Install with:
  apt-get install squid

The external authentication program that authenticates your proxy users against LDAP is now available as a module in the Squid package.

(parts of) /etc/squid.conf
  ....
  http_port proxy.intra.example.com:3128
  ....
  auth_param basic program /usr/lib/squid/ldap_auth -b ou=People,dc=intra,dc=example,dc=com -h ldap.intra.example.com -f "(&(objectClass=posixAccount)(uid=%s))" 
  auth_param basic realm Squid proxy-caching web server
  auth_param basic children 30
  auth_param basic credentialsttl 2 hours
  ....
  acl SquidLdapUsers proxy_auth REQUIRED
  # You may put the line above just before the line
  "acl CONNECT method CONNECT" in the squid.conf file.
  ....
  http_access allow SquidLdapUsers
  # You might put the line above just below the line:
  # INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
  ....


You can check the authentication program by hand:

Just type on the prompt:
  /usr/lib/squid/ldap_auth -b ou=People,dc=intra,dc=example,dc=com 192.168.1.12

Now input "username<SPACE>password<ENTER>" pairs. The authentication module
will answer back with OK or ERR. End the session with Control-D.


Start the proxyserver
  /etc/init.d/squid start