Autodir HOWTO

Venkata Ramana Enaganti


Revision History
Revision 1.042007-5-25Revised by: VRE
Minor updates
Revision 1.032006-09-15Revised by: GaMA
Review requested by author.
Revision 1.022004-12-25Revised by: VRE
Minor updates
Revision 1.002004-09-23Revised by: VRE
Initial release, reviewed by Rahul Sundaram at TLDP
Revision 0.322004-09-13Revised by: VRE
New sections like requirements and others.
Revision 0.102004-06-24Revised by: VRE
second draft
Revision 0.92004-06-11Revised by: VRE
first draft

This HOWTO is about the Autodir installation, configuration and other issues related to Autodir. The Autodir system is often applied for making home directories available in an easy way.

Table of Contents
1. Introduction
1.1. Copyright and License
1.2. Disclaimer
1.3. Feedback
1.4. New Versions of this Document
1.5. Credits / Contributors
2. Before going into the details...
3. Why not pam_mkhomedir?
4. Where can Autodir be used?
5. What Autodir is not
6. Differences between Autodir and Autofs
7. How it works
8. Some definitions
9. Directory organization in the real base directory
10. Virtual directory expiration
11. Backup support
12. Backup program requirements
13. Module options
14. Autodir requirements
15. Autofs kernel module
16. Importing user and group accounts
17. Getting Autodir
18. Managing home directories
18.1. Base directories for autohome
18.2. Directory organization
18.3. Misc suboptions for autohome
18.4. Example
19. Managing group directories
20. Autodir options
21. Backup options
22. Examples
23. RPM specific
24. Further Information

1. Introduction

Autodir offers a simple and effective means to create directories like home directories in a transparent manner. It relies on the autofs protocol for its operation.

This document explains how to create directories on demand using Autodir in a transparent way to the applications. This document also explains using the transparent backup feature that is possible with Autodir, without system downtime for backup purpose; this applies for all directories managed by Autodir.

1.1. Copyright and License

This document, Autodir HOWTO, is copyrighted (c) 2004 by Venkata Ramana Enaganti. This work is licensed under the Creative Commons Attribution License. To view a copy of this license, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Linux is a registered trademark of Linus Torvalds.

1.2. Disclaimer

No liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors and inaccuracies, that could be damaging to your system. Proceed with caution, and although this is highly unlikely, the author(s) do not take any responsibility.

All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark. Naming of particular products or brands should not be seen as endorsements.

1.3. Feedback

Feedback is most certainly welcome for this document. Send your additions, comments and criticisms to the following email address : .

1.4. New Versions of this Document

The latest version of this HOWTO will be made available from

1.5. Credits / Contributors

In this document, I have the pleasure of acknowledging for language and technical review work:

2. Before going into the details...

After releasing intraperson beta, I started working on a administration guide that deals with the administration aspects of intraPerson. For more details check But I was stuck with one simple thing. It is easy to create users in LDAP - at least I think so - but how to create home directories for those users in LDAP wherever those LDAP accounts are imported?

I found some solutions, but I was not satisfied as every solution has serious drawbacks. After leafing through the Autofs documents and hacking a bit, I came to the conclusion that the Autofs protocol might offer a much better solution to this challenge.

The result is Autodir, based on the Autofs protocol.

3. Why not pam_mkhomedir?

The PAM module pam_mkhomedir uses Pluggable Authentication Module architecture for its operation. As such, there are some limitations associated with it. For instance:

Finally, Autodir is much wider in scope and supports many more interesting features.

4. Where can Autodir be used?

5. What Autodir is not

Autodir can create directories but it does not remove them once user and/or group entries are removed from the system accounts database. Use custom made scripts from cron for this.

6. Differences between Autodir and Autofs

Issue arises when you are already using the Autofs package, handling the mounts of (home) directories. Let's look at the differences between the two packages:

7. How it works

Autodir uses modules to get specific functionality. The core Autodir implements generic functionality that modules can exploit and add specific functionality to.

At any moment only one module can be added to Autodir. If there are two modules, for example autohome and autogroup, then two processes of Autodir should be created so that each process can have one of the required modules attached to it.

For further explanation I chose the autohome module which handles transparent home directory creation.


  • The autohome module creates user home directories on demand if these do not exist already.

  • It is assumed that user accounts exists, but the accompanying home directories do not - either because these accounts were created with the -M option with useradd or because these accounts were imported from LDAP, NIS or some other external database for which home directories are yet to be created.

  • It also assumed for this explanation only that all user home directories are expected to be in the /home directory.


Keep it Simple: Some fine details are intentionally kept aside to make the explanation easy to understand.

First the autofs file system is mounted on the /home directory by Autodir. The Linux kernel is informed that /home is managed by a user space application, Autodir, from now on.


