[e73e361] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[6732c094] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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[e73e361] | 4 | <!ENTITY % general-entities SYSTEM "../../general.ent">
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| 5 | %general-entities;
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| 6 | ]>
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| 7 |
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| 8 | <sect1 id="postlfs-devices" xreflabel="About Devices">
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| 9 | <?dbhtml filename="devices.html"?>
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| 10 |
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| 11 | <sect1info>
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| 12 | <date>$Date$</date>
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| 13 | </sect1info>
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| 14 |
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| 15 | <title>About Devices</title>
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| 16 |
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| 17 | <indexterm zone="postlfs-devices">
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| 18 | <primary sortas="e-etc-udev-rules">/etc/udev/rules.d</primary>
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| 19 | </indexterm>
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| 20 |
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[81a73ed8] | 21 | <para>
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| 22 | Although most devices needed by packages in BLFS and beyond are set up
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| 23 | properly by <application>udev</application> using the default rules
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| 24 | installed by LFS in <filename
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| 25 | class="directory">/etc/udev/rules.d</filename>, there are cases where
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| 26 | the rules must be modified or augmented.
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| 27 | </para>
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[e73e361] | 28 |
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| 29 | <para condition="html" role="usernotes">User Notes:
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| 30 | <ulink url="&blfs-wiki;/aboutdevices"/></para>
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| 31 |
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| 32 | <sect2>
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| 33 | <title>Multiple Sound Cards</title>
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| 34 |
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[81a73ed8] | 35 | <para>
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| 36 | If there are multiple sound cards in a system, the "default"
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| 37 | sound card becomes random. The method to establish sound card order
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| 38 | depends on whether the drivers are modules or not. If the sound card
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| 39 | drivers are compiled into the kernel, control is via kernel command line
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| 40 | parameters in <filename>/boot/grub/grub.cfg</filename>. For example,
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| 41 | if a system has both an FM801 card and a SoundBlaster PCI card, the
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| 42 | following can be appended to the command line:
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| 43 | </para>
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[e73e361] | 44 |
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| 45 | <screen><literal>snd-fm801.index=0 snd-ens1371.index=1</literal></screen>
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| 46 |
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[81a73ed8] | 47 | <para>
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| 48 | If the sound card drivers are built as modules, the order can be
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| 49 | established in the <filename>/etc/modprobe.conf</filename> file with:
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| 50 | </para>
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[e73e361] | 51 |
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| 52 | <screen><literal>options snd-fm801 index=0
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| 53 | options snd-ens1371 index=1</literal></screen>
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| 54 |
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| 55 | </sect2>
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| 56 |
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[d20c871] | 57 | <sect2 id="usb-device-issues">
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| 58 | <title>USB Device Issues</title>
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| 59 |
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[81a73ed8] | 60 | <para>
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| 61 | USB devices usually have two kinds of device nodes associated with them.
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| 62 | </para>
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| 63 |
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| 64 | <para>
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| 65 | The first kind is created by device-specific drivers (e.g.,
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| 66 | usb_storage/sd_mod or usblp) in the kernel. For example, a USB mass
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| 67 | storage device would be /dev/sdb, and a USB printer would be
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| 68 | /dev/usb/lp0. These device nodes exist only when the device-specific
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| 69 | driver is loaded.
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| 70 | </para>
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| 71 |
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| 72 | <para>
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| 73 | The second kind of device nodes (/dev/bus/usb/BBB/DDD, where BBB is
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| 74 | the bus number and DDD is the device number) are created even if the
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| 75 | device doesn't have a kernel driver. By using these "raw" USB device
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| 76 | nodes, an application can exchange arbitrary USB packets with the
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| 77 | device, i.e., bypass the possibly-existing kernel driver.
