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+  <title>Customizing your login on Purdue CS computers (WIP, but updated) — Luke T. Shumaker</title>
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+<header><a href="/">Luke T. Shumaker</a> » <a href=/blog>blog</a> » purdue-cs-login</header>
+<article>
+<h1
+id="customizing-your-login-on-purdue-cs-computers-wip-but-updated">Customizing
+your login on Purdue CS computers (WIP, but updated)</h1>
+<blockquote>
+<p>This article is currently a Work-In-Progress. Other than the one
+place where I say “I’m not sure”, the GDM section is complete. The
+network shares section is a mess, but has some good information.</p>
+</blockquote>
+<p>Most CS students at Purdue spend a lot of time on the lab boxes, but
+don’t know a lot about them. This document tries to fix that.</p>
+<p>The lab boxes all run Gentoo.</p>
+<h2 id="gdm-the-gnome-display-manager">GDM, the Gnome Display
+Manager</h2>
+<p>The boxes run <code>gdm</code> (Gnome Display Manager) 2.20.11 for
+the login screen. This is an old version, and has a couple behaviors
+that are slightly different than new versions, but here are the
+important bits:</p>
+<p>System configuration:</p>
+<ul>
+<li><code>/usr/share/gdm/defaults.conf</code> (lower precidence)</li>
+<li><code>/etc/X11/gdm/custom.conf</code> (higher precidence)</li>
+</ul>
+<p>User configuration:</p>
+<ul>
+<li><code>~/.dmrc</code> (more recent versions use
+<code>~/.desktop</code>, but Purdue boxes aren’t running more recent
+versions)</li>
+</ul>
+<h3 id="purdues-gdm-configuration">Purdue’s GDM configuration</h3>
+<p>Now, <code>custom.conf</code> sets</p>
+<pre><code>BaseXsession=/usr/local/share/xsessions/Xsession
+SessionDesktopDir=/usr/local/share/xsessions/</code></pre>
+<p>This is important, because there are <em>multiple</em> locations that
+look like these files; I take it that they were used at sometime in the
+past. Don’t get tricked into thinking that it looks at
+<code>/etc/X11/gdm/Xsession</code> (which exists, and is where it would
+look by default).</p>
+<p>If you look at the GDM login screen, it has a “Sessions” button that
+opens a prompt where you can select any of several sessions:</p>
+<ul>
+<li>Last session</li>
+<li>1. MATE (<code>mate.desktop</code>;
+<code>Exec=mate-session</code>)</li>
+<li>2. CS Default Session (<code>default.desktop</code>;
+<code>Exec=default</code>)</li>
+<li>3. Custom Session (<code>custom.desktop</code>;
+<code>Exec=custom</code>)</li>
+<li>4. FVWM2 (<code>fvwm2.desktop</code>; <code>Exec=fvwm2</code>)</li>
+<li>5. gnome.desktop (<code>gnome.desktop</code>;
+<code>Exec=gnome-session</code>)</li>
+<li>6. KDE (<code>kde.desktop</code>, <code>Exec=startkde</code>)</li>
+<li>Failsafe MATE (<code>ShowGnomeFailsafeSession=true</code>)</li>
+<li>Failsafe Terminal (<code>ShowXtermFailsafeSession=true</code>)</li>
+</ul>
+<p>The main 6 are configured by the <code>.desktop</code> files in
+<code>SessionDesktopDir=/usr/local/share/xsessions</code>; the last 2
+are auto-generated. The reason <code>ShowGnomeFailsafeSession</code>
+correctly generates a Mate session instead of a Gnome session is because
+of the patch
+<code>/p/portage/*/overlay/gnome-base/gdm/files/gdm-2.20.11-mate.patch</code>.</p>
+<p>I’m not sure why Gnome shows up as <code>gnome.desktop</code> instead
+of <code>GNOME</code> as specified by <code>gnome.desktop:Name</code>. I
+imagine it might be something related to the aforementioned patch, but I
+can’t find anything in the patch that looks like it would screw that up;
+at least not without a better understanding of GDM’s code.</p>
+<p>Which of the main 6 is used by default (“Last Session”) is configured
+with <code>~/.dmrc:Session</code>, which contains the basename of the
+associated <code>.desktop</code> file (that is, without any directory
+part or file extension).</p>
+<p>Every one of the <code>.desktop</code> files sets
+<code>Type=XSession</code>, which means that instead of running the
+argument in <code>Exec=</code> directly, it passes it as arguments to
+the <code>Xsession</code> program (in the location configured by
+<code>BaseXsession</code>).</p>
+<h4 id="xsession">Xsession</h4>
+<p>So, now we get to read
+<code>/usr/local/share/xsessions/Xsession</code>.</p>
+<p>Before it does anything else, it:</p>
+<ol type="1">
+<li><code>. /etc/profile.env</code></li>
+<li><code>unset ROOTPATH</code></li>
+<li>Try to set up logging to one of <code>~/.xsession-errors</code>,
+<code>$TMPDIR/xses-$USER</code>, or <code>/tmp/xses-$USER</code> (it
+tries them in that order).</li>
+<li><code>xsetroot -default</code></li>
+<li>Fiddles with the maximum number of processes.</li>
+</ol>
+<p>After that, it handles these 3 “special” arguments that were given to
+it by various <code>.desktop</code> <code>Exec=</code> lines:</p>
+<ul>
+<li><code>failsafe</code>: Runs a single xterm window. NB: This is NOT
+run by either of the failsafe options. It is likey a vestiage from a
+prior configuration.</li>
+<li><code>startkde</code>: Displays a message saying KDE is no longer
+available.</li>
+<li><code>gnome-session</code>: Displays a message saying GNOME has been
+replaced by MATE.</li>
+</ul>
+<p>Assuming that none of those were triggered, it then does:</p>
+<ol type="1">
+<li><code>source ~/.xprofile</code></li>
+<li><code>xrdb -merge ~/.Xresources</code></li>
+<li><code>xmodmap ~/.xmodmaprc</code></li>
+</ol>
+<p>Finally, it has a switch statement over the arguments given to it by
+the various <code>.desktop</code> <code>Exec=</code> lines:</p>
+<ul>
+<li><code>custom</code>: Executes <code>~/.xsession</code>.</li>
+<li><code>default</code>: Executes <code>~/.Xrc.cs</code>.</li>
+<li><code>mate-session</code>: It has this whole script to start DBus,
+run the <code>mate-session</code> command, then cleanup when it’s
+done.</li>
+<li><code>*</code> (<code>fvwm2</code>): Runs
+<code>eval exec "$@"</code>, which results in it executing the
+<code>fvwm2</code> command.</li>
+</ul>
+<h2 id="network-shares">Network Shares</h2>
+<p>Your data is on various hosts. I believe most undergrads have their
+data on <code>data.cs.purdue.edu</code> (or just <a
+href="https://en.wikipedia.org/wiki/Data_%28Star_Trek%29"><code>data</code></a>).
