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