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Date: Mon, 23 Jul 2012 15:28:14 +0100
From: Glen Birnie <glen.birnie@bbc.co.uk>
To: <users@httpd.apache.org>
Message-ID: <CC331E0E.9B02%glen.birnie@bbc.co.uk>
Thread-Topic: httpd 2.2.21: mod_cache incorrectly returns HTTP 200 for
 nonexistent reverse-proxied content
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Subject: [users@httpd] httpd 2.2.21: mod_cache incorrectly returns HTTP 200
 for nonexistent
 reverse-proxied content

We have seen an issue in httpd 2.2.21 where mod_cache incorrectly returns
HTTP 200 responses instead of HTTP 404 where:

- Reverse proxying is configured.
- ProxyErrorOverride is set to On.
- mod_disk_cache is set up to cache content proxied from mod_proxy.
- A local ErrorDocument 404 URL is configured.
- The error document URL is requested and the contents cached by
mod_disk_cache.
- A nonexistent URL is routed to the proxy destination, the destination
proxy returns a 404, but mod_cache returns the configured ErrorDocument
content with an incorrect 200 response.

We have done some searching of the list archives and bug database but have
not seen reports of any similar problems.

---

Steps to reproduce:

- Create an error image and write to the document root (we assume URI
/error.jpg in the examples below).

- Setup a mod_disk_cache directory (we assume
/data/httpcache/httpd/cache-root) with the necessary permissions.

- Use configuration akin to the following (set "proxy.destination" /
"web.proxy" as appropriate):

<IfModule !mod_cache.c>
LoadModule cache_module modules/mod_cache.so
</IfModule>

<IfModule !mod_disk_cache.c>
LoadModule disk_cache_module modules/mod_disk_cache.so
</IfModule>

<IfModule !mod_rewrite.c>
LoadModule rewrite_module modules/mod_rewrite.so
</IfModule>

<IfModule !mod_proxy.c>
LoadModule proxy_module modules/mod_proxy.so
</IfModule>

<IfModule mod_cache.c>
  <IfModule mod_disk_cache.c>
    CacheRoot /data/httpcache/httpd/cache-root
    CacheDirLevels 3
    CacheDirLength 1
    CacheMinFileSize 0
    CacheMaxFileSize 1000000
  </IfModule>
</IfModule>

CacheEnable disk /

ProxyErrorOverride On
ProxyPassReverse / http://proxy.destination/

# Configure this if the destination server resides behind a proxy
#ProxyRemote http://proxy.destination/ http://web.proxy:80

<Location /proxystuff/>
  RewriteEngine On
  RewriteRule /proxystuff/error$ /error.jpg [L]
  RewriteRule /proxystuff/get http://proxy.destination%{REQUEST_URI} [P,L]

  # This image should reside in the document root,
  # i.e. return a HTTP 200 when requested
  ErrorDocument 404 /error.jpg
</Location>


- Request the error image e.g.:

curl -Lsv -H 'Pragma: ' -o /dev/null http://localhost/error.jpg

This should get written into the cache:

htcacheclean -a -D -p /data/httpcache/httpd/cache-root


- Request a nonexistent proxied URL e.g.:

curl -Lsv -H 'Pragma: ' -o /dev/null http://localhost/proxystuff/get

An incorrect HTTP 200 response is returned instead of the expected HTTP 404.
Note this does not happen if the CacheEnable directive is disabled.

It appears that mod_cache is returning a cached 404 errordocument response
and assuming that (because this exists) a 200 should be issued, rather than
forwarding the 404 response from the destination proxy server.


Glen


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