527 Railgun Error (retired)

Cloudflare (retired): an error between Cloudflare and the origin's Railgun listener.

527 at a glance

Code: 527
Name: Railgun Error (retired)
Class: 5xx Server error
Status: Non-standard used by Cloudflare
Documented by: Cloudflare
Cacheable by default: No by default. Caches only store responses with unrecognised status codes if explicit freshness (Cache-Control max-age) is given.

What 527 means

527 was returned when Cloudflare's Railgun WAN-optimisation product could not connect to or got an error from the Railgun listener next to the origin. Cloudflare retired Railgun in 2024, so new occurrences should not happen; if you still see 527, the zone or documentation is outdated.

What typically causes a 527

Legacy Railgun deployments (listener down, port blocked, TLS mismatch). No longer applicable after Railgun's retirement.

Fix it on the server

Remove any leftover Railgun configuration; use Argo Smart Routing or Cloudflare Tunnel for similar benefits.

Fix it as a client or visitor

Report it to the site owner.

Example response

HTTP/1.1 527
Server: cloudflare
Content-Type: text/html