HTTP 101 Switching Protocols vs 520 Web Server Returned an Unknown Error
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HTTP 101 (Switching Protocols) is a 1xx Informational response, while 520 (Web Server Returned an Unknown Error) is a 5xx Server Error response. 101 indicates that the server understands the Upgrade header field request and indicates which protocol it is switching to. In contrast, 520 means that cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
Description
The server understands the Upgrade header field request and indicates which protocol it is switching to.
When You See It
When upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
How to Fix
This is normal behavior during protocol upgrades. Ensure your client supports the target protocol.
Description
Cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
When You See It
When Cloudflare receives an empty, unknown, or malformed response from the origin.
How to Fix
Check origin server logs. Ensure responses include proper headers. Verify SSL configuration.
Key Differences
101 is a 1xx Informational response, while 520 is a 5xx Server Error response.
HTTP 101: The server understands the Upgrade header field request and indicates which protocol it is switching to.
HTTP 520: Cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
You encounter 101 when when upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
You encounter 520 when when Cloudflare receives an empty, unknown, or malformed response from the origin.
When to Use Which
For 101 (Switching Protocols): This is normal behavior during protocol upgrades. Ensure your client supports the target protocol. For 520 (Web Server Returned an Unknown Error): Check origin server logs. Ensure responses include proper headers. Verify SSL configuration.