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The Microsoft Windows operating system has a number of features that impose specific challenges to interoperability with the operating systems on which Samba is implemented. This chapter deals explicitly with the mechanisms Samba-3 (version 3.0.8 and later) uses to overcome one of the key challenges in the integration of Samba servers into an MS Windows networking environment. This chapter deals with identity mapping (IDMAP) of Windows security identifiers (SIDs) to UNIX UIDs and GIDs.
To ensure sufficient coverage, each possible Samba deployment type is discussed. This is followed by an overview of how the IDMAP facility may be implemented.
The IDMAP facility is of concern where more than one Samba server (or Samba network client) is installed in a domain. Where there is a single Samba server, do not be too concerned regarding the IDMAP infrastructure the default behavior of Samba is nearly always sufficient. Where mulitple Samba servers are used it is often necessary to move data off one server and onto another, and that is where the fun begins!
Where user and group account information is stored in an LDAP directory every server can have the same
consistent UID and GID for users and groups. This is achieved using NSS and the nss_ldap tool. Samba
can be configured to use only local accounts, in which case the scope of the IDMAP problem is somewhat
reduced. This works reasonably well if the servers belong to a single domain, and interdomain trusts
are not needed. On the other hand, if the Samba servers are NT4 domain members, or ADS domain members,
or if there is a need to keep the security name-space separate (i.e., the user
DOMINICUS\FJones must not be given access to the account resources of the user
FRANCISCUS\FJones[4] free from inadvertent cross-over, close attention should be given
to the way that the IDMAP facility is configured.
The use of IDMAP is important where the Samba server will be accessed by workstations or servers from more than one domain, in which case it is important to run winbind so it can handle the resolution (ID mapping) of foreign SIDs to local UNIX UIDs and GIDs.
The use of the IDMAP facility requires the execution of the winbindd upon Samba startup.
There are four basic server deployment types, as documented in the chapter on Server Types and Security Modes.
A standalone Samba server is an implementation that is not a member of a Windows NT4 domain, a Windows 200X Active Directory domain, or a Samba domain.
By definition, this means that users and groups will be created and controlled locally, and the identity of a network user must match a local UNIX/Linux user login. The IDMAP facility is therefore of little to no interest, winbind will not be necessary, and the IDMAP facility will not be relevant or of interest.
Samba-3 can act as a Windows NT4 PDC or BDC, thereby providing domain control protocols that are compatible with Windows NT4. Samba-3 file and print sharing protocols are compatible with all versions of MS Windows products. Windows NT4, as with MS Active Directory, extensively makes use of Windows SIDs.
Samba-3 domain member servers and clients must interact correctly with MS Windows SIDs. Incoming Windows SIDs must be translated to local UNIX UIDs and GIDs. Outgoing information from the Samba server must provide to MS Windows clients and servers appropriate SIDs.
A Samba member of a Windows networking domain (NT4-style or ADS) can be configured to handle
identity mapping in a variety of ways. The mechanism it uses depends on whether or not
the winbindd daemon is used and how the winbind functionality is configured.
The configuration options are briefly described here:
Where winbindd is not used Samba (smbd)
uses the underlying UNIX/Linux mechanisms to resolve the identity of incoming
network traffic. This is done using the LoginID (account name) in the
session setup request and passing it to the getpwnam() system function call.
This call is implemented using the name service switch (NSS) mechanism on
modern UNIX/Linux systems. By saying "users and groups are local,"
we are implying that they are stored only on the local system, in the
/etc/passwd and /etc/group respectively.
For example, when the user BERYLIUM\WambatW tries to open a
connection to a Samba server the incoming SessionSetupAndX request will make a
system call to look up the user WambatW in the
/etc/passwd file.
This configuration may be used with standalone Samba servers, domain member servers (NT4 or ADS), and for a PDC that uses either an smbpasswd or a tdbsam-based Samba passdb backend.
In this situation user and group accounts are treated as if they are local accounts. The only way in which this differs from having local accounts is that the accounts are stored in a repository that can be shared. In practice this means that they will reside in either an NIS-type database or else in LDAP.
