[debian-edu-commits] [Debian Wiki] Update of "DebianEdu/Documentation/Bookworm/Architecture" by WolfgangSchweer

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The "DebianEdu/Documentation/Bookworm/Architecture" page has been changed by WolfgangSchweer:
https://wiki.debian.org/DebianEdu/Documentation/Bookworm/Architecture

Comment:
initial content (copy from the bullseye page w/ some adjustments)

New page:
<<Anchor(Architecture)>>
= Architecture =

<<TableOfContents(3)>>

<<Anchor(Architecture--Network)>>
== Network ==

This section of the document describes the network architecture and services provided by a Skolelinux installation.

{{attachment:Debian_Edu_Network.png|The Debian Edu network topology|width=1024}}

The figure is a sketch of the assumed network topology. The default setup of a Skolelinux network assumes that there is one (and only one) main server, while allowing the inclusion of both normal workstations and LTSP servers (with associated thin clients and/or diskless workstations). The number of workstations can be as large or small as you want (starting from none to a lot). The same goes for the LTSP servers, each of which is on a separate network so that the traffic between the clients and the LTSP server doesn't affect the rest of the network services. LTSP is explained in detail in [[DebianEdu/Documentation/Bookworm/HowTo/NetworkClients|the related HowTo]] chapter.

The reason that there can only be one main server in each school network is that the main server provides DHCP, and there can be only one machine doing so in each network. It is possible to move services from the main server to other machines by setting up the service on another machine, and subsequently updating the DNS configuration, pointing the DNS alias for that service to the right computer.

In order to simplify the standard setup of Skolelinux, the Internet connection runs over a separate router, also called gateway. See the [[DebianEdu/Documentation/Bookworm/Requirements#Internet_router|Internet router]] chapter for details how to set up such a gateway if it is not possible to configure an existing one as needed.

=== The default network setup ===

DHCP on the main server serves the 10.0.0.0/8 network, providing a PXE boot menu where you can choose whether to install a new
server/workstation, boot a thin client or a diskless workstation, run memtest, or boot from the local hard disk.

This is designed to be modified; for details, see [[DebianEdu/Documentation/Bookworm/HowTo/NetworkClients|the related HowTo]] chapter.

DHCP on the LTSP servers only serves a dedicated network on the second interface (192.168.0.0/24 and 192.168.1.0/24 are preconfigured options) and should seldom need to be changed.

The configuration of all subnets is stored in LDAP.

=== Main server ===

A Skolelinux network needs one main server (hostname "tjener" which is Norwegian and means "server") which per default has the IP address 10.0.2.2 and is installed by selecting the Main Server profile. It's possible (but not required) to also select and install the LTSP Server and Workstation profiles in addition to the Main Server profile.

=== Services running on the main server ===
With the exception of the control of the thin clients, all services are initially set up on one central computer (the main server). For performance reasons, the LTSP server(s) should be separate (though it is possible to install both the Main Server and LTSP Server profiles on the same machine). All services are allocated a dedicated DNS-name and are offered exclusively over IPv4. The allocated DNS name makes it easy to move individual services from the main server to a different machine, by simply stopping the service on the main server, and changing the DNS configuration to point to the new location of the service (which should be set up on that machine first, of course).

To ensure security all connections where passwords are transmitted over the network are encrypted, so no passwords are sent over the network as plain text.

Below is a table of the services that are set up by default in a Skolelinux network and the DNS name of each service. If possible all configuration files will refer to the service by name (without the domain name) thus making it easy for schools to change either their domain (if they have an own DNS domain) or the IP addresses they use.


