Using AutoFS with Active Directory

This article shows how to use AutoFS and Kerberos to automatically mount shared folders on a Microsoft Windows Server via Common Internet File System (CIFS).

First, we need to create a shared folder on the Windows Server. As a Windows administrator you probably already know how to do that, but for the sake of this article you can use the following PowerShell script as an example.

It will create a folder at C:\_shared and enable sharing on it for the groups Administrator (full access) and Users (read only access). This means that it requires an authenticated user to access the share.

# Create a directory that we want to share
New-Item 'C:\_shared' -ItemType Directory

# Share the directory
New-SMBShare -Name _shared -Path C:\_shared -FullAccess Administrators -ReadAccess Users

First, install CIFS dependencies:

$ sudo dnf install -y cifs-utils
$ sudo yum install -y cifs-utils
$ sudo apt-get install -y cifs-utils

Now make sure you have a valid ticket-granting ticket for your user:

$ kinit Administrator@AD.VM
Password for Administrator@AD.VM:
$ klist
Ticket cache: KCM:1730800500:40268
Default principal: Administrator@AD.VM

Valid starting       Expires              Service principal
03/08/2022 13:06:07  03/08/2022 23:06:07  krbtgt/AD.VM@AD.VM
    renew until 03/15/2022 13:06:07

When we have all prerequisites in place, we can move forward by creating the target folder and finally mounting the shared directory.

$ mkdir _shared
$ sudo mount -t cifs -o "user=Administrator@AD.VM,cruid=Administrator@ad.vm,sec=krb5,vers=3,multiuser" // _shared

Parameters explanation:

  • sec=krb5 simply says that we want to use Kerberos protocol to authenticate the user against the CIFS share

  • user defines the user that we want to authenticate as

  • cruid is the user whose credential cache will be used for authentication

  • vers=3 tells the cifs module to use Window’s SMB protocol v3 or later

  • multiuser means that the credentials of the user who is currently accessing the share will be used to control access

See also

You can read man mount.cifs(8) for more information.

In order to let autofs mount the folder automatically, we need to use a Kerberos keytab. Lets create a new user cifs that would be used to mount the CIFS share. We need to generate a keytab for this user and copy it to /etc/krb5.keytab on the Linux machine.

Import-Module ActiveDirectory

$password = "Secret123" | ConvertTo-SecureString -AsPlainText -Force
New-AdUser -Name "cifs" -UserPrincipalName "cifs@ad.vm" -AccountPassword $password -Enabled $true -PasswordNeverExpires $true -CannotChangePassword $true
ktpass -mapuser "cifs@ad.vm" -princ "cifs@AD.VM" -pass "Secret123" -ptype KRB5_NT_PRINCIPAL -crypto ALL -out C:\krb5.keytab

Once the keytab is created and present on the hosts, we can create a new autofs map. It will create a new mount point at /_shared.

Import-Module ActiveDirectory

New-ADOrganizationalUnit -Name "autofs" -Path "dc=ad,dc=vm"

New-ADObject -Name auto.master -Path 'ou=autofs,dc=ad,dc=vm' -Type nisMap -OtherAttributes @{'nisMapName'='auto.master'}
New-ADObject -Name '/-' -Path 'cn=auto.master,ou=autofs,dc=ad,dc=vm' -Type nisObject -OtherAttributes @{'nisMapName'='/-'; 'nisMapEntry'=''}

New-ADObject -Name -Path 'ou=autofs,dc=ad,dc=vm' -Type nisMap -OtherAttributes @{'nisMapName'=''}
New-ADObject -Name /_shared -Path ',ou=autofs,dc=ad,dc=vm' -Type nisObject -OtherAttributes @{'nisMapName'='/_shared'; 'nisMapEntry'='-fstype=cifs,sec=krb5,user=cifs@AD.VM,vers=3,multiuser ://'}
dn: ou=autofs,dc=ad,dc=vm
objectClass: top
objectClass: organizationalUnit
ou: autofs

dn: cn=auto.master,ou=autofs,dc=ad,dc=vm
objectClass: top
objectClass: automountMap
cn: auto.master
nisMapName: auto.master

dn: cn=/-,cn=auto.master,ou=autofs,dc=ad,dc=vm
objectClass: top
objectClass: automount
nisMapName: /-
cn: /-

objectClass: top
objectClass: automountMap

dn: cn=/_shared,,ou=autofs,dc=ad,dc=vm
objectClass: top
objectClass: automount
nisMapName: /_shared
nisMapEntry: -fstype=cifs,sec=krb5,user=cifs@AD.VM,vers=3,multiuser ://
cn: /_shared

Then we need to restart autofs, access the mount point and check that it has been successfully mounted.

$ sudo systemctl enable autofs
$ sudo systemctl restart autofs
$ cd /_shared
$ mount
... on /_shared type autofs (rw,relatime,fd=5,pgrp=36940,timeout=300,minproto=5,maxproto=5,direct,pipe_ino=72103)
// on /_shared type cifs (rw,relatime,vers=3.1.1,sec=krb5,cruid=0,cache=strict,multiuser,uid=0,noforceuid,gid=0,noforcegid,addr=,file_mode=0755,dir_mode=0755,soft,nounix,serverino,mapposix,noperm,rsize=4194304,wsize=4194304,bsize=1048576,echo_interval=60,actimeo=1,user=cifs@AD.VM)