NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.
As security vulnerabilities are discovered, the affected software must be updated in order to limit any potential security risks. If the software is part of a package within a Red Hat Enterprise Linux distribution that is currently supported, Red Hat,
Inc. is committed to releasing updated packages that fix the vulnerability as soon as possible. Often, announcements about a given security exploit are accompanied with a patch (or source code that fixes the problem). This patch is then applied to the
Red Hat Enterprise Linux package, tested by the Red Hat quality assurance team, and released as an errata update. However, if an announcement does not include a patch, a Red Hat developer works with the maintainer of the software to fix the problem. Once
the problem is fixed, the package is tested and released as an errata update.
If an errata update is released for software used on your system, it is highly recommended that you update the effected packages as soon as possible to minimize the amount of time the system is potentially vulnerable.
41.5.1. Updating Packages
When updating software on a system, it is important to download the update from a trusted source. An attacker can easily rebuild a package with the same version number as the one that is supposed to fix the problem but with a different security exploit
and release it on the Internet. If this happens, using security measures such as verifying files against the original RPM does not detect the exploit. Thus, it is very important to only download RPMs from trusted sources, such as from Red Hat, Inc. and
check the signature of the package to verify its integrity.
Red Hat offers two ways to find information on errata updates:
Listed and available for download on Red Hat Network
Listed and unlinked on the Red Hat Errata website
Note
Beginning with the Red Hat Enterprise Linux product line, updated packages can be downloaded only from Red Hat Network. Although the Red Hat Errata website contains updated information, it does not contain the actual packages for download.
41.5.1.1. Using Red Hat Network
Red Hat Network allows the majority of the update process to be automated. It determines which RPM packages are necessary for the system, downloads them from a secure repository, verifies the RPM signature to make sure they have not been tampered with,
and updates them. The package install can occur immediately or can be scheduled during a certain time period.
Red Hat Network requires a System Profile for each machine to be updated. The System Profile contains hardware and software information about the system. This information is kept confidential and is not given to anyone else. It
is only used to determine which errata updates are applicable to each system, and, without it, Red Hat Network can not determine whether a given system needs updates. When a security errata (or any type of errata) is released, Red Hat Network sends an
email with a description of the errata as well as a list of systems which are affected. To apply the update, use the Red Hat Update Agent or schedule the package to be updated through the website
https://rhn.redhat.com.
Tip
Red Hat Enterprise Linux includes the Red Hat Network Alert Notification Tool, a convenient panel icon that displays visible alerts when there is an update for a registered Red Hat Enterprise Linux system.
Refer to the following URL for more information about the applet: https://rhn.redhat.com/help/basic/applet.html
Important
Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 41.5.1.5, “Applying the Changes” for general instructions about applying the changes
made by an errata update.
41.5.1.2. Using the Red Hat Errata Website
When security errata reports are released, they are published on the Red Hat Errata website available at https://www.redhat.com/security/. From this page, select the product and version for your
system, and then select security at the top of the page to display only Red Hat Enterprise Linux Security Advisories. If the synopsis of one of the advisories describes a package used on your system, click on the
synopsis for more details.
The details page describes the security exploit and any special instructions that must be performed in addition to updating the package to fix the security hole.
To download the updated package(s), click on the link to login to Red Hat Network, click the package name(s) and save to the hard drive. It is highly recommended that you create a new directory, such as
/tmp/updates, and save all the downloaded packages to it.
41.5.1.3. Verifying Signed Packages
All Red Hat Enterprise Linux packages are signed with the Red Hat, Inc. GPG key. GPG stands for GNU Privacy Guard, or GnuPG, a free software package used for ensuring the authenticity of distributed files. For example, a private
key (secret key) held by Red Hat locks the package while the public key unlocks and verifies the package. If the public key distributed by Red Hat does not match the private key during RPM verification, the package may have been altered and therefore
cannot be trusted.
The RPM utility within Red Hat Enterprise Linux automatically tries to verify the GPG signature of an RPM package before installing it. If the Red Hat GPG key is not installed, install it from a secure, static location, such as an Red Hat Enterprise
Linux installation CD-ROM.
Assuming the CD-ROM is mounted in /mnt/cdrom, use the following command to import it into the keyring (a database of trusted keys on the system):
rpm --import /mnt/cdrom/RPM-GPG-KEY
To display a list of all keys installed for RPM verification, execute the following command:
rpm -qa gpg-pubkey*
For the Red Hat key, the output includes the following:
gpg-pubkey-db42a60e-37ea5438
To display details about a specific key, use the rpm -qi command followed by the output from the previous command, as in this example:
rpm -qi gpg-pubkey-db42a60e-37ea5438
It is extremely important to verify the signature of the RPM files before installing them to ensure that they have not been altered from the Red Hat, Inc. release of the packages. To verify all the downloaded packages at once, issue the following
command:
rpm -K /tmp/updates/*.rpm
For each package, if the GPG key verifies successfully, the command returns gpg OK. If it doesn't, make sure you are using the correct Red Hat public key, as well as verifying the source of the content.
Packages that do not pass GPG verifications should not be installed, as they may have been altered by a third party.
After verifying the GPG key and downloading all the packages associated with the errata report, install the packages as root at a shell prompt.
41.5.1.4. Installing Signed Packages
Installation for most packages can be done safely (except kernel packages) by issuing the following command:
rpm -Uvh /tmp/updates/*.rpm
For kernel packages use the following command:
rpm -ivh /tmp/updates/<kernel-package>
Replace <kernel-package> in the previous example with the name of the kernel RPM.
Once the machine has been safely rebooted using the new kernel, the old kernel may be removed using the following command:
rpm -e <old-kernel-package>
Replace <old-kernel-package> in the previous example with the name of the older kernel RPM.
Note
It is not a requirement that the old kernel be removed. The default boot loader, GRUB, allows for multiple kernels to be installed, then chosen from a menu at boot time.
Important
Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 41.5.1.5, “Applying the Changes” for general instructions about applying the changes
made by an errata update.
41.5.1.5. Applying the Changes
After downloading and installing security errata via Red Hat Network or the Red Hat 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.
Note
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.
Applications
User-space applications are any programs that can be initiated by a system user. Typically, such applications are used only when a user, script, or automated task utility launches them and they do not persist for long periods of time.
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 Red Hat 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:
/sbin/service <service-name> restart
In the previous example, replace <service-name> with the name of the service, such as sshd.
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:
