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+ <h1>Udev on Fedora</h1>
+ <h2>by Harald Hoyer</h2>
+ <p>
+ This document tries to reveal the secrets of udev and how it works on Fedora.
+ </p>
+ <p>
+ udev was developed by Greg Kroah-Hartman <greg@kroah.com> with much
+ help from Dan Stekloff <dsteklof@us.ibm.com>, Kay Sievers <kay.sievers@vrfy.org>,
+ and many others.
+ </p>
+ <p>
+ The <a href="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html">udev homepage</a> and the
+ Linux-hotplug-devel mailing list <a href="https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel">https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel</a> are the main development sources.
+ </p><p>
+ </p>
+
+ <h2>What Does Udev Do?</h2>
+ <p>
+ udev provides a dynamic device directory containing only the files for
+ actually present devices. It creates or removes device node files usually
+ located in the <tt class="filename">/dev/</tt> directory, or it renames network interfaces.
+ </p><p>
+ As part of the hotplug subsystem, udev is executed if a kernel device
+ is added or removed from the system. On device creation, udev reads
+ the sysfs directory of the given device to collect device attributes
+ like label, serial number or bus device number. These attributes may
+ be used as keys to determine a unique name for the device. udev maintains
+ a database for devices present on the system.
+ On device removal, udev queries its database for the name of the device
+ file to be deleted.
+ </p><p>
+ udev gets called by hotplug, if a module is loaded, and a device is added
+ or removed. udev looks in <tt class="filename">/sys/</tt>, if the driver provides a "dev" file, which
+ contains the major and minor number for a device node to communicate with
+ the driver. After looking in the udev rules (in the <tt class="filename">/etc/udev/rules.d/</tt> directory), which
+ specify the device node filename and symlinks, a device node is created
+ in <tt class="filename">/dev/</tt> with the permissions, which are specified in <tt class="filename">/etc/udev/permissions.d/</tt>.
+ </p>
+ After device node creation, removal, or network device renaming, udev
+ executes the programs in the directory tree under <tt class="filename">/etc/dev.d/</tt>. The
+ name of a program must end with the <tt class="filename">.dev</tt> suffix, to be recognized.
+ In addition to the hotplug environment variables, DEVNAME is exported
+ to make the name of the created node or the name the network device is
+ renamed to, available to the executed program. The programs in every
+ directory are sorted in lexical order, while the directories are
+ searched in the following order:
+ <ul>
+ <li><tt class="filename">/etc/dev.d/$(DEVNAME)/*.dev</tt></li>
+ <li><tt class="filename">/etc/dev.d/$(SUBSYSTEM)/*.dev</tt></li>
+ <li><tt class="filename">/etc/dev.d/default/*.dev</tt></li>
+ </ul>
+
+ <h2>How is Udev Integrated on Fedora?</h2>
+ <h3><tt class="command">initrd</tt> / <tt class="command">initfs</tt></h3>
+ <p>
+ <tt class="command">mkinitrd</tt> copies <tt class="filename">/sbin/udev.static</tt>
+ to the <tt class="command">initrd</tt> <tt class="filename">/sbin/udev</tt> and symlinks it to
+ <tt class="filename">/sbin/udevstart</tt>.
+ </p><p>
+After the kernel boots, it executes the nash script of the <tt class="command">initrd</tt>. This
+mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>. Instead of hotplug <tt class="command">/sbin/udev</tt> is
+called in the <tt class="command">initrd</tt> phase. udevstart creates all device nodes for the
+devices, which are compiled in the kernel and for the modules, which
+are loaded by nash. </p>
+
+<h4>Problems</h4>
+
+The whole udev and hotplug infrastructure is
+not available in <tt class="command">initrd</tt>. Thus no hotplug scripts, udev rules, and
+permissions and no <tt class="filename">/etc/dev.d/</tt> scripts are executed for any hotplug
+event, which is sent from the kernel.
+
+<h3><tt class="filename">rc.sysinit</tt></h3>
+ <p> First, if SELinux is loaded and enabled,
+the context of <tt class="filename">/dev/</tt> is set. <tt class="filename">rc.sysinit</tt> calls <tt class="filename">/sbin/start_udev</tt>.
+<tt class="filename">start_udev</tt> mounts a tmpfs filesystem on <tt class="filename">/dev/</tt>, if there is none already
+mounted. Then it creates some device nodes, which need module
+autoloading, or where there is no kernel module. After that
+<tt class="command">/sbin/udevstart</tt> is called again, which simulates the hotplug events in
+the <tt class="command">initrd</tt> phase, to apply the whole udev rules and permissions. After
+that <tt class="filename">rc.sysinit</tt> parses the ouput of <tt class="filename">/sbin/kmodule</tt> and loads every
+module. This should provide device nodes for all hardware found on your
+computer. </p>
+ <h3>Console User Permissions</h3>
+ <p>
+<tt class="filename">/etc/dev.d/default/pam_console.dev</tt> is called whenever a device node is
+created and calls <tt class="filename">/sbin/pam_console_setowner</tt> with the filename (and an
+optional symlink) of the device node. This sets the permissions for
+console users like specified in <tt class="filename">/etc/security/console.perms</tt>. </p>
+ <h2>Customizing Udev on Fedora</h2>
+ <p>
+ Read the manpage of udev and udevinfo.
+ Please try not to modify the files of RPM packages.
+ </p>
+ <h3>New Rules</h3>
+ <p>
+New rules should be placed in a file, which ends in <tt class="filename">.rules</tt> in
+<tt class="filename">/etc/udev/rules.d/</tt>. Please do not use <tt class="filename">50-udev.rules</tt>. The supported and
+preferred way is to create rules without the "NAME" tag and only
+create "SYMLINK"s. </p><p>
+ A nice document describing how to write rules can be found on <a href="http://www.reactivated.net/udevrules.php">http://www.reactivated.net/udevrules.php</a>.
+ </p>
+
+<h3>Permissions</h3>
+New permissions should be placed in a file, which ends in
+<tt class="filename">.permissions</tt> in <tt class="filename">/etc/udev/permissions.d/</tt>. Please do not use
+<tt class="filename">50-udev.permissions</tt>.
+
+<h3>But I Really Want My Device Node!</h3>
+ <p>
+ Put them in <tt class="filename">/etc/udev/devices/</tt>, and they will get copied to <tt class="filename">/dev/</tt>. <a href="https://bugzilla.redhat.com/bugzilla">File a bugzilla entry</a>, if you think that should be done per default.
+ </p>
+
+ <h2>Updating to udev Without <tt class="filename">/dev/</tt></h2>
+ <p>
+ The steps to upgrade without Anaconda or a rescue CD are (NOT recommended):
+ </p>
+
+ <ul>
+ <li>start from a kernel-2.6
+ </li><li>Make sure <tt class="filename">/sys/</tt> is mounted
+ </li><li>Install the latest <tt class="filename">initscripts</tt> package
+ </li><li>Install the latest <tt class="filename">udev</tt> package
+ </li><li>Execute <tt class="command">/sbin/start_udev</tt>
+ </li><li>Install the latest <tt class="filename">mkinitrd</tt> package
+ </li><li>Install the latest <tt class="filename">kernel</tt> package
+ </li><li>Or execute <tt class="command">mkinitrd</tt> for your existing kernel(s)
+ </li></ul>
+
+ <h2>Udev without <tt class="command">initrd</tt></h2>
+
+ <p>Install Fedore Core as usual and reboot. Execute the following commands
+ </p>
+
+<pre class="screen">
+<tt class="command">
+mkdir /tmp/dev
+mount --move /dev /tmp/dev
+sbin/MAKEDEV null console zero
+mount --move /tmp/dev /dev
+</tt></pre>
+ Install your kernel without an <tt class="command">initrd</tt>. Reboot.
+ <p>
+ You will get some SELinux errors, and syslogd will not work as expected.
+ </p>
+ <h2>Current Problems on Fedora</h2>
+ <p>
+ <a href="http://bugzilla.redhat.com/bugzilla/buglist.cgi?short_desc_type=allwordssubstr&component=udev&bug_status=ASSIGNED&bug_status=MODIFIED&bug_status=NEEDINFO&bug_status=NEW&bug_status=REOPENED&bug_status=VERIFIED&bug_severity=high&bug_severity=low&bug_severity=normal&bug_severity=security&bug_severity=translation&long_desc_type=allwordssubstr&bug_file_loc_type=allwordssubstr&status_whiteboard_type=allwordssubstr&fixed_in_type=allwordssubstr&devel_whiteboard_type=allwordssubstr&keywords_type=allwords&cust_facing=YES&emailassigned_to1=1&emailtype1=exact&emailreporter2=1&emailtype2=exact&bugidtype=include&chfieldto=Now&cmdtype=doit&remaction=run&namedcmd=blank&order=Reuse+same+sort+as+last+time&field0-0-0=noop&type0-0-0=noop">All open bugs for <code class="filename">udev</code></a>
+ </p>
+ <h3>Nvidia</h3>
+ <p>Quick solution: If you do not need rhgb, just load the nvidia module in <tt class="filename">/etc/rc.local</tt>
+ </p><p>
+ If you have udev >= 032-5, load the nvidia module:
+ </p>
+<pre class="screen">
+<tt class="command">
+cp -a /dev/nvidia* /etc/udev/devices
+chown root.root /etc/udev/devices/nvidia*
+</tt></pre>
+
+ <p>The Bugzilla for this problem is <strike><a href="https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=133900">133900</a></strike>.</p>
+ <h3>Palm Pilot</h3>
+ <p>
+ If you have udev >= 032-5, execute the command:
+ </p>
+<pre class="screen">
+<tt class="command">
+ln -s ttyUSB1 /etc/udev/devices/pilot
+</tt></pre>
+
+ <h3>ISDN</h3>
+ <p>
+ If you have udev >= 032-5:
+ </p>
+<pre class="screen">
+<tt class="command">
+/sbin/MAKEDEV -d /etc/udev/devices isdn
+</tt></pre>
+ </pre>
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