April 16, 2011, 2:25 p.m.
posted by lime
Any discussion about authentication is fruitless without including the topic of users. After all, the users are the ones being authenticated. As early as Chapter 2 we have seen some basic utilities that can be used to create a user account (i.e., the smbpasswd tool). However, we have not really talked about what the smbpasswd and similar utilities actually do. Here we break the discussion into two parts. We cover the various ways that user account information can be stored, followed by a explanation of the user management tools provided by Samba. First, we expand upon the discussion of Windows SIDs that we started in Chapter 1.
A Windows security identifier (SID) is a collection of numbers combined into a binary blob that uniquely identifies an object such as a user, group, or computer. The common string representations of a SID is written as:
It is impossible to determine what type of object the SID represents by its value alone. It could be a user or a group or something else. Windows (and Samba) provide calls to convert this SID to a name and to obtain its type.
The structure of a SID can be broken down into four parts:
The last 32-bit number in the list of subauthorities is referred to as a relative identifier (RID). To be completely accurate, each 32-bit number of the subauthority list is a RID, but generally people use the term only to refer to the last number in the list. In this example, the RID is 1003.
Removing the RID from the original SID leaves us with an identifier that represents the SID's security domain. A security domain is not necessarily the same thing as an authentication domain (as discussed in Chapter 1), although there is a relationship between the two. In our example, the security domain would be S-1-5-21-3489264249-1556752242-1837584028. Each Windows host has a machine SID that defines its local domain. On domain controllers, the domain SID is identical to the local machine SID.
You can view Samba's local machine SID by logging on as root and running the net command from a shell prompt.
root# net getlocalsid SID for domain RAIN is: S-1-5-21-3489264249-1556752242-1837584028
The concepts of local and foreign security domains do not neatly match up to Unix hosts, which have one authentication domain, based on an entry in /etc/passwd. Even when a network of hosts is configured to be part of an NIS domain (which should not be confused with a Microsoft domain), there is no distinction between users within the NIS map and those existing in the local /etc/passwd file.
From the Windows GUI, the distinction between local and remote domains can be seen from the initial logon box. Figure shows the drop-down list of domains available when logging onto a Windows XP client. The local Administrator account is distinguished from the Administrator account in a foreign domain by prefacing the username with either the local machine's name (LETTUCE\Administrator) or the name of the foreign domain (AD\Administrator). These are two different user accounts with different SIDs, even though they share the same login name of Administrator.
Selecting the domain when logging onto a Windows XP client
In addition to the local machine security domain, all Windows and Samba hosts are expected to support the S-1-5-32 domain, which is called BUILTIN. Groups existing within this domain have predefined RIDs. For example, the BUILTIN\Administrators group has a SID of S-1-5-32-544 and the BUILTIN\Users group's SID is S-1-5-32-545.
More information on SIDs and authentication domains is presented in Chapters 9 and 10.
Samba exposes Unix objectsfiles, printers, users and groupsin a way that Windows clients understand. It is necessary, however, for Samba to store some additional attributes for users beyond the information in /etc/passwd. These attributes, such as the LanMan and NT password hashes, the user's SID, and a home directory UNC path, are maintained in what is referred to as a passdb backend. This storage facility can currently take one of three forms:
The passdb backend parameter is a global option whose value is in the form name:argument[,argument]. The Samba code for passdb is written such that new storage modules can be written by the community. However, in this chapter, we concern ourselves with only three, which are distributed as part of the core Samba source code: smbpasswd, tdbsam, and ldapsam. Because each passdb module has its own list of supported options, we discuss possible argument values later, after we have covered each backend in depth. Frequently, arguments can be omitted in order to rely on the passdb module's default behavior. If no backend is specified in smb.conf, Samba defaults to using an smbpasswd file.
