The /etc/fstab file gives a list of all possible mounts that happen either automatically on boot or manually for removable storage devices. Each mount point is described by a line with several space-separated fields:
device to mount: this can be a local partition (hard drive, CD-ROM) or a remote filesystem (such as NFS).
This field is frequently replaced with the unique ID of the filesystem (which you can determine with blkid device) prefixed with UUID=. This guards against a change in the name of the device in the event of addition or removal of disks, or if disks are detected in a different order.
mount point: this is the location on the local filesystem where the device, remote system, or partition will be mounted.
type: this field defines the filesystem used on the mounted device. ext3, vfat, ntfs, reiserfs, xfs are a few examples.
BACK TO BASICS NFS, a network filesystem
NFS is a network filesystem; under Linux, it allows transparent access to remote files by including them in the local filesystem.
A complete list of known filesystems is available in the mount(8) man page. The swap special value is for swap partitions; the auto special value tells the mount program to automatically detect the filesystem (which is especially useful for disk readers and USB keys, since each one might have a different filesystem);
options: there are many of them, depending on the filesystem, and they are documented in the mount man page. The most common are
rw or ro, meaning, respectively, that the device will be mounted with read/write or read-only permissions.
noauto deactivates automatic mounting on boot.
user authorizes all users to mount this filesystem (an operation which would otherwise be restricted to the root user).
defaults means the group of default options: rw, suid, dev, exec, auto, nouser and async, each of which can be individually disabled after defaults by adding nosuid, nodev and so on to block suid, dev and so on. Adding the user option reactivates it, since defaults includes nouser.
backup: this field is almost always set to 0. When it is 1, it tells the dump tool that the partition contains data that is to be backed up.
check order: this last field indicates whether the integrity of the filesystem should be checked on boot, and in which order this check should be executed. If it is 0, no check is conducted. The root filesystem should have the value 1, while other permanent filesystems get the value 2.
Example 8.6. Example /etc/fstab file:
# /etc/fstab: static file system information.
#
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
# / was on /dev/sda1 during installation
UUID=c964222e-6af1-4985-be04-19d7c764d0a7 / ext3 errors=remount-ro 0 1
# swap was on /dev/sda5 during installation
UUID=ee880013-0f63-4251-b5c6-b771f53bd90e none swap sw 0 0
/dev/scd0 /media/cdrom0 udf,iso9660 user,noauto 0 0
/dev/fd0 /media/floppy auto rw,user,noauto 0 0
arrakis:/shared /shared nfs defaults 0 0
The last entry in this example corresponds to a network filesystem (NFS): the /shared/ directory on the arrakis server is mounted at /shared/ on the local machine. The format of the /etc/fstab file is documented on the fstab(5) man page.
GOING FURTHER Auto-mounting
The am-utils package provides the amd auto-mounting utility, able to mount removable media on demand when a user attempts to access their usual mount point. It will unmount these devices when no process is accessing them any longer.
Other auto-mounting utilities exist, such as automount in the autofs package.
Note also that GNOME, KDE, and other graphical desktop environments work together with the hal (Hardware Abstraction Layer) system, and can automatically mount removable media when they are connected.
8.9.6. locate and updatedb
The locate command can find the location of a file when you only know part of the name. It sends a result almost instantaneously, since it consults a database that stores the location of all the files on the system; this database is updated daily by the updatedb command (executed by the /etc/cron.daily/find script).
Since anybody can use locate, it is important to ensure hidden files are not revealed to the user. This is why the updatedb command runs with the limited permission of the nobody user, which is a classic pattern on Unix systems for this kind of task. Furthermore, the administrator can configure some directories to be skipped by simply listing them in the PRUNEDPATHS variable in /etc/updatedb.conf.
The slocate package goes even further, by replacing the locate command with a more secure version that only returns the names of files accessible to the user who employs it.
8.10. Compiling a Kernel
The kernels provided by Debian include the largest possible number of features, as well as the maximum of drivers, in order to cover the broadest spectrum of existing hardware configurations. This is why some users prefer to recompile the kernel in order to only include what they specifically need. There are two reasons for this choice. First, it may be to optimize memory consumption, since the kernel code, even if it is never used, occupies memory for nothing (and never “goes down” on the swap space, since it is actual RAM that it uses), which can decrease overall system performance. A locally compiled kernel can also limit the risk of security problems since only a fraction of the kernel code is compiled and run.
NOTE Security updates
If you choose to compile your own kernel, you must accept the consequences: Debian can not ensure security updates for your custom kernel. By keeping the kernel provided by Debian, you benefit from updates prepared by the Debian Project's security team.
Recompilation of the kernel is also necessary if you want to use certain features that are only available as patches (and not included in the standard kernel version).
8.10.1. Introduction and Prerequisites
Debian manages the kernel in the form of a package, which is not how kernels have traditionally been compiled and installed. Specific tools have therefore been developed for this purpose. They allow easy creation of a Debian package from Linux kernel sources, possibly adding patches along the way. Since the kernel remains under the control of the packaging system, it can then be removed cleanly, or deployed on several machines. Furthermore, the scripts associated with these packages automate the interaction with the bootloader.
To compile a Linux kernel the Debian way, you will need to use the tools included in the kernel-package package. Furthermore, the configuration step for the kernel requires the libncurses5-dev package. Finally, the fakeroot package will enable creation of the Debian package without using administrator's rights.