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Table of Contents
You've come a long way now. You have pretty much mastered Samba-3 for most uses it can be put to. Up until now, you have cast Samba-3 in the leading role, and where authentication was required, you have used one or another of Samba's many authentication backends (from flat text files with smbpasswd to LDAP directory integration with ldapsam). Now you can design a solution for a new Abmas business. This business is running Windows Server 2003 and Active Directory, and these are to stay. It's time to master implementing Samba and Samba-supported services in a domain controlled by the latest Windows authentication technologies. Let's get started this is leading edge.
Abmas has continued its miraculous growth; indeed, nothing seems to be able to stop its diversification into multiple (and seemingly unrelated) fields. Its latest acquisition is Abmas Snack Foods, a big player in the snack-food business.
With this acquisition comes new challenges for you and your team. Abmas Snack Foods is a well-developed business with a huge and heterogeneous network. It already has Windows, NetWare, and Proprietary UNIX, but as yet no Samba or Linux. The network is mature and well-established, and there is no question of its chosen user authentication scheme being changed for now. You need to take a wise new approach.
You have decided to set the ball rolling by introducing Samba-3 into the network gradually, taking over key services and easing the way to a full migration and, therefore, integration into Abmas's existing business later.
You've promised the skeptical Abmas Snack Foods management team that you can show them how Samba can ease itself and other Open Source technologies into their existing infrastructure and deliver sound business advantages. Cost cutting is high on their agenda (a major promise of the acquisition). You have chosen Web proxying and caching as your proving ground.
Abmas Snack Foods has several thousand users housed at its head office and multiple regional offices, plants, and warehouses. A high proportion of the business's work is done online, so Internet access for most of these users is essential. All Internet access, including for all regional offices, is funneled through the head office and is the job of the (now your) networking team. The bandwidth requirements were horrific (comparable to a small ISP), and the team soon discovered proxying and caching. In fact, they became one of the earliest commercial users of Microsoft ISA.
The team is not happy with ISA. Because it never lived up to its marketing promises, it underperformed and had reliability problems. You have pounced on the opportunity to show what Open Source can do. The one thing they do like, however, is ISA's integration with Active Directory. They like that their users, once logged on, are automatically authenticated against the proxy. If your alternative to ISA can operate completely seamlessly in their Active Directory domain, it will be approved.
This is a hands-on exercise. You build software applications so that you obtain the functionality Abmas needs.
The key requirements in this business example are straightforward. You are not required to do anything new, just to replicate an existing system, not lose any existing features, and improve performance. The key points are:
Internet access for most employees
Distributed system to accommodate load and geographical distribution of users
Seamless and transparent interoperability with the existing Active Directory domain
Functionally, the user's Internet Explorer requests a browsing session with the Squid proxy, for which it offers its AD authentication token. Squid hands off the authentication request to the Samba-3 authentication helper application called ntlm_auth. This helper is a hook into winbind, the Samba-3 NTLM authentication daemon. Winbind enables UNIX services to authenticate against Microsoft Windows domains, including Active Directory domains. As Active Directory authentication is a modified Kerberos authentication, winbind is assisted in this by local Kerberos 5 libraries configured to check passwords with the Active Directory server. Once the token has been checked, a browsing session is established. This process is entirely transparent and seamless to the user.
Enabling this consists of:
Preparing the necessary environment using preconfigured packages
Setting up raw Kerberos authentication against the Active Directory domain
Configuring, compiling, and then installing the supporting Samba-3 components
Tying it all together
You are a stranger in a strange land, and all eyes are upon you. Some would even like to see you fail. For you to gain the trust of your newly acquired IT people, it is essential that your solution does everything the old one did, but does it better in every way. Only then will the entrenched positions consider taking up your new way of doing things on a wider scale.
First, your system needs to be prepared and in a known good state to proceed. This consists of making sure that everything the system depends on is present and that everything that could interfere or conflict with the system is removed. You will be configuring the Squid and Samba-3 packages and updating them if necessary. If conflicting packages of these programs are installed, they must be removed.
The following packages should be available on your Red Hat Linux system:
In the case of SUSE Linux, these packages are called:
If the required packages are not present on your system, you must install them from the vendor's installation media. Follow the administrative guide for your Linux system to ensure that the packages are correctly updated.
If the requirement is for interoperation with MS Windows Server 2003, it will be necessary to ensure that you are using MIT Kerberos version 1.3.1 or later. Red Hat Linux 9 ships with MIT Kerberos 1.2.7 and thus requires updating.
Heimdal 0.6 or later is required in the case of SUSE Linux. SUSE Enterprise Linux Server 8 ships with Heimdal 0.4. SUSE 9 ships with the necessary version.
If Samba and/or Squid RPMs are installed, they should be updated. You can build both from source.
Locating the packages to be un-installed can be achieved by running:
root#rpm -qa | grep -i sambaroot#rpm -qa | grep -i squid
The identified packages may be removed using:
root# rpm -e samba-common
The systems Kerberos installation must be configured to communicate with your primary Active Directory server (ADS KDC).
