Linux Automount Howto

Posted on 4:15 PM by Bharathvn

his file describes the autofs automounter, how to configure it, and
points out some problems to avoid.
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Table of Contents


1. Introduction

1.1 Revision History
1.2 Automount - what and why?
1.3 Types of automounting

2. Administrative

2.1 Copyright information
2.2 Disclaimer

3. Installation

4. Configuration

5. The long wait for unmounting

6. Questions

6.1 I don't see /auto/floppy, or whatever mountpoint I'm looking for.
6.2 How do I see what's mounted?
6.3 I put in a win95 disk ("vfat") and it was autodetected as only a regular FAT disk.
6.4 My filesystem
6.5 What happens if I make / the directory for the automounter
6.6 Can I have two map files on the same directory?
6.7 I'm using SuSE 6.0 and needed
6.8 How do I set the permissions and ownership for the filesystem (ie)FAT.
6.9 How to browse the content of an autofs directory, when sub-dirs are not currently mounted ? Philippe Froidevaux
6.10 Who do I thank for autofs?
6.11 Where can I learn more about automounting?
6.12 where to contact the developpers" ?.Is there a mailing-list for autofs ?

7. Acknowledgements,feedback and dedication



______________________________________________________________________

1. Introduction

1.1. Revision History


· Versions below 1.5 - Authored by Don.

· Version 1.5 - Added the copyright and other minor details.Rahul
Sundaram took over maintainance.

· Version 1.5.1 - Added details to the question about VFAT.

· Version 1.5.2 - Revision history and other minor corrections.

· Version 1.6 - Added a few questions and answers.

1.2. Automount - what and why?

Automounting is the process where mounting and unmounting of certain
filesystems is done automatically by a daemon. If the filesystem is
unmounted, and a user attempts to access it, it will be automatically
(re)mounted. This is especially useful in large networked environments
and for crossmounting filesystems between a few machines (especially
ones which are not always online). It may also be very useful for
removable devices, or a few other uses, such as easy switching between
a forced-on ascii conversion mount of a dos filesystem and a forced-
off ascii conversion mount of the same dos fs. If you are new to
Linux and dont understand what mounting and deamons are,then refer to
some documentation regarding this.


1.3. Types of automounting

There are two types of automounters in linux; AMD and autofs. AMD is
the automount daemon, and supposedly works like the SunOS AMD. It is
implemented in user space, meaning it's not part of the kernel. It's
not necessary for the kernel to understand automounting if you NFS
mount to the local host, through the AMD daemon, which routes all
automount filesystem traffic through the NFS system. Autofs is a newer
system assisted by the kernel, meaning that the kernel's filesystem
code knows where the automount mount points are on an otherwise normal
underlying fs, and the automount program takes it from there. Only
autofs will be described in this mini-howto.


2. Administrative

2.2. Disclaimer

Although I have tried to do my best to bring out this howto in a good
form,I am not responsible for any damage due to the actions taken
based upon the information contained in this document. It is
impossible to test the things under all the configurations, so
probably some of the hints given in this document may be incorrect and
may not work on your system. In case you find anything wrong, let me
know it first.I will rewrite it as soon as possible.

This document is provided ``as is''. I put great effort into writing
it as accurately as I could, but you use the information contained in
it at your own risk. In no event shall I be liable for any damages
resulting from the use of this work.


3. Installation

Autofs is implemented in kernel-space, so your kernel must have
support compiled in. All versions of the kernel starting from 2.2.xx
supports autofs.
The automount program and its configuration files are also necessary;
using the rpms. The RedHat distribution has this package available as
part of the installation.


4. Configuration

Installing the RPM packages will get you to this point easily enough,
but here's the part you might not be sure about if you haven't done
this before.


There are two files in /etc, one called auto.master and one called
auto.misc. A sample auto.master looks like this:

/auto /etc/auto.misc --timeout=60



The first entry is not the mount point. It's where the set of mount
points (found in the second entry) are going to be. The third option
says that the mounted filesystems can try to unmount themselves 60
seconds after use. You will have to stop using the disk before
unmounting it.

Auto.misc is a "map file". The map file can have any name; this one is
named auto.misc because it originally controlled /misc. Multiple map
files can be defined in auto.master. My auto.misc looks like this:

kernel -ro,soft,intr ftp.kernel.org:/pub/linux
cd -fstype=iso9660,ro :/dev/cdrom
zip -fstype=auto :/dev/hdd4
floppy -fstype=vfat :/dev/fd0



The first column (the "key") is the mount point. In this case it would
be /auto/floppy or whatever. The middle set are the options; read the
mount manpage for details on this. And the last column specifies where
the fs comes from. The "kernel" entry is supposed to be an NFS mount.
The : on all the other lines means its a local device.



5. The long wait for unmounting

Some of you may be eyeing that 60 second timeout and thinking, that's
a long time to wait to eject a floppy.. Maybe I'll just sync the disks
and pop it out mounted and nobody will notice. Let me suggest saner
alternatives. First of all, you can change the timeout. But that
could be a little inefficient; telling the system to unmount stuff
after only 15 seconds or whatever. Depending on your setup, you may be
able to simply run the umount command as a normal user. But there is
actually a way to ask the automount program to umount. If you send
(with the program kill) the signal SIGUSR1 to the automount process,
it will unmount everything it can. But before people start making
unmount buttons on their window managers, there's a little problem.

