Frequently Asked Questions about CentOS in general

1. Why does CentOS exist?

CentOS exists to provide a free enterprise class computing platform to anyone who wishes to use it. CentOS 4, 5 and 6 are built from publicly available open source SRPMS provided by a prominent North American Enterprise Linux vendor, often referred to as "Upstream" or "The Upstream Vendor (TUV)". CentOS conforms fully with the upstream vendors redistribution policies and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.). CentOS is designed for people who need an enterprise class OS without the cost or support of the prominent North American Enterprise Linux vendor. Neither the CentOS Project (we who build CentOS) nor any version of CentOS is affiliated with, produced by, or supported by the prominent North American Enterprise Linux vendor. Neither does our software contain the upstream vendor's product ... although it is built from the same open source SRPMS as the upstream enterprise products.

2. How long after Red Hat publishes a fix does it take for CentOS to publish a fix?

Our goal is to have individual RPM packages available on the mirrors within 72 hours of their release, and normally they are available within 24 hours. Occasionally packages are delayed for various reasons. On rare occasions packages may be built and pushed to the mirrors but not available via yum. (This is because yum-arch has not been run on the master mirror. This may happen when issues with upstream packages are discovered shortly after their release, and if releasing the package would break it's functionality.)

Update Sets (see this FAQ) will have Security Errata released was stated above, while the BugFix and Enhancement errata are actually tested more rigorously and released after the new ISO for the Update Set is produced. The goal for release of a new point release or update set is four to eight weeks after the release by upstream. For more details see the explanation of The CentOS Rebuild and Release Process

During the period between the upstream release of a new minor version and the CentOS release intermediate updates will be published in a Continuous Release (CR) repository.

3. How do I get Updates?

CentOS ships with an application called yum that serves as our recommended tool for performing updates and package installation. Please see PackageManagement, PackageManagement/Yum, and this guide for Managing software with Yum.

A version of up2date is also provided that does not connect to the upstream vendor, but instead connects to the CentOS Network (CN) for updates. Starting with CentOS 4, yum will import the key, but only after the user sees the key and can verify it is authentic.

There is a separate FAQ question concerning how to import the CentOS RPM signature key.

Note on using yum: Starting with CentOS-4 we use a newer version of yum (as compared to CentOS-3), so there is a new metadata system that has the hdr files compressed in a single file (and not lots of small header files).

4. Many RPMs still contain the name redhat, rhel, or rh. Shouldn't these be changed?

This question has an answer on its own page: CentOS Goals.

Additional Text by JohnnyHughes:

• The upstream vendor is using open source (mostly GPL) software in their business model. They take software that other people write (Gnome.org, X.org, KDE.org, OpenOffice.org to name a few). They repackage the source files into RPM format for redistribution. Because they chose an open source model to obtain the software they distribute, they must provide their source code to others. That is how the GPL works. The upstream vendor provides much added value by creating the Source RPMS and distributing them. They also fix problems in software and provide feedback to the software developers ... this is what makes open source software work. The CentOS Project takes the publicly available source packages (SRPMS) provided by the upstream vendor and creates binary (installable) packages for use by anyone who wants to use it. Some packages contain Trademark information and the upstream vendor has specifically created a guide to redistribute software built from their

  publicly available sources. You can read about it here. We support the upstream vendors Trademark rights and strive to be in full compliance with those guidelines. The CentOS project is not interested in taking credit for work done by others, so where possible we will leave all vendor file names as they are. If we must make any changes to a package (due to trademark restrictions, to setup a configuration file, etc.), it will have a .centosx in the filename (the x is the CentOS version ... 3 for CentOS-3.x, 4 for CentOS-4.x, etc.) As do most of the other rebuild projects, we change the kernel SRPM and do not label it .centosx. This is because the kernel needs to be exactly the same name to allow 3rd party modules to function.

  I would like to thank the upstream vendor for making the SRPMS available in the manner that they do. There are several other enterprise vendors who do not make their source as readily available. Their product is excellent (or we would not rebuild it as CentOS), as is their support. If you need the support services that they provide, we highly recommend their enterprise product I would also like to make sure there is no confusion concerning the CentOS Project and the upstream vendor. The CentOS Project is not supported by or affiliated with the upstream vendor in any way. The upstream vendor does not recommend or support any of our offerings in any way.

5. Does CentOS change the upstream Source RPMs?

No. CentOS' key mandate for our base and updates repositories is NOT extending or enhancing packages or features beyond those supplied by the upstream Source RPM's. CentOS strives intentionally to be a simple binary-functional clone for our users. CentOS does offer other (optional) repositories called extras, addons, contribs, and centosplus that do offer added functionality. There is a Wiki page about the various CentOS repositories and their purposes.

