The Relationship Between Cloudflare 521 Errors and "Orange Cloud / Gray Cloud" Settings

When using Cloudflare,521 Error (Web Server Is Down)This is a common yet easily misjudged issue. Many sites experience this scenario: DNS resolution works normally,server (computer)Seems online, but switching to gray cloud access works fine. However, once...Enable Orange Cloud Proxy and immediately report 521In fact, this is not Cloudflare not a fault in itself, but ratherThe origin server cannot establish a normal connection with Cloudflare in proxy mode.Understanding the differences in access paths between orange clouds and gray clouds is key to identifying and resolving 521 errors.

I. The True Meaning of Cloudflare's 521 Error

1.1 What Does the 521 Error Actually Mean?

Cloudflare for 521 Error The definition is:
Cloudflare can receive visitor requests but fails when attempting to establish a TCP connection with the origin server.
This typically means:

• The connection from Cloudflare to the origin server was refused or could not be established.
• The problem occurred when Network / Firewall / Web Service Layer
• DNS resolution itself is functioning normally.

1.2 521 does not equate to "server downtime."

The following situationswill trigger 521::

take	Is it possible?
The server has truly crashed.	be
Firewall blocks Cloudflare IP	be
The web service is not listening on the port.	be
Only direct IP connections are permitted.	be
WAF / Security Plugin Interception	be

The key issue is not whether the server is online, but whether Cloudflare is permitted to connect.

II. The Fundamental Difference Between Orange Clouds and Gray Clouds

2.1 What Are Orange Clouds and Gray Clouds?

exist Cloudflare In DNS, each record has a cloud status:

state of affairs	demonstrate	Actual meaning
Orange Cloud	Proxied	Enable Cloudflare Proxy
Gray Clouds	DNS only	DNS resolution only, direct connection to origin server

It's aSwitch for routing through CloudflareThe

2.2 Access Paths in Orange Cloud State

Access path:Visitor → Cloudflare node → Origin server

At this moment:

• The IP address seen by the origin server is a Cloudflare node.
• The firewall must allow Cloudflare IP ranges.
• The web service must be listening on the port.

2.3 Access Paths in Gray Cloud State

Access path:Visitor → Origin Server (Direct Connection)

At this moment:

• Cloudflare does not participate in forwarding.
• No Cloudflare error pages will appear.
• Nor will a 521 error occur.

III. Why Does 521 Only Appear in the Orange Cloud State?

The reason is very clear:The 521 error occurs when Cloudflare attempts to connect to the origin server.

• Gray Cloud: Cloudflare not connecting to origin server → Impossible 521
• Orange Cloud: Cloudflare must connect to the origin server → If it fails, it returns a 521 error.

Therefore, during troubleshooting, whenever "Access to Gray Cloud is normal; switching to Orange Cloud immediately triggers error 521.In such cases, it can generally be determined that the problem lies inOrigin Server Configuration for Proxy Accessthe service itself, not DNS or Cloudflare.

IV. High Frequency of 521 in the Orange Cloud Context

4.1 Firewall Blocking Cloudflare IPs (Most Common)

show off

• Gray Cloud access is normal.
• Orange Cloud Immediately 521
• server (computer)No requests or only rejection records in the log

proper practice

• Clearance Cloudflare Official IP Segment
• The allowed ports must include at least:
  • 80(HTTP)
  • 443(HTTPS)

Cloudflare IP List is required.Regular synchronizationDo not manually set a fixed IP address.

4.2 Web service is not listening on a public network port

Common mistakes include:

• Listen only 127.0.0.1
• Not monitored 80 / 443
• The web service has not started or has terminated abnormally.

Recommended Approach (General, Safe):

• monitoring 0.0.0.0:80
• monitoring 0.0.0.0:443

4.3 Misunderstanding "Hide Origin Server IP"

The following practicesVery likely to lead to 521::

• Block all non-local IP addresses
• Block overseas IP addresses
• Only allow access from your own IP address

Most Cloudflare nodes are overseas IPs and will be blocked as well.

4.4 Security Plugins / WAF False Positives

including but not limited to:

• Baota Firewall
• System-Level Protection Rules
• Third-party security plug-in

AllRules based on IP, ASN, and countryAll instances require verification whether Cloudflare has been mistakenly blocked.

V. Practical Process for Locating 521 Using Orange Cloud / Gray Cloud

5.1 Step 1: Switch to Gray Cloud for origin server verification

1. Logging in to the Cloudflare Console
2. DNS → Find the corresponding record
3. switch to Grey Cloud (DNS only)
4. Wait 1–2 minutes before accessing the website.

• Accessible → Original site functioning normally
• Unreachable → Issue with the origin server itself

5.2 Step Two: Switch Back to Orange Cloud

1. Cut back Orange Cloud (Proxied)
2. refresh page

If it appears immediately 521The issue can be confirmed to have occurred at Cloudflare → Origin ServerThe

5.3 Step Three: Troubleshoot in Sequence

Recommended Order:

1. Does the firewall allow Cloudflare IP + ports 80/443?
2. Web Service Listening Status
3. Security Plugin / WAF
4. System and Web Logs

VI. Recommendations for Proper Use of Orange Cloud

6.1 Firewall Layer

• Do not directly block overseas IP addresses.
• Not only individual IP addresses are permitted
• Should:
  • Allow Cloudflare IP
  • Entrust your security strategy to Cloudflare

6.2 Web Services Layer

• Monitor public network ports normally
• Do not impose crude restrictions based on source IP addresses.
• utilization CF-Connecting-IP Obtain real visitor IP addresses

6.3 Operational Recommendations

• Gray Cloud is used solely for troubleshooting.
• Long-term use of Orange Cloud in production environments
• After each security policy change, Orange Cloud access must be tested.

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