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Customizing your login on Purdue CS computers (WIP, but updated)
=============================================
---
date: "2015-02-06"
---
> 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.
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.
The lab boxes all run Gentoo.
## GDM, the Gnome Display Manager
The boxes run `gdm` (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:
System configuration:
- `/usr/share/gdm/defaults.conf` (lower precidence)
- `/etc/X11/gdm/custom.conf` (higher precidence)
User configuration:
- `~/.dmrc` (more recent versions use `~/.desktop`, but Purdue boxes
aren't running more recent versions)
### Purdue's GDM configuration
Now, `custom.conf` sets
BaseXsession=/usr/local/share/xsessions/Xsession
SessionDesktopDir=/usr/local/share/xsessions/
This is important, because there are *multiple* 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
`/etc/X11/gdm/Xsession` (which exists, and is where it would look by
default).
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:
- Last session
- 1\. MATE (`mate.desktop`; `Exec=mate-session`)
- 2\. CS Default Session (`default.desktop`; `Exec=default`)
- 3\. Custom Session (`custom.desktop`; `Exec=custom`)
- 4\. FVWM2 (`fvwm2.desktop`; `Exec=fvwm2`)
- 5\. gnome.desktop (`gnome.desktop`; `Exec=gnome-session`)
- 6\. KDE (`kde.desktop`, `Exec=startkde`)
- Failsafe MATE (`ShowGnomeFailsafeSession=true`)
- Failsafe Terminal (`ShowXtermFailsafeSession=true`)
The main 6 are configured by the `.desktop` files in
`SessionDesktopDir=/usr/local/share/xsessions`; the last 2 are
auto-generated. The reason `ShowGnomeFailsafeSession` correctly
generates a Mate session instead of a Gnome session is because of the
patch
`/p/portage/*/overlay/gnome-base/gdm/files/gdm-2.20.11-mate.patch`.
I'm not sure why Gnome shows up as `gnome.desktop` instead of `GNOME`
as specified by `gnome.desktop:Name`. 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.
Which of the main 6 is used by default ("Last Session") is configured
with `~/.dmrc:Session`, which contains the basename of the associated
`.desktop` file (that is, without any directory part or file
extension).
Every one of the `.desktop` files sets `Type=XSession`, which means
that instead of running the argument in `Exec=` directly, it passes it
as arguments to the `Xsession` program (in the location configured by
`BaseXsession`).
#### Xsession
So, now we get to read `/usr/local/share/xsessions/Xsession`.
Before it does anything else, it:
1. `. /etc/profile.env`
2. `unset ROOTPATH`
3. Try to set up logging to one of `~/.xsession-errors`,
`$TMPDIR/xses-$USER`, or `/tmp/xses-$USER` (it tries them in that
order).
4. `xsetroot -default`
5. Fiddles with the maximum number of processes.
After that, it handles these 3 "special" arguments that were given to
it by various `.desktop` `Exec=` lines:
- `failsafe`: Runs a single xterm window. NB: This is NOT run by
either of the failsafe options. It is likey a vestiage from a
prior configuration.
- `startkde`: Displays a message saying KDE is no longer available.
- `gnome-session`: Displays a message saying GNOME has been replaced by MATE.
Assuming that none of those were triggered, it then does:
1. `source ~/.xprofile`
2. `xrdb -merge ~/.Xresources`
3. `xmodmap ~/.xmodmaprc`
Finally, it has a switch statement over the arguments given to it by
the various `.desktop` `Exec=` lines:
- `custom`: Executes `~/.xsession`.
- `default`: Executes `~/.Xrc.cs`.
- `mate-session`: It has this whole script to start DBus, run
the `mate-session` command, then cleanup when it's done.
- `*` (`fvwm2`): Runs `eval exec "$@"`, which results in it executing
the `fvwm2` command.
## Network Shares
Your data is on various hosts. I believe most undergrads have their
data on `data.cs.purdue.edu` (or just
[`data`](https://en.wikipedia.org/wiki/Data_%28Star_Trek%29)). Others
have theirs on [`antor`](http://swfanon.wikia.com/wiki/Antor) or
[`tux`](https://en.wikipedia.org/wiki/Tux) (that I know of).
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, `data` 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 `BASENAME-NUMBER.cs.purdue.edu`.
For example, `data-25.cs.purdu.edu` is 128.10.25.13.
They use [AutoFS](https://www.kernel.org/pub/linux/daemons/autofs/)
quite extensively. The maps are generated dynamically by
`/etc/autofs/*.map`, which are all symlinks to
`/usr/libexec/amd2autofs`. As far as I can tell, `amd2autofs` is
custom to Purdue. Its source lives in
`/p/portage/*/overlay/net-fs/autofs/files/amd2autofs.c`. The name
appears to be a misnomer; seems to claim to dynamically translate from
the configuration of
[Auto Mounter Daemon (AMD)](http://www.am-utils.org/) to AutoFS, but
it actually talks to NIS. It does so using the `yp` 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.
There are 3 mountpoints using AutoFS: `/homes`, `/p`, and
`/u`. `/homes` creates symlinks on-demand from `/homes/USERNAME` to
`/u/BUCKET/USERNAME`. `/u` mounts NFS shares to `/u/SERVERNAME`
on-demand, and creates symlinks from `/u/BUCKET` to
`/u/SERVERNAME/BUCKET` on-demand. `/p` mounts on-demand various NFS
shares that are organized by topic; the Xinu/MIPS tools are in
`/p/xinu`, the Portage tree is in `/p/portage`.
I'm not sure how `scratch` works; it seems to be heterogenous between
different servers and families of lab boxes. Sometimes it's in `/u`,
sometimes it isn't.
This 3rd-party documentation was very helpful to me:
<http://www.linux-consulting.com/Amd_AutoFS/> 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.
### Lore
[`lore`](https://en.wikipedia.org/wiki/List_of_Star_Trek:_The_Next_Generation_characters#Lore)
Lore is a SunOS 5.10 box running on Sun-Fire V445 (sun4u) hardware.
SunOS is NOT GNU/Linux, and sun4u is NOT x86.
Instead of `/etc/fstab` it is `/etc/mnttab`.
|
