varnish or squid reverse proxy behind corp proxy - reverse-proxy

Question

How to configure varnish(or if someone can hint me to squid im fine) to cache requests from backend, but connect to backend through http_proxy
So I try to:
backend default {
.host = "10.1.1.1";
.port = "8080";
}
backend corp_proxy {
.host = "proxy";
.port = "8080";
}
sub vcl_recv {
# Happens before we check if we have this in cache already.
#
# Typically you clean up the request here, removing cookies you don't need,
# rewriting the request, etc.
set req.backend_hint = corp_proxy;
set req.url ="http://" + req.http.host + req.url;
}

Answer 1

Varnish (or other web caching proxies) caches a request, based on its cache-related headers (like Cache-Control).
Unfortunately, many web applications don't set these headers correctly. So we should use a more aggressive approach to cache some well-known items, e.g., pictures, .js or .css files.
Moreover, this line set req.url ="http://" + req.http.host + req.url; is not required, because Varnish sends the request, as is, to your designated backend.
Here is my recommended config:
backend corp_proxy {
.host = "proxy";
.port = "8080";
}
sub vcl_recv {
// Determine backend
if ( req.http.host ~ ".example.com" ) {
set req.backend_hint = corp_proxy;
// Determine cacheable items
if( req.url ~ "\.(css|js|jpg|jpeg|png|gif|ico) {
unset req.http.Cookie;
unset req.http.Cache-Control;
}
}
}
sub vcl_backend_response {
if( bereq.http.host ~ ".example.com" ) {
if (bereq.url ~ "\.(css|jsjpg|jpeg|png|gif|ico)") {
set beresp.ttl = 20m; // I opt for 20 minutes of caching by your mileage may vary
}
}