Mírate:
http://www.linuxdoc.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/chap5sec39.html/etc/host.conf
# Lookup names via DNS first then fall back to /etc/hosts.
Order bind, hosts
# We have machines with multiple IP addresses.
multi on
# Check for IP address spoofing.
Nospoof on
The order option indicates the order of services. The sample entry specifies that the resolver library should first consult the name server to resolve a name and then check the /etc/hosts file. It is recommended to set the resolver library to first check the name server, bind and then the hosts file (hosts) for better performance and security on all your servers. Of course you must have the DNS/BIND software installed or this configuration will not work.
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Si te refieres a que no use squid para determinadas direcciones, entonces mírate el manual que yo no sé cómo configurarlo:
http://squid.visolve.com/squid24s1/contents.htmSuerte.