Apache reverse proxy not preserving headers

Question

Does anyone know if there is something strange going on? It seems that my reverse proxy setup, doesn't preserve the correct caching headers from the original virtualhost.

Let me explain...

I am currently using PHP Slim to create an API for my web app. It has its own caching mechanisms, which are setup and running fine. So, when visiting the resource api.example.com/info for example, I have a Last-Modified that is set, which allows it to be cached (it doesn't change very often).

As such, the headers looks something like:

Request URL:http://api.example.com/info
Request Method:GET
Status Code:304 Not Modified

Request Headers

Accept:text/html,application/xhtml xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
Cache-Control:max-age=0
Connection:keep-alive
Host:api.example.com
If-Modified-Since:Mon, 04 Oct 2010 08:00:52  1100
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.56 Safari/536.5

Response Headers

Connection:Keep-Alive
Date:Tue, 19 Jun 2012 22:34:45 GMT
Keep-Alive:timeout=5, max=100
Server:Apache/2.2.21 (Win64) PHP/5.3.8
Vary:Accept-Encoding

Which shows that the resource returns with a 304 - it's been retrieved from cache. Great!

However, the web app accesses this from a reverse proxy (cross domain issues). When accessing the exact same resource, this time from app.example.com/api/info, it returns with a 200 code. Its also noteably slower to load, which makes me think something isn't working.

This time, the headers are

Request URL:http://app.example.com/api/info
Request Method:GET
Status Code:200 OK

Request Headers

Accept:text/html,application/xhtml xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
Cache-Control:max-age=0
Connection:keep-alive
Host:app.example.com
If-Modified-Since:Mon, 04 Oct 2010 08:00:52 GMT
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.56 Safari/536.5

Response Headers

Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:1959
Content-Type:application/json
Date:Tue, 19 Jun 2012 22:41:01 GMT
Keep-Alive:timeout=5, max=100
Last-Modified:Mon, 04 Oct 2010 08:00:52 GMT
Server:Apache/2.2.21 (Win64) PHP/5.3.8
Vary:Accept-Encoding
X-API-Version:v1.0
X-Powered-By:PHP/5.3.8, Slim

Any ideas? Sorry about the mass of header info above - didn't want to leave out any information.

Answer 1

When i compare your both example request, I see some more differeneces between them.

e.g. the second response (throught the proxy) includes an

Content-Encoding:gzip

Line. Maybe your reverse proxy adds an gzip compression filter before delivering the results. What could mean: you primary resource did not change and but the proxy changes the content by compressing it, and so it labels this as "new content".

Another difference is in your request: in your first request you send:

If-Modified-Since:Mon, 04 Oct 2010 08:00:52 +1100

but in your second request the timeoffset +1100 is missing, so you`re sending another modification time.