Nginx - Rate limit when origin server response code is 401 - node.js

Question

I would like nginx to rate limit by user-ip when the origin server responds with a 401 status code. How would I go about this. I already have a limit_req_zone setup for normal API calls which looks something like this: limit_req_zone $binary_remote_addr zone=api:10m rate=5r/s; but I would like to further rate limit offenders that make unauthorized calls to my API end-points.
Edit:
I did try mapping the response status 401 to ip addresses and rate-limit based on the mapped variable but that doesn't seem to do anything. See code below.
map $status $limit {
default '';
401 $binary_remote_addr;
}
limit_req_zone $limit zone=api:10m rate=5r/s;
location /api {
limit_req zone=api burst=5;
...
}

Answer 1

This is quite tricky because the $status variable is empty when declaring the limit_req_zone. The $status is only known after nginx has processed the request. For example after a proxy_pass directive.
The closest I could get to achieve rate limiting by status is doing the following:
...
...
...
limit_req_zone $binary_remote_addr zone=api:10m rate=5r/s;
...
...
...
server {
location /mylocation {
proxy_intercept_errors on;
proxy_pass http://example.org;
error_page 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 421 422 423 424 426 428 429 431 451 500 501 502 503 504 505 506 507 508 510 511 #custom_error;
}
location #custom_error {
limit_req zone=api burst=5 nodelay;
return <some_error_code>;
}
}
...
The drawback is that this way you must return a different status code then the proxy pass response.