Automount mini-Howto
don@sabotage.org
v0.3, 22 October 1998
This 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 Automount - what and why
1.2 Types of automounting
2. Installation
3. Configuration
4. That long wait to unmount
5. Questions
5.1 I don't see /auto/floppy, or whatever mountpoint I'm looking for.
5.2 How do I see what's mounted?
5.3 I put in a win95 disk ("vfat") and it autodetected it as only a regular FAT disk.
5.4 My filesystem
5.5 Who do I thank for autofs?
5.6 Where can I learn more about automounting?
______________________________________________________________________
1. Introduction
1.1. Automount - what and why
Automouting 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.
1.2. 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. Installation
Because autofs is implemented in kernel-space, your kernel must have
support compiled in. In 2.0.xx it is an experimental option, but
appears to be quite stable. In 2.1.xx (and presumably 2.2.xx) it is
not experimental.
The automount program and its configuration files are also necessary;
using the rpms (from RedHat, as part of the install) is a great way to
go. The automount program should be started by an rc script under the
/etc/rc.d/init.d directory . The rpm installs this, but you will need
to make sure it gets started, either by linking it from your rc?.d
directory, using Redhat's control-panel, or on another distribution by
getting the thing started anyway you care to. Non-rpm distros will
have to do whatever's applicable to their system. And don't look too
hard at what the rc script does; if you're reading this howto you
probably don't want to know.
3. Configuration
Installing the RPM's 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. My 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. They can't unmount if being used, of course.
Auto.misc is a "map file". 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.
4. That long wait to unmount
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. 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 an 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: (thanks for the suggestions)
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...
5. Questions
5.1. 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. 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")
5.2. 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.
5.3. I put in a win95 disk ("vfat") and it autodetected it as only a
regular FAT disk.
This is not a problem with automount. As of this writing, the "auto"
fs type does not attempt a vfat mount before it successfully mounts an
msdos filesystem. VFAT is the Win95 and WinNT long filenames crammed
into a FAT/MSDOS filesystem.
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. I'm sure that if
anyone wants to go to the effort of finding the owner(s) of the mount
program, your comments would be welcome. So for now this means that
you can't mount vfat unless you give up the ability to autodetect all
other fs's. Hopefully it will be fixed soon. In the mean time, feel
free to create multiple mount points with different fs types
specified.
5.4. My filesystem /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.
5.5. Who do I thank for autofs?
Not me. I didn't have anything to do with it. I just wanted to bring
everyone's attention to what a great job had been done with autofs,
and how easy it is to use. Compared to the original perpetrators of
AMD (Hint, they sell an overpriced unice with prehistoric versions of
free tools) the autofs is very well documented and the implementors
have my sincere thanks. Everything is stamped copyright Transmeta so
sorry I can't provide a credits list.
5.6. Where can I learn more about automounting?
There's a autofs tutorial at <
http://www.linuxhq.com/lg/issue24/nielsen.html>. See also am-utils at
<http://www.cs.columbia.edu/~ezk/am-utils>
(Thanks for these URLs)