Support Samba file server as a domain member on IPA client

Introduction

Samba is a free software that implements various aspects of SMB protocol and Active Directory infrastructure. Apart from the networking file system that SMB is well known for, Samba provides services to resolve user and group identities for resources accessible via SMB. SMB protocol identity model is based on a Windows NT concept of security identifiers (SIDs) and access control lists (ACLs) which is not directly compatible with a concept of identities employed in POSIX environment model. Thus, Samba suite serves as a translation layer between the two environments.

Active Directory is an extension of Windows NT identity model where identity information is stored in a database exposed to the world via a combination of LDAP and SMB protocols, with authentication provided with both password (NTLMSSP) and Kerberos methods. Systems in Active Directory are organized into logical groups, domains, where some nodes, domain controllers, are used to store domain-specific information and others, domain members, utilize the information via SMB, LDAP, and Kerberos protocols.

SMB protocol has a mechanism for encapsulating and channeling through itself other types of requests, expressed as an access to “files” over a specialized share IPC$. There are multiple interfaces provided by a typical domain controller and domain member servers, most well-known ones are LSA (local security authority, documented in MS-LSAD and MS-LSAT) and NETLOGON remote protocol (documented in MS-NRPC). LSA remote procedure calls are used, among other needs, for retrieving identity information about SIDs and their relationship to other objects. NETLOGON, as its name suggests, is utilized for authentication in a domain environment, across domains, and across forests of domains.

In a traditional domain member set up, the member machine has no possession of a particular user credentials. Instead, it relies on its own connection to its own domain controller to identify a user and to proxy a user’s authentication to the domain controller of the domain a user belongs to. In case a user is performing a remote authentication using Kerberos, a remote system has to present a Kerberos ticket to the domain member’s SMB service, like with any other Kerberos services.

To operate as a domain member in a FreeIPA domain, thus, Samba needs a FreeIPA master to be configured as a domain controller and a FreeIPA client needs to be configured in a specific way to allow Samba to talk to a domain controller. This document overviews a set of implementation tasks to achieve the domain member operation. Most of these tasks are related to FreeIPA components but some of changes required do belong to Samba itself.

Domain member configuration overview

Samba suite, when running as a domain member, starts two daemons:

  • smbd, the main process which handles network connections, file system operations, and remote procedure calls like LSA and NETLOGON. Each connection is handled by a separate smbd child;

  • winbindd, a process to perform identity resolution for all configured and known domains. Active connection to a domain is handled by a separate winbindd child. winbindd processes connect to domain controllers and perform required LSA and NETLOGON operations against them. Normally, authentication of a user from a trusted domain is delegated to the domain member’s own domain controller which then forwards it further.

Both smbd and winbindd daemons rely on a number of pluggable components to abstract out various aspects of their operations. For smbd, there are pluggable modules to represent file system operations. It also uses so-called PASSDB interface to convert SIDs to POSIX identities and back — this interface might be optional on a domain member. In some special cases smbd also directly resolves a name of a user associated with the authenticated connection using standard POSIX API for name resolution (getpwnam() and similar calls). All other identity resolution operations it delegates to winbindd.

winbindd uses a set of identity mapping modules collectively called ‘idmap modules’ in Samba terminology. Each idmap module represents a strategy to map SIDs to corresponding POSIX IDs. Since SID name space in Active Directory is common for all kind of objects and POSIX ID name space is separate for users and groups, with both POSIX ID name spaces being smaller than a common SID name space, there exist multiple approaches to perform the translation. A choice of a translation method is tightly connected with a specific deployment configuration. ID mapping module should be coordinated with a PASSDB module (if one is defined) and how an operating system represents the POSIX users and groups.

To communicate with its domain controller, Samba needs to know own machine account information. Machine account is an account in Active Directory that has its name derived from a NetBIOS machine name (due to Windows NT past) post-fixed with a :math:`` sign, e.g. MY-MACHINE. Password for the machine account is the same as the one used to derive Kerberos keys for the host/.. and cifs/.. principals of the same host. In Active Directory all Kerberos principals associated with the host (service principal names, SPNs) share the same Kerberos keys. Thus, Samba needs to known a clear text password for the machine account and it can derive all Kerberos keys for itself based on that knowledge. The clear text password knowledge is also important for the case of machine account password rotation.