||||||'''Table of services'''||
|| '''Service description''' || '''Common name''' || '''DNS service name''' ||
|| Centralised Logging || rsyslog  || syslog ||
|| Domain Name Service || DNS (BIND) || domain ||
|| Automatic Network Configuration of Machines || DHCP || bootps ||
|| Clock Synchronisation || NTP || ntp ||
|| Home Directories via Network File System || SMB / NFS || homes ||
|| Electronic Post Office || IMAP (Dovecot)  || postoffice ||
|| Directory Service || OpenLDAP || ldap ||
|| User Administration || GOsa² || ---  ||
|| Web Server || Apache/PHP || www ||
|| Central Backup || sl-backup, slbackup-php || backup ||
|| Web Cache || Proxy (Squid) || webcache ||
|| Printing || CUPS || ipp ||
|| Secure Remote Login || OpenSSH || ssh ||
|| Automatic Configuration || CFEngine || cfengine ||
|| LTSP Server/s || LTSP || ltsp ||
|| Machine and Service Surveillance with Error Reporting, plus Status and History on the Web. Error Reporting by email ||Munin, Icinga and Sitesummary || sitesummary ||

Personal files for each user are stored in their home directories, which are made available by the server. Home directories are accessible from all machines, giving users access to the same files regardless of which machine they are using. The server is operating system agnostic, offering access via NFS for Unix clients and via SMB2/SMB3 for other clients.

By default email is set up for local delivery (i.e. within the school) only, though email delivery to the wider Internet may be set up if the school has a permanent Internet connection. Clients are set up to deliver mail to the server (using 'smarthost'), and users can [[DebianEdu/Documentation/Bookworm/HowTo/Users#Using_email|access their personal mail]] through IMAP.

All services are accessible using the same username and password, thanks to the central user database for authentication and authorisation.

To increase performance on frequently accessed sites a web proxy that caches files locally (Squid) is used. In conjunction with blocking web-traffic in the router this also enables control of Internet access on individual machines.

Network configuration on the clients is done automatically using DHCP. All types of clients can be connected to the private 10.0.0.0/8 subnet and will get according IP addresses; LTSP clients should be connected to the corresponding LTSP server via the separate subnet 192.168.0.0/24 (this is to ensure that the network traffic of the LTSP clients doesn't interfere with the rest of the network services).

Centralised logging is set up so that all machines send their syslog messages to the server. The syslog service is set up so that it only accepts incoming messages from the local network.

By default the DNS server is set up with a domain for internal use only (*.intern), until a real ("external") DNS domain can be set up. The DNS server is set up as caching DNS server so that all machines on the network can use it as the main DNS Server.

Pupils and teachers have the ability to publish websites. The web server provides mechanisms for authenticating users, and for limiting access to individual pages and subdirectories to certain users and groups. Users will have the ability to create dynamic web pages, as the web server will be programmable on the server side.

Information on users and machines can be changed in one central location, and is made accessible to all computers on the network automatically. To achieve this a centralised directory server is set up. The directory will have information on users, user groups, machines and groups of machines. To avoid user confusion there won't be any difference between file groups and network groups. This implies that groups of machines which are to form network groups will use the same namespace as user groups.

Administration of services and users will mainly be via the web, and follow established standards, functioning well in the web browsers which are part of Skolelinux. The delegation of certain tasks to individual users or user groups will be made possible by the administration systems.

In order to avoid certain problems with NFS, and to make it simpler to debug problems, the different machines need synchronised clocks. To achieve this the Skolelinux server is set up as a local Network Time Protocol (NTP) server, and all workstations and clients are set up to synchronise with the server. The server itself should synchronise its clock via NTP against machines on the Internet, thus ensuring the whole network has the correct time.

Printers are connected where convenient, either directly onto the main network, or connected to a server, workstation or LTSP server. Access to printers can be controlled for individual users according to the groups they belong to; this will be achieved by using quota and access control for printers.

=== LTSP server(s) ===

A Skolelinux network can have many LTSP servers, which are installed by selecting the LTSP Server profile.

The LTSP servers are set up to receive syslog from thin clients and workstations, and forward these messages to the central syslog recipient. 

