Something

anc-proxy-01:~# cat /etc/approx/approx.conf 
...
debian          http://ftp.us.debian.org/debian
debian-security http://security.debian.org
ubuntu          http://archive.ubuntu.com/ubuntu
ubuntu-security http://security.ubuntu.com/ubuntu
...

anc-proxy-01:~# cat /etc/squid/squid.conf
...
url_rewrite_program /usr/lib/squid/jesred
...

anc-proxy-01:~# cat /etc/jesred.acl
...
# Uncomment the following to rewrite all URLs from all sources:
0.0.0.0/0
...

anc-proxy-01:~# cat /etc/jesred.rules 
...
#Debian
regex ^http://ftp.us.debian.org/debian/(.*)\.(.*) http://192.168.50.3:9999/debian/\1.\2
regex ^http://security.debian.org/(.*)\.(.*) http://192.168.50.3:9999/debian-security/\1.\2
#Ubuntu
regex ^http://archive.ubuntu.com/ubuntu/(.*)\.(.*) http://192.168.50.3:9999/ubuntu/\1.\2
regex ^http://security.ubuntu.com/ubuntu/(.*)\.(.*) http://192.168.50.3:9999/ubuntu-security/\1.\2
...