Strictly speaking, MIT Kerberos version 1.3.4 currently gives the best results, although the current default Red Hat MIT version 1.2.7 gives acceptable results unless you are using Windows 2003 servers.
Officially, neither MIT (1.3.4) nor Heimdal (0.63) Kerberos needs an /etc/krb5.conf
file in order to work correctly. All ADS domains automatically create SRV records in the
DNS zone Kerberos.REALM.NAME for each KDC in the realm. Since both
MIT and Heimdal, KRB5 libraries default to checking for these records, so they
automatically find the KDCs. In addition, krb5.conf allows
specifying only a single KDC, even if there is more than one. Using the DNS lookup
allows the KRB5 libraries to use whichever KDCs are available.
Procedure 12.1. Kerberos Configuration Steps
If you find the need to manually configure the krb5.conf, you should edit it
to have the contents shown in ???. The final fully qualified path for this file
should be /etc/krb5.conf.
The following gotchas often catch people out. Kerberos is case sensitive. Your realm must
be in UPPERCASE, or you will get an error: “Cannot find KDC for requested realm while getting
initial credentials”. Kerberos is picky about time synchronization. The time
according to your participating servers must be within 5 minutes or you get an error:
“kinit(v5): Clock skew too great while getting initial credentials”.
Clock skew limits are, in fact, configurable in the Kerberos protocols (the default is
5 minutes). A better solution is to implement NTP throughout your server network.
Kerberos needs to be able to do a reverse DNS lookup on the IP address of your KDC.
Also, the name that this reverse lookup maps to must either be the NetBIOS name of
the KDC (i.e., the hostname with no domain attached) or the
NetBIOS name followed by the realm. If all else fails, you can add a
/etc/hosts entry mapping the IP address of your KDC to its
NetBIOS name. If Kerberos cannot do this reverse lookup, you will get a local error
when you try to join the realm.
You are now ready to test your installation by issuing the command:
root# kinit [USERNAME@REALM]
You are asked for your password, which you should enter. The following is a typical console sequence:
root# kinit ADMINISTRATOR@LONDON.ABMAS.BIZ
Password for ADMINISTRATOR@LONDON.ABMAS.BIZ:
Make sure that your password is accepted by the Active Directory KDC.
Example 12.1. Kerberos Configuration File: /etc/krb5.conf
[libdefaults]
default_realm = LONDON.ABMAS.BIZ
[realms]
LONDON.ABMAS.BIZ = {
kdc = w2k3s.london.abmas.biz
}
root# klist -e
shows the Kerberos tickets cached by the system.
Samba must be configured to correctly use Active Directory. Samba-3 must be used, since it has the necessary components to interface with Active Directory.
Procedure 12.2. Securing Samba-3 With ADS Support Steps
Download the latest stable Samba-3 for Red Hat Linux from the official Samba Team FTP site. The official Samba Team RPMs for Red Hat Fedora Linux contain the ntlm_auth tool needed, and are linked against MIT KRB5 version 1.3.1 and therefore are ready for use.
The necessary, validated RPM packages for SUSE Linux may be obtained from the SerNet FTP site that is located in Germany. All SerNet RPMs are validated, have the necessary ntlm_auth tool, and are statically linked against suitably patched Heimdal 0.6 libraries.
Using your favorite editor, change the /etc/samba/smb.conf
file so it has contents similar to the example shown in ???.
i Next you need to create a computer account in the Active Directory. This sets up the trust relationship needed for other clients to authenticate to the Samba server with an Active Directory Kerberos ticket. This is done with the “net ads join -U [Administrator%Password]” command, as follows:
root# net ads join -U administrator%vulcon
Your new Samba binaries must be started in the standard manner as is applicable to the platform you are running on. Alternatively, start your Active Directory-enabled Samba with the following commands:
root#smbd -Droot#nmbd -Droot#winbindd -B
We now need to test that Samba is communicating with the Active Directory domain; most specifically, we want to see whether winbind is enumerating users and groups. Issue the following commands:
root# wbinfo -t
checking the trust secret via RPC calls succeeded
This tests whether we are authenticating against Active Directory:
root# wbinfo -u
LONDON+Administrator
LONDON+Guest
LONDON+SUPPORT_388945a0
LONDON+krbtgt
LONDON+jht
LONDON+xjht
This enumerates all the users in your Active Directory tree:
root# wbinfo -g
LONDON+Domain Computers
LONDON+Domain Controllers
LONDON+Schema Admins
LONDON+Enterprise Admins
LONDON+Domain Admins
LONDON+Domain Users
LONDON+Domain Guests
LONDON+Group Policy Creator Owners
LONDON+DnsUpdateProxy
This enumerates all the groups in your Active Directory tree.
