Squid Parent Proxy

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

Note, with this configuration, if the parent cache fails or becomes unreachable, then every request will result in an error message.

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

In case you want to be able to use direct connections when all the parents go down you should use a different approach:

prefer_direct off

Don't use never_direct as in the first example but instead use prefer_direct