HowTos for networked clients

Introduction to thin clients and diskless workstations

One generic term for both thin clients and diskless workstations is LTSP client.

(!) Starting with Bullseye, LTSP is quite different from the previous versions. This concerns both setup amd maintenance. As a main difference, the SquashFS image for diskless workstations is now generated from the LTSP server filesystem. Also, thin clients are no longer supported by default.
See the LTSP homepage for details.

Thin client

A thin client setup enables an ordinary PC to function as an (X-)terminal, where all software runs on the LTSP server. This means that this machine boots via PXE without using a local client hard drive.

Diskless workstation

A diskless workstation runs all software locally. The client machines boot directly from the LTSP server without a local hard drive. Software is administered and maintained on the LTSP server, but it runs on the diskless workstation. Home directories and system settings are stored on the server too. Diskless workstations are an excellent way of reusing older (but powerful) hardware with the same low maintenance cost as with thin clients.

Unlike workstations diskless workstations run without any need to add them with GOsa².

LTSP client firmware

LTSP client boot will fail if the client's network interface requires a non-free firmware. A PXE installation can be used for troubleshooting problems with netbooting a machine; if the Debian Installer complains about a missing XXX.bin file then non-free firmware has to be added to the LTSP server's initrd.

In this case execute the following commands on the LTSP server.

# First get information about firmware packages
apt update && apt search ^firmware-
 
# Decide which package has to be installed for the network interface(s). 
# Most probably this will be firmware-linux-nonfree.
apt -y -q install <package name>
 
# copy the new initrd to the server's tftpboot directory and update the SqushFS image.
ltsp image /

LTSP client type selection

Each LTSP server has two ethernet interfaces: one configured in the main 10.0.0.0/8 subnet (which is shared with the main server), and another forming a local subnet (a separate subnet for each LTSP server).

On the main subnet the complete PXE menu is provided; the separate subnet for each LTSP server allows only diskless and thin LTSP client selection.

Using the default PXE menu on the main subnet 10.0.0.0/8, a machine could be started as diskless workstation or thin client.

Configuring the PXE menu

The PXE configuration is generated using the script debian-edu-pxeinstall. It allows some settings to be overridden using the file /etc/debian-edu/pxeinstall.conf with replacement values.

Configuring the PXE installation

The PXE installation option is by default available to anyone able to PXE boot a machine. To password protect the PXE installation options, a file /var/lib/tftpboot/menupassword.cfg can be created with content similar to this:

MENU PASSWD $4$NDk0OTUzNTQ1NTQ5$7d6KvAlVCJKRKcijtVSPfveuWPM$

The password hash should be replaced with an MD5 hash for the desired password.

The PXE installation will inherit the language, keyboard layout and mirror settings from the settings used when installing the main-server, and the other questions will be asked during installation (profile, popcon participation, partitioning and root password). To avoid these questions, the file /etc/debian-edu/www/debian-edu-install.dat can be modified to provide preselected answers to debconf values. Some examples of available debconf values are already commented in /etc/debian-edu/www/debian-edu-install.dat. Your changes will be lost as soon as debian-edu-pxeinstall is used to recreate the PXE-installation environment. To append debconf values to /etc/debian-edu/www/debian-edu-install.dat during recreation with debian-edu-pxeinstall, add the file /etc/debian-edu/www/debian-edu-install.dat.local with your additional debconf values.

More information about modifying PXE installations can be found in the Installation chapter.

Adding a custom repository for PXE installations

For adding a custom repository add something like this to /etc/debian-edu/www/debian-edu-install.dat.local:

#add the skole projects local repository
d-i     apt-setup/local1/repository string      http://example.org/debian stable main contrib non-free
d-i     apt-setup/local1/comment string         Example Software Repository
d-i     apt-setup/local1/source boolean         true
d-i     apt-setup/local1/key    string          http://example.org/key.asc

and then run /usr/sbin/debian-edu-pxeinstall once.

Use a different LTSP client network

192.168.0.0/24 is the default LTSP client network if a machine is installed using the LTSP profile. If lots of LTSP clients are used or if different LTSP servers should serve both i386 and amd64 chroot environments the second preconfigured network 192.168.1.0/24 could be used as well. Edit the file /etc/network/interfaces and adjust the eth1 settings accordingly. Use ldapvi or any other LDAP editor to inspect DNS and DHCP configuration.

Add LTSP chroot to support 32-bit-PC clients

FIXME add information for thin client chroot

Changing network settings

The debian-edu-config package comes with a tool which helps in changing the network from 10.0.0.0/8 to something else. Have a look at /usr/share/debian-edu-config/tools/subnet-change. It is intended for use just after installation on the main server, to update LDAP and other files that need to be edited to change the subnet.