Squid uses the ntlm_auth helper build with Samba-3. You may test ntlm_auth with the command:
root# /usr/bin/ntlm_auth --username=jht
password: XXXXXXXX
You are asked for your password, which you should enter. You are rewarded with:
root# NT_STATUS_OK: Success (0x0)
The ntlm_auth helper, when run from a command line as the user “root”, authenticates against your Active Directory domain (with the aid of winbind). It manages this by reading from the winbind privileged pipe. Squid is running with the permissions of user “squid” and group “squid” and is not able to do this unless we make a vital change. Squid cannot read from the winbind privilege pipe unless you change the permissions of its directory. This is the single biggest cause of failure in the whole process. Remember to issue the following command (for Red Hat Linux):
root#chgrp squid /var/cache/samba/winbindd_privilegedroot#chmod 750 /var/cache/samba/winbindd_privileged
For SUSE Linux 9, execute the following:
root#chgrp squid /var/lib/samba/winbindd_privilegedroot#chmod 750 /var/lib/samba/winbindd_privileged
For Squid to benefit from Samba-3, NSS must be updated to allow winbind as a valid route to user authentication.
Edit your /etc/nsswitch.conf file so it has the parameters shown
in ???.
Example 12.2. Samba Configuration File: /etc/samba/smb.conf
Example 12.3. NSS Configuration File Extract File: /etc/nsswitch.conf
passwd: files winbind shadow: files group: files winbind
Procedure 12.3. Squid Configuration Steps
If your Linux distribution is SUSE Linux 9, the version of Squid supplied is already enabled to use the winbind helper agent. You can therefore omit the steps that would build the Squid binary programs.
Squid, by default, runs as the user nobody. You need to
add a system user squid and a system group
squid if they are not set up already (if the default
Red Hat squid rpms were installed, they will be). Set up a
squid user in /etc/passwd
and a squid group in /etc/group if these aren't there already.
You now need to change the permissions on Squid's var
directory. Enter the following command:
root# chown -R squid /var/cache/squid
Squid must also have control over its logging. Enter the following commands:
root#chown -R chown squid:squid /var/log/squidroot#chmod 770 /var/log/squid
Finally, Squid must be able to write to its disk cache! Enter the following commands:
root#chown -R chown squid:squid /var/cache/squidroot#chmod 770 /var/cache/squid
The /etc/squid/squid.conf file must be edited to include the lines from
??? and ???.
You must create Squid's cache directories before it may be run. Enter the following command:
root# squid -z
Finally, start Squid and enjoy transparent Active Directory authentication. Enter the following command:
root# squid
Example 12.4. Squid Configuration File Extract /etc/squid.conf [ADMINISTRATIVE PARAMETERS Section]
cache_effective_user squid cache_effective_group squid
Example 12.5. Squid Configuration File extract File: /etc/squid.conf [AUTHENTICATION PARAMETERS Section]
auth_param ntlm program /usr/bin/ntlm_auth \
--helper-protocol=squid-2.5-ntlmssp
auth_param ntlm children 5
auth_param ntlm max_challenge_reuses 0
auth_param ntlm max_challenge_lifetime 2 minutes
auth_param basic program /usr/bin/ntlm_auth \
--helper-protocol=squid-2.5-basic
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours
acl AuthorizedUsers proxy_auth REQUIRED
http_access allow all AuthorizedUsers
Microsoft Windows networking protocols permeate the spectrum of technologies that Microsoft Windows clients use, even when accessing traditional services such as Web browsers. Depending on whom you discuss this with, this is either good or bad. No matter how you might evaluate this, the use of NTLMSSP as the authentication protocol for Web proxy access has some advantages over the cookie-based authentication regime used by all competing browsers. It is Samba's implementation of NTLMSSP that makes it attractive to implement the solution that has been demonstrated in this chapter.
The development of the ntlm_auth module was first discussed in many Open Source circles in 2002. At the SambaXP conference in Goettingen, Germany, Mr. Francesco Chemolli demonstrated the use of ntlm_auth during one of the late developer meetings that took place. Since that time, the adoption of ntlm_auth has spread considerably.
The largest report from a site that uses Squid with ntlm_auth-based authentication support uses a dual processor server that has 2 GB of memory. It provides Web and FTP proxy services for 10,000 users. Approximately 2,000 of these users make heavy use of the proxy services. According to the source, who wishes to remain anonymous, the sustained transaction load on this server hovers around 140 hits/sec. The following comments were made with respect to questions regarding the performance of this installation:
[In our] EXTREMELY optimized environment . . . [the] performance impact is almost [nothing]. The “almost” part is due to the brain damage of the ntlm-over-http protocol definition. Suffice to say that its worst-case scenario triples the number of hits needed to perform the same transactions versus basic or digest auth[entication].
You would be well-advised to recognize that all cache-intensive proxying solutions demand a lot of memory. Make certain that your Squid proxy server is equipped with sufficient memory to permit all proxy operations to run out of memory without invoking the overheads involved in the use of memory that has to be swapped to disk.