This configuration may be used with standalone Samba servers, domain member servers (NT4 or ADS), and for a PDC that uses either an smbpasswd or a tdbsam-based Samba passdb backend.
There are many sites that require only a simple Samba server or a single Samba server that is a member of a Windows NT4 domain or an ADS domain. A typical example is an appliance like file server on which no local accounts are configured and winbind is used to obtain account credentials from the domain controllers for the domain. The domain control can be provided by Samba-3, MS Windows NT4, or MS Windows Active Directory.
Winbind is a great convenience in this situation. All that is needed is a range of
UID numbers and GID numbers that can be defined in the smb.conf file. The
/etc/nsswitch.conf file is configured to use winbind,
which does all the difficult work of mapping incoming SIDs to appropriate UIDs and GIDs.
The SIDs are allocated a UID/GID in the order in which winbind receives them.
This configuration is not convenient or practical in sites that have more than one Samba server and that require the same UID or GID for the same user or group across all servers. One of the hazards of this method is that in the event that the winbind IDMAP file becomes corrupted or lost, the repaired or rebuilt IDMAP file may allocate UIDs and GIDs to different users and groups from what was there previously with the result that MS Windows files that are stored on the Samba server may now not belong to the rightful owners.
The IDMAP_RID facility is new to Samba version 3.0.8. It was added to make life easier for a number of sites that are committed to use of MS ADS, that do not apply an ADS schema extension, and that do not have an installed an LDAP directory server just for the purpose of maintaining an IDMAP table. If you have a single ADS domain (not a forest of domains, and not multiple domain trees) and you want a simple cookie-cutter solution to the IDMAP table problem, then IDMAP_RID is an obvious choice.
This facility requires the allocation of the idmap uid and the
idmap gid ranges, and within the idmap uid
it is possible to allocate a subset of this range for automatic mapping of the relative
identifier (RID) portion of the SID directly to the base of the UID plus the RID value.
For example, if the idmap uid range is 1000-100000000
and the idmap backend = idmap_rid:DOMAIN_NAME=1000-50000000, and
a SID is encountered that has the value S-1-5-21-34567898-12529001-32973135-1234,
the resulting UID will be 1000 + 1234 = 2234.
In this configuration winbind resolved SIDs to UIDs and GIDs from
the idmap uid and idmap gid ranges specified
in the smb.conf file, but instead of using a local winbind IDMAP table, it is stored
in an LDAP directory so that all domain member machines (clients and servers) can share
a common IDMAP table.
It is important that all LDAP IDMAP clients use only the master LDAP server because the
idmap backend facility in the smb.conf file does not correctly
handle LDAP redirects.
The use of LDAP as the passdb backend is a smart solution for PDC, BDC, and domain member servers. It is a neat method for assuring that UIDs, GIDs, and the matching SIDs are consistent across all servers.
The use of the LDAP-based passdb backend requires use of the PADL nss_ldap utility or an equivalent. In this situation winbind is used to handle foreign SIDs, that is, SIDs from standalone Windows clients (i.e., not a member of our domain) as well as SIDs from another domain. The foreign UID/GID is mapped from allocated ranges (idmap uid and idmap gid) in precisely the same manner as when using winbind with a local IDMAP table.
The nss_ldap tool set can be used to access UIDs and GIDs via LDAP as well as via Active Directory. In order to use Active Directory, it is necessary to modify the ADS schema by installing either the AD4UNIX schema extension or using the Microsoft Services for UNIX version 3.5 or later to extend the ADS schema so it maintains UNIX account credentials. Where the ADS schema is extended, a Microsoft Management Console (MMC) snap-in is also installed to permit the UNIX credentials to be set and managed from the ADS User and Computer Management tool. Each account must be separately UNIX-enabled before the UID and GID data can be used by Samba.
Microsoft Windows domain security systems generate the user and group SID as part
of the process of creation of an account. Windows does not have a concept of the UNIX UID or a GID; rather,
it has its own type of security descriptor. When Samba is used as a domain controller, it provides a method
of producing a unique SID for each user and group. Samba generates a machine and a domain SID to which it
adds an RID that is calculated algorithmically from a base value that can be specified
in the smb.conf file, plus twice (2x) the UID or GID. This method is called “algorithmic mapping”.
