Nginx not caching requests coming through ELB - caching

Question

I've got nginx sitting between ELBs. I've got a couple application pools behind an ELB that nginx passes traffic back to and I want to cache static content. My problem is nginx doesn't appear to be caching any responses. This is the cache configuration:
proxy_cache_path /usr/share/nginx/cache/app levels=1:2 keys_zone=cms-cache:8m max_size=1000m inactive=600m;
proxy_temp_path /usr/share/nginx/cache/;
location / {
proxy_pass http://cms-pool;
proxy_cache cms-cache;
proxy_cache_valid 200 302 60m;
proxy_cache_valid 404 1m;
}
After some reading I found there might be some headers causing the issue but after hiding the obvious ones I had no luck and ended up breaking the application since I was hiding all the backend cookies. These are the headers I tried removing:
proxy_ignore_headers Cache-Control Expires Set-Cookie;
proxy_hide_header Cache-Control;
proxy_hide_header Set-Cookie;
I'm at a loss right now as to why requests are not being cached, here's an output from curl of the headers that came through with the above header configuration (the cookies and such were set from nginx/elb in front of nginx):
Accept-Ranges: bytes
Connection: keep-alive
Content-length: 6821
Content-Type: text/html
Date: Sun, 16 Nov 2014 19:25:41 GMT
ETag: W/"6821-1415964130000"
Expires: Thu, 01 Jan 1970 00:00:00 GMT
HTTP/1.1 200 OK
Last-Modified: Fri, 14 Nov 2014 11:22:10 GMT
Pragma: no-cache
Server: nginx/1.7.6
Set-Cookie: AWSELB=4BB7AB49169E74EC05060FB9839BD30C2CB1D0E43D90837DC593EB2BA783FB372E90B6F6F575D13C6567102032557C76E00B1F5DB0B520CF929C3B81327C1D259A9EA5C73771C4EA3DB6390EB40484EDF56491135B;PATH=/
Set-Cookie: frontend=CgAAi1Ro+jUDNkZYAwMFAg==; path=/
Update I found that the above wasn't entirely accurate as there's a 302 that directs a user to login which hits another backend that doesn't have static resources, as such the headers above are coming from the login backend. I adjusted the URI to point to just the images but no caching is occurring. I'm using the following location block:
location /app/images {
proxy_pass http://cms-pool/app/images;
proxy_cache cms-cache;
proxy_cache_valid 200 302 60m;
proxy_cache_valid 404 1m;
proxy_ignore_headers Cache-Control Expires Set-Cookie;
proxy_hide_header Cache-Control;
proxy_hide_header Set-Cookie;
}
These are the headers which are coming through now:
Accept-Ranges: bytes
Connection: keep-alive
Content-length: 12700
Content-Type: image/png
Date: Mon, 17 Nov 2014 09:25:38 GMT
ETag: "0cd80ce9afecf1:0"
HTTP/1.1 200 OK
Last-Modified: Wed, 12 Nov 2014 17:05:06 GMT
Server: nginx/1.7.6
Set-Cookie: AWSELB=4BB7AB49169E74EC05060FB9839BD30C2CB1D0E43D638163025E92245C6C6E40197CA48C5B22F3E8FDA53365109BC1C16C808322881855C100D4AC54E5C0EC6CDE91B96151F66369C7B697B04D2C08439274033D81;PATH=/
Set-Cookie: tscl-frontend=CgAK1FRpvxI4b0bQAwMEAg==; path=/
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Answer 1

This is caused by a wobbly implementation of HTTP caching headers in what's behind ELB.
Per RFC 2616 (HTTP 1.1) :
Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the
request/response chain.
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache". No new Pragma directives will be
defined in HTTP.
Note: because the meaning of "Pragma: no-cache as a response
header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response
The Pragma: no-cache header doesn't mean anything in an HTTP reply because it's behaviour is unspecified by RFCs.
But as nginx acts as a (reverse) proxy in your case, it will honor the header as if it was Cache-Control: no-cache header to maintain compatibility with HTTP 1.0 protocol's Pragma header defined in RFC 1945.
It will also pass it through client's response headers as it doesn't have to suppose anything about the actual meaning of it.
So either correct this bad implementation or append Pragma header to proxy_ignore_headers and proxy_hide_header directives.