passdb backend = smbpasswd
We have seen the structure of an entry from an smbpasswd file earlier in this chapter. Although the file's format changed between Samba 1.9 and 2.0, smbpasswd is the original account storage mechanism used by Samba and still the recommended solution for most standalone servers. Additional storage facilities were not officially supported until Samba 3.0.[*] The structure of an smbpasswd entry is:
The fields are defined as follows:
The following example configures Samba to use an smbpasswd text file for account storage:
The file's default location is set at compile time and can be determined by entering smbd -b | grep SMB_PASSWD_FILE. If you wish to assign a different location, append a colon and the desired absolute path to the smbpasswd module name:
passdb backend = smbpasswd:/etc/smbpasswd
passdb backend = tdbsam
The TDB passdb backend, named tdbsam, expands upon the list of user attributes supported by the smbpasswd backend. TDbsam is the recommended method for storing accounts for a single Samba primary domain controller that does not share its users and groups with any Samba backup domain controllers. The full discussion of Samba domains is provided in Chapter 9. For now, it is sufficient to understand that a TDbsam is a database variant of smbpasswd with support for a richer set of attributes.
The default tdbsam database filename is passdb.tdb and is located in the /usr/local/samba/private directory. For custom Samba installations, you can determine this location by running smbd -b | grep PRIVATE_DIR. If you wish to change that location at runtime, TDbsam accepts, as its only argument, the absolute path to a tdb file:
passdb backend = tdbsam:/etc/passdb.tdb
passdb backend = ldapsam
The third officially supported passdb module is the ldapsam backend. A complete discussion of LDAP is beyond the scope of this book. If you are interested in LDAP and directory services, a recommended resource is LDAP System Administration, by Gerald Carter (O'Reilly). The remainder of this section assumes a basic level of comfort with LDAP directories and the OpenLDAP software in particular. If you are using a directory server from a different vendor, the examples should prove easy to adapt.
When you consider the ldapsam backend, the first thing to do is to become familiar with the schema. There are two auxiliary classes and one structural object class that will be encountered in relation to users and groups:
All the necessary attributes and object classes are defined in an OpenLDAP 2.x compatible schema file named samba.schema located in the examples/LDAP directory of the Samba source distribution. In this same location are schema files for other directory services as well, although these may not be up to date. Make sure that you include or import the appropriate schema file into your LDAP server's configuration. Be aware that Samba's OpenLDAP schema file requires you include the nis.schema, inetorgperson.schema, and cosine.schema files first.
Remember that the LanMan (sambaLMPassword) and NT (sambaNTPassword) password hashes stored in the sambaSamAccount object are plain-text equivalents and should never be made readable to users. Access control rules should restrict these attributes to administrative users only, such as Samba's ldap admin dn distinguished name (discussed a few paragraphs ahead). The following ACLs in OpenLDAP's slapd.conf file protect the passwords from normal users but allow them to be read and modified by Samba:
## protect the samba password hashes access to attr=sambaNTPassword,sambaLMPassword by cn=smbadmin,ou=people,dc=example,dc=com write by * none
For performance reasons, the directory service should support fast equality searches on the uid, cn, sambaSID, gidNumber, uidNumber, and displayName attributes. Newer Samba releases (beginning with 3.0.23) also use a substring matching rule on the sambaSID attribute. To effect this performance enhancement, add the following indexes (or their equivalents) to the server's database section, if any are missing.
## Samba's index settings for OpenLDAP's slapd.conf index uid,cn,displayName,memberUid eq index uidNumber,gidNumber eq index sambaSID eq,sub
Finally, it may be necessary to restart your directory server and/or rerun indexing tools to get it to recognize the changes.
Begin configuring smb.conf by setting up the connection parameters, starting with the LDAP server's URI in the passdb backend value.
[global] passdb backend = ldapsam:ldap://localhost/
By default, all LDAP requests are sent to the directory in an unencrypted form. Unless the master LDAP server and Samba are running on the same machine, it is highly recommended that you take steps to secure the LDAP traffic from eavesdropping. Even when an LDAP replica is running locally on the Samba host, any referrals going back to the master LDAP server must still be encrypted.