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| 78 | </para>
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| 79 |
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| 80 | <para>
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| 81 | Access to raw USB device nodes is needed when a userspace program is
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| 82 | acting as a device driver. However, for the program to open the device
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| 83 | successfully, the permissions have to be set correctly. By default, due
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| 84 | to security concerns, all raw USB devices are owned by user root and
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| 85 | group usb, and have 0664 permissions (the read access is needed, e.g.,
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| 86 | for lsusb to work and for programs to access USB hubs). Packages (such
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| 87 | as SANE and libgphoto2) containing userspace USB device drivers also
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| 88 | ship udev rules that change the permissions of the controlled raw USB
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| 89 | devices. That is, rules installed by SANE change permissions for known
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| 90 | scanners, but not printers. If a package maintainer forgot to write
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| 91 | a rule for your device, report a bug to both BLFS (if the package is
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| 92 | there) and upstream, and you will need to write your own rule.
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| 93 | </para>
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| 94 |
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| 95 | <para>
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| 96 | There is one situation when such fine-grained access control with
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| 97 | pre-generated udev rules doesn't work. Namely, PC emulators such as KVM,
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| 98 | QEMU and VirtualBox use raw USB device nodes to present arbitrary USB
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| 99 | devices to the guest operating system (note: patches are needed in order
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| 100 | to get this to work without the obsolete /proc/bus/usb mount point
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| 101 | described below). Obviously, maintainers of these packages cannot know
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| 102 | which USB devices are going to be connected to the guest operating
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| 103 | system. You can either write separate udev rules for all needed USB
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| 104 | devices yourself, or use the default catch-all "usb" group, members
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[8558044] | 105 | of which can send arbitrary commands to all USB devices.
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[81a73ed8] | 106 | </para>
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| 107 |
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| 108 | <para>
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| 109 | Before Linux-2.6.15, raw USB device access was performed not with
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| 110 | /dev/bus/usb/BBB/DDD device nodes, but with /proc/bus/usb/BBB/DDD
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| 111 | pseudofiles. Some applications (e.g., VMware Workstation) still use only
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| 112 | this deprecated technique and can't use the new device nodes. For them to
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| 113 | work, use the "usb" group, but remember that members will have
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| 114 | unrestricted access to all USB devices. To create the fstab entry for
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| 115 | the obsolete usbfs filesystem:
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| 116 | </para>
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[d20c871] | 117 |
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| 118 | <screen><literal>usbfs /proc/bus/usb usbfs devgid=14,devmode=0660 0 0</literal></screen>
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| 119 |
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[81a73ed8] | 120 | <note>
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| 121 | <para>
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| 122 | Adding users to the "usb" group is inherently insecure, as they can
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| 123 | bypass access restrictions imposed through the driver-specific USB
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| 124 | device nodes. For instance, they can read sensitive data from USB
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| 125 | hard drives without being in the "disk" group. Avoid adding users
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| 126 | to this group, if you can.
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| 127 | </para>
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| 128 | </note>
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[d20c871] | 129 |
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| 130 | </sect2>
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| 131 |
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[e73e361] | 132 | <sect2>
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[826bacb5] | 133 | <title>Udev Device Attributes</title>
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[9f12e36] | 134 |
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[81a73ed8] | 135 | <para>
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| 136 | Fine-tuning of device attributes such as group name and permissions
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| 137 | is possible by creating extra <application>udev</application> rules,
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| 138 | matching on something like this. The vendor and product can be found by
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| 139 | searching the <filename class='directory'>/sys/devices</filename>
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| 140 | directory entries or using <command>udevadm info</command> after the
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| 141 | device has been attached. See the documentation in the current
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| 142 | <application>udev</application> directory of <filename
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| 143 | class='directory'>/usr/share/doc</filename> for details.
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| 144 | </para>
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[9f12e36] | 145 |
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[fdf8b6d0] | 146 | <screen><literal>SUBSYSTEM=="usb_device", SYSFS{idVendor}=="05d8", SYSFS{idProduct}=="4002", \
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| 147 | GROUP:="scanner", MODE:="0660"</literal></screen>
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| 148 |
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[81a73ed8] | 149 | <note>
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| 150 | <para>
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| 151 | The above line is used for descriptive purposes only. The
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| 152 | scanner <application>udev</application> rules are put into place when
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| 153 | installing <xref linkend='sane'/>.
