Squid Parent Proxy
From KlavoWiki
To enable Squid to upstream edit the file squid.conf with the following setting.
cache_peer proxy.some-isp.com parent 8080 0 no-query no-digest never_direct allow all
To enable Squid to upstream to multiple proxies in order.
cache_peer proxy.isp1.com parent 8080 0 no-query weight=1 cache_peer proxy.isp2.com parent 8080 0 no-query weight=2