Use the ldaps:// URI in the passdb backend option if you wish to connect using LDAP over SSL. However, using StartTLS is the recommended method for configuring data privacy when communicating with an LDAP directory. In this case, the ldap:// URI suffices. To enable StartTLS support, add the following setting to the [global] section:
ldap ssl = start_tls
It is possible to include multiple LDAP URIs in a single-quoted string for purposes of fault tolerance or load balancing. If there are two servers, ldap1 and ldap2, which are replicas of the directory, we can configure Samba to use one in case the other is unavailable. The list of servers is passed on to the underlying LDAP client libraries, which handle the actual network connection details and any failover behavior. The ldap ssl parameter is included here to reiterate the need to secure all communication with the directory service; its value, however, specifies the use of StartTLS instead of SSL:
passdb backend = ldapsam:"ldap://ldap1/ ldap://ldap2/" ldap_ssl = start_tls
Samba treats LDAP as another storage facility for users and groups. Thus all of the user's attributes are retrieved from the directory when a SMB/CIFS connection request must be authenticated. When configuring the directory service access control settings, we restricted the password hashes to be readable only by Samba itself when using its ldap admin dn distinguished name to bind to the server:
ldap admin dn = cn=smbadmin,ou=people,dc=example,dc=com
The password associated with this privileged DN is stored in clear text separately in secrets.tdb. The smbpasswd command can store these credentials interactively (-W option) or on a command line (-w option). Here we have chosen to enter it interactively so that the password will not be displayed in the output of ps:
root# smbpasswd -W Setting stored password for "cn=smbadmin,ou=people,dc=example,dc=com" in secrets.tdb New SMB password: <enter password> Retype new SMB password: <re-enter password>
The final bit of information that Samba requires for ldapsam is the set of base suffixes used to query and store users and groups. The top-level suffix is specified by the ldap suffix option. This DN should be the parent of the other smb.conf search suffixes, which are specified by the following options:
The ldap suffix should be specified first in smb.conf and should be a full DN. The remaining search suffixes should be defined relative to the ldap suffix value. In order to support a directory name space such as the directory information tree (DIT) shown in Figure, we would add the following parameters to Samba's configuration:
[global] ldap suffix = dc=example,dc=com ldap user suffix = ou=people ldap machine suffix = ou=people ldap group suffix = ou=group ldap idmap suffix = ou=idmap
It is possible to define different machine and user suffixes. If you do so, the server's LDAP NSS module must search both bases when querying for a posixAccount. As one of the Samba developers has said, "Machines are people too." The nss_ldap library from PADL software (http://www.padl.com) supports this by enabling the library's RFC2307bis extensions (pass the --enable-rfc2307bis option to the nss_ldap configure script when compiling) and then defining multiple nss_base_passwd directives in its configuration file (usually /etc/ldap.conf). The complete details of PADL's nss_ldap configuration is beyond the scope of this discussion. For more information, please refer to PADL's web site and the documentation included with its software.
To finish off the section, Figure lists the LDAP-related parameters supported in smb.conf. Samba and LDAP integration are revisited in Chapters 9 and 10.
A username map is a mechanism for translating a login name sent by a client in the session setup request to a local Unix username. It's independent of any passdb backend. The most common use is to handle usernames that differ between Windows and Unix. Usernames on Windows systems can violate the limits placed by Unix systems on length and characters used, so sometimes the user account must be stored on the Unix or Linux system, and recognized by Samba as a different name from the one sent by a Windows system to authenticate the user.
This mapping feature comes in two forms: username map uses a file lookup, whereas username map script relies upon an external command to perform the search. Both smb.conf parameters are summarized in Figure.
The username map option requires the absolute path to a map file on the server. Frequently this file is named smbusers and is stored in the same location as smb.conf. No default mapping file is distributed with Samba, although some vendors may choose to do so. The username map option must be explicitly set as shown here:
[global] username map = /usr/local/samba/lib/smbusers
This map file contains entries in the form of:
map_to = map_from
The map_to value is a single Unix username. The map_from value may be a list of values that include:
Processing of the username map file continues until either the complete map has been parsed or until an entry prefixed by an exclamation point (!) matches successfully. Any lines beginning with a hash (#) or semicolon (;) character are ignored as comments. If no match is found at all, the original username is unchanged.
As an example, to map a Windows user of Lee.Zard to a Unix login name of lizard, we could use a single entry such as:
!lizard = Lee.Zard
The righthand values are compared in a case-insensitive fashion, so this example would succeed regardless of whether the user logs on with Lee.Zard or LEE.ZARD. The beginning ! prevents smbd from continuing to look for additional matches for lizard after this entry is parsed.
