I am running httpd 2.2.3 with mod_cache in reverse proxy mode. The content being
cached has a max-age of 0 so the cache is allowed to store it but must always
validate it with the origin. If one does an unconditional HEAD on a piece of
content that has been cached on the edge, you get a "HTTP 304 Not Modified"
instead of a 200.

Here is the headers from the origin in response to a first time request,

HTTP/1.1 200 OK
Date: Thu, 21 Dec 2006 21:05:11 GMT
Server: Apache
Last-Modified: Fri, 01 Dec 2006 22:35:51 GMT
ETag: "4809a-4f-9f42dfc0"
Accept-Ranges: bytes
Content-Length: 79
Cache-Control: max-age=0, no-transform
Expires: Thu, 21 Dec 2006 21:06:39 GMT
svct: D=27819 usec
Content-Type: text/html
Connection: close

Below is the HEAD request followed by the reply.

HEAD /testing/basam/test.html HTTP/1.1
Host: edge.stream.aol.com

HTTP/1.1 304 Not Modified
Date: Thu, 21 Dec 2006 21:06:28 GMT
Server: Apache
Connection: close
ETag: "4809a-4f-9f42dfc0"
Expires: Thu, 21 Dec 2006 21:07:56 GMT
Cache-Control: max-age=0

Below is the debug output from apache for the HEAD request.

[Thu Dec 21 21:06:32 2006] [debug] mod_disk_cache.c(477): disk_cache: Recalled
cached URL info header http://edge.stream.aol.com:80/testing/basam/test.html?
[Thu Dec 21 21:06:32 2006] [debug] mod_disk_cache.c(750): disk_cache: Recalled
headers for URL http://edge.stream.aol.com:80/testing/basam/test.html?
[Thu Dec 21 21:06:32 2006] [debug] cache_storage.c(272): Cached response for
/testing/basam/test.html isn't fresh.  Adding/replacing conditional request headers.
[Thu Dec 21 21:06:32 2006] [debug] mod_cache.c(129): Adding CACHE_SAVE filter
for /testing/basam/test.html
[Thu Dec 21 21:06:32 2006] [debug] mod_cache.c(136): Adding CACHE_REMOVE_URL
filter for /testing/basam/test.html
[Thu Dec 21 21:06:32 2006] [debug] mod_proxy_http.c(54): proxy: HTTP:
canonicalising URL //origin.stream.aol.com/testing/basam/test.html
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(1378): [client 127.0.0.1] proxy:
http: found worker http://origin.stream.aol.com/ for
http://origin.stream.aol.com/testing/basam/test.html [Thu Dec 21 21:06:32 2006]
[debug] mod_proxy.c(756): Running scheme http handler (attempt 0)
[Thu Dec 21 21:06:32 2006] [debug] mod_proxy_http.c(1662): proxy: HTTP: serving
URL http://origin.stream.aol.com/testing/basam/test.html
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(1798): proxy: HTTP: has acquired
connection for (origin.stream.aol.com)
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(1858): proxy: connecting
http://origin.stream.aol.com/testing/basam/test.html to origin.stream.aol.com:80
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(1951): proxy: connected
/testing/basam/test.html to origin.stream.aol.com:80
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(2045): proxy: HTTP: fam 2 socket
created to connect to origin.stream.aol.com
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(2141): proxy: HTTP: connection
complete to 10.1.10.15:80 (origin.stream.aol.com)
[Thu Dec 21 21:06:32 2006] [debug] mod_proxy_http.c(1541): proxy: header only
[Thu Dec 21 21:06:32 2006] [debug] mod_cache.c(502): cache:
/testing/basam/test.html not cached. Reason: HTTP HEAD request
[Thu Dec 21 21:06:32 2006] [debug] proxy_util.c(1816): proxy: HTTP: has released
connection for (origin.stream.aol.com)