPs that minimise the detection signals which trigger both 403 blocks and intentional 520 responses on protected targets. Coverage details in the residential proxy overview; high-anonymity configuration details in the high-anonymity proxy guide.

Conclusion

Error 520 is Cloudflare's catch-all for an origin server that responded with something it could not process — an empty response, malformed headers, an abruptly closed connection, or a crash mid-response. It is not a standard HTTP code, and it only appears on Cloudflare-proxied sites. The CF-RAY ID on the error page is your fastest diagnostic tool: it lets you correlate the exact request across Cloudflare's analytics and your origin server logs without guessing.

The four fixes that resolve most 520 cases are: whitelist Cloudflare's IP ranges in your firewall, set TCP keepalive timeout above 300 seconds, audit and reduce cookie/header sizes, and check origin resource usage during the error window. For developers running scraping pipelines, exponential backoff retry logic and residential IP rotation reduce both the incidence and impact of 520 errors from Cloudflare-protected targets.

Frequently Asked Questions