+Others have theirs on <a
+href="http://swfanon.wikia.com/wiki/Antor"><code>antor</code></a> or <a
+href="https://en.wikipedia.org/wiki/Tux"><code>tux</code></a> (that I
+know of).</p>
+<p>Most of the boxes with tons of storage have many network cards; each
+with a different IP; a single host’s IPs are mostly the same, but with
+varying 3rd octets. For example, <code>data</code> is 128.10.X.13. If
+you need a particular value of X, but don’t want to remember the other
+octets; they are individually addressed with
+<code>BASENAME-NUMBER.cs.purdue.edu</code>. For example,
+<code>data-25.cs.purdu.edu</code> is 128.10.25.13.</p>
+<p>They use <a
+href="https://www.kernel.org/pub/linux/daemons/autofs/">AutoFS</a> quite
+extensively. The maps are generated dynamically by
+<code>/etc/autofs/*.map</code>, which are all symlinks to
+<code>/usr/libexec/amd2autofs</code>. As far as I can tell,
+<code>amd2autofs</code> is custom to Purdue. Its source lives in
+<code>/p/portage/*/overlay/net-fs/autofs/files/amd2autofs.c</code>. The
+name appears to be a misnomer; seems to claim to dynamically translate
+from the configuration of <a href="http://www.am-utils.org/">Auto
+Mounter Daemon (AMD)</a> to AutoFS, but it actually talks to NIS. It
+does so using the <code>yp</code> interface, which is in Glibc for
+compatibility, but is undocumented. For documentation for that
+interface, look at the one of the BSDs, or Mac OS X. From the comments
+in the file, it appears that it once did look at the AMD configuration,
+but has since been changed.</p>
+<p>There are 3 mountpoints using AutoFS: <code>/homes</code>,
+<code>/p</code>, and <code>/u</code>. <code>/homes</code> creates
+symlinks on-demand from <code>/homes/USERNAME</code> to
+<code>/u/BUCKET/USERNAME</code>. <code>/u</code> mounts NFS shares to
+<code>/u/SERVERNAME</code> on-demand, and creates symlinks from
+<code>/u/BUCKET</code> to <code>/u/SERVERNAME/BUCKET</code> on-demand.
+<code>/p</code> mounts on-demand various NFS shares that are organized
+by topic; the Xinu/MIPS tools are in <code>/p/xinu</code>, the Portage
+tree is in <code>/p/portage</code>.</p>
+<p>I’m not sure how <code>scratch</code> works; it seems to be
+heterogenous between different servers and families of lab boxes.
+Sometimes it’s in <code>/u</code>, sometimes it isn’t.</p>
+<p>This 3rd-party documentation was very helpful to me: <a
+href="http://www.linux-consulting.com/Amd_AutoFS/"
+class="uri">http://www.linux-consulting.com/Amd_AutoFS/</a> It’s where
+Gentoo points for the AutoFS homepage, as it doesn’t have a real
+homepage. Arch just points to FreshMeat. Debian points to
+kernel.org.</p>
+<h3 id="lore">Lore</h3>
+<p><a
+href="https://en.wikipedia.org/wiki/List_of_Star_Trek:_The_Next_Generation_characters#Lore"><code>lore</code></a></p>
+<p>Lore is a SunOS 5.10 box running on Sun-Fire V445 (sun4u) hardware.
+SunOS is NOT GNU/Linux, and sun4u is NOT x86.</p>
+<p>Instead of <code>/etc/fstab</code> it is
+<code>/etc/mnttab</code>.</p>
+
+</article>
+<footer>
+<p>The content of this page is Copyright © 2015 <a href="mailto:lukeshu@lukeshu.com">Luke T. Shumaker</a>.</p>
+<p>This page is licensed under the <a href="https://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA 4.0</a> license.</p>
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