Securing a Samba File and Print Server

Samba Security Modes

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 security mode you choose will depend on your environment and what you need the Samba server to accomplish.

Security = User

This section will reconfigure the Samba file and print server, from the section called “Samba File Server” and the section called “Samba 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 you chose the Samba Server task 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 restart smbd
sudo restart nmbd

Now when connecting to the shared directories or printers you should be prompted for a username and password.

[Note]

If you choose to map a network drive to the share you can check the Reconnect at Logon check box, which will require you to only enter the username and password once, at least until the password changes.

Share Security

There are several options available to increase the security for each individual shared directory. Using the [share] example, this section will cover some common options.

Groups

Groups define a collection of computers or users which 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 the section called “Adding and Deleting Users”.

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, if you wished to define a group named sysadmin in a certain section of the /etc/samba/smb.conf, you would do so by entering the group name as @sysadmin.

File Permissions

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, if you have defined a Samba share called share and wish to give read-only permissions to the group of users known as qa, but wanted to allow writing to the share by the group called sysadmin and the user named vincent, then you could edit the /etc/samba/smb.conf file, and 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 the user has been given explicit administrative permissions to.

For example, if you wanted to give the user melissa administrative permissions to the share example, you would edit the /etc/samba/smb.conf file, and 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 restart smbd
sudo restart nmbd
[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 Ubuntu 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 on a separate partition. If /srv, or wherever you have configured your share path, 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 you may or may not want.

Now from a Windows client you should notice the new file permissions are implemented. See the acl and setfacl man pages for more information on POSIX ACLs.

Samba AppArmor Profile

Ubuntu comes with the AppArmor security module, which provides mandatory access controls. The default AppArmor profile for Samba will need to be adapted to your configuration. For more details on using AppArmor see the section called “AppArmor”.

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 and reload it:

sudo aa-enforce /usr/sbin/smbd
cat /etc/apparmor.d/usr.sbin.smbd | sudo apparmor_parser -r

You should now be able 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 you configure Samba to share. Also, any errors will be logged to /var/log/syslog.

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