Configuring a Squid Server to authenticate off Active Directory

By Adrian Chadd

Basic Concepts

In this example, a Squid installation will use the Samba ntlm_auth helper to authenticate against an Windows Active Directory. The server will be joined to the Active Directory domain and other services can use the ntlm_auth helper to authenticate users (but be out of the scope of this document.)

Environment

• Windows Server 2003 AD
• Ubuntu Dapper installation
• Squid-2.6
• Kerberos 5
• Samba + Winbind
• NTP server running on AD controller

Packages to install

• samba (3)
• ntp-server (Kerberos requires time-synchronised machines)
• krb5-doc, krb5-config, krb5-user, libkerb53, libkadm55 (Kerberos related user libraries)
• winbind

Files to modify

/etc/krb5.conf

[logging]
   default = FILE:/var/log/krb5libs.log
   kdc = FILE:/var/log/krb5kdc.log
   admin_server = FILE:/var/log/ksadmind.log

[libdefaults]
   default_realm = DOMAIN.COM.AU.
   dns_lookup_realm = false
   dns_lookup_kdc = false
   ticket_lifetime = 24h
   forwardable = yes
   default_tgs_enctypes = DES-CBC-CRC DES-CBC-MD5 RC4-HMAC
   default_tkt_enctypes = DES-CBC-CRC DES-CBC-MD5 RC4-HMAC
   preferred_enctypes = DES-CBC-CRC DES-CBC-MD5 RC4-HMAC 

[realms]
   DOMAIN.COM.AU = {
           kdc = ad-master.domain.com.au.:88
           admin_server = ad-master.domain.com.au.:749
           default_domain = domain.
   } 

[domain_realm]
   .domain. = DOMAIN.COM.AU.
   domain. = DOMAIN.COM.AU. 

[kdc]
   profile = /var/kerberos/krb5kdc/kdc.conf

[appdefaults]
   pam = {
           debug = false
           ticket_lifetime = 36000
           renew_lifetime = 36000
           forwardable = true
           krb4_convert = false
   }

/etc/samba.smb.conf

[global]
        netbios name = SERVERNAME
        workgroup = DOMAIN
        realm = DOMAIN.COM.AU
        server string = Domain Proxy Server
        encrypt passwords = yes
        security = ADS
        password server = ad-master.domain.com.au
        log level = 3
        log file = /var/log/samba/%m.log
        max log size = 50
        socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
        printcap name = /etc/printcap
        preferred master = No
        dns proxy = No
        ldap ssl = no
        idmap uid = 10000-20000
        idmap gid = 10000-20000
        winbind use default domain = yes
        cups options = raw

/var/kerberos/krb5kdc/kdc.conf

[kdcdfefaults]
        acl_file = /var/kerberos/krb5kdc/kadm5.acl
        dict_file = /usr/share/dict/words
        admin_keytab = /var/kerberos/krb5kdc/kadm5.keytab
        v4_mode = noreauth

[libdefaults]
        default_realm = DOMAIN.

[realms]
        DOMAIN. = {
                master_key_type = des-cbc-crc
                supported_enctypes = des3-hmac-sha1:normal arcfour-hmac:normal des-hmac-sha1:normal des-cbc-md5:normal des-cbc-crc:normal des-cbc-crc:v4 des-cbc-crc:afs3 
             }

/var/kerberos/krb5kdc/kadm5.acl

*/admin@EXAMPLE.COM     *

Configure NTP time synchronisation

The server must time synchronise against the AD clock - so configure ntpd to sync against the same time source as the AD server is. You must do this step or random authentication failures will occur!

Joining the server to the AD domain

Once the files have been initialised, join the server to the Active Directory by using an AD account with sufficient privilege:

# kinit <admin user>@<fulldomain>

eg

kinit chadda@DOMAIN.COM.AU.

You may need to do this a couple of times - it may take a while and fail; so try it once again.

Now, to do the actual join:

# net ads join -U <admin user>@<fulldomain>

eg

# net ads join -U chadda@DOMAIN.COM.AU.

This will also take some time and may need to be repeated. It should eventually tell you that the server successfully joined the domain.

Next, restart samba and winbind, ie

# /etc/init.d/samba restart
# /etc/init.d/winbind restart 

'wbinfo' can tell you whether winbind has successfully negotiated and joined the network:

• wbinfo -t will check whether the trust exists

• wbinfo -u will list the users in the domain

Fix Ownership

By default Squid will run as a non-root user; but the directory with the winbind socket (perhaps /var/run/winbindd_privileged/) is generally owned by root and only readable by people in the root group or user.

Don't try changing the ownership of the directory - it'll just revert at reboot. Instead you need to run Squid as the group root.

cache_effective_group root

Configuring Squid to use the Samba 3 ntlm_auth program for authentication

Finally, configure Squid to talk to the NTLM authentication helper:

auth_param ntlm program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-ntlmssp
auth_param ntlm children 10
#auth_param ntlm max_challenge_reuses 0
#auth_param ntlm max_challenge_lifetime 2 minutes
#auth_param ntlm use_ntlm_negotiate off
auth_param basic program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-basic
auth_param basic children 5
auth_param basic realm Domain Proxy Server
auth_param basic credentialsttl 2 hours
auth_param basic casesensitive off
authenticate_cache_garbage_interval 10 seconds

# Credentials past their TTL are removed from memory
authenticate_ttl 0 seconds

Make sure you add the relevant ACL statements to force proxy authentication, eg:

acl lcl src 192.168.0.0/16
acl auth  proxy_auth REQUIRED

http_access allow lcl auth
http_access deny all
miss_access allow all
icp_access deny all

The ntlm authentication helper will start logging authentication attempts (success and failure) to the cache.log file.

CategoryConfigExample