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| 154 | </para>
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| 155 | </note>
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[9f12e36] | 156 |
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[e73e361] | 157 | </sect2>
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| 158 |
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| 159 | <!--
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| 160 | <sect2>
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| 161 | <title>Multiple Network Interfaces</title>
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| 162 |
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| 163 |
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| 164 | </sect2>
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| 165 | -->
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| 166 |
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[fe142cc] | 167 | <!-- These instructions are invalid on systemd because we don't use bootscripts.
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| 168 | In addition, we should probably verify that these are valid on SysV too.-->
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| 169 |
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| 170 | <sect2 revision="sysv">
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[e73e361] | 171 | <title>Devices for Servers</title>
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| 172 |
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[81a73ed8] | 173 | <para>
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| 174 | In some cases, it makes sense to disable
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| 175 | <application>udev</application> completely and create static devices.
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| 176 | Servers are one example of this situation. Does a server need the
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| 177 | capability of handling dynamic devices? Only the system administrator
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| 178 | can answer that question, but in many cases the answer will be no.
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| 179 | </para>
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| 180 |
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| 181 | <para>
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| 182 | If dynamic devices are not desired, then static devices must be
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| 183 | created on the system. In the default configuration, the
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| 184 | <filename>/etc/rc.d/rcS.d/S10udev</filename> boot script mounts a
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| 185 | <systemitem class="filesystem">tmpfs</systemitem> partition over the
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| 186 | <filename class="directory">/dev</filename> directory. This problem can
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| 187 | be overcome by mounting the root partition temporarily:
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| 188 | </para>
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| 189 |
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| 190 | <warning>
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| 191 | <para>
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| 192 | If the instructions below are not followed carefully, your
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| 193 | system could become unbootable.
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| 194 | </para>
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| 195 | </warning>
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[9f12e36] | 196 |
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[e73e361] | 197 |
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| 198 | <screen><userinput>mount --bind / /mnt
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| 199 | cp -a /dev/* /mnt/dev
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[f41f8b6] | 200 | rm /etc/rc.d/rcS.d/{S10udev,S50udev_retry}
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[e73e361] | 201 | umount /mnt</userinput></screen>
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| 202 |
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[81a73ed8] | 203 | <para>
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| 204 | At this point, the system will use static devices upon the next
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| 205 | reboot. Create any desired additional devices using
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| 206 | <command>mknod</command>.
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| 207 | </para>
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| 208 |
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| 209 | <para>
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| 210 | If you want to restore the dynamic devices, recreate the
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| 211 | <filename>/etc/rc.d/rcS.d/{S10udev,S50udev_retry}</filename> symbolic
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| 212 | links and reboot again. Static devices do not need to be removed
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| 213 | (console and null are always needed) because they are covered by the
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| 214 | <systemitem class="filesystem">tmpfs</systemitem> partition. Disk
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| 215 | usage for devices is negligible (about 20–30 bytes per entry.)
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| 216 | </para>
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[e73e361] | 217 |
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| 218 | </sect2>
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| 219 |
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[e104a36] | 220 | <sect2 id="dev-dvd">
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| 221 | <title>Devices for DVD Drives</title>
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| 222 |
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[81a73ed8] | 223 | <para>
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[8558044] | 224 | If the initial boot process does not set up the
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[81a73ed8] | 225 | <systemitem>/dev/dvd</systemitem> device properly, it can
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| 226 | be installed using the following modification to the default udev rules.
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| 227 | As the <systemitem class="username">root</systemitem> user, run:
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| 228 | </para>
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[e104a36] | 229 |
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| 230 | <screen><userinput>sed '1d;/SYMLINK.*cdrom/ a\
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| 231 | KERNEL=="sr0", ENV{ID_CDROM_DVD}=="1", SYMLINK+="dvd", OPTIONS+="link_priority=-100"' \
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| 232 | /lib/udev/rules.d/60-cdrom_id.rules > /etc/udev/rules.d/60-cdrom_id.rules</userinput></screen>
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| 233 |
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| 234 | </sect2>
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| 235 |
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[e73e361] | 236 | </sect1>
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