HTTP 429 Too Many Requests vs 521 Web Server Is Down
HTTP 429 (Too Many Requests) is a 4xx Client Error response, while 521 (Web Server Is Down) is a 5xx Server Error response. 429 indicates that the user has sent too many requests in a given time (rate limiting). The response should include a Retry-After header. In contrast, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The user has sent too many requests in a given time (rate limiting). The response should include a Retry-After header.
When You See It
When hitting API rate limits or making too many requests too quickly.
How to Fix
Check the Retry-After header. Implement exponential backoff. Consider caching responses.
Description
Cloudflare-specific. The origin server refused the connection.
When You See It
When Cloudflare can't establish a TCP connection to the origin server.
How to Fix
Check that the origin server is running and the firewall allows Cloudflare IPs.
Key Differences
429 is a 4xx Client Error response, while 521 is a 5xx Server Error response.
HTTP 429: The user has sent too many requests in a given time (rate limiting). The response should include a Retry-After header.
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 429 when when hitting API rate limits or making too many requests too quickly.
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 429 (Too Many Requests): Check the Retry-After header. Implement exponential backoff. Consider caching responses. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.