Please note:
 * LTSP diskless workstations are using the programs installed on the server.
 * The client root filesystem is provided using NFS. After each modification to the LTSP server the related image has to be re-generated; run {{{debian-edu-ltsp-install --diskless_workstation yes}}} on the LTSP server.

=== Thin clients ===
A thin client setup enables ordinary PCs to function as (X-)terminals. This means that the machine boots directly from the server using PXE without using the local client hard drive. The thin client setup now uses !X2Go, because LTSP has dropped support.

Thin clients are a good way to still make use of very old (mostly 32-bit) machines as they effectively run all programs on the LTSP server. This works as follows: the service uses DHCP and TFTP to connect to the network and boot from the network. Next, the file system is mounted from the LTSP server using NFS, and finally the !X2Go client is started.

=== Diskless workstations ===

A diskless workstation runs all software on the PC without a locally installed operating system. This means that client machines boot via PXE without running software installed on a local hard drive.

Diskless workstations are an excellent way of using powerful hardware with the same low maintenance cost as with thin clients. Software is administered and maintained on the server with no need for local installed software on the clients. Home directories and system settings are stored on the server too. 

=== Networked clients ===

The term "networked clients" is used in this manual to refer to both thin clients and diskless workstations, as well as computers running macOS or Windows.

== Administration ==

All the Linux machines that are installed with the Skolelinux installer will be administrable from a central computer, most likely the server. It will be possible to log in to all machines via SSH, and thereby have full access to the machines. As root one needs to run {{{kinit}}} first to get a Kerberos TGT.

All user information is kept in an LDAP directory. Updates of user accounts are made against this database, which is used by the clients for user authentication.

=== Installation ===

Currently there are two kinds of installation media images: netinst and BD. Both images can also be booted from USB sticks.

The aim is to be able to install a server from any type of medium once, and install all other clients over the network by booting from the network. 

Only the netinstall image needs access to the Internet during installation.

The installation should not ask any questions, with the exception of desired language, location, keyboard layout and machine profile (Main Server, Workstation, LTSP Server, …). All other configuration will be set up automatically with reasonable values, to be changed from a central location by the system administrator subsequent to the installation.

<<Anchor(Architecture--File_system_access_configuration)>>
=== File system access configuration ===

Each Skolelinux user account is assigned a section of the file system on the file server. This section (home directory) contains the user's configuration files, documents, email and web pages. Some of the files should be set to have read access for other users on the system, some should be readable by everyone on the Internet, and some should not be accessible for reading by anyone but the user.

To ensure that all disks that are used for user directories or shared directories can be uniquely named across all the computers in the installation, they can be mounted as {{{/skole/host/directory/}}}. Initially, one directory is created on the file server, {{{/skole/tjener/home0/}}}, in which all the user accounts are created. More directories may then be created when needed to accommodate 
particular user groups or particular patterns of usage.

To enable shared access to files under the normal UNIX permissions system, users need to be in supplementary shared groups (such as "students") as well as the personal primary group that they're in by default. If users have an appropriate umask to make newly created items group-accessible (002 or 007), and if the directories they're working in are setgid to ensure the files inherit the correct group-ownership, the result is controlled file sharing between the members of a group.

The initial access settings for newly created files are a matter of policy. The Debian default umask is 022 (which would not allow group-access as described above), but Debian Edu uses a default of 002 - meaning that files are created with read access for everybody, which can later be removed by explicit user action. This can alternatively be changed (by editing `/etc/pam.d/common-session`) to a umask of 007 - meaning read access is initially blocked, necessitating user action to make them accessible.  The first approach encourages knowledge sharing, and makes the system more transparent, whereas the second method decreases the risk of unwanted spreading of sensitive information. The problem with the first solution is that it is not apparent to the users that the material they create will be accessible to all other users. They can only detect this by inspecting other users' directories and seeing that their files are readable. The problem with the second solution is that few people are likely to make their files accessible, even if they do not contain sensitive information and the content would be helpful to inquisitive users who want to learn how others have solved particular problems (typically configuration issues).

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