mirror of
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af8a78dddd
git-svn-id: svn://svn.linuxfromscratch.org/BLFS/trunk/BOOK@19660 af4574ff-66df-0310-9fd7-8a98e5e911e0
193 lines
8.4 KiB
XML
193 lines
8.4 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../../general.ent">
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%general-entities;
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]>
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<sect1 id="postlfs-devices" xreflabel="About Devices">
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<?dbhtml filename="devices.html"?>
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<sect1info>
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<othername>$LastChangedBy$</othername>
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<date>$Date$</date>
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</sect1info>
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<title>About Devices</title>
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<indexterm zone="postlfs-devices">
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<primary sortas="e-etc-udev-rules">/etc/udev/rules.d</primary>
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</indexterm>
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<para>Although most devices needed by packages in BLFS and beyond are set up
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properly by <application>udev</application> using the default rules installed
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by LFS in <filename class="directory">/etc/udev/rules.d</filename>, there are
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cases where the rules must be modified or augmented.</para>
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<para condition="html" role="usernotes">User Notes:
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<ulink url="&blfs-wiki;/aboutdevices"/></para>
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<sect2>
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<title>Multiple Sound Cards</title>
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<para>If there are multiple sound cards in a system, the "default"
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sound card becomes random. The method to establish sound card order
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depends on whether the drivers are modules or not. If the sound card
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drivers are compiled into the kernel, control is via kernel command line
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parameters in <filename>/boot/grub/grub.cfg</filename>. For example,
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if a system has both an FM801 card and a SoundBlaster PCI card, the
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following can be appended to the command line:</para>
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<screen><literal>snd-fm801.index=0 snd-ens1371.index=1</literal></screen>
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<para>If the sound card drivers are built as modules, the order can be
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established in the <filename>/etc/modprobe.conf</filename> file
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with:</para>
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<screen><literal>options snd-fm801 index=0
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options snd-ens1371 index=1</literal></screen>
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</sect2>
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<sect2 id="usb-device-issues">
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<title>USB Device Issues</title>
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<para>USB devices usually have two kinds of device nodes associated with
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them.</para>
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<para>The first kind is created by device-specific drivers (e.g.,
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usb_storage/sd_mod or usblp) in the kernel. For example, a USB mass storage
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device would be /dev/sdb, and a USB printer would be /dev/usb/lp0. These
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device nodes exist only when the device-specific driver is loaded.</para>
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<para>The second kind of device nodes (/dev/bus/usb/BBB/DDD, where BBB is
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the bus number and DDD is the device number) are created even if the device
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doesn't have a kernel driver. By using these "raw" USB device nodes, an
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application can exchange arbitrary USB packets with the device, i.e.,
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bypass the possibly-existing kernel driver.</para>
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<para>Access to raw USB device nodes is needed when a userspace program is
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acting as a device driver. However, for the program to open the device
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successfully, the permissions have to be set correctly. By default, due to
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security concerns, all raw USB devices are owned by user root and group
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usb, and have 0664 permissions (the read access is needed, e.g., for lsusb
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to work and for programs to access USB hubs). Packages (such as SANE and
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libgphoto2) containing userspace USB device drivers also ship udev rules
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that change the permissions of the controlled raw USB devices. That is, rules
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installed by SANE change permissions for known scanners, but not printers.
