[squid-users] please help me to organize hard scheme on squid

hello, squid-users.

i want make two parent proxy on my squid. first (main) proxy have some
limitations on content, which returns 403 (forbidden) on the request. this is
filters by url, concrete filters lists is unknown. second proxy have no
limitation, but cost of this (second) proxy very higher that cost of main
proxy. i want to configure squid work by next algorithm:
0. my squid receive request from my user;
1. my squid sends this request to main proxy;
2. if this request is ok, user will receive data;
3. if main proxy returns 403 - forbidden, then my squid will try to
"reorganize" url of request by my script and will try again;
4. if request will receive 403 again, then my squid must make this request to
the second proxy.
4a. as a variant: if request will receive 403 again, then my squid must
download required data directly, BUT not via default route, by using alias of
one of interfaces.

default route is static and follows to main proxy, second proxy have other
static route for one ip address only.

how i can make this configuration on my squid? changes of squid source code is
undesirable, but possible. just say what files i must patch by hands.

my system: FreeBSD 4.7-STABLE, squid-2.4_9 from ports.

-- 
Dmitry Sukhodoev, network administrator of bingo.ru, icq#550315