Samba 4.17 Features added/changed

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Samba 4.17 is Upcoming Release.

Samba 4.17.0rc5

Release Announcements

Release Notes for 4.17.0rc5
September 6, 2022

This is the fifth release candidate of Samba 4.17. This is *not* intended for production environments and is designed for testing purposes only. Please report any defects via the Samba bug reporting system at

Samba 4.17 will be the next version of the Samba suite.



SMB Server performance improvements

The security improvements in recent releases (4.13, 4.14, 4.15, 4.16), mainly as protection against symlink races, caused performance regressions for meta data heavy workloads.

With 4.17 the situation improved a lot again:

  • Pathnames given by a client are devided into dirname and basename. The amount of syscalls to validate dirnames is reduced to 2 syscalls (openat, close) per component. On modern Linux kernels (>= 5.6) smbd makes use of the openat2() syscall with SOLVE_NO_SYMLINKS, in order to just use 2 syscalls (openat2, close) for the whole dirname.
  • Contended path based operations used to generate a lot of unsolicited wakeup events causing thundering herd problems, which lead to masive latencies for some clients. These events are now avoided in order to provide stable latencies and much higher throughput of open/close operations.

Configure without the SMB1 Server

It is now possible to configure Samba without support for the SMB1 protocol in smbd. This can be selected at configure time with either of the options:


By default (without either of these options set) Samba is configured to include SMB1 support (i.e. --with-smb1-server is the default). When Samba is configured without SMB1 support, none of the SMB1 code is included inside smbd except the minimal stub code needed to allow a client to connect as SMB1 and immediately negotiate the selected protocol into SMB2 (as a Windows server also allows).

None of the SMB1-only smb.conf parameters are removed when configured without SMB1, but these parameters are ignored by the smbd server. This allows deployment without having to change an existing smb.conf file.

This option allows sites, OEMs and integrators to configure Samba to remove the old and insecure SMB1 protocol from their products.

Note: that the Samba client libraries still support SMB1 connections even when Samba is configured as --without-smb1-server. This is to ensure maximum compatibility with environments containing old SMB1 servers.

Bronze bit and S4U support now also with MIT Kerberos 1.20

In 2020 Microsoft Security Response Team received another Kerberos-related report. Eventually, that led to a security update of the CVE-2020-17049, Kerberos KDC Security Feature Bypass Vulnerability, also known as a 'Bronze Bit'. With this vulnerability, a compromised service that is configured to use Kerberos constrained delegation feature could tamper with a service ticket that is not valid for delegation to force the KDC to accept it.

With the release of MIT Kerberos 1.20, Samba AD DC is able able to mitigate the ‘Bronze Bit’ attack. MIT Kerberos KDC's KDB (Kerberos Database Driver) API was changed to allow passing more details between KDC and KDB components. When built against MIT Kerberos, Samba AD DC supports MIT Kerberos 1.19 and 1.20 versions but 'Bronze Bit' mitigation is provided only with MIT Kerberos 1.20.

In addition to fixing the 'Bronze Bit' issue, Samba AD DC now fully supports S4U2Self and S4U2Proxy Kerberos extensions.

Note: the default (Heimdal-based) KDC was already fixed in 2021, see BUG #14642

Resource Based Constrained Delegation (RBCD) support

Samba AD DC built with MIT Kerberos 1.20 offers RBCD support now. With MIT Kerberos 1.20 we have complete RBCD support passing Sambas S4U testsuite.

samba-tool delegation got the 'add-principal' and 'del-principal' subcommands in order to manage RBCD.

To complete RBCD support and make it useful to Administrators we added the Asserted Identity [1] SID into the PAC for constrained delegation. This is available for Samba AD compiled with MIT Kerberos 1.20.

Note: the default (Heimdal-based) KDC does not support RBCD yet.
Kerberos Constrained Delegation Overview

Customizable DNS listening port

It is now possible to set a custom listening port for the builtin DNS service, making easy to host another DNS on the same system that would bind to the default port and forward the domain-specific queries to Samba using the custom port. This is the opposite configuration of setting a forwarder in Samba.

It makes possible to use another DNS server as a front and forward to Samba.

Dynamic DNS updates may not be proxied by the front DNS server when forwarding to Samba. Dynamic DNS update proxying depends on the features of the other DNS server used as a front.

CTDB changes

  • When Samba is configured with both --with-cluster-support and --systemd-install-services then a systemd service file for CTDB will be installed.
  • ctdbd_wrapper has been removed. ctdbd is now started directly from a systemd service file or init script.
  • The syntax for the ctdb.tunables configuration file has been relaxed. However, trailing garbage after the value, including comments, is no longer permitted. Please see ctdb-tunables(7) for more details.

Operation without the (unsalted) NT password hash

When Samba is configured with 'nt hash store = never' then Samba will no longer store the (unsalted) NT password hash for users in Active Directory. (Trust accounts, like computers, domain controllers and inter-domain trusts are not impacted).

In the next version of Samba the default for 'nt hash store' will change from 'always' to 'auto', where it will follow (behave as 'nt hash store = never' when 'ntlm auth = disabled' is set.

Security-focused deployments of Samba that have eliminated NTLM from their networks will find setting 'ntlm auth = disabled' with 'nt hash store = always' as a useful way to improve compliance with best-practice guidance on password storage (which is to always use an interated hash).

Note: that when 'nt hash store = never' is set, then arcfour-hmac-md5 Kerberos keys will not be available for users who subsequently change their password, as these keys derive their values from NT hashes. AES keys are stored by default for all deployments of Samba with Domain Functional Level 2008 or later, are supported by all modern clients, and are much more secure.

