as i know squid listents to all interfaces by the defaults. u must define clients (acl) and allow them to access internet. squid allow only localhost by the default.
do u ask differencies between M$ ISA and squid?
squid is a only cache proxy server. it is not complex as ISA.
if u wanna do like M$ ISA configuration, i can advice u to use squid in transparent mode. so u can configure iptables and squid together. also u wont have to configure browsers of clients.
here is necessary lines for squid.conf for transparent proxy:
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
here is iptables redirect command for transparent proxy:
iptables -t nat -A PREROUTING -s local_net/subnet -p tcp --dport 80 -j REDIRECT --to-port 3128
by the default squid listents to port 3128.
all http requests come from local network will be redirected to the 3128 port of linux. squid will check only http requests in transparent modes. other protocols must be controlled by iptables.
and another advice. use squidGuard to block addware warez porn and etc... it works well here.
www.squid-cache.org
www.squidguard.org
www.netfilter.org