/!\ Note that changing to one of the subnets already used elsewhere in Debian Edu will not work. 192.168.0.0/24 and 192.168.1.0/24 are already set up as LTSP client networks. Changing to these subnets will require manual editing of configuration files to remove duplicate entries.

There is no easy way to change the DNS domain name. Changing it would require changes to both the LDAP structure and several files in the main server file system. There is also no easy way to change the host and DNS name of the main server (tjener.intern). To do so would also require changes to LDAP and files in the main-server and client file system. In both cases the Kerberos setup would have to be changed, too.

LTSP in detail

LTSP client configuration

Run man ltsp.conf to have a look at available configuration options. Or read it online: https://ltsp.org/man/ltsp.conf/

Desktop autoloader

This tool preloads the default Desktop environment (and programs of your choice). It is only useful for diskless clients. The setup is site specific, also some technical skills are required.

At least two files need to be edited. Available <editor> choices are: vi, nano, mcedit.

If the setup is complete, update the NBD image running ltsp-update-image and test it.

Sound with LTSP clients

LTSP thin clients use networked audio to pass audio from the server to the clients.

LTSP diskless workstations handle audio locally.

Use printers attached to LTSP clients

Connecting Windows machines to the network

For Windows clients the Windows domain "SKOLELINUX" is available to be joined. A special service called Samba, installed on the main server, enables Windows clients to store profiles and user data, and also authenticates the users during the login.

/!\ Joining a domain with a Windows client requires the steps described in the Debian Edu Bullseye Samba Howto.

Windows will sync the profiles of domain users on every Windows login and logout. Depending on how much data is stored in the profile, this could take some time. To minimise the time needed, deactivate things like local cache in browsers (you can use the Squid proxy cache installed on the main server instead) and save files into the H: volume rather than under "My Documents".

Remote Desktop

Choosing the LTSP server profile or the combined server profile also installs the xrdp and x2goserver packages.

Xrdp

Xrdp uses the Remote Desktop Protocol to present a graphical login to a remote client. Microsoft Windows users can connect to the LTSP server running xrdp without installing additional software - they simply start a Remote Desktop Connection on their Windows machine and connect.

Additionally, xrdp can connect to a VNC server or another RDP server.

Xrdp comes without sound support; to compile the required modules this script could be used.

 #!/bin/bash
 # Script to compile / recompile xrdp PulseAudio modules.
 # The caller needs to be root or a member of the sudo group.
 # Also, /etc/apt/sources.list must contain a valid deb-src line.
 set -e
  if [[ $UID -ne 0 ]] ; then  
     if ! groups | egrep -q sudo ; then
         echo "ERROR: You need to be root or a sudo group member."
         exit 1
     fi
 fi
 if ! egrep -q  ^deb-src /etc/apt/sources.list ; then
     echo "ERROR: Make sure /etc/apt/sources.list contains a deb-src line."
     exit 1
 fi
 TMP=$(mktemp -d)
 PULSE_UPSTREAM_VERSION="$(dpkg-query -W -f='${source:Upstream-Version}' pulseaudio)"
 XRDP_UPSTREAM_VERSION="$(dpkg-query -W -f='${source:Upstream-Version}' xrdp)"
 sudo apt -q update
 # Get sources and build dependencies:
 sudo apt -q install dpkg-dev
 cd $TMP
 apt -q source pulseaudio xrdp
 sudo apt -q build-dep pulseaudio xrdp
 # For pulseaudio 'configure' is all what is needed:
 cd pulseaudio-$PULSE_UPSTREAM_VERSION/
 ./configure
 # Adjust pulseaudio modules Makefile (needs absolute path)
 # and build the pulseaudio modules.
 cd $TMP/xrdp-$XRDP_UPSTREAM_VERSION/sesman/chansrv/pulse/
 sed -i 's/^PULSE/#PULSE/' Makefile
 sed -i "/#PULSE_DIR/a \
 PULSE_DIR = $TMP/pulseaudio-$PULSE_UPSTREAM_VERSION" Makefile
 make
 # Copy modules to Pulseaudio modules directory, adjust rights.
 sudo cp *.so /usr/lib/pulse-$PULSE_UPSTREAM_VERSION/modules/
 sudo chmod 644 /usr/lib/pulse-$PULSE_UPSTREAM_VERSION/modules/module-xrdp*
 # Restart xrdp, now with sound enabled.
 sudo service xrdp restart

X2Go

X2Go enables you to access a graphical desktop on the LTSP server over both low bandwidth and high bandwidth connections from a PC running Linux, Windows or macOS. Additional software is needed on the client side, see the X2Go wiki for more information.

Please note that the killer package should best be removed on the LTSP server if X2Go is used, see 890517.

Available Remote Desktop clients

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