influencing beresp.cacheable on the backend

[David Birdsong]

How can I influence bereps.cacheable in a backend such that it will
evaluate to False?

I set Expires and Cache-Control in the backend, this is what the
backend generates:
HTTP/1.1 200 OK
Server: nginx/0.7.65
Date: Mon, 12 Apr 2010 21:48:25 GMT
Content-Type: image/gif
Content-Length: 43
Last-Modified: Mon, 28 Sep 1970 06:00:00 GMT
Connection: close
Expires: Thu, 01 Jan 1970 00:00:01 GMT
Cache-Control: no-cache

>From the man page:
"""
A response is considered cacheable if it is valid (see above), the
HTTP status code is 200, 203, 300, 301, 302, 404 or 410 and it has a
non-zero time-to-live when Expires and Cache-Control headers are taken
into account.
"""

And yet this object is getting cached in varnish.  Here is my vcl_fetch:


sub vcl_fetch {
  if (beresp.http.Set-Cookie) {
    unset beresp.http.Set-Cookie;
  }

  if (beresp.cacheable) {
    unset beresp.http.expires;
    set beresp.ttl = 1h;

    if (beresp.status >= 300 && beresp.status <= 399) {
      set beresp.ttl = 10m;
    }

    if (beresp.status  >= 399) {
      set beresp.ttl = 0s;
    }
  }

  remove beresp.http.X-Varnish-IP;
  remove beresp.http.X-Varnish-Port;
}