forcing cache under original url

[David Birdsong]

I'm trying to wrap my brain around the VCL needed to do what I'm
thinking of, but it'd be helpful to know up front if what I'm trying
to achieve isn't possible.

I'm hoping to provide a backend of last resort for certain types of
requests for the cases of
1. an actual down backend
2. where obj.status != 200 && req.request == "GET" && req.url ~
"\.(gif|jpg|swf|css|js|png|jpg|jpeg|gif|png|tiff|tif|svg|swf|ico|css|js|vsd|doc|ppt|pps|xls|pdf|mp3|mp4|m4a|ogg|mov|avi|wmv|sxw|zip|gz|bz2|tgz|tar|rar|odc|odb|odf|odg|odi|odp|ods|odt|sxc|sxd|sxi|sxw|dmg|torrent|deb|msi|iso|rpm)

#2 means i can't rely on probes to set my backend to the backend of last resort.

here's the pertinent vcl vcl_fetch

sub vcl_fetch {
        # force minimum ttl of 300 seconds
        if (obj.ttl <  604800s) {
                set obj.ttl = 604800s;
        }
        if (obj.status != 200)
          if (req.restarts > 0) {
            deliver;
          }
          if (req.request == "GET" && req.url ~
"\.(gif|jpg|swf|css|js|png|jpg|jpeg|gif|png|tiff|tif|svg|swf|ico|css|js|vsd|doc|ppt|pps|xls|pdf|mp3|mp4|m4a|ogg|mov|avi|wmv|sxw|zip|gz|bz2|tgz|tar|rar|odc|odb|odf|odg|odi|odp|ods|odt|sxc|sxd|sxi|sxw|dmg|torrent|deb|msi|iso|rpm)$")
{
            restart;
          } else {
            deliver;
          }
        }
}

sub vcl_recv {
        if (req.restarts == 0) {
          if (req.http.host ~ "^img3\.(.*)$") {
                  set req.backend = b0;
           } else {
                  error 750 "invalid host header";
          }
        }
 if (req.request == "GET" && req.url ~
"\.(gif|jpg|swf|css|js|png|jpg|jpeg|gif|png|tiff|tif|svg|swf|ico|css|js|vsd|doc|ppt|pps|xls|pdf|mp3|mp4|m4a|ogg|mov|avi|wmv|sxw|zip|gz|bz2|tgz|tar|rar|odc|odb|odf|odg|odi|odp|ods|odt|sxc|sxd|sxi|sxw|dmg|torrent|deb|msi|iso|rpm)$")
{
          if (! req.backend.healthy && req.restarts == 0) {
            set req.backend = last_resort_backend;
          }
            lookup;
        }
       lookup;
}

I'm able to get varnish to re-send a failed request to
last_resort_backend.  I'm still working on the necessary VCL to follow
the 2 redirects that the last_resort_backend will send -I'd like
varnish to complete these and not pass them on to the client.

 What I'd like to do is instruct it to cache the static file under the
original url, not the url of the last 302.

My inexperience using varnish has me thinking about having to set the
req.url for first the 301 and then again for the 302 combined with
restarts, but this made me think that I'd have to store the original
url somewhere and set it again.  Is this type of thing possible?

Apologies if this is not clear.