Instalar o modulo:
apt-get install libpam-mysql

após editar vim /etc/pam.d/squid
E incluir
#%PAM-1.0
auth sufficient /lib/security/pam_mysql.so user=root passwd=senhamysql host=localhost db=squid table=squidauth usercolumn=username passwdcolumn=password crypt=0 sqllog=0
account required /lib/security/pam_mysql.so user=root passwd=senhamysql host=localhost db=squid table=squidauth usercolumn=username passwdcolumn=password crypt=0 sqllog=0

Após criar a tabela squid no mysql (create table squid) e incluir:

SET SQL_MODE=”NO_AUTO_VALUE_ON_ZERO”;
CREATE TABLE IF NOT EXISTS `squidauth` (
`username` varchar(255) NOT NULL default ”,
`password` varchar(255) NOT NULL default ”,
`name` varchar(255) NOT NULL default ”,
`created` datetime NOT NULL default ‘0000-00-00 00:00:00’,
`modified` datetime NOT NULL default ‘0000-00-00 00:00:00’,
`active` tinyint(1) NOT NULL default ‘1’,
PRIMARY KEY (`username`),
KEY `username` (`username`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 COMMENT=’Virtual Mailboxes’;
INSERT INTO `squidauth` (`username`, `password`, `name`, `created`, `modified`, `active`) VALUES
(‘ulisses’, ‘123345’, ‘ulisses’, ‘0000-00-00 00:00:00’, ‘0000-00-00 00:00:00’, 1);