We've got an upstream returning X-Accel-Redirect responses to an internal location (which itself proxies to another upstream). Something like:

upstream fileserver {
server foo;
}

upstream backend {
server bar;
}

server {
location /filestore {
internal;
proxy_cache filestore_cache;
proxy_pass http://fileserver;
}

# responses from backend include header:
# X-Accel-Redirect: /filestore/<pathname>
location / {
proxy_set_header Host $host;
proxy_pass http://backend;
}
}

Both upstreams return Last-Modified and ETag headers. We want to keep the ETag and Last-Modified from the original backend (not the fileserver), so we changed the X-Accel-Redirect upstream to:

location /filestore {
internal;

set $first_etag $upstream_http_etag;
set $first_last_modified $upstream_http_last_modified;
add_header ETag $first_etag;
add_header Last-Modified $first_last_modified;

proxy_cache filestore_cache;
proxy_pass http://fileserver;
}

This appears to work OK, because a response returns the ETag and Last-Modified we'd expect:
ETag: "ce79a35523521ae64290db620e1073b1ea145497"
Expires: Fri, 09 Oct 2015 05:46:35 GMT
Last-Modified: Fri, 17 Jan 2014 05:20:40 GMT

But if we send a matching:
If-None-Match: "ce79a35523521ae64290db620e1073b1ea145497"
or:
If-Modified-Since: Fri, 17 Jan 2014 05:20:40 GMT
request header, then we get a HTTP 200 response, not a 304 Not Modified as we'd hope for.

I assume that this is because we're relying on the ETag and Last-Modified being set from temporary variables rather than the ones from the fileserver upstream. Is there any way we can work around this to pass through the ETag and Last-Modified from the backend sending X-Accel-Redirect responses whilst still supporting conditional requests and 304 responses?

Thanks,
Tom

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