Do not bother too much about the autofs file system if you do not understand about it. Just think of it as a special kind of file system, similar to memory based file systems but with some additional special properties.

                                |  Linux Kernel  |
                                  /        \
                                 /          \
                                /            \
                               /              \
   +-------------+       +--------+       +------------+        +-----------------+
   | Application |------>| /home  |<----->|  Autodir   |<------>| autohome module |
   +-------------+       +--------+       +------------+        +-----------------+
                              \                                          /
                               \  +----------------+                    /
                                +-|   /autohome    |<------------------+

Whenever an application or daemon needs access to a user's home directory, for example /home/userhome1, they directly enter into /home/userhome1 to access it. The kernel, which notices this, informs Autodir if the userhome1 directory does not yet exist already in /home.

Autodir, in turn, passes this request to the autohome module. The autohome module does not touch the /home directory. Instead it manages real home directories somewhere else, for example in /autohome as shown in the above figure.

The autohome module creates a real home directory if it does not exist in the /autohome directory. After it is successfully created or failed to be created, whatever the outcome, autohome reports back to Autodir. When the directory creation task has completed successfully, the path to real home directory is provided to Autodir.

If the autohome module reports success, Autodir creates userhome1 directory under /home and mounts the real home directory from /autohome on it. At the end of the process, Autodir informs the kernel whether the whole operation was successful or not. Accordingly, the kernel allows applications to enter the directory, or, in case of failure, it reports that no such directory exists.

8. Some definitions

Before going further it is better to understand the following terms to simplify explanation.

Virtual directories

These directories do not exist on disk. Instead these are created and deleted on demand in memory. If the system reboots all these directories vanish. In the previous figure, all directories under /home are virtual directories.

Virtual base directory

This is the directory that holds all Virtual directories. This directory does exist on disk and therefore it remains even after reboot. In the previous figure /home is virtual base directory.

Real directories

These are the directories that actually reside on the disk. Even after reboot, these remain intact. In the previous figure all directories created under /autohome are real directories.

Real base directory

This is the directory that holds all real directories. In the above figure /autohome is real base directory.

Each virtual directory is mapped to a real directory. This means that whatever is written to or modified in the virtual directory is actually sent to the real directory.

On reboot of the system real directories and their content remain intact. But virtual directories are again created on demand, exactly as they were before.

Virtual directories are removed if these are not used for a specified period of time, and created again if necessary. When a Virtual directory is removed, the backup program is started on the corresponding real directory - if backup is configured.


Applications should access only virtual directories. Real directories are hidden from applications. Only the root user can see them. There is one exception: backup programs always access the real directories only.

9. Directory organization in the real base directory

Why should there be any special organization in the real base directory? If we just create all real directories in one real base directory there could be a performance penalty when there are a large number of real directories to be created. File systems like ext2/ext3 are not optimized for this kind of flat directory structure.

It would be much better if the real base directory is divided into more subdirectories, or even to divide these subdirectories again into more subdirectories. And in the final subdirectories actual home directories are kept.

There are three types of directory organization:

level 0

Actually no organization. All home directories are created directly under real base directory.

level 1

The Real base directory is divided into more subdirectories. The subdirectory names are derived from the first character of the final directory to be created. For example, if the user1 directory is to be created, first a directory named 'u' is created under real base directory. Then in that subdirectory the actual directory user1 is created as /<real_base_directory>/u/user1.

level 2

Same as level 1 organization but after the first level of subdirectories, a second level of subdirectories is created. The names here are the first two characters of the final directory to be created. For example, for user user1, as in the above example, the /<real_base_directory>/u/us/user1 directory is created.

10. Virtual directory expiration

When an application tries to access a virtual directory in a virtual base directory, Autodir creates the virtual directory in it if it does not exist already and mounts the real directory on it from the real base directory. Once this is done, a timer is started. If the virtual directory is not accessed from the virtual base directory by any application for the specified period of time, this directory is removed and the corresponding real directory in the real base directory is marked for backup.

The time period to wait for expiration can be given through a command line option to Autodir.

11. Backup support

Autodir supports backup program launching when a virtual directory is removed after a period of inactivity. Removal of the virtual directory is itself an assurance that no other application can access the content and modify it.

Just like there is wait time for expiring a virtual directory, for backup Autodir also waits during some time after the expiry of the virtual directory, prior to starting the backup. This time period can be configured through a command line option to Autodir.

By design, backup programs are expected to operate on real directories but not on virtual directory. If the backup program try to access a virtual directory, then Autodir assumes some regular application is in need of that directory and the backup program is killed, even if the virtual directory accessing process is the backup program itself.

A separate backup process for each real directory is used. The backup program can be given arguments of real directories on which to operate.


Backup support is independent of any particular module being used. It is applicable to all modules with Autodir.

ImportantBackup = real!

Backup programs should never access virtual directory or virtual base directory.

