Web Filtering On Squid 3 With QuintoLabs Content Security 1.4 And Windows Active Directory Integration

This HOWTO will show you how to set up a Squid proxy server deployed on CentOS or RedHat 6 Linux with web and content filtering done by QuintoLabs Content Security with proxy users transparently authenticated by Windows 2008 R2 based Active Directory. This is the work in progress and all comments are welcomed. The HOWTO is targeted at novice users and may sometimes seem too thorough for more advanced gurus. No compilation magic will be involved in our setup so any system administrator accustomed to Windows will be able to easily follow the instructions. :)

We will use VMWare's Workstation 8 to setup the staging test lab to make sure everything works before deploying it on the production servers and real hardware. The chosen virtualization platform does not really matter much and any other virtualization solution can be successfully used too.



The test lab will contain three machines - a domain controller running Microsoft Windows 2008 R2 server, a user machine running Windows 7 and a proxy machine running CentOS 6 with Squid Proxy 3 coming from standard repository. I presume you have ISOs of Windows Server R2, Windows 7 and RedHat (CentOS) 6 Linuxes somewhere around you. I also presume installation of Microsoft Windows and Active Directory is not a problem for you.

The domain we are creating is called example.lan. The IP addresses in use come from standard network used in NAT networking in VMWare Workstation - Name of the Domain Controller will be dc.example.lan, proxy will be called proxy.example.lan and client workstation client.example.lan.


Step 1. Install Domain Controller

First, perform basic installation of Microsoft Windows and do the initial post install configuration.

  1. Create a new virtual machine for domain controller. Put the Windows Server ISO into the virtual CD drive and install it. Set the Administrator's password to "[email protected]" (without quotes).

  2. After installation finishes set the static IP address of the server to, subnet mask, default gateway Set the preferred DNS server to Start the web browser and navigate to your favorite web site to test that Internet connection is working properly.

  3. Set the time zone on dc.example.lan to the one you live in.

  4. Change the name of the computer to "dc" (without quotes) so that after installation of the Active Directory and DNS server later our domain controller has the FQDN address of dc.example.lan.

Now install Active Directory.

  1. Click Start - Administrative Tools - Server Manager and run the Roles Wizard to add the "Active Directory Domain Services" role.

  2. When role wizard finishes, open the command prompt and run "dcpromo.exe" to set up the New Domain in a New Forest. Specify example.lan as the FQDN name of the forest root domain. Leave Forest and Domain functional levels on "Windows Server 2003", mark DNS server as the additional option for this domain controller. When installation wizard complains about "A delegation for this DNS server cannot be created because the authoritative parent zone cannot be found or it does not run Windows DNS server" just say Yes to continue the installation and install a new instance of DNS server on this domain controller. Wait a little until installation is finished. Then reboot the VM.

Now set up static IP address information for dc.example.lan and proxy.example.lan hosts.

  1. Using DNS management snap-in at dc.example.lan create a primary IPv4 Reverse DNS Lookup zone for network id 192.168.28, set Replication Scope "to all DNS servers running on domain controllers in this domain: example.lan" and allow both nonsecure and secure dynamic updates.

  2. Add new pointer (PTR) record for the dc.example.lan and check using nslookup that dc.example.lan can be successfully resolved into its IP address ( and vice versa.

  3. Add Host (A) and Pointer (PTR) records for the proxy.example.lan and check using nslookup that proxy.example.lan can be successfully resolved into its IP address ( and vice versa.


Step 2. Install Windows 7 Client Machine

Now create a VM that will act as a machine for the users of the proxy. Give VM a name - client.example.lan, set up Windows 7 as you normally do, join the machine to the example.lan domain created on Step 1 and make a bunch of domain users that will act as proxy clients.


Step 3. Install CentOS Proxy Machine

Create a new virtual machine named proxy.example.lan. Ensure the network adapter is set to "NAT" mode. Start the VM and follow the steps of the CentOS install wizard mostly accepting the defaults. Configure machine hostname as "proxy.example.lan" and root password as "[email protected]" (without quotation marks). Wait a little until the installation is complete and then reboot the system.

NOTE 1: For the purpose of this howto I have SELinux explicitly disabled in /etc/selinux/config.

NOTE 2: For the purpose of this howto I have firewall explicitly disabled by typing #chkconfig iptables off and #chkconfig ip6tables off in the root terminal and restarting the proxy.example.lan VM.


