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 CentOS, Inc. and check the signature of the package to verify its integrity.

CentOS offers two ways to find information on errata updates:

• Listed and unlinked on the CentOS Errata website

  Beginning with the Community Enterprise Linux product line, updated packages can be downloaded only from CentOS Network. Although the CentOS Errata website contains updated information, it does not contain the actual packages for download.

  Using Automatic Updates with RHN Classic

  Warning: Deprecate Feature

  Automatic system updates are only available using RHN Classic, which basis subscription consumption on access to content repository channels. RHN Classic is available as a convenience for customer environments with legacy systems which have not updated to Certificate-Based CentOS Network.

  The update and content stream is different for Certificate-Based CentOS Network, so automatic updates are not used.

  The new Certificate-Based CentOS Network and the differences between Certificate-Based CentOS Network and RHN Classic are described in Product Subscriptions and Entitlements.

  RHN Classic 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.

  RHN Classic requires a system profile for each machine, which 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, RHN Classic can not determine whether a given system needs updates. When a security errata (or any type of errata) is released, RHN Classic 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 CentOS Update Agent or schedule the package to be updated through the RHN Classic Subscription Management area of the Customer Portal.

  Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to "Applying the Changes" for general instructions about applying the changes made by an errata update.

  Using the CentOS Errata Website

  When security errata reports are released, they are published on the CentOS Errata website available at http://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 Community 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 CentOS 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.

  Verifying Signed Packages

  All Community Enterprise Linux packages are signed with the CentOS, 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 CentOS locks the package while the public key unlocks and verifies the package. If the public key distributed by CentOS does not match the private key during RPM verification, the package may have been altered and therefore cannot be trusted.

  The RPM utility within Community Enterprise Linux automatically tries to verify the GPG signature of an RPM package before installing it. If the CentOS GPG key is not installed, install it from a secure, static location, such as an Community 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-redhat-release

  To display a list of all keys installed for RPM verification, execute the following command:

  rpm -qa gpg-pubkey*

  For the CentOS key, the output includes the following:

  gpg-pubkey-37017186-45761324

  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-37017186-45761324

  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 CentOS, 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 CentOS 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.

  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.

  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.

  Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to "Applying the Changes" for general instructions about applying the changes made by an errata update.

  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.

  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 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

  
  

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  ^[10] Source: http://www.cert.org

  ^[11] Source: http://www.cert.org/stats/

  ^[12] Source: http://www.newsfactor.com/perl/story/16407.html

  ^[13] Source: http://www.sans.org/security-resources/mistakes.php