CautionTime off

The backup feature is not much use if the virtual directories are being accessed by applications all the time.

12. Backup program requirements

Autodir demands some extra requirements from the backup program being used: when the backup is working on the real directory, with corresponding expired virtual directory, and that virtual directory is requested again by an application while the backup is running, the backup process is killed. First a SIGTERM is sent to gracefully stop it. But if it does not shut down in time - and it has one second to do this - a SIGKILL will be sent, which is guaranteed to stop the backup.

NoteOnly when the backup has stopped the application is given access to the requested virtual directory.


Whatever backup is used, it should be able to recover from this signal gracefully, not causing unrecoverable side effects.

One more important issue is the environment in which the backup runs. All backup programs run as root. But at the same time all unnecessary root privileges are taken away using POSIX capabilities. In other words these backup programs can read any file or directory that belongs to any user on the system and nothing more than that. Other than that, the backup process behaves like an ordinary user level process.

13. Module options

There are two kinds of options that can be passed to Autodir. In the first type, options are for autodir itself and are common irrespective of which module is used. The other type of options are specific to the module being used. These options are called suboptions and are passed to the module being used; they are different from the main -o option. This is similar to the suboptions used with the mount command.

For example, suboptions to the example module autohome can be passed as follows:

-o 'realpath=/tmp/autohome,level=2,noskel'

Here realpath, level and noskel are suboptions for autohome module.

14. Autodir requirements

15. Autofs kernel module

Autodir uses the autofs kernel module for its operation. The kernel module autofs must be loaded before starting autodir.

This can be done as the root user, using the modprobe command as follows:

# modprobe autofs

16. Importing user and group accounts

If user and group accounts reside in a centralized database these must be imported before starting Autodir. How to do this is out of the scope of this HOWTO. There are a number of documents which explain in a clear way how to do this.

17. Getting Autodir

At this moment Autodir is available in tar and rpm formats. More information can be found at

After downloading the source, follow these simple steps to install :

NoteNo go?

The configure script checks for the required libraries. If these are not present it will stop.

18. Managing home directories

This section will explain how to configure Autodir so that user home directories are created on demand. For this purpose the autohome module, which deals with specifics of home directory creation, is used.

To load the autohome module with Autodir, use the -m option. For example, -m /usr/lib/autodir/

NoteUser/home matching

When an application tries to access a home directory, that home directory is used to check if there is any user with the same user name as the directory name being accessed. If a user name exists, then the home directory is created. Otherwise the message "no such file or directory" is reported back to the application.

NoteUser accounts

Autohome does not deal with the creation of user accounts on local systems, in LDAP or in any other database. It only deals with creating home directories once these accounts exist and are imported to the local system from databases like LDAP and NIS.


It is worth mentioning one limitation of the autohome module. It expects that user name and home directory are related to each other. For example, for user user1 the home directory should be /home/user1 or /some/directory/name/user1 but not /some/directory/name/userhome1. This can be supported but it will be a burden on system resources as each password entry has to be examined from first to last.

NoteKnowing when not to use autohome

If the existing user password database is such that user home directories are distributed under different base directories, for example /home/class1/user1, /home/class2/user2332, then autohome configuration becomes complicated and is not recommended.

18.1. Base directories for autohome

The next step in the setup procedure is to decide where the virtual base directory and real base directory for home directory creation will be located.

What are the virtual base directory and the real base directory in the context of the autohome module?

This all depends on how user accounts are created. If a user account created for user name user1 with home directory /home/user1 then /home will become the virtual base directory.

Then what is the real base directory? This can be any directory. The only thing that you need to keep in mind is that there should be enough space, as all actual files are stored here instead of in the virtual base directory.

In most server configurations /home is a separate partition. But if /home is the virtual base directory, then files are not stored in that directory! The solution is not to mount a partition on /home but instead mount it somewhere else and make it the real base directory.

The Autodir option -d is used to specify the virtual base directory. For example: autodir -d /home assumes that /home is the virtual base directory.

It is somewhat tricky to specify the real base directory. The real base directory is managed by the autohome module so this option must be passed to the module through module suboptions. If the real base directory is /var/autohome then it is specified with the option -o as -o realpath=/var/autohome.

18.2. Directory organization

Refer to directory organization under the real base directory for a detailed explanation of this topic.

autohome does support this kind of organization. The suboption used to specify the desired directory organization the level suboption, for instance: -o level=2.

18.3. Misc suboptions for autohome

The suboption skel can be used if the skeleton path is not the default value /etc/skel: -o skel=/some/other/dir.

The suboption noskel can be used with -o to indicate not to copy any skeleton files to the home directories when these are created.

18.4. Example

First, import your user accounts from a centralized database, for instance from LDAP.

Next, the autofs kernel module must be loaded into the Linux kernel. This can be done as described in autofs kernel module section.

