The Debian Administrator's Handbook

3.1.3. Integration with Other Linux/Unix Machines

Finally, NFS and NIS, both included, guarantee interaction with Unix systems. NFS ensures file server functionality, while NIS creates user directories. The BSD printing layer, used by most Unix systems, also allows sharing of print queues.

Figure 3.1. Coexistence of Debian with MacOS, Windows and Unix systems

3.2. How To Migrate

In order to guarantee continuity of the services, each computer migration must be planned and executed according to the plan. Whatever the operating system used, this principle never changes.

3.2.1. Survey and Identify Services

As simple as it seems, this step is essential. A serious administrator truly knows the principal roles of each server, but such roles can change, and sometimes experienced users may have installed “wild” services. Knowing that they exist will at least allow you to decide what to do with them, rather than delete them haphazardly.

For this purpose, it is wise to inform your users of the project before migrating the server. To involve them in the project, it may be useful to install the most common free software programs on their desktops prior to migration, which they will come across again after the migration to Debian; OpenOffice.org and the Mozilla suite are the best examples here.

3.2.1.1. Network and Processes

The nmap tool (in the package with the same name) will quickly identify Internet services hosted by a network connected machine without even requiring to log in to it. Simply call the following command on another machine connected to the same network:

$ nmap mirlaine

Starting Nmap 5.00 ( http://nmap.org ) at 2010-07-29 16:36 CET

Interesting ports on mirlaine (192.168.1.99):

Not shown: 1694 closed ports

PORT    STATE SERVICE

22/tcp  open  ssh

79/tcp  open  finger

111/tcp open  rpcbind

Nmap done: 1 IP address (1 host up) scanned in 0.19 seconds

ALTERNATIVE Use netstat to find the list of available services

On a Linux machine, the netstat -tupan command will show the list of active or pending TCP sessions, as well UDP ports on which running programs are listening. This facilitates identification of services offered on the network.

GOING FURTHER IPv6

Some network commands may work either with IPv4 (the default usually) or with IPv6. This is especially the case with the nmap and netstat commands, but also others, such as route or ip. The convention is that this behavior is enabled by the -6 command-line option.

If the server is a Unix machine offering shell accounts to users, it is interesting to determine if processes are executed in the background in the absence of their owner. The command ps auxw displays a list of all processes with their user identity. By checking this information against the output of the who command, which gives a list of logged in users, it is possible to identify wild servers or programs running in the background. Looking at crontabs (tables listing automatic actions scheduled by users) will often provide interesting information on functions fulfilled by the server (a complete explanation of cron is available in Section 9.7, “Scheduling Tasks with cron and atd”).

In any case, it is essential to backup your servers: this allows recovery of information after the fact, when users will report specific problems due to the migration.

3.2.2. Backing up the Configuration

It is wise to retain the configuration of every service identified in order to be able to install the equivalent on the updated server. The strict minimum is to print the configuration files and make a backup copy of them.

For Unix machines, the configuration files are usually found in /etc/, but they may be located in a sub-directory of /usr/local/. This is the case if a program has been installed from sources, rather than with a package. One may also find them, in some cases, under /opt/.

For data managing services (such as databases), it is strongly recommended to export them to a standard format that will be easily imported by the new software. Such a format is usually in text mode and documented; it may be, for example, an SQL dump for a database, or an LDIF file for an LDAP server.

Figure 3.2. Database backups

Each server software is different, and it is impossible to detail all existing cases. See the new and current software documentation to identify the exportable (thus, re-importable) portions and those which will require manual manipulation. Reading this book will clarify the configuration of the main Linux server programs.

3.2.3. Taking Over an Existing Debian Server