The username map script allows an administrator to define an external command that will be invoked, rather than reading a map file directly. It provides the flexibility to store the maps in directory services such as LDAP. The mapping command must accept a username as its sole parameter (provided by smbd) and must return a single login name to standard output if any mapping is necessary. Without getting into too many details, the following example uses the OpenLDAP ldapsearch tool to query a directory service based on the common name (cn) attribute and maps the user to the login name provided by the uid attribute.
#!/bin/sh ldapsearch -x -LLL -h ldapsrv1 -b "dc=example,dc=com" \ -s sub "(&(cn=$1)(objectclass=posixAccount))" \| grep uid: | cut -d: -f 2 | sed 's/^\s*//'
Assuming that this script is named ldapmapuser.sh, we can instruct smbd to make use of it by adding the following line to smb.conf:
[global] username map script = /usr/local/bin/ldapmapuser.sh
In this way, it is possible to integrate the username map functionality with the ldapsam passdb backend.
The point at which the mapping occurs is dependent on the value of the security parameter. In the context of our current discussion around security = user, the map is queried before the user is authenticated. To illustrate the consequence, assume that we have the following entry in a username map file:
root = administrator
If a user attempts to connect to our standalone server with a login name of Administrator, the password supplied must match the one for root in Samba's configured passdb backend.
When Samba is configured as a domain member server (security = domain or security = ads), the map is applied after a user has been authenticated by a domain controller. The means that when a user connects as DOMAIN\administrator, she must provide the actual password for that account even though the account may be eventually mapped to the local superuser account. The process becomes even more complicated with the presence of winbindd. Therefore the remainder of the discussion of username maps and member servers is saved until Chapter 10.
Samba provides a set of tools for manipulating user accounts stored in its passdb. The Samba developers have designed these tools to work in the same manner, regardless of which passdb module is used. For this reason, our discussion can focus on the tool without worrying about where or how the information is stored.
The two main user management tools are smbpasswd and pdbedit . The former is the original tool for setting user passwords in an smbpasswd file. During the Samba 3.0 development cycle, it was thought that this tool would be superseded by pdbedit. However, this has not yet happened, and pdbedit is considered by some as the example of how not to build a command-line interface. In Chapter 9, we explore how to use MS-RPC tools such as the Windows NT 4.0 User Manager for Domains and MMC plug-ins to manage users and group from Windows clients. At the moment, these two command-line utilities are what we have to work with.
The smbpasswd tool has two basic categories of functions:
Local user management breaks down further into:
In previous chapters, you've seen examples of adding a new user by passing a login name to the -a argument. It is also possible to feed the new password to the tool on standard input using the -s option, which can be very useful for shell scripts. Here is an example that adds a user named smitty and assigns a password of "cat." The reason for the complicated syntax is to answer both prompts output by the smbpasswd command to request the password. Remember that the Unix user smitty must already exist.
root# (echo "cat"; echo "cat" ) | smbpasswd -s -a smitty Added user smitty.
To later manually change this user's password, run the smbpasswd command again, but this time without the -a option. In this example, we enter the new password interactively rather than using the -s option again:
root# smbpasswd smitty New SMB password: <enter new password> Retype new SMB password: <re-enter new password>
The password is verified by comparing both input strings. If both match, the new password is set. Otherwise you will see an error message stating, Mismatch - password unchanged.
An account can be disabled to prevent the user from logging on. Disabling a user's account sets the D flag in the account control flags. (Refer to Figure in the earlier section on the smbpasswd file format for an overview of these flags.) The following lines disable smitty's account (-d option) and then reenable it (-e option):
root# smbpasswd -d smitty Disabled user smitty. root# smbpasswd -e smitty Enabled user smitty.
When the account is no longer necessary, we can delete this user from our passdb using the -x option and passing it the account name. This command has no effect on the user's Unix account in /etc/passwd.
root# smbpasswd -x smitty Deleted user smitty.
Figure summarizes the command-line options available to root when running smbpasswd.