For example, if a user has a UID of 4321, and the algorithmic RID base has a value of 1000, the RID will
be 1000 + (2 x 4321) = 9642. Thus, if the domain SID is
S-1-5-21-89238497-92787123-12341112, the resulting SID is
S-1-5-21-89238497-92787123-12341112-9642.
The foregoing type of SID is produced by Samba as an automatic function and is either produced on the fly
(as is the case when using a passdb backend = [tdbsam | smbpasswd]), or may be stored
as a permanent part of an account in an LDAP-based ldapsam.
ADS uses a directory schema that can be extended to accommodate additional account attributes such as UIDs and GIDs. The installation of Microsoft Service for UNIX 3.5 will expand the normal ADS schema to include UNIX account attributes. These must of course be managed separately through a snap-in module to the normal ADS account management MMC interface.
Security identifiers used within a domain must be managed to avoid conflict and to preserve itegrity. In an NT4 domain context, the PDC manages the distribution of all security credentials to the backup domain controllers (BDCs). At this time the only passdb backend for a Samba domain controller that is suitable for such information is an LDAP backend.
BDCs have read-only access to security credentials that are stored in LDAP. Changes in user or group account information are passed by the BDC to the PDC. Only the PDC can write changes to the directory.
IDMAP information can be written directly to the LDAP server so long as all domain controllers have access to the master (writable) LDAP server. Samba-3 at this time does not handle LDAP redirects in the IDMAP backend. This means that it is is unsafe to use a slave (replicate) LDAP server with the IDMAP facility.
Anyone who wishes to use winbind will find the following example configurations helpful.
Remember that in the majority of cases winbind is of primary interest for use with
domain member servers (DMSs) and domain member clients (DMCs).
Two common configurations are used:
Networks that have an NT4 PDC (with or without BDCs) or a Samba PDC (with or without BDCs).
Networks that use MS Windows 200x ADS.
NT4 Domain Member Server smb.con is a simple example of an NT4 DMS
smb.conf file that shows only the global section.
The use of winbind requires configuration of NSS. Edit the /etc/nsswitch.conf
so it includes the following entries:
... passwd: files winbind shadow: files winbind group: files winbind ... hosts: files [dns] wins ...
The use of DNS in the hosts entry should be made only if DNS is used on site.
The creation of the DMS requires the following steps:
Create or install an smb.conf file with the above configuration.
Execute:
root# net rpc join -UAdministrator%password
Joined domain MEGANET2.
The success of the join can be confirmed with the following command:
root# net rpc testjoin
Join to 'MIDEARTH' is OK
A failed join would report an error message like the following:
root# net rpc testjoin
[2004/11/05 16:34:12, 0] utils/net_rpc_join.c:net_rpc_join_ok(66)
Join to domain 'MEGANET2' is not valid
Start the nmbd, winbind, and smbd daemons in the order shown.
The procedure for joining an ADS domain is similar to the NT4 domain join, except the smb.conf file
will have the contents shown in ADS Domain Member Server smb.conf
ADS DMS operation requires use of kerberos (KRB). For this to work, the krb5.conf
must be configured. The exact requirements depends on which version of MIT or Heimdal Kerberos is being
used. It is sound advice to use only the latest version, which at this time are MIT Kerberos version
1.3.5 and Heimdal 0.61.
The creation of the DMS requires the following steps:
Create or install an smb.conf file with the above configuration.
Edit the /etc/nsswitch.conf file as shown above.
root# net ads join -UAdministrator%password
Joined domain BUTTERNET.
The success or failure of the join can be confirmed with the following command:
root# net ads testjoin
Using short domain name -- BUTTERNET
Joined 'GARGOYLE' to realm 'BUTTERNET.BIZ'
An invalid or failed join can be detected by executing:
root# net ads testjoin
GARGOYLE$@'s password:
[2004/11/05 16:53:03, 0] utils/net_ads.c:ads_startup(186)
ads_connect: No results returned
Join to domain is not valid
The specific error message may differ from the above because it depends on the type of failure that
may have occurred. Increase the log level to 10, repeat the test,
and then examine the log files produced to identify the nature of the failure.
