Applying the Changes

After downloading and installing security errata via CentOS Network or the CentOS errata website, it is important to halt usage of the older software and begin using the new software. How this is done depends on the type of software that has been updated. The following list itemizes the general categories of software and provides instructions for using the updated versions after a package upgrade.

In general, rebooting the system is the surest way to ensure that the latest version of a software package is used; however, this option is not always available to the system administrator.

Once such a user-space application is updated, halt any instances of the application on the system and launch the program again to use the updated version.

Kernel

The kernel is the core software component for the Community Enterprise Linux operating system. It manages access to memory, the processor, and peripherals as well as schedules all tasks.

Because of its central role, the kernel cannot be restarted without also stopping the computer. Therefore, an updated version of the kernel cannot be used until the system is rebooted.

Shared Libraries

Shared libraries are units of code, such as glibc, which are used by a number of applications and services. Applications utilizing a shared library typically load the shared code when the application is initialized, so any applications using the updated library must be halted and relaunched.

To determine which running applications link against a particular library, use the lsof command as in the following example:

lsof /usr/lib/libwrap.so*

This command returns a list of all the running programs which use TCP wrappers for host access control. Therefore, any program listed must be halted and relaunched if the tcp_wrappers package is updated.

SysV Services

SysV services are persistent server programs launched during the boot process. Examples of SysV services include sshd, vsftpd, and xinetd.

Because these programs usually persist in memory as long as the machine is booted, each updated SysV service must be halted and relaunched after the package is upgraded. This can be done using the Services Configuration Tool or by logging into a root shell prompt and issuing the /sbin/service command as in the following example:

service <service-name> restart

In the previous example, replace <service-name> with the name of the service, such as sshd.

Refer to Network Configuration for more information on the Services Configuration Tool.

xinetd Services

Services controlled by the xinetd super service only run when a there is an active connection. Examples of services controlled by xinetd include Telnet, IMAP, and POP3.

Because new instances of these services are launched by xinetd each time a new request is received, connections that occur after an upgrade are handled by the updated software. However, if there are active connections at the time the xinetd controlled service is upgraded, they are serviced by the older version of the software.

To kill off older instances of a particular xinetd controlled service, upgrade the package for the service then halt all processes currently running. To determine if the process is running, use the ps command and then use the kill or killall command to halt current instances of the service.

For example, if security errata imap packages are released, upgrade the packages, then type the following command as root into a shell prompt:

ps -aux | grep imap

This command returns all active IMAP sessions. Individual sessions can then be terminated by issuing the following command:

kill <PID>

If this fails to terminate the session, use the following command instead:

kill -9 <PID>

In the previous examples, replace <PID> with the process identification number (found in the second column of the ps command) for an IMAP session.

To kill all active IMAP sessions, issue the following command:

killall imapd