Error Code 521 — Causes, Fixes, and Prevention (Complete Guide)

Introduction

Few things are more frustrating for website owners and visitors alike than encountering an error message instead of a working website. One such common and confusing error is “Web Server Is Down – Error Code 521.” This error often appears when using Cloudflare as a CDN or security service, and it can cause panic—especially if your website is business-critical.

Error 521 means that Cloudflare is unable to connect to your origin web server. While Cloudflare itself is working, the server hosting your website is either refusing connections or is completely offline.

In this comprehensive guide, we’ll explore what Error Code 521 means, why it happens, how to fix it step by step, and how to prevent it from happening again. Whether you’re a beginner website owner or an experienced system administrator, this article will help you understand and resolve Error 521 with confidence.

What Is Error Code 521?

Error 521: Web Server Is Down is a Cloudflare-specific error that occurs when:

Cloudflare successfully reaches its own network
But cannot establish a TCP connection to your website’s origin server

In simple terms:

Cloudflare is online, but your server is not responding to Cloudflare’s requests.

How Error 521 Works (Behind the Scenes)

When a visitor accesses your website:

The request first goes to Cloudflare
Cloudflare forwards the request to your origin server
Your server is expected to respond
If the server refuses or fails to respond, Cloudflare returns Error 521

This means the issue is almost always on the origin server, not Cloudflare.

How Error 521 Looks to Visitors

Visitors typically see a page like this:


Web server is down
Error code 521
Visit cloudflare.com for more information.

Additional details often include:

Browser: Working
Cloudflare: Working
Host: Error

This diagnostic breakdown is Cloudflare’s way of saying:
“The problem is on your server, not ours.”

Common Causes of Error Code 521

Understanding the cause is the key to fixing the issue. Below are the most common reasons why Error 521 occurs.

1. Origin Server Is Offline

The most common cause is that your web server is completely down.

Possible reasons include:

Server crash
Hosting provider outage
Power failure
Kernel panic
Resource exhaustion (CPU, RAM, disk)

If the server is not running, Cloudflare has nothing to connect to.

2. Web Server Is Not Accepting Connections

Even if the server is technically “up,” it may be refusing connections due to:

Apache or Nginx service stopped
PHP-FPM not running
Server overload
Max connections reached

Cloudflare attempts to connect but gets no response or a rejection.

3. Firewall Blocking Cloudflare IPs

A very common misconfiguration.

Your firewall (server firewall, hosting firewall, or security plugin) may be:

Blocking Cloudflare IP ranges
Rate-limiting Cloudflare
Treating Cloudflare as a malicious source

When this happens, Cloudflare is explicitly denied access.

4. Incorrect Server IP in Cloudflare DNS

If your DNS settings point to the wrong IP address:

Cloudflare sends traffic to the wrong server
That server does not respond
Error 521 occurs

This often happens after:

Server migrations
Hosting changes
IP address updates

5. SSL/TLS Misconfiguration

SSL issues can indirectly cause Error 521, especially when:

Cloudflare SSL mode is set to Full or Full (Strict)
The origin server has no valid SSL certificate
The certificate is expired or misconfigured

The server may refuse the secure connection.

6. Server Software Crashes

Critical services may crash without taking the entire server down:

Apache/Nginx crash
PHP process failure
Database lockups affecting response

From Cloudflare’s perspective, the server is unreachable.

7. DDoS Protection or Security Rules

Some servers block traffic automatically when they detect:

Too many connections
Suspicious IP patterns
Bot-like behavior

Cloudflare traffic can mistakenly trigger these protections.

How to Fix Error Code 521 (Step-by-Step)

Let’s walk through practical solutions, starting with the fastest checks and moving to more advanced fixes.

Step 1: Check If Your Server Is Online

What to Do

Try accessing your website directly via server IP
Use SSH to connect to your server
Check your hosting control panel

If You Cannot Access the Server

Your server is likely down
Contact your hosting provider immediately
Check hosting status pages for outages

No Cloudflare fix will work until the server is online.

Step 2: Restart Web Server Services

If the server is reachable, restart critical services.

Common Services to Restart

Apache
Nginx
PHP-FPM
LiteSpeed

This resolves many temporary crashes and memory issues.

