[squid-users] Limitting particular group to specific sites (not working perfectly)

In my squid.conf, i am trying to grant access ONLY to a set of predefined sites for a group of users (those who are member of limitedsurfers). They are not allowed to access any other thing from the Internet. The following is the acl which i created
 
All my other rules are working perfectly...

Squid Version: 2.6 Stable
OS: CentOS 5.2

First from authentication rule;
auth_param basic program /usr/lib/squid/pam_auth
external_acl_type unix_group %LOGIN /usr/lib/squid/squid_unix_group
acl limited_surfers_acl external unix_group limitedsurfers

Then the particular acl;
acl limited_sites dstdomain "/etc/squid/include-files/limited_site.squid"
http_access allow limited_surfers_acl limited_sites
http_access deny limited_surfers_acl
deny_info ERR_LIMITED_SURFERS limited_surfers_acl

Now the situation is;
It is perfectly granting access to the sited listed in the limited_site.squid file

But when i try to access some other site, it will ask the username/password for 3 times (even when we give the correct username/pass) then only it is denying the request.

Why it is happening so?

I have almost the same kind of rule like this for a particular list of IP's instead of users. That is working perfect for allowing and denying.

Can anybody help me in this case...

Thanks in Advance...

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