[Samba] basic LDAP authentication to Samba share from existing directory

Brent Busby brent at jfi.uchicago.edu
Wed Jun 29 15:58:53 MDT 2011

We have an existing LDAP directory in which users have UNIX passwords that are 
used for a variety of different services.  We'd like to keep as close to having 
a single synchronized password service as possible, but we've run into an issue.

There seem to be two ways of doing this, neither of which seem helpful:

(1) "ldapsam"
>From looking at the Samba documentation that's available, it looks like there 
is no possibility of true password synchronization between NT passwords and 
UNIX.  (Please correct me if that's not so -- I'd really like to be wrong!)  
You setup the samba.schema on the LDAP server, which gives you the 
sambaNTPassword objectClass (among others), and that stores the clients' 
Windows password.  They still have regular UNIX password capability from the 
inetorgperson.schema.  These are two separate password fields, provided by two 
different schemas, both belonging to the same user's LDAP account.  Basically, 
you've got two account systems in the same user's LDAP data, completely 
separate.  (Is all this true so far?)

You use the ldapsam passdb backend to connect to Samba to your LDAP server, and 
when a Windows machine wants to change its NT password, it can use that backend 
to do it.

None of this seems to be helping get any closer to allowing Windows clients to 
authenticate off of the same password database as our UNIX services....  
There's a utility called smbldap-populate, but all this seems to do is go 
through an existing user database and give the users the new Samba object 
classes if they don't have them.  It doesn't really translate their UNIX 
passwords into NT passwords and fill them in, does it?

(2) "pam_unix"
On the other hand, there is a more apocryphal (and dangerous) way to do this, 
which does what we want, but is completely insecure:

You can setup Samba to use pam_unix to authenticate, so that it is using the 
local UNIX security stack rather than its own ldapsam passdb, and then setup 
PAM to do LDAP auth at the UNIX level (the same way you would if you were 
setting the machine up to allow LDAP login for SSH or some other such UNIX 

The reason that's insecure is because since PAM doesn't know what to do with an 
encrypted NT password, it is necessary to setup both the Windows clients and 
the smb.conf on the Samba server for "encrypted passwords = no", which then 
makes it so that even if you're doing secure LDAP over SSL/TLS, you're still 
screwed because your passwords get sent from the Windows clients in cleartext.  
So you get:

WINDOWS -> cleartext -> SAMBA -> ldap ssl/tls encrypted -> LDAP

It's only encrypted for part of the trip, which isn't good enough at all.

This method does however let you authenticate Windows clients directly off of 
an existing UNIX password database in LDAP, and works perfectly if you don't 
mind having passwords flying around in the clear on your LAN.

Does anyone have any suggestions on this?  I've poured over literally reams of 
Samba and LDAP documentation in the past week or so, looking for an answer to 
this.  It hasn't helped that most of the documentation seems to be aimed at 
setting up Samba as a full scale NT Primary Domain Controller, with domain 
membership for machines and the whole nine yards.  Many of these documents are 
much more elaborate than is (hopefully!) necessary for just doing LDAP password 
auth, and it's not clear from reading them how much of what is being described 
is required for basic authentication, and how much is just the writer taking 
advantage of everything Samba can do in one configuration.  (Some of these 
howtos are thirty or forty pages long.)  Also, many of them presume that you're 
starting from scratch, and that you don't have any existing users, and you're 
free to implement an LDAP namespace from an empty tree.

Is there any way to LDAP-authenticate Samba from an existing user database with 
their existing UNIX passwords, without resorting to implementing a full PDC 
setup, or requiring that the Windows side use cleartext passwords, or ending up 
with two separate password fields (UNIX and NT)?  (The later option almost 
seems to remove some of the motivation for using LDAP at all, since you end up 
with double-signon.

Help and comments appreciated!

+ Brent A. Busby         +      The New JFI Computing Web Site:
+ Sr. UNIX Systems Admin +      http://jficomputing.uchicago.edu/
+ University of Chicago  +
+ Physical Sciences Div. +      For problem reports and requests:
+ James Franck Institute +      email:  sysadmin at jfi.uchicago.edu

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