HTML-style rate limit responses

Issuance Tech
1

Hi all,

We’ve gotten some reports from people who have gotten an odd style of 429 error message from Let’s Encrypt recently, related to the “Overall Requests” limits described at https://letsencrypt.org/docs/rate-limits/. Those limits are implemented using Nginx’s limit_req. There were two specific problems:

  1. We were serving HTML, when ACME clients (rightly) expect JSON.
  2. We were serving 429s to clients that stayed below the documented limits.

To fix (2), we recently made two changes. We removed the “nodelay” parameter from limit_req, so Nginx will queue a small number of requests rather than rejecting them immediately. And we increased the “burst” parameter from 5 to 40. After making these changes, the number of Nginx-style 429s we serve dropped dramatically.

We are also working on a config change to fix (1) and return JSON for these errors. Thanks to everyone who reported!

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