We're having the same problem:

upstream sent too big header while reading response header from upstream

Where:

upstream: "passenger:unix:/passenger_helper_server:"

I tried every one of proxy_buffers, proxy_busy_buffers_size, proxy_buffer_size, fastcgi_buffers, fastcgi_busy_buffers_size, fastcgi_buffer_size, large_client_header_buffers.

Isn't this error message coming from nginx, complaining about the size of the header returned from passenger? Given that, surely there's some way I can tell nginx to allow a larger header size?

Any suggestions?

Cheers,
David.