
There are two security levels available to the Common Internet Filesystem (CIFS) network protocol user-level and share-level. Samba's security mode implementation allows more flexibility, providing four ways of implementing user-level security and one way to implement share-level:
security = user: requires clients to supply a username and password to connect to shares. Samba user accounts are separate from system accounts, but the libpam-smbpass package will sync system users and passwords with the Samba user database.
security = domain: this mode allows the Samba server to appear to Windows® clients as a Primary Domain Controller (PDC), Backup Domain Controller (BDC), or a Domain Member Server (DMS). See the section called “Samba as a Domain Controller” for further information.
security = ADS: allows the Samba server to join an Active Directory® domain as a native member. See the section called “Samba Active Directory Integration” for details.
security = server: this mode is left over from before Samba could become a member server, and, due to some security issues, should not be used. See the Server Security section of the Samba guide for more details.
security = share: allows clients to connect to shares without supplying a username and password.
The preferred security mode depends on the environment and what the Samba server needs to accomplish.
This section will reconfigure the Samba file and print server, from the section called “Samba File Server” and the Print Server, to require authentication.
First, install the libpam-smbpass package, which will sync the system users to the Samba user database:
sudo apt-get install libpam-smbpass
Note
If the Samba Server task was chosen during installation, libpam-smbpass is already installed.
Edit /etc/samba/smb.conf
, and in the
[share] section change:
guest ok = no
Finally, restart Samba for the new settings to take effect:
sudo /etc/init.d/samba restart
Now when connecting to the shared directories or printers, there will be a prompt for a username and password.
Note
To map a network drive to the share, “Reconnect at Logon” should be checked, which will require the username and password to be entered just once — at least until the password changes.
There are several options available to increase the security for each shared directory. Using the [share] example, this section will cover some common options.
Groups define a collection of computers or users who have a common level of access to particular network resources and offer a level of granularity in controlling access to such resources. For example, if a group qa is defined and contains the users freda, danika, and rob, and a second group support is defined and consists of users danika, jeremy, and vincent, then certain network resources configured to allow access by the qa group will subsequently enable access by freda, danika, and rob, but not jeremy or vincent. Since the user danika belongs to both the qa and support groups, she will be able to access resources configured for access by both groups, whereas all other users will have only access to resources explicitly allowing the group they are part of.
By default, Samba looks for the local system groups
defined in /etc/group
to determine which users belong to
which groups. For more information on adding and removing users from groups,
see Basics.
When defining groups in the Samba configuration
file, /etc/samba/smb.conf
, the recognized syntax is to
preface the group name with an "@" symbol. For example, to define a group named
sysadmin in a certain section of the
/etc/samba/smb.conf
, the group name would be entered as
@sysadmin.
File permissions define the explicit rights a computer or user has to a
particular directory, file, or set of files. Such permissions may be defined by
editing the /etc/samba/smb.conf
file and specifying the
explicit permissions of a defined file share.
For example, for a defined Samba share called
share and the need to give read-only permissions to the group of users known as
qa, while allowing write permissions to the
share by the group called sysadmin and the
user named vincent, then the
/etc/samba/smb.conf
file could be edited to add the
following entries under the [share] entry:
read list = @qa write list = @sysadmin, vincent
Another possible Samba permission is to declare administrative permissions to a particular shared resource. Users having administrative permissions may read, write, or modify any information contained in the resource where they have been given explicit administrative permissions.
For example, to give the user melissa
administrative permissions to the share
example, the /etc/samba/smb.conf
file would be edited to
add the following line under the [share] entry:
admin users = melissa
After editing /etc/samba/smb.conf
, restart
Samba for the changes to take effect:
sudo /etc/init.d/samba restart
Note
For the read list and write list to work, the Samba security mode must not be set to security = share.
Now that Samba has been configured to limit which groups have access to the shared directory, the filesystem permissions need to be updated.
Traditional Linux™ file permissions do not map well to
Windows NT Access Control Lists®
(ACLs). Fortunately POSIX™
ACLs are available on Kubuntu servers providing more
fine-grained control. For example, to enable ACLs on
/srv
an EXT3™ filesystem, edit
/etc/fstab
adding the acl option:
UUID=66bcdd2e-8861-4fb0-b7e4-e61c569fe17d /srv ext3 noatime,relatime,acl 0 1
Then remount the partition:
sudo mount -v -o remount /srv
Note
The above example assumes /srv
is on a separate partition.
If /srv
— or wherever the share path is configured
— is part of the /
partition, a reboot may be
required.
To match the Samba configuration above, the
sysadmin group will be given read, write, and execute
permissions to /srv/samba/share
, the
qa group will be given read and execute permissions, and
the files will be owned by the username melissa. Enter the
following in a terminal:
sudo chown -R melissa /srv/samba/share/ sudo chgrp -R sysadmin /srv/samba/share/ sudo setfacl -R -m g:qa:rx /srv/samba/share/
Note
The setfacl command above gives
execute permissions to all files in the
/srv/samba/share
directory, which may or may not be
desirable.
A Windows® client will show that the new file permissions are implemented. See the acl and setfacl man pages for more information on POSIX™ ACLs.
Kubuntu comes with the AppArmor security module, which provides mandatory access controls. The default AppArmor profile for Samba will need to be adapted to the proper configuration. For more details on using AppArmor, please refer to the wiki.
There are default AppArmor profiles for
/usr/sbin/smbd
and /usr/sbin/nmbd
,
the Samba daemon binaries, as part of the
apparmor-profiles packages. To install the package
from a terminal prompt, enter:
sudo apt-get install apparmor-profiles
Note
This package contains profiles for several other binaries.
By default the profiles for smbd and nmbd are in complain mode, allowing Samba to work without modifying the profile, and only logging errors. To place the smbd profile into enforce mode and have Samba work as expected, the profile will need to be modified to reflect any directories that are shared.
Edit /etc/apparmor.d/usr.sbin.smbd
, adding information for
[share] from the file server example:
/srv/samba/share/ r, /srv/samba/share/** rwkix,
Now place the profile into enforce mode and reload it:
sudo aa-enforce /usr/sbin/smbd cat /etc/apparmor.d/usr.sbin.smbd | sudo apparmor_parser -r
It is now possible to read, write, and execute files in the shared directory as
normal, and the smbd binary will have access to only
the configured files and directories. Be sure to add entries for each directory
that Samba is configured to share. Any errors will
be logged to /var/log/syslog
.
For in depth Samba configurations, see the Samba HOWTO Collection.
The guide is also available in printed format.
O'Reilly's Using Samba is also a good reference.
Chapter 18 of the Samba HOWTO Collection is devoted to security.
For more information on Samba and ACLs, see the Samba ACLs page.