If smbpasswd is a tool for day-to-day administrative tasks, pdbedit is more akin to a low-level database editor. Overall, its syntax can be cryptic at times, but it does provide three major functions not supported by the smbpasswd command:
The first two features are more related to Samba domain controller functionality, and so are discussed in full detail in Chapter 9. The last is covered here, because without the translation support between passdb storage formats, tasks such as converting from an smbpasswd file to tdbsam would be time-consuming and extremely error-prone.
pdbedit's option naming is a bit confusing at first. The import option (-i) reads in from one backend, whereas the export option (-e) writes to another. Each command-line switch accepts a passdb backend value as its argument. So to convert from smbpasswd to a TDbsam backend, you would run the following command as root:
root# pdbedit -ismbpasswd:/tmp/smbpasswd -etdbsam:/tmp/passdb.tdb Importing account for root...ok Importing account for kong...ok <remaining output deleted>
The complaint with Samba in regard to user accounts is that its user passwords must be maintained separately from the from the Unix or Linux system passwords. To help alleviate the pain of managing multiple passwords for each user, Samba provides a mechanism to synchronize the user's Unix password entry when a CIFS client requests that the LanMan and NT password hashes be changed. Of course, this solution does not help when the user changes the password by means other than the SMB/CIFS protocol, such as using the passwd command or writing to the passdb storage directly using pdbedit.
The only prerequisite of using this feature is for the root user to able to reset a user's password without knowing the old password. The reason for this requirement is that the client encrypts the new password with the old password hash as the key. The clear text of the old password is never sent. Password hashes are one-way, so there is no way to derive the clear text of the password from the old password hash.
The smbd daemon currently supports three mechanisms for changing a user's Unix password:
The simplest option of the three, the ldap password sync option (sometimes called ldap passwd sync), instructs smbd to send a ModifyPassword extended request to the directory service, which then updates the userPassword attribute on behalf of the user. This option currently works only when Samba is using the ldapsam passdb module and when the LDAP directory service is running a recent version of OpenLDAP. To enable password synchronization, with all these prerequisites in place, add ldap password sync = yes to the [global] section of smb.conf.
If you can't make use of this optimal solution, the next option is to enable the unix password sync option and then choose which of the first two mutually exclusive password change mechanisms you wish to use. Relying on an external program is the older method. In this case, you must define a value for the passwd program parameter and then specify a passwd chat conversation string.[*]
The chat value is a special string generally called an "expect string" or "chat string"; it lists pairs of strings in which the first of each pair is the text that you expect the external program to output, and the second is the text that the external program expects the user to enter. With an expect string, an automated system can interact with a program that was designed for a human user. In this case, Samba is pretending to be the root user and is interacting with a password change program. The Samba expect string is case-insensitive and can contain wildcards (*) to eat a variable number of characters when evaluating the output from the program in the passwd program parameter. Remember that the passwd program executable is run as root, so be sure to pass the Unix user name (%u) as a command-line argument, or else you will be stuck just changing root's credentials.
The following example works on most recent versions of Linux from Novell or Red Hat:
[global] encrypt passwords = yes unix password sync = yes passwd program = /usr/bin/passwd %u passwd chat = *New*password* %n\n\ *Reenter*new*password* %n\n\ *Passwd*changed*
Deriving passwd chat values is not extremely difficult. This one was developed by examining the output from running /usr/bin/passwd from a shell prompt, as shown here:
root# passwd lizard Changing password for lizard. New Password: Reenter New Password: Password changed.
Notice that the expect string collapses the first line of output to a single * character.
The pam password change Boolean parameter replaces the invocation of an external command with a series of calls to the system's PAM library. The passwd chat parameters plays the same role as before, providing a means by which smbd is able to interact with the PAM password change interface. This requires that the Samba PAM service has been correctly configured in either /etc/pam.conf or /etc/pam.d/samba. The following is a basic PAM password change stack that performs strengths checks on the new password, and finally hands it off to the pam_unix.so library to actually update the user's credentials:
password required pam_pwcheck.so nullok password required pam_unix2.so nullok use_first_pass use_authtok
Next, we can update the previous example to make use of the new PAM configuration file:
[global] encrypt passwords = yes unix password sync = yes pam password change = yes passwd chat = *New*password* %n\n\ *Reenter*new*password* %n\n\ *Passwd*changed*
If desired, password strength checking can be performed using an external utility specified by the check password script parameter. This directive should point to a tool or script that accepts the new password as its single argument and returns 0 for valid passwords and a nonzero value if the strength check fails.
Figure summarizes all of the password synchronization options we have discussed in this section.