6. Is there a Contrib area?

Yes. Please see the Wiki page on Contributing to the CentOS project

7. What is CentOS's relationship with Red Hat®, Inc. or RHEL?

There is none, nada, zip, zero. CentOS-x is NOT a Red Hat®, Inc. or Fedora™ Core affiliated product or project.

The CentOS Project is not affiliated with or supported by Red Hat®, Inc or the Fedora™ Project.

CentOS-x is NOT supported in any way by Red Hat®, Inc. or the Fedora™ Project.

CentOS-x is NOT Red Hat® Linux, it is NOT Fedora™ Core. It is NOT Red Hat® Enterprise Linux. It is NOT RHEL. CentOS-x does NOT contain Red Hat® Linux, Fedora™ Core, or Red Hat® Enterprise Linux.

CentOS is built from publicly available open source SRPMS. See About CentOS if you have any questions.

8. Where can I get package XyZ.rpm for CentOS?

The Wiki has a page about the CentOS and other ''friendly'' repositories. Chances are good that one of those repositories has the package you are looking for.

9. What architectures are supported?

The following architectures are supported by each version of CentOS:

CentOS 4 currently supports x86, x86_64, s390, s390x and ia64. ppc (PowerPC), alpha (DEC Alpha) and sparc are released in beta for CentOS 4.

CentOS 5 currently supports x86 and x86_64. ia64, ppc (PowerPC) and sparc are being developed.

CentOS 6 currently supports x86 and x86_64.

10. Why does YUM complain about missing a GPG key under CentOS? or I just installed CentOS and yum keeps reporting that the correct GPG key is not installed. How do I install it?

These are answered in the the CentOS 4 and the CentOS 5 sections of the FAQ.

11. The upstream provider offers Enterprise Linux in several flavors, AS, ES, WS, PWS, etc. Which one is CentOS like?

CentOS is built from the the publicly provided AS Enterprise Sources, although all of the above versions are built from the same sources. AS is either a larger subset of packages (than PWS and WS) or has advanced Kernel parameters supporting larger number of processors or memory (as compared to ES).

With the upstream provider, AS supports some IBM architectures not supported by the other versions (ES, PWS, WS). CentOS is built like the AS version.

Consult: What release am I running?

12. How can I easily compare what major package versions are in CentOS 4, CentOS 5 and CentOS 6?

On the CentOS Distro Page at DistroWatch.com you can compare Major packages and All tracked packages. DistroWatch is an good resource for comparing Linux and BSD distributions.

13. What are all the CentOS repositories (directories) and what is each one for?

addons

Contains packages required in order to build the main Distribution or packages produced by SRPMS built in the main Distribution, but not included in the main Red Hat package tree (mysql-server in CentOS-3.x falls into this category). Packages contained in the addons repository should be considered essentially a part of the core distribution, but may not be in the main Red Hat Package tree.

apt

(CentOS-4 only) Contains all the apt RPMS for the CentOS site. This is where you would point if you want to use apt to do updates. Apt has issues with distros that use multiple libraries, so is only available for the i386 distro.

centosplus

Packages contributed by CentOS Developers and the Users. These packages might replace rpm's included in the core Distribution. You should understand the implications of enabling and using packages from this repository.

contrib

Packages contributed by the CentOS Users, which do not overlap with any of the core Distribution packages. These packages have not been tested by the CentOS developers, and may not track upstream version releases very closely.

docs

Contains manuals and release notes for CentOS

extras

Packages built and maintained by the CentOS developers, that add functionality to the core distribution. These packages have undergone some basic testing, should track upstream release versions fairly closely and will never replace any core distribution package.

isos

Contains the ISOs for download. On the main CentOS mirror sites ISOs can not be downloaded directly, but we provide a Bittorent file for downloading. On external public mirrors, ISOs may or may not be directly downloadable (at the discretion of the mirror owner).

os

Contains the base OS tree that is on the Main ISO files.

updates

Contains updates released for the CentOS distro.

CR

Contains updates from the next point-release which went through basic QA but have not yet been released as a new CentOS point release.

debuginfo

Contains RPMs with debugging symbols generated when the primary packages are built. No repo config is provided. These packages are found at http://debuginfo.centos.org/

testing

This repository is a proving grounds for packages potentially on their way to CentOSPlus and CentOS Extras. They may or may not replace core CentOS packages, and are not guaranteed to function properly. These packages build and install, but are waiting for feedback from testers as to functionality and stability. Packages in this repository will come and go during the development period, so it should not be left enabled or used on production systems without due consideration. No repo config is provided by default. CentOS-Testing.repo can be put in /etc/yum.repos.d. Contribute to CentOS by reporting problems or successes on the CentOS Developer's list. Use with caution.

