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The command-line examples above suggest a tool based on curl is possible. In fact, a tool that implements HTTP conditional GET can be downloaded from GitHub. It's a bash script called cget that caches the HTTP response header along with the resource. In this way, subsequent requests can provide the appropriate request headers. If the server supports conditional GET, and the resource has not changed since the previous GET (as indicated by HTTP 304), the script accesses the resource from cache.

Let's use the script to illustrate HTTP conditional GET (as we did with curl above). Here's how to fetch and cache a metadata file:

$ echo $MD_LOCATION
http://md.incommon.org/InCommon/InCommon-metadata.xml
$ cget.sh -H $MD_LOCATION
HTTP/1.1 200 OK
Date: Tue, 30 Dec 2014 19:28:30 GMT
Server: Apache
Last-Modified: Mon, 29 Dec 2014 20:24:24 GMT
ETag: "110328-b28945-50b60a9050e00"
Accept-Ranges: bytes
Content-Length: 11700549
Connection: close
Content-Type: application/samlmetadata+xml

Subsequent requests will produce HTTP 304 responses as long as the metadata file does not change:

$ cget.sh -H $MD_LOCATION
HTTP/1.1 304 Not Modified
Date: Tue, 30 Dec 2014 19:29:01 GMT
Server: Apache
Connection: close
ETag: "110328-b28945-50b60a9050e00"

Later versions of Shibboleth (at least IdP 2.2 and SP 2.4) implement HTTP conditional GET (and more) so the above script is not particularly useful unless you're running something other than Shibboleth. For instance, simpleSAMLphp does everything except HTTP conditional GET, so users of simpleSAMLphp might find the above script useful.

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