The knowledge of the machine account password is recorded in a special database, secrets.tdb, during the process of a machine join to the domain. For FreeIPA client the join process is different from the one Samba uses for Active Directory, thus we need to seed the machine account password separately to enrolling FreeIPA client. Note that FreeIPA machine enrollment does not allow to share clear text machine account password as it is not recorded anywhere.

Domain controller side configuration overview

See samba-domain-controller for the details of how Samba domain controller is set up and configured in FreeIPA.

Changes required on domain member

In order to configure the domain member part of Samba suite, following steps need to be preformed. These steps are implemented as an installer utility ipa-client-samba and are provided here for documentation purpose only.

Assumptions:

  • At least one of IPA masters is configured as a trust controller using

    ipa-adtrust-install. This is required to enable a hybrid SMB domain where Samba domain controller would understand Samba domain members enrolled via IPA tools but will not be able to enroll them any other way.

  • A client host is enrolled into IPA, with a fully-qualified hostname, ${hostname}. Additional elements that will be referred to below:

    ${realm} : IPA domain’s realm

    ${netbios_name} : NetBIOS name of a domain, whether it is IPA or a trusted Active Directory domain

    ${machine_name} : NetBIOS name of the client where Samba domain member is being deployed

Next steps should be performed on the client itself. With the support for Samba domain member enabled, IPA masters allow creation of the required records with the host credentials (host/${hostname}).

# kinit -k

  1. Retrieve information about Security Identifier and NetBIOS name of the IPA domain:

    # kinit -k
# ipa trustconfig-show --raw
  cn: ipa.realm
  ipantsecurityidentifier: S-1-5-21-570121326-3336757064-1157332047
  ipantflatname: ipa
  ipantdomainguid: be06e132-876a-4f9c-aed4-ef2dc1de8118
  ipantfallbackprimarygroup: cn=Default SMB Group,cn=groups,cn=accounts,dc=ipa,dc=realm

    In the output above,

    cn : IPA domain’s realm, ${realm}, in lower case

    ipantsecurityidentifier : IPA domain’s SID (security identifier)

    ipaflatname : IPA domain’s NetBIOS name, ${netbios_name}, also known as the flat name in Active Directory

    ipantdomainguid : IPA domain’s globally unique identifier (GUID)

  2. Retrieve ID range information for the IPA domain and other trusted domains:

    # ipa idrange-find --raw
----------------
2 ranges matched
----------------
  cn: AD.DOMAIN_id_range
  ipabaseid: 967000000
  ipaidrangesize: 200000
  ipabaserid: 0
  ipanttrusteddomainsid: S-1-5-21-1356309095-650748730-1613512775
  iparangetype: ipa-ad-trust

  cn: IPA.REALM_id_range
  ipabaseid: 1536000000
  ipaidrangesize: 200000
  ipabaserid: 1000
  ipasecondarybaserid: 100000000
  iparangetype: ipa-local
----------------------------
Number of entries returned 2
----------------------------

    From the output above, ipabaseid and ipaidrangesize attributes are used to define ranges for Samba configuration. Samba requires to have IDMAP ranges set for specific domains. For each such range, a pair of (range start, range end) values will need to be calculated:

    ${range_id_min} = ipabaseid
${range_id_max} = ipabaseid + ipaidrangesize - 1

  3. Add Samba-specific Kerberos service principal using ipa service-add-smb command. This command runs a sequence of operations on IPA master that create an LDAP object for the Samba service Kerberos principal with required LDAP object classes and attributes. Some of the attributes have to be set at the creation time because they are auto-generated when the object is added, thus a sequence of ipa service-add and ipa service-mod commands cannot be used instead.