Start the nmbd, winbind, and smbd daemons in the order shown.
The idmap_rid facility is a new tool that, unlike native winbind, creates a
predictable mapping of MS Windows SIDs to UNIX UIDs and GIDs. The key benefit of this method
of implementing the Samba IDMAP facility is that it eliminates the need to store the IDMAP data
in a central place. The downside is that it can be used only within a single ADS domain and
is not compatible with trusted domain implementations.
This alternate method of SID to UID/GID mapping can be achieved using the idmap_rid
plug-in. This plug-in uses the RID of the user SID to derive the UID and GID by adding the
RID to a base value specified. This utility requires that the parameter
“allow trusted domains = No” be specified, as it is not compatible
with multiple domain environments. The idmap uid and
idmap gid ranges must be specified.
The idmap_rid facility can be used both for NT4/Samba-style domains and Active Directory.
To use this with an NT4 domain, do not include the realm parameter; additionally, the
method used to join the domain uses the net rpc join process.
An example smb.conf file for and ADS domain environment is shown in ADS
Domain Member smb.conf using idmap_rid.
Example 14.3. ADS Domain Member smb.conf using idmap_rid
In a large domain with many users it is imperative to disable enumeration of users and groups.
For example, at a site that has 22,000 users in Active Directory the winbind-based user and
group resolution is unavailable for nearly 12 minutes following first startup of
winbind. Disabling enumeration resulted in instantaneous response.
The disabling of user and group enumeration means that it will not be possible to list users
or groups using the getent passwd and getent group
commands. It will be possible to perform the lookup for individual users, as shown in the following procedure.
The use of this tool requires configuration of NSS as per the native use of winbind. Edit the
/etc/nsswitch.conf so it has the following parameters:
... passwd: files winbind shadow: files winbind group: files winbind ... hosts: files wins ...
The following procedure can use the idmap_rid facility:
Create or install an smb.conf file with the above configuration.
Edit the /etc/nsswitch.conf file as shown above.
Execute:
root# net ads join -UAdministrator%password
Using short domain name -- KPAK
Joined 'BIGJOE' to realm 'CORP.KPAK.COM'
An invalid or failed join can be detected by executing:
root# net ads testjoin
BIGJOE$@'s password:
[2004/11/05 16:53:03, 0] utils/net_ads.c:ads_startup(186)
ads_connect: No results returned
Join to domain is not valid
The specific error message may differ from the above because it depends on the type of failure that
may have occurred. Increase the log level to 10, repeat the test,
and then examine the log files produced to identify the nature of the failure.
Start the nmbd, winbind, and smbd daemons in the order shown.
Validate the operation of this configuration by executing:
root# getent passwd administrator
administrator:x:1000:1013:Administrator:/home/BE/administrator:/bin/bash
The storage of IDMAP information in LDAP can be used with both NT4/Samba-3-style domains and ADS domains. OpenLDAP is a commonly used LDAP server for this purpose, although any standards-complying LDAP server can be used. It is therefore possible to deploy this IDMAP configuration using the Sun iPlanet LDAP server, Novell eDirectory, Microsoft ADS plus ADAM, and so on.
An example is for an ADS domain is shown in ADS Domain Member Server using LDAP.
Example 14.4. ADS Domain Member Server using LDAP
In the case of an NT4 or Samba-3-style domain the realm is not used, and the
command used to join the domain is net rpc join. The above example also demonstrates
advanced error-reporting techniques that are documented in Reporting Bugs.