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If a package maintainer forgot to write a rule for your device,
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report a bug to both BLFS (if the package is there) and upstream, and
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you will need to write your own rule.</para>
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<para>There is one situation when such fine-grained access control with
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pre-generated udev rules doesn't work. Namely, PC emulators such as KVM,
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QEMU and VirtualBox use raw USB device nodes to present arbitrary USB
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devices to the guest operating system (note: patches are needed in order to
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get this to work without the obsolete /proc/bus/usb mount point described
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below). Obviously, maintainers of these packages cannot know which USB
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devices are going to be connected to the guest operating system. You can
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either write separate udev rules for all needed USB devices yourself, or
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use the default catch-all "usb" group, members of which can send
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arbitrary commands to all USB devices. </para>
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<para>Before Linux-2.6.15, raw USB device access was performed not with
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/dev/bus/usb/BBB/DDD device nodes, but with /proc/bus/usb/BBB/DDD
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pseudofiles. Some applications (e.g., VMware Workstation) still use only
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this deprecated technique and can't use the new device nodes. For them to
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work, use the "usb" group, but remember that members will have unrestricted
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access to all USB devices. To create the fstab entry for the obsolete
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usbfs filesystem:</para>
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<screen><literal>usbfs /proc/bus/usb usbfs devgid=14,devmode=0660 0 0</literal></screen>
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<note><para>Adding users to the "usb" group is inherently insecure, as they
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can bypass access restrictions imposed through the driver-specific USB
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device nodes. For instance, they can read sensitive data from USB hard drives
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without being in the "disk" group. Avoid adding users to this group, if
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you can.</para></note>
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</sect2>
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<sect2>
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<title>Udev Device Attributes</title>
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<para>Fine-tuning of device attributes such as group name and permissions
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is possible by creating extra <application>udev</application> rules,
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matching on something like this. The vendor and product can be found by
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searching the <filename class='directory'>/sys/devices</filename> directory
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entries or using <command>udevadm info</command> after the device has been
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attached. See the documentation in the current
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<application>udev</application> directory of
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<filename class='directory'>/usr/share/doc</filename> for details.</para>
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<screen><literal>SUBSYSTEM=="usb_device", SYSFS{idVendor}=="05d8", SYSFS{idProduct}=="4002", \
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GROUP:="scanner", MODE:="0660"</literal></screen>
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<note><para>The above line is used for descriptive purposes only. The
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scanner <application>udev</application> rules are put into place when
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installing <xref linkend='sane'/>.</para></note>
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</sect2>
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<!--
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<sect2>
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<title>Multiple Network Interfaces</title>
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</sect2>
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-->
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<sect2>
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<title>Devices for Servers</title>
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<para>In some cases, it makes sense to disable
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<application>udev</application> completely and create static devices.
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Servers are one example of this situation. Does a server need the
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capability of handling dynamic devices? Only the system administrator can
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answer that question, but in many cases the answer will be no.</para>
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<para>If dynamic devices are not desired, then static devices must be
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created on the system. In the default configuration, the
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<filename>/etc/rc.d/rcS.d/S10udev</filename> boot script mounts a
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<systemitem class="filesystem">tmpfs</systemitem> partition over the
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<filename class="directory">/dev</filename> directory. This problem can be
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overcome by mounting the root partition temporarily:</para>
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<warning><para>If the instructions below are not followed carefully, your
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system could become unbootable.</para></warning>
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<screen><userinput>mount --bind / /mnt
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cp -a /dev/* /mnt/dev
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rm /etc/rc.d/rcS.d/{S10udev,S50udev_retry}
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umount /mnt</userinput></screen>
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<para>At this point, the system will use static devices upon the next
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reboot. Create any desired additional devices using
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<command>mknod</command>.</para>
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<para>If you want to restore the dynamic devices, recreate the
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<filename>/etc/rc.d/rcS.d/{S10udev,S50udev_retry}</filename> symbolic
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links and reboot again. Static devices do not need to be removed (console
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and null are always needed) because they are covered by the <systemitem
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class="filesystem">tmpfs</systemitem> partition. Disk usage for devices is
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negligible (about 20–30 bytes per entry.)</para>
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</sect2>
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<sect2 id="dev-dvd">
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<title>Devices for DVD Drives</title>
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<para>If the initial boot process does not set up the
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<systemitem>/dev/dvd</systemitem> device properly, it can
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be installed using the following modification to the default udev rules.
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As the <systemitem class="username">root</systemitem> user, run:</para>
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<screen><userinput>sed '1d;/SYMLINK.*cdrom/ a\
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KERNEL=="sr0", ENV{ID_CDROM_DVD}=="1", SYMLINK+="dvd", OPTIONS+="link_priority=-100"' \
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/lib/udev/rules.d/60-cdrom_id.rules > /etc/udev/rules.d/60-cdrom_id.rules</userinput></screen>
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</sect2>
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</sect1>
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