    # ipa service-add-smb <hostname> [<NetBIOS name>]

  4. Generate a random pre-defined password for the machine account that will be used for both Samba-specific Kerberos service princiapl and for Samba machine account. The generated password has to follow few rules to be usable by Samba. In particular, it has to be encoded in UTF-16. Samba Python bindings provide two methods to allow the password generation, generate_random_machine_password() and generate_random_password(). While the former call is what is needed, it returns munged UTF-16 which is not readable by net changesecretpwd -f utility. Thus, the latter call is used instead. Its output is limited to ASCII characters but still should be strong enough for a machine account password. The code used by the ipa-client-samba utility is equivalent for the following call:

    # python3 -c 'import samba; print(samba.generate_random_password(128, 255))'

  5. Retrieve the Kerberos key for cifs/<hostname> service using pre-defined password for the key. The domain controller must know RC4-HMAC hash of the domain member machine account in order to allow NetLogon ServerAuthenticate3 operation. ServerAuthenticate3 call needs an AES session key which is calculated based on an RC4-HMAC of the machine account credential according to MS-NRPC section 3.1.4.3.1. The code used by the ipa-client-samba utility is equivalent for the following call:

    # ipa-getkeytab -p cifs/<hostname> -k /etc/samba/samba.keytab -P \
                -e aes128-cts-hmac-sha1-96,aes256-cts-hmac-sha1-96,arcfour-hmac

    Note that in the call above three encryption types were passed explicitly. The reason for that is to allow to pass RC4-HMAC encryption type request through Kerberos library used by ipa-getkeytab in FIPS mode. ipa-getkeytab utility uses Kerberos encryption types internally. If RC4-HMAC is not allowed for use by the system-wide crypto policy, it will not be specified in the list of default encryption types. If ipa-getkeytab utility gets -e option, it overrides rather than amends the list of the default encryption types, thus forcing to specify the whole set of encryption types explicitly.

  6. Create Samba config as /etc/samba/smb.conf on the client:

    [global]
 # Limit number of forked processes to avoid SMBLoris attack
 max smbd processes = 1000
 # Use dedicated Samba keytab. The key there must be synchronized
 # with Samba tdb databases or nothing will work
 dedicated keytab file = FILE:/etc/samba/samba.keytab
 kerberos method = dedicated keytab
 # Set up logging per machine and Samba process
 log file = /var/log/samba/log.%m
 log level = 1
 # We force 'member server' role to allow winbind automatically
 # discover what is supported by the domain controller side
 server role = member server
 realm = IPA.REALM
 netbios name = ${machine_name}
 workgroup = ${netbios_name}
 # Local writable range for IDs not coming from IPA or trusted domains
 idmap config * : range = 0 - 0
 idmap config * : backend = tdb
 idmap config ${netbios_name} : range = ${range_id_min} - ${range_id_max}
 idmap config ${netbios_name} : backend = sss

    In the config above two IDMAP configurations were defined:

    • for the IPA domain the range from ipabaseid to ipabaseid + ipaidrangesize - 1 is used. IDMAP backend configuration says that the range is served by SSSD, using idmap_sss module. idmap_sss module is provided by sssd-winbind-idmap package.

    • for all unknown domains a local ‘tdb’ IDMAP backend and a range that doesn’t conflict with IPA domain is used. In fact, this has to be choosen carefully, especially if IPA setup already integrates with Active Directory and other ranges defined for AD domains. In such case one needs to define separate idmap config FOO : range and idmap config FOO : backend options per each AD domain that is served through IPA trust to Active Directory the same way as for idmap config IPA. The values there should come from the corresponding ID ranges for AD domains.

  7. Defining access to specific shares can be done with a normal Samba write list option. An example below grants access to share shared to everyone in IPA admins group. The group membership resolution will be done by SSSD. It is recommended to use POSIX ACLs tools to set up access controls on the local file system instead of directly setting them in the Samba configuration as this gives more flexibility. Also, one needs to make sure that the POSIX path specified in the share actually allows write access to the users or groups from the write list:

    [shared]
  path = /srv/shared
  read only = No
  write list = @admins

[homes]
  browsable = no
  writable = yes

  8. Samba configuration has to use the same Security Identifier for the domain as is used by the IPA domain controller. The original value is retrieved in step 1 as ipantsecurityidentifier. This information is not stored in the smb.conf. Instead, it is stored in the binary databases managed by Samba. It can be set through net setdomainsid command:

    # net setdomainsid ${ipantsecurityidentifier}

  9. For SMB protocol, BUILTIN\Guests group has always to be mapped to a local POSIX groups. It is typically mapped to a local nobody group. This is required in all recent Samba releases:

    # net groupmap add sid=S-1-5-32-546 unixgroup=nobody type=builtin

  10. Before using Samba, it needs to know the machine account credentials. Unfortunately, it is only possible to change the machine account credentials when Samba is already enrolled into domain or set it when it is being enrolled with net [ads|rpc] join command. Since IPA client host is enrolled using an alternative method, the join command cannot be used and internal binary databases do not contain correct values that allow Samba to see itself as an enrolled one.