Where MIT kerberos is installed (version 1.3.4 or later), edit the /etc/krb5.conf
file so it has the following contents:
[logging]
default = FILE:/var/log/krb5libs.log
kdc = FILE:/var/log/krb5kdc.log
admin_server = FILE:/var/log/kadmind.log
[libdefaults]
default_realm = SNOWSHOW.COM
dns_lookup_realm = false
dns_lookup_kdc = true
[appdefaults]
pam = {
debug = false
ticket_lifetime = 36000
renew_lifetime = 36000
forwardable = true
krb4_convert = false
}
Where Heimdal kerberos is installed, edit the /etc/krb5.conf
file so it is either empty (i.e., no contents) or it has the following contents:
[libdefaults]
default_realm = SNOWSHOW.COM
clockskew = 300
[realms]
SNOWSHOW.COM = {
kdc = ADSDC.SHOWSHOW.COM
}
[domain_realm]
.snowshow.com = SNOWSHOW.COM
Samba cannot use the Heimdal libraries if there is no /etc/krb5.conf file.
So long as there is an empty file, the Heimdal kerberos libraries will be usable. There is no
need to specify any settings because Samba, using the Heimdal libraries, can figure this out automatically.
Edit the NSS control file /etc/nsswitch.conf so it has the following entries:
... passwd: files ldap shadow: files ldap group: files ldap ... hosts: files wins ...
You will need the PADL nss_ldap
tool set for this solution. Configure the /etc/ldap.conf file so it has
the information needed. The following is an example of a working file:
host 192.168.2.1 base dc=snowshow,dc=com binddn cn=Manager,dc=snowshow,dc=com bindpw not24get pam_password exop nss_base_passwd ou=People,dc=snowshow,dc=com?one nss_base_shadow ou=People,dc=snowshow,dc=com?one nss_base_group ou=Groups,dc=snowshow,dc=com?one ssl no
The following procedure may be followed to effect a working configuration:
Configure the smb.conf file as shown above.
Create the /etc/krb5.conf file as shown above.
Configure the /etc/nsswitch.conf file as shown above.
Download, build, and install the PADL nss_ldap tool set. Configure the
/etc/ldap.conf file as shown above.
Configure an LDAP server and initialize the directory with the top-level entries needed by IDMAP, shown in the following LDIF file:
dn: dc=snowshow,dc=com objectClass: dcObject objectClass: organization dc: snowshow o: The Greatest Snow Show in Singapore. description: Posix and Samba LDAP Identity Database dn: cn=Manager,dc=snowshow,dc=com objectClass: organizationalRole cn: Manager description: Directory Manager dn: ou=Idmap,dc=snowshow,dc=com objectClass: organizationalUnit ou: idmap
Execute the command to join the Samba DMS to the ADS domain as shown here:
root# net ads testjoin
Using short domain name -- SNOWSHOW
Joined 'GOODELF' to realm 'SNOWSHOW.COM'
Store the LDAP server access password in the Samba secrets.tdb file as follows:
root# smbpasswd -w not24get
Start the nmbd, winbind, and smbd daemons in the order shown.
Follow the diagnositic procedures shown earlier in this chapter to identify success or failure of the join. In many cases a failure is indicated by a silent return to the command prompt with no indication of the reason for failure.
The use of this method is messy. The information provided in the following is for guidance only and is very definitely not complete. This method does work; it is used in a number of large sites and has an acceptable level of performance.
An example smb.conf file is shown in ADS Domain Member Server using
RFC2307bis Schema Extension Date via NSS.
The DMS must be joined to the domain using the usual procedure. Additionally, it is necessary to build and install the PADL nss_ldap tool set. Be sure to build this tool set with the following:
./configure --enable-rfc2307bis --enable-schema-mapping make install
The following /etc/nsswitch.conf file contents are required:
... passwd: files ldap shadow: files ldap group: files ldap ... hosts: files wins ...
The /etc/ldap.conf file must be configured also. Refer to the PADL documentation
and source code for nss_ldap to specific instructions.
The next step involves preparation of the ADS schema. This is briefly discussed in the remaining part of this chapter.
The Microsoft Windows Service for UNIX (SFU) version 3.5 is available for free download from the Microsoft Web site. You will need to download this tool and install it following Microsoft instructions.
Instructions for obtaining and installing the AD4UNIX tool set can be found from the Geekcomix Web site.