Chapter 14. Identity Mapping (IDMAP)
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## Chapter 14. Identity Mapping (IDMAP)

### John H. Terpstra

Samba Team

`<jht@samba.org>`

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 multiple 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.

## Samba Server Deployment Types and IDMAP

There are four basic server deployment types, as documented in the chapter on Server Types and Security Modes.

### Standalone Samba Server

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.

### Domain Member Server or Domain Member Client

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:

Winbind is not used; users and groups are local:

  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.

Winbind is not used; users and groups resolved via NSS:

  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.

Winbind/NSS with the default local IDMAP table:

  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.

Winbind/NSS uses RID based IDMAP:

  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`.

Winbind with an NSS/LDAP backend-based IDMAP facility:

  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.

Winbind with NSS to resolve UNIX/Linux user and group IDs:

  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.

### Primary Domain Controller

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 integrity. 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.

### Backup Domain Controller

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.

## Examples of IDMAP Backend Usage

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).

### Default Winbind TDB

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-Style Domains (Includes Samba Domains)

NT4 Domain Member Server smb.con is a simple example of an NT4 DMS `smb.conf` file that shows only the global section.

Example 14.1. NT4 Domain Member Server smb.conf

# Global parameters
`[global]`
`workgroup = MEGANET2`
`security = DOMAIN`
`idmap uid = 10000-20000`
`idmap gid = 10000-20000`
`template primary group = "Domain Users"`
`template shell = /bin/bash`

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:

1. Create or install an `smb.conf` file with the above configuration.

2. 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

3. Start the `nmbd, winbind,` and `smbd` daemons in the order shown.

#### ADS Domains

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

Example 14.2. ADS Domain Member Server smb.conf

# Global parameters
`[global]`
`workgroup = BUTTERNET`
`netbios name = GARGOYLE`
`realm = BUTTERNET.BIZ`
`security = ADS`
`template shell = /bin/bash`
`idmap uid = 500-10000000`
`idmap gid = 500-10000000`
`winbind use default domain = Yes`
`winbind nested groups = Yes`
`printer admin = "BUTTERNET\Domain Admins"`

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:

1. Create or install an `smb.conf` file with the above configuration.

2. Edit the `/etc/nsswitch.conf` file as shown above.

3. Execute:

       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.

4. Start the `nmbd`, `winbind`, and `smbd` daemons in the order shown.

### IDMAP_RID with Winbind

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

# Global parameters
`[global]`
`workgroup = KPAK`
`netbios name = BIGJOE`
`realm = CORP.KPAK.COM`
`server string = Office Server`
`security = ADS`
`allow trusted domains = No`
`idmap backend = idmap_rid:KPAK=500-100000000`
`idmap uid = 500-100000000`
`idmap gid = 500-100000000`
`template shell = /bin/bash`
`winbind use default domain = Yes`
`winbind enum users = No`
`winbind enum groups = No`
`winbind nested groups = Yes`
`printer admin = "Domain Admins"`

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:

1. Create or install an `smb.conf` file with the above configuration.

2. Edit the `/etc/nsswitch.conf` file as shown above.

3. 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.

4. Start the `nmbd`, `winbind`, and `smbd` daemons in the order shown.

5. Validate the operation of this configuration by executing:

       root#  getent passwd administrator
       administrator:x:1000:1013:Administrator:/home/BE/administrator:/bin/bash

### IDMAP Storage in LDAP Using Winbind

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

# Global parameters
`[global]`
`workgroup = SNOWSHOW`
`netbios name = GOODELF`
`realm = SNOWSHOW.COM`
`server string = Samba Server`
`security = ADS`
`log level = 1 ads:10 auth:10 sam:10 rpc:10`
`ldap admin dn = cn=Manager,dc=SNOWSHOW,dc=COM`
`ldap idmap suffix = ou=Idmap`
`ldap suffix = dc=SNOWSHOW,dc=COM`
`idmap backend = ldap:ldap://ldap.snowshow.com`
`idmap uid = 150000-550000`
`idmap gid = 150000-550000`
`template shell = /bin/bash`
`winbind use default domain = Yes`

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

### Note

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:

1. Configure the `smb.conf` file as shown above.

2. Create the `/etc/krb5.conf` file as shown above.

3. Configure the `/etc/nsswitch.conf` file as shown above.

4. Download, build, and install the PADL nss_ldap tool set. Configure the `/etc/ldap.conf` file as shown above.

5. 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

6. 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'

7. Store the LDAP server access password in the Samba `secrets.tdb` file as follows:

       root#  smbpasswd -w not24get

8. 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.

### IDMAP and NSS Using LDAP from ADS with RFC2307bis Schema Extension

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.

Example 14.5. ADS Domain Member Server using RFC2307bis Schema Extension Date via NSS

# Global parameters
`[global]`
`workgroup = BOBBY`
`realm = BOBBY.COM`
`security = ADS`
`idmap uid = 150000-550000`
`idmap gid = 150000-550000`
`template shell = /bin/bash`
`winbind cache time = 5`
`winbind use default domain = Yes`
`winbind trusted domains only = Yes`
`winbind nested groups = Yes`

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.

#### IDMAP, Active Directory, and MS Services for UNIX 3.5

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.

#### IDMAP, Active Directory and AD4UNIX

Instructions for obtaining and installing the AD4UNIX tool set can be found from the Geekcomix Web site.

[4] Samba local account mode results in both `DOMINICUS\FJones` and `FRANCISCUS\FJones` mapping to the UNIX user `FJones`.

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