Redirecting to the next proxy (PAC files)

[Robert Shilston <robert@xxxxxxxxxxxx>]

Hi,

Is there a way to configure Squid or any other HTTP proxy to reject 
requests such that browsers then use the next proxy that's been supplied 
in their PAC file?  It looks like HTTP response 306 would have been 
perfect, except that browsers don't implement it.

Firefox and IE seem to hop proxy if the proxy replies with an invalid 
HTTP response, but Safari doesn't (it seems to try the primary proxy 
server for every request, and fail to the second if the first doesn't 
open the connection request).  And, it's also a bit nasty to use invalid 
protocol responses to trigger legitimate behaviour. 

So, can anyone suggest how I can persuade a browser to hop to the next 
proxy server?

If you're wondering why I want to do this, it's because the primary 
proxy server will perform some form of non-critical authentication / 
user validation, and the second proxy server is DIRECT.  So, once the 
user is validated, the server would hand them off to run freely directly 
over the internet.

Thanks,


Rob