Step 3.1 - Configure Networking

CentOS 6 deployed as VM usually does not have network subsystem enabled by default. In order to enable networking we need to do the following:

  1. Set the static IP address in /etc/sysconfig/network-scripts/ifcfg-eth0

  2. Set the default gateway in /etc/sysconfig/network

  3. Point to the DNS server at dc.example.lan by editing /etc/resolv.conf


Restart your network subsystem by

# /etc/init.d/network restart 

or by just restarting the virtual machine. After restart confirm that the network functions correctly by typing the following commands in the terminal and watching for any error outputs

$ping -c 3

Finally update the VM

# yum update 

and install needed prerequisites for the next steps:

# yum install bind-utils


Step 3.2 - Configure Network Time Synchronization (NTP)

To perform successful Kerberos authentication system time on proxy.domain.lan must be synchronized with system time on dc.example.lan. The easiest way to do that is to install network synchronization server and point it to the domain controller.

  1. Install NTP server:

    # yum install ntp
  2. Make it start automatically at system boot time

    #chkconfig ntpd on
  3. Open /etc/ntp.conf and add the name of the domain controller:

  4. To perform initial time sync stop the service

    #service ntpd stop
    and run the manual sync command
    #ntpdate -b dc.example.lan
    Then start the NTP service again
    #service ntpd start

NOTE: If you get "Clock skew too great while getting initial credentials" later while running the kinit utility then the sync was probably not successful and you are advised to check that the server name in /etc/ntp.conf is correct and restart the ntpd service. Log files at /var/log/messages may contain more information about the reason of unsuccessful synchronization.


Step 3.3 - Install Kerberos

All needed Kerberos packages are installed by default in CentOS. But to ensure you really have all the needed Kerberos packages on the proxy.example.lan type

# yum install krb5-workstation krb5-libs

Kerberos configuration is stored in /etc/krb5.conf, open it with the text editor and change the contents to the following:

 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

 default_realm = EXAMPLE.LAN
 dns_lookup_realm = false
 dns_lookup_kdc = false
 ticket_lifetime = 24h
 renew_lifetime = 7d
 forwardable = true
 default_tgs_enctypes = rc4-hmac
 default_tkt_enctypes = rc4-hmac
 permitted_enctypes = rc4-hmac

  kdc = dc.example.lan
  admin_server = dc.example.lan
  default_domain = example.lan

 .example.lan = EXAMPLE.LAN
 example.lan = EXAMPLE.LAN

To test that Kerberos authentication is set up correctly type in the root terminal

# kinit [email protected]

The command should complete without errors. The command

# klist 

should print the info about acquired Kerberos ticket.

Ticket cache: FILE:/tmp/krb5cc_0
Default principal: [email protected]

Valid starting     Expires            Service principal
12/07/11 11:07:58  12/07/11 21:08:00  krbtgt/[email protected]
    renew until 12/14/11 11:07:58

Now reboot the VM.


Step 3.4 - Install Samba and Join the proxy.example.lan to Active Directory

NOTE: this step heavily relies on RedHat 6 Documentation side at http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/index.html, especially on books "Deployment Guide" and "Enterprise Identity Management Guide".

In order to be able to transparently authenticate clients of Squid proxy using their Active Directory's credentials the server with Squid must be joined into the Active Directory domain (i.e. example.lan). The easiest way to do it is to run Samba on the proxy machine.

  1. Install Samba by typing

    # yum install samba
  2. Make it always start at boot time

    # chkconfig smb on
    # chkconfig nmb on
  3. Open the /etc/samba/smb.conf and ensure its contents look like this (this is the result of running testparm -s on proxy.example.lan):

    	workgroup = EXAMPLE
    	realm = EXAMPLE.LAN
    	server string = Samba Server Version %v
    	security = ADS
    	log file = /var/log/samba/log.%m
    	max log size = 50
    	cups options = raw
    	comment = Home Directories
    	read only = No
    	browseable = No
    	comment = All Printers
    	path = /var/spool/samba
    	printable = Yes
    	browseable = No

    Note we set workgroup to EXAMPLE, realm to EXAMPLE.LAN (capital letters) and security to ads.

  4. Restart Samba services

    #service smb restart
    #service nmb restart

If you now browse the network from the client.example.lan you should see a proxy machine in the EXAMPLE workgroup.

Now join the machine proxy.example.lan into the Active Directory:

  1. Stop Samba services by typing

    # service smb stop
    #service nmb stop
  2. Initialize Kerberos subsystem on proxy.example.lan by typing

    # kinit [email protected]
    , it should ask for password, and complete without errors.
  3. Ensure you got a correct Kerberos ticket by typing

    # klist
  4. Join Active Directory by typing

    # net ads join -S dc.example.lan -U Administrator%[email protected]
    . You should get something like this as an output:


  5. Open "Users and Computers" snap in on dc.example.lan and ensure the proxy computer account now present in the "Computers" tree node


  6. Reboot the proxy.example.lan VM.

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By: sichent