Step 3: Check Firewall Rules

Allow Cloudflare IP Ranges

Cloudflare publishes a list of IP ranges that must be whitelisted.

Make sure:

Server firewall allows all Cloudflare IPs
Hosting firewall (CSF, UFW, iptables) is not blocking them
Security plugins (WordPress, etc.) are not blocking Cloudflare

Failure to whitelist Cloudflare is one of the top causes of Error 521.

Step 4: Verify DNS Settings in Cloudflare

What to Check

Cloudflare DNS A record points to the correct origin IP
No old or incorrect IP addresses
Only one active A record (unless intentionally load-balanced)

Tip

If you recently migrated servers, DNS errors are extremely common.

Step 5: Temporarily Disable Cloudflare

This is a diagnostic step, not a permanent solution.

How to Do It

Pause Cloudflare
Or set DNS records to “DNS Only” (gray cloud)

What This Tells You

If the site works without Cloudflare → issue is Cloudflare ↔ server communication
If the site still doesn’t work → server issue unrelated to Cloudflare

Step 6: Check Server Resource Usage

Error 521 often appears when servers are overloaded.

Look For

CPU at 100%
RAM fully used
Disk space full
Too many concurrent connections

Solutions

Upgrade hosting plan
Optimize database
Enable caching
Reduce plugin usage (for CMS sites)

Step 7: Review SSL/TLS Configuration

Recommended Cloudflare Settings

SSL Mode: Full (Strict) (only if origin has valid SSL)
Ensure certificate is:
- Valid
- Not expired
- Correctly installed

If unsure, temporarily switch to Full (not Strict) to test.

Step 8: Check Logs

Logs provide definitive answers.

Important Logs

Web server error logs
Firewall logs
System logs
Cloudflare dashboard events

Look for:

Connection refusals
IP blocks
Service crashes

Preventing Error Code 521 in the Future

Once fixed, prevention is crucial.

1. Monitor Server Uptime

Use monitoring tools to detect outages early:

Uptime monitors
Server health alerts
Resource usage notifications

2. Always Whitelist Cloudflare IPs

Whenever:

You install a firewall
You add a security plugin
You migrate servers

Re-check Cloudflare IP whitelisting.

3. Keep Server Software Updated

Outdated software can:

Crash unexpectedly
Have compatibility issues
Trigger security blocks

Regular updates reduce risk.

4. Use Proper Hosting for Your Traffic Level

Shared hosting may not handle:

High traffic
API calls
Bots
Crawlers

Upgrade when needed.

5. Implement Caching and Optimization

Reduce server load by:

Enabling page caching
Using object caching
Optimizing images
Reducing database queries

Less load = fewer 521 errors.

Error 521 vs Other Cloudflare Errors

Understanding the difference helps troubleshooting.

Error Code	Meaning
520	Unknown error from origin
521	Server refuses connection
522	Connection timed out
523	Origin unreachable
524	Timeout after connection

521 specifically means refusal or no response, not just slowness.

Is Error 521 Dangerous?

Error 521 itself is not a security threat, but:

It hurts SEO
Increases bounce rates
Causes revenue loss
Damages user trust

Search engines may de-rank sites that are frequently unavailable.

Frequently Asked Questions (FAQ)

Is Error 521 Cloudflare’s Fault?

No. In nearly all cases, the origin server is responsible.

Can Error 521 Fix Itself?

Sometimes, if caused by temporary overload.
But recurring 521 errors require action.

Does Error 521 Affect SEO?

Yes. Prolonged downtime negatively impacts:

Crawlability
Rankings
User experience signals

Should I Disable Cloudflare Permanently?

No. Cloudflare improves security and performance.
Fix the server issue instead.

Final Thoughts

Error Code 521: Web Server Is Down can be alarming, but it’s also one of the most fixable Cloudflare errors once you understand what’s happening.

The key takeaway is simple:

Cloudflare is doing its job — your server is not responding.

By checking server availability, whitelisting Cloudflare IPs, fixing firewall rules, verifying DNS settings, and monitoring resources, you can resolve Error 521 quickly and prevent it from returning.

A stable server combined with proper Cloudflare configuration ensures your website remains fast, secure, and always accessible.

TIPS RECIPES

jeudi 15 janvier 2026

Web server is down Error code 521