Finally, also note that password history in Active Directory is stored in nTPwdHistory using a series of NT hash values. Therefore the full password history feature is not available in this mode.

To provide some protection against password re-use previous Kerberos hash values (the current, old and older values are already stored) are used, providing a history length of 3.

There is one small limitation of this workaround: Changing the sAMAccountName, userAccountControl or userPrincipalName of an account can cause the Kerberos password salt to change. This means that after *both* an account rename and a password change, only the current password will be recognised for password history purposes.

Python API for smbconf

Samba's smbconf library provides a generic frontend to various configuration backends (plain text file, registry) as a C library. A new Python wrapper, importable as 'samba.smbconf' is available. An additional module, 'samba.samba3.smbconf', is also available to enable registry backend support. These libraries allow Python programs to read, and optionally write, Samba configuration natively.

JSON support for smbstatus

It is now possible to print detailed information in JSON format in the smbstatus program using the new option --json. The JSON output covers all the existing text output including sessions, connections, open files, byte-range locks, notifies and profile data with all low-level information maintained by Samba in the respective databases.

Protected Users security group

Samba AD DC now includes support for the Protected Users security group introduced in Windows Server 2012 R2. The feature reduces the attack surface of user accounts by preventing the use of weak encryption types. It also mitigates the effects of credential theft by limiting credential lifetime and scope.

The protections are intended for user accounts only, and service or computer accounts should not be added to the Protected Users group. User accounts added to the group are granted the following security protections:

  • NTLM authentication is disabled.
  • Kerberos ticket-granting tickets (TGTs) encrypted with RC4 are not issued to or accepted from affected principals. Tickets encrypted with AES, and service tickets encrypted with RC4, are not affected by this restriction.
  • The lifetime of Kerberos TGTs is restricted to a maximum of four hours.
  • Kerberos constrained and unconstrained delegation is disabled.

If the Protected Users group is not already present in the domain, it can be created with 'samba-tool group add'. The new '--special' parameter must be specified, with 'Protected Users' as the name of the group. An example command invocation is:

samba-tool group add 'Protected Users' --special

or against a remote server:

samba-tool group add 'Protected Users' --special -H ldap:// -U Administrator

The Protected Users group is identified in the domain by its having a RID of 525. Thus, it should only be created with samba-tool and the '--special' parameter, as above, so that it has the required RID to function correctly.


LanMan Authentication and password storage removed from the AD DC

The storage and authentication with LanMan passwords has been entirely removed from the Samba AD DC, even when "lanman auth = yes" is set.

smb.conf changes

 Parameter Name                          Description     Default
 --------------                          -----------     -------
 dns port                                New default     53
 fruit:zero_file_id                      New default     yes
 nt hash store                           New parameter   always
 smb1 unix extensions                    Replaces "unix extensions"
 volume serial number                    New parameter   -1
 winbind debug traceid                   New parameter   no


  • Ralph Boehme <>
  • BUG #15126: acl_xattr VFS module may unintentionally use filesystem permissions instead of ACL from xattr.
  • BUG #15153: Missing SMB2-GETINFO access checks from MS-SMB2
  • BUG #15161: assert failed: !is_named_stream(smb_fname)") at ../../lib/util/fault.c:197.
  • Volker Lendecke <>
  • BUG #15126: acl_xattr VFS module may unintentionally use filesystem permissions instead of ACL from xattr.
  • BUG #15161: assert failed: !is_named_stream(smb_fname)") at ../../lib/util/fault.c:197.
  • Stefan Metzmacher <>
  • BUG #15159: Cross-node multi-channel reconnects result in SMB2 Negotiate returning NT_STATUS_NOT_SUPPORTED.
  • Noel Power <>
  • BUG #15160: winbind at info level debug can coredump when processing


  • Anoop C S <>
  • BUG #15157: Make use of glfs_*at() API calls in vfs_glusterfs.


  • Jeremy Allison <>
  • BUG #15128: Possible use after free of connection_struct when iterating smbd_server_connection->connections.
  • Christian Ambach <>
  • BUG #15145: `net usershare add` fails with flag works with --long but fails with -l.
  • Ralph Boehme <>
  • BUG #15126: acl_xattr VFS module may unintentionally use filesystem permissions instead of ACL from xattr.
  • Stefan Metzmacher <>
  • BUG #15125: Performance regression on contended path based operations.
  • BUG #15148: Missing READ_LEASE break could cause data corruption.
  • Andreas Schneider <>
  • BUG #15141: libsamba-errors uses a wrong version number.
  • Joseph Sutton <>
  • BUG #15152: SMB1 negotiation can fail to handle connection errors.


  • Jeremy Allison <>
  • BUG #15143: New filename parser doesn't check veto files smb.conf parameter.
  • BUG #15144: 4.17.rc1 still uses symlink-race prone unix_convert()
  • BUG #15146: Backport fileserver related changed to 4.17.0rc2
  • Jule Anger <>
  • Volker Lendecke <>
  • BUG #15146: Backport fileserver related changed to 4.17.0rc2
  • Stefan Metzmacher <>
  • BUG #15125: Performance regression on contended path based operations
  • BUG #15146: Backport fileserver related changed to 4.17.0rc2
  • Andreas Schneider <>
  • BUG #15140: Fix issues found by coverity in smbstatus json code
  • BUG #15146: Backport fileserver related changed to 4.17.0rc2