Adding and Removing Packages

Proper maintenance of the different modules that reside on your system helps ensure the reliability of the services you offer. An important step in the configuration of a server is to ensure that only required packages are installed on a system. This has the dual benefit of minimizing resources consumed as well as reducing the possible exposure to exploits in unused packages that tend to be forgotten and therefore not adequately maintained.

How do you know what minimal subcomponents are required to support the main tasking of your server? As a first approximation, this is handled for you by the YaST utility. YaST and the underlying subsystems maintain a list of interdependencies between different packages. If an attempt is made to install or remove a component that is required by a module you need, YaST will warn you.

Installing a Package

The process of adding and removing packages using YaST is fairly simple. As an example, the following walks through the creation of a Domain Name Services (DNS) server. The first step is to build an SLES server and then install the DNS software. Configuring the DNS service is somewhat more complicated and will be covered in Chapter 8, "Network Services."

Continuing with the desire to run only applications that are absolutely necessary, install a server with SLES and choose to install a minimum system. This installs SLES, YaST, and a number of other utilities that make the system easier to administer. To convert this base system into a DNS server, you must add the DNS server software (Bind).

Because the newly installed system is running a base install of SLES, the X Windows System is not available. Sign onto the machine using the root account and start YaST by typing yast at the command line. Figure 2.1 shows the ncurses main menu for the YaST tool. You navigate through the tool by using the tab and arrow keys, and when available, highlighted characters represent Alt-keystroke shortcuts. In Figure 2.1, the Install and Remove Software option has been selected.

Pressing Enter at this stage brings up the Install and Remove submenu. By default, the selection screen is filtered by RPM groups. Because the leftmost column for these items is blank in Figure 2.3, you know that they are not installed on this system. If an item is already installed on one of your servers, a lowercase i will appear on the left side.

Figure 2.3. YaST installed software filtered by RPM group.

To find the packages already installed on this system, change the filtering option. You can change the filter to show the packages by Selection group by pressing Alt-F and choosing Selections from the drop-down menu, as in Figure 2.4.

Figure 2.4. Changing the Filter setting to present items by choosing Selections.

After you choose Selections and press Enter, a window appears showing the package groups currently installed (see Figure 2.5). As you can see from the figure, only the base SLES system and YaST are installed on this system.

Figure 2.5. Installed components for a minimum configuration of SLES.

To install the DNS components required, use the arrow keys to scroll down to the appropriate option and press the plus sign (+) or spacebar to select the package (see Figure 2.6). You can then tab over to the OK button or press Alt-O to continue.

Figure 2.6. The DHCP and DNS Server option is selected for installation.

You are then presented with the items contained in the package, as shown in Figure 2.7. If required, it is possible to scroll through these items and remove those not needed. In the current example, simply accept the whole package, select Accept (Alt-A), and continue the installation.

Figure 2.7. YaST installation package subcomponent listing.

From this stage on, you are prompted for the appropriate media required for the packages selected (see Figure 2.8). If the media is not readily available or if you decide to stop at this point, you can abort the installation by pressing Alt+R. If you do this, the requested changes will not be committed to the current configuration.

Figure 2.8. YaST prompt for the appropriate installation media.

Figure 2.9 shows the progress information of the installation. As each subcomponent is installed, the size, description, and files are displayed.

Figure 2.9. YaST installation progress screen.

After all the required components are copied to the server, the appropriate changes are made to the configurations files. As explained earlier, YaST delegates these tasks to the SuSEconfig tool. As each subsystem is affected, YaST logs SuSEconfig's progress, as you can see in Figure 2.10.

Figure 2.10. YaST delegates configuration changes to SuSEconfig.

You have now successfully installed the DHCP and DNS package on the server. Before these services can be used, they must be configured. These tasks will be explained in Chapter 8. We will now investigate how unwanted applications can be removed.

Removing a Package or Subcomponents

The steps required to remove components of the server are similar to those used to install them. In the previous example, you installed the software required for creating a DNS server. At the same time you installed the DNS software, you also installed a DHCP server environment. Due to their interoperability, it is not uncommon to host these applications on the same server. However, since you expect to place this DNS server so that it is accessible from the Internet at large, it would not be prudent to host the DHCP environment on the same server.

To remove the DHCP components, start YaST and navigate to the Install and Remove Software submenu. Change the filtering option to Selections (as you did during the install phase). In addition to the software listed in Figure 2.5, you now have the DHCP and DNS server package installed, as indicated by the letter I in the left column (see Figure 2.11).

Figure 2.11. Verification that the DHCP and DNS server packages are installed.

To remove the DHCP components, mark the package for update. Highlighting the DHCP and DNS menu option allows you to type a greater-than sign (>) or press the spacebar and select the Update function (see Figure 2.12).

Figure 2.12. The Update option is selected for the DHCP and DNS Server package.

Selecting OK from this screen reveals the list of subcomponents making up the package. This same screen was presented during the install (refer to Figure 2.7). In this case, however, you can select any of the subcomponents for removal. This can be done by entering a hyphen (-) next to the package.

The question of which one to remove first is handled by YaST itself. As components are selected for removal, YaST checks for any dependencies and presents an appropriate warning if necessary. In Figure 2.13, the DHCP common components were chosen first for removal. After you select them, YaST presents a list of the dependent components. As you can see, dhcp-relay and dhcp-server require that the common components be present. By canceling this step and choosing the other dependent packages, you can ensure a clean uninstall.

Figure 2.13. Component removal dependency warning.

After all the appropriate DHCP components have been marked for removal (see Figure 2.14), you can select the Accept option. YaST then proceeds to remove the identified software and invokes SuSEconfig to manage the changes in the system configuration. At this stage, you should now have a minimum system configuration hosting a single network server process: DNS. We will discuss the configuration of DNS in Chapter 8.

Figure 2.14. Identification of components to be removed.

You have seen how YaST can simplify the process of adding and removing application packages on a system. When a system environment is properly configured for its tasking, few changes will be made.

As a system matures and the user community's use of a machine increases, it is not uncommon for a system to run low on resources. The following section examines how you can add devices to a system to solve some of these issues.