    Instead, until a support for ‘offline’ enrollment is added, the following procedure has to be used. The procedure employs low-level tools to manipulate Samba TDB databases:

    # tdbtool /var/lib/samba/private/secrets.tdb store SECRETS/MACHINE_LAST_CHANGE_TIME/${netbios_name} '2\00'
# tdbtool /var/lib/samba/private/secrets.tdb store SECRETS/MACHINE_PASSWORD/${netbios_name} '2\00'
# net changesecretpw -f

    ${netbios_name} value in the calls above corresponds to the IPA domain’s NetBIOS name. net changesecretpw -f call will require entering the password generated at the step 4.

  11. Start Samba systemd services. At least smb.service and winbind.service services has to be started because Samba cannot function without both of them in newer releases. winbindd daemon is an integral part of Samba and all fallback code for the cases when winbindd was not running in some configurations was removed from smbd daemon in newer Samba releases.

    # systemctl start smb winbind

Example of using Samba file server on IPA client

Once ipa-client-samba utility was used to configure Samba services, the shares were added and systemd services smb.service and winbind.service were started, one can access a Samba share as a user from IPA domain. Below is an example from the test run of ipatests/test_integration/test_smb.py done by PR CI.

# kinit athena
Password for athena@IPA.TEST:
# mkdir -p /mnt/athena
# mount -t cifs //replica0.ipa.test/homes  /mnt/athena -o user=athena,sec=krb5i
# dd  count=1024 bs=1K if=/dev/zero of=/mnt/athena/athena.dat
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0339479 s, 30.9 MB/s
# findmnt -t cifs
TARGET      SOURCE                    FSTYPE OPTIONS
/mnt/athena //replica0.ipa.test/homes cifs   rw,relatime,vers=3.1.1,sec=krb5,cruid=0i,cache=strict,username=athena,uid=0,noforceuid,gid=0,noforcegid,addr=192.168.121.171,file_mode=0755,dir_mode=0755,soft,nounix,serverino,mapposix,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=60,actimeo=1
# ls -laZ /mnt/athena/athena.dat
 -rwxr-xr-x 1 root root ? 1048576 Jun 19 11:45 /mnt/athena/athena.dat
# smbstatus

Samba version 4.10.4
PID     Username     Group        Machine                                   Protocol Version  Encryption           Signing
----------------------------------------------------------------------------------------------------------------------------------------
17249   athena       athena       192.168.121.43 (ipv4:192.168.121.43:46286) SMB3_11           -                    AES-128-CMAC

Service      pid     Machine       Connected at                     Encryption   Signing
---------------------------------------------------------------------------------------------
IPC$         17249   192.168.121.43 Wed Jun 19 11:45:46 AM 2019 UTC  -            AES-128-CMAC
athena       17249   192.168.121.43 Wed Jun 19 11:45:46 AM 2019 UTC  -            AES-128-CMAC

No locked files

# umount -a -t cifs
# smbclient -k //replica0.ipa.test/homes -c 'allinfo athena.dat'
 altname: athena.dat
 create_time:    Wed Jun 19 11:45:46 AM 2019 UTC
 access_time:    Wed Jun 19 11:45:46 AM 2019 UTC
 write_time:     Wed Jun 19 11:45:46 AM 2019 UTC
 change_time:    Wed Jun 19 11:45:46 AM 2019 UTC
 attributes: A (20)
 stream: [::$DATA], 1048576 bytes
 # kdestroy -A

Notes about unfinished Samba work

Since changes on Samba side apply for both domain controller and domain member, unfinished work is reflected in a single place only. Please see samba-domain-controller for details.