The automount process is run by root, and it will only accept signals
from root. Half of the reason you're probably doing automounting is so
you can mount and unmount without being root. It would be easy to make
a suid-root C program which does the dirty deed. However, by using
sudo it is possible to allow users to send the proper kill signal. The
only problem is that sudo will not let you use to process subcommands,
which you would have to do to find the current PID. You should have a
program called killall, which will let you do this:
ALL ALL=NOPASSWD:/usr/bin/killall -USR1 automount


Otherwise, you would have to allow your users to send -SIGUSR1 to all
processes. That has various effects on programs; it will recycle some
window managers, but kills xemacs. So here's hoping there's no buffer
overruns in killall...


6. Questions



6.1. I don't see /auto/floppy, or whatever mountpoint I'm looking
for.

If automount is setup properly, whatever mount point you're looking
for will be there if you try and use it, even though you don't see it
when not in use. If you're browsing the directory with a graphical
tool, you may need to type in the name manually; most programs will
try what you give it, and the drive will be mounted before it notices.
Unfortunately not being able to choose from the available invisible
mount points is probably the major drawback of autofs. If it really
bugs you, edit the configuration files. (Hint, the ones that end in
.c for "configuration")

One workaround several people have tried is to create symbolic links
to where automount will create something once it's mounted. This will
likely prevent the program from complaining a directory doesn't exist
(if the mount works, that is) but careless directory listings will
cause filesystems to be mounted.


6.2. How do I see what's mounted?

The df command. mount with no options will do the same, plus show the
options its mounted with.



6.3. I put in a win95 disk ("vfat") and it was autodetected as only a
regular FAT disk.

This is not a problem with automount.The "auto" fs type does not
attempt a vfat mount before it successfully mounts an MS-DOS
filesystem. VFAT is an extension of the basic FAT filesystem inorder
to provide Windows 95 and Windows NT with long filenames.

According to one of the authors of mount, since mount is only a
wrapper around a system call which must specify the filesystem type,
it's still the responsibility of the user to come up with the fs type.
Having mount take a list of filesystems to try in order, rather than
the current "heuristic" is under consideration. Some users have simply
not compiled msdos into the kernel; this prevents it from being tested
prior to vfat. This will work for most people; a few actually need
msdos fs and there is actually a work around. You have to copy the
/proc/filesystems as /etc/filesystems and edit it to change the order
such that vfat appears before msdos.(Thanks Mark)

Ariel(aslinux At dsgml.com) writes

" o make mount try vfat before fat, just create or edit the file
/etc/filesystems


List, in order of priority, what filesystems you want the 'auto' fs
type to try.

Create the file with cp /proc/filesystems /etc/filesystems.

Edit the list to change the order. Put fs types that are detected with
great confidence such as ext2 (which means they are checked very
quickly), and those that are more common for you first. Just put vfat
before msdos and you're all set. Make sure to put both, in case you're
mounting something that has no vfat.

Mine looks like:

ext2 vfat msdos iso9660

****

I use a timeout of 1 second for removable devices. Create separate
maps, separated by the timeout you need.

You're thinking 1 second? That wastes a lot of resources - but it
doesn't. Remember that the system only unmounts when it's no longer
in use.

So a 1 second unmount means, as soon as no one is using the device,
it's unmounted.

Also, be very sure to put 'sync' as an option for the floppy!

i.e.

floppy -fstype=auto,sync,user,umask=002,gid=floppy :/dev/fd0"

That should make clear the answer.


6.4. My filesystem /auto/grumblesmurf is mounted and kill -SIGUSR1
won't unmount it.

It's being used by something. Root probably can't manually unmount it
either. If you're the one who caused it to be mounted (i.e. it can't
be someone else using it) look around for a shell that might be in
that directory. If there are none, look for something else
(particularly something that might have gone though that directory
like a directory browser) that might have left an invisible foot in
the door so to speak. If you've given up looking, try using the fuser
program.


6.5. What happens if I make / the directory for the automounter

I dont recommend it.If you want /grumblesmurf, then I suggest a
symbolic link. It would be much safer.


6.6. Can I have two map files on the same directory?

Not as far as I know. Try using one map file, with specific options
for individual entries.


6.7. I'm using SuSE 6.0 and needed ---timeout instead of --timeout

Another solution to "timeout not working" problems would be to add a
-t time option to the autofs script.

6.8. How do I set the permissions and ownership for the filesystem
(ie)FAT.

Check the man page for mount for some of the options, such as setting
the uid=value or umask=value options. One option that appears to be
missing for FAT filesystems is mode=value. Sorry. Check in with the
people who do mounting.


6.9. Philippe Froidevaux writes


" The author of autofs say "no way"

a possible hack is (from the newsgroup) :

"don't mount to /home/blauser but to /mount/.auto/blauser then add
a link /home/blauser -> /mount/.auto/blauser

now you can browse into /home and see the users-home, it then gets
mounted when you enter it.

problem with this: it isn't so clean and needs work for each user.

another solution is to use 'amd' in place of 'autofs', but amd is less
performant".