If /home is to be used for home directories then /home will become the virtual directory; this is specified to autodir with the -d /home option.

Assuming that the autohome module is located in /usr/lib/autodir/, this module can be loaded with autodir as -m /usr/lib/autodir/ Note that the full path for the module is given.

The actual location of the real home directories is given with the realpath suboption. If it is /autohome, the location can be specified as realpath=/autohome.

With all these options autodir can be started as:

# autodir -d /home 						\
		-m /usr/lib/autodir/ 		\
		-o 'realpath=/autohome'			 	\

Once Autodir is started, initially the /home directory will be empty. Whether Autodir is working properly can be tested by changing directories to one of the home directories, as the root user or as the owner of the home directory.

19. Managing group directories

The autogroup module is used for creating group directories on demand for common group access. It can be used with Samba, for example, to dynamically create shared directories for a group of people.


The autogroup module checks for the requested directory in valid groups from the system group database.

TipUsing autogroup for the creation of home directories

The autogroup module can be used to create home directories as well, provided that user private groups exist for each user. This way all group and home directories can be created in one place with one module. However, no skeleton files will be copied and the autogroup suboption nopriv should not be used.

The autogroup configuration is the same as the autohome module, but unlike autohome, the virtual base directory can be placed anywhere and any name can be given to it. It is not dictated by system accounts.

The module autogroup can be used with Autodir using the -m option. For example, -m /usr/lib/autodir/

All suboptions explained in managing home directories are the same for autogroup, except skel and noskel, as these are meaningless for the autogroup module. Additionally, there are other suboptions specific to autogroup. These are given below.


Some Linux installations use user private groups. If directories for these groups are not to be created, then use this suboption.

20. Autodir options

In this section some of the options to Autodir are explained. Backup options are explained in the backup section.


Specifies the virtual base directory. If this path does not exist, it will be created. An absolute path is expected for this option.


Expiration timeout for virtual directories. For more details refer to virtual directory expiration.


Module to be used with Autodir. Currently autohome and autogroup are available. The full path to the module is expected.


All suboptions that are to be passed to module are given here. This option passing syntax is similar to that of the mount command with its -o option. See specific module sections for more info.


Run in the foreground and log all messages to the console. For debugging purpose and to see how Autodir works.


This option expects a path name to a file in which Autodir will write its process id.


Help about all options supported by Autodir.


Version information about Autodir.

21. Backup options

These options are passed to Autodir to request backup support.


This is the main option to specify the backup program path and arguments to it. The path given should be an absolute path, otherwise Autodir does not accept it.


Whenever a virtual directory is not used for a period of time, it is assumed inactive and it is unmounted. After unmounting the directory, whether to launch the backup immediately or to wait some more time is decided with this option. It takes arguments in seconds. It is the minimum time to wait before starting backup after virtual directory expiration. It should not exceed one day.


This is the priority to be given to the backup process. This is in the range of 1 to 40 inclusive. Lower value means higher priority and vice versa. The default value is 30.


This restricts the maximum number of backup processes at any given time. The default is 150.

CautionUsing quotes

The argument for the -b option is includes the absolute backup program path as well as its own arguments. Therefore it is recommended to use single quotes around this argument.

The -b option takes a path to executable file as well as arguments to it. However, the arguments are interpreted for a sequence of %x characters and replaced with predefined strings as follows:


Replaced with the virtual directory name.


Replaced with an absolute path to the real directory.


Replaced with host name.


Others are fed to strftime. See the man page for strftime for more information.

22. Examples

# autodir -d /home 						\
		-m /usr/lib/autodir/ 		\
		-t 1000						\
		-f 						\
		-o 'realpath=/autohome,level=1,skel=/etc/skel' 	\
		-l /var/run/

# autodir -d /home						\
		-m /usr/lib/autodir/			\
		-t 300						\
		-b '/bin/tar cf /tmp/%N%F.tar %L'		\
		-w 600						\
		-o 'realpath=/tmp/autohome,level=2,noskel'	\
		-l /var/run/

# autodir -d /var/abase/ 					\
		-m /usr/lib/autodir/		\
		-t 300						\
		-b '/bin/tar cf /tmp/%N%F.tar %L'		\
		-w 86400					\
		-o 'nopriv,nosetgid,realpath=/var/realbase,level=0' 

23. RPM specific

Autodir can be installed from rpms as follows:

# rpm -ivh autodir-0.28-4.i386.rpm

When installed from rpms, two startup scripts are provided: /etc/rc.d/init.d/autohome and /etc/rc.d/init.d/autogroup. The first one is for starting Autodir with the autohome module, the second for starting with the autogroup module.

The script configuration files /etc/sysconfig/autohome and /etc/sysconfig/autogroup can be used to specify what options can be passed to Autodir.

24. Further Information