See the Repositories page for more information.

14. What is the versioning/release scheme of CentOS and how does it compare to the upstream vendor?

The upstream vendor has three versions of enterprise Linux that CentOS rebuilds from the freely available SRPMS (see About CentOS for the details). So, the major CentOS releases are CentOS 4, CentOS 5 and CentOS 6. The upstream vendor releases security updates as required by circumstances. CentOS releases rebuilds of security updates as soon as possible. Usually within 24 hours (our stated goal is with 72 hours, but we are usually much faster).

The upstream vendor also releases numbered update sets for Version 4, Version 5 and Version 6 of their product (Currently EL 4 update 9, EL 5 update 7 and EL 6.2) 2 to 4 times per year. There are new ISOs from the upstream vendor provided for these update sets. Update sets will be completed as soon as possible after the vendor releases their version. For more details see the explanation of The CentOS Rebuild and Release Process

CentOS follows these conventions as well, so CentOS 4.9 correlates with EL 4 update 9 and CentOS 5.7 correlates with EL 5 update 7, CentOS 6.2 correlates to EL 6 update 2, etc.

One thing some people have problems understanding is that if you have any CentOS-x product and update it, you will be updated to the latest CentOS-x.y version.

The same is true for CentOS-4, CentOS-5 and CentOS-6. If you update any CentOS-4 (CentOS-5 or CentOS-6) product, you will be updated to the latest CentOS-4.y (CentOS-5.y or CentOS-6.y)version.

Any point release is just a "snapshot" with previous updates, plus the latest batch of new upstream updates, rolled into a new [base] repo with an initially empty [updates] repo. Unless you have defeated the $baserelease mechanism and the conventional symbolic link for the major release (currently 6) pointing to the current point release (6.x) upgrades between point releases happen "automagically". Old point releases are never supported. If you want/need to "freeze" at an old point release you are on your own.

This is exactly the same behavior as the upstream product. Let's assume that the latest EL4 product is update 8. If you install the upstream original EL4 CDs (the ones before any update set) and upgrade via their up2date, you will have latest update set installed (EL4 update 9 in our example). Since all updates within a major release (CentOS 4, CentOS 5 or CentOS 6) always upgrade to the latest version when updates are performed (thus mimicking upstream behavior), only the latest version is maintained in each main tree on the CentOS mirrors.

There is a CentOS Vault containing old CentOS trees. This vault is a picture of the older tree when it was removed from the main tree, and does not receive updates. It should only be used for reference.

15. How do I download and burn the CentOS isos?

You can download the latest CentOS ISOs from here: CentOS Downloads

After you download the ISOs, you should check the MD5 sums (or better sha1 or sha265 sums) of the ISO file(s) that you downloaded against the published md5sum (again sha1 or sha265 sums) list in the ISO directory. If the values match, the download is good ... if they do not match, the file was not downloaded correctly, and you need to get the file over again. Bittorrent downloads are best, because they do an MD5sum check as part of the download process. Generally the DVD media is preferable as it avoids switching media during installation, and can be used via the pre-defined [c5-media] repo definition for local installation of additional packages via yum. Some environments block P2P file sharing including Bittorrent, in which case direct download is the only viable option. The mirrors that support it can be found via the "Downloads/Mirrors" drop-down on the menu bar at the top of Home or Forum pages. Then pick the link Current CentOS Public Mirror List, pick your geographic region, and look down the column labeled "Direct DVD Downloads" to find a suitable mirror near you.

Once you have verified the checksums of the ISO, you know you have a good download. Now you can burn the ISO to a disk. If you have k3b (CentOS-4 users do, all other CentOS users do not by default) I recommend you use it. You want to use the Tools -> CD -> Burn CD Image or Tools -> DVD -> Burn DVD ISO Image option to write the ISO file to a CD/DVD.

More information on how to verify and burn a CD ISO image can be found at http://www.centos.org/docs/5/html/CD_burning_howto.html.

Once the CD is burned, you should be able to boot from it. The last check you need to do is to verify the media. This will verify that the writing of the ISO to your media happened correctly. There will be a Check Media option after you select your keyboard and language.

If your media passes this check (make sure to check each disc for multiple media sets), you have a fully working installable media. If it fails this check, but passed the md5 (or sha1 or sha265 sum) check above, then the problem is with the burned media. Try burning on new media at a slower speed, if possible.

All CentOS ISOs that we release have been checked, so if the checksums that you have match, the ISOs should burn clean and pass the media checks. If they do not, the problem is almost always a bad media write to CD/DVD.

If you would rather buy your CentOS ISOs already burned, please see our official CentOS CD/DVD Vendors page. These official CentOS vendors donate a portion of each CD/DVD sale directly to the CentOS Project. You get a tested ISO ready to use ... we get money ... does it get any better than that

16. Can I add X to my server after install? or I installed from the Server ISO and it didn't install X, how do I install X?

The easiest way to install X (and a GUI system) is to use the `yum groupinstall` feature. First you can see all the yum groups available with the command: yum grouplist You can install X and Gnome or KDE like this: yum groupinstall "X Window System" "GNOME Desktop Environment" or yum groupinstall "X Window System" "KDE (K Desktop Environment)" You may also want to add some other groups from the list like `"Graphical Internet" or "Office/Productivity"`

Note: If you are running CentOS 5, yum groupinstall "GNOME Desktop Environment" may complain about a missing libgaim.so.0. This is a known bug. Please see CentOS-5 FAQ for details.

17. What are the Maximum number CPUs, Maximum filesystem size, Minimum / Maximum Memory and other specifications associated with the different CentOS versions?

See this page: CentOS Product Page

18. There is no mp3 support in CentOS! Or is there? How about other multimedia formats?

Yes. There is no mp3 support in CentOS, as there is no mp3 support in the distributions from upstream. It's not included because of legal (patent) issues. You either can encode your music files to ogg vorbis, which is supported in CentOS or install mp3 support from a third party repository, namely rpmforge. For example: If you want mp3 support for xmms, then install xmms-mp3 from that repository.

The same is true for several other multimedia formats (codecs, for example gstreamer plugins) and multimedia players like xine or mplayer. These aren't packaged with CentOS because of legal issues, but you may find those in the rpmforge repository.

19. What is the support ''end of life'' for each CentOS release?

20. Where can I get the latest version of XyZ.rpm for CentOS? I cannot find it anywhere.

CentOS is an Enterprise-class operating system and as such is more about stability and long-term support than cutting edge. Major package versions are retained throughout the life cycle of the product. This is generally what Enterprise wants and affords developers a stable base on which to develop without fear that bespoke applications will break every time something gets upgraded to the latest and greatest, but ultimately buggy version or the API changes breaking backwards compatibility.

So no, you will generally NOT find the very latest versions of various packages included in an Enterprise-class operating system such as CentOS. It's a feature not a deficiency.

Security patches and bug fixes are backported into the shipped version. See here for details: http://www.redhat.com/advice/speaks_backport.html

Please also read about the Wiki article Software Installation from Source.

21. How do I install or update on a system with no network connection?

See the Wiki article Creating Update Media. The technique described there also works with DVD installation media. The key technique is

yum --disablerepo=\* --enablerepo=c5-media <yum_command_and_argument(s)>

22. I installed the x86_64 version, so why do I have i386 packages, and can I get rid of them?

CentOS follows the upstream source in this respect, as it does in general, and the x86_64 installation by default will install iX86 32-bit packages on a 64-bit installation for compatibility purposes. Many server system administrators (and some desktop users) want a pure 64-bit system and so remove all 32-bit packages. This can be accomplished as follows:

yum remove \*.i\?86

To keep any 32-bit packages from being installed in future updates, edit your /etc/yum.conf and add the line:

exclude = *.i?86

Be aware that 32-bit applications, including some browser plugins that may only be available in 32-bit versions, will no longer work after this procedure.

You may also want to do this:

yum reinstall \*

The reason is that sometimes the /usr/share/ items (shared between BOTH packages) get removed when removing the 32-bit RPM packages.

23. How do I provide appropriate information about my system when asking questions?

When asking questions on a forum, mailing list, or IRC, it is important to provide correct and complete information about your system. Good general guidance can be found in the Readme First post on the CentOS fora, and in the classic guide "How To Ask Questions The Smart Way" by open source developer and philosopher Eric S. Raymond.

To aid in the process a script getinfo.sh has been developed to collect system information for a variety of classes of questions. Please click on the link, select the text and save the script to a file, for example "getinfo.sh", change the permissions to make it executable and run it:

cat - > getinfo.sh # Note: Ctrl-D is EOF (or right-click and save)
chmod +x getinfo.sh
./getinfo.sh

If you right-click the script link and save it, the Wiki will unfortunately save it with DOS "Ctrl-M" line terminations. Similar problems may occur if the script is saved on a Windows system. Running "dos2unix" (available via "yum install dos2unix") on the file will correct the problem. So will the following commands if dos2unix is not available due to the problem being worked:

mv getinfo.sh getinfo.