Udev and Rule Locations
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Video Transcription
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>> Hey Cyberians, welcome back to
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the Linux plus course here at Cybrary.
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I'm your instructor Rob Gills.
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In today's lesson, we're going to be
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discussing udev and udev rules.
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Upon completion of today's lesson,
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you are going to be able to understand
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the purpose of udev,
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we're going to talk about how
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udev and udev rules work, and then finally,
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we're going to locate udev rules files
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during some demo time at the end of the lesson.
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Udev is actually short for userspace dev,
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and it acts as a device manager for the Linux kernel.
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Udev is started in the background at boot time,
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and it runs as a daemon.
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On SysVinit or system five systems,
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you're going to find that at rc5 or rc3.d/udev.
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On our systemd systems,
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it's a service, so it's just systemd-udevd.service.
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Now udev is actually supposed to sit
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there and listen to kernel notifications,
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information from the kernel
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when it finds a hardware device.
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This comes from the kernel in
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the form of something called a uevent.
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So the hardware device is placed in/sys by the kernel,
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uvent is generated and then
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>> what happens is udev manages
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>> the device files in/dev by
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using the information that it gets from that sysfs,
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the information that's in this sys directory and it
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configures devices based on configuration rules,
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which we'll look at later.
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Now here's an example of this.
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Just to make this a little bit clearer, hopefully,
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the kernel module is going to detect
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the device and this could be at boot time or it
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could just be because the device is
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added and the kernel is going to place
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that device entry in/sys for
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that device and generate a uevent.
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That uevent turn gets picked up by
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udevd which responds to
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the uevent and it matches that event against
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this configuration files or
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rules about how to handle that,
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how to create the device.
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If the device matches a rule,
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udev acts on the uevent
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and configures the device based on
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the rule creating an entry in the dev file system.
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Udev has to match
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the device information from uevent against the rule.
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Well, where are the rules found?
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There's a few places that you
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>> can find these udev rules.
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>> The default user system default,
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our lowest priority is /usr/lib/udev/rules.d.
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Now u as a system administrator
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can create custom rules to
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override or create a higher priority
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and specify how you want devices to be handled.
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If you put these rules in sudev/rules.d directory.
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Finally there's another place run/udev/rules.d,
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these are non-persistent
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>> custom rules that are created by
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>> applications on the fly when
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they're utilizing or working with a device.
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The rules in these directories are
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all handled in dictionary order,
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which means numbers first,
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higher numbers override lower numbers,
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and then by names alphabetically.
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Let's have a closer look at all of
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this with some demo time.
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Here we are in our demo environment.
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Today we're going to be working in Ubuntu.
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Let's first take a look at some of the default rules.
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We can do this by doing an ls on /usr/lib/udev/rules.d.
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In here we can see every possible conceivable rule
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for the system that handle uevents.
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These are pretty much all of the kind of
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things that you might need to
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handle in terms of hardware and kernel operation.
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If we open one of these files,
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we can see a little bit more about them.
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We'll do a /usr/lib/udev/rules.d.
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I don't know, let's do it 99 systemb rules.
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What we find is that that's pretty wickedly complex.
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They're not all this bad.
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A simpler one we can look at would be,
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let's take a look at 40-usbmedia.player.rules.
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This is a little bit more easy to read.
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Just tells us about what it's going to use
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for USB media plugged into the system.
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We can also take a look at the custom rules.
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For example, if we do an ls on etc/udev/rules.d,
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these are the custom rules,
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the highest priority that override
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everything else is configured by default on the system.
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One of them we can look at would be like,
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let's say less/60-vbox [NOISE]. Oh, sorry.
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No, we're going to have to
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>> pipe the whole thing in here.
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>> We'll do [NOISE] less on
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etc/udev/rules.d60-vboxdrv.rules and then we can
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see here that this is
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just a rule that handles
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how Virtual Box works with USB drives.
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I guess the question that this
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brings us to is how often you
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>> have to change these rules.
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>> Hopefully as little as possible.
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But if you do have to modify rules,
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make sure that you do it in etc/udev/rules.d.
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Again, this is the highest priority we'll override.
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Then generally you want to set them at something low,
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like 10 low for rules.
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But with that, we've reached the end of this lesson.
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In this lesson, we covered
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the purpose of the udev device manager,
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we talked a little bit about how udev and udev rules
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work and then we talked about how to locate udev rules.
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Thank you so much for being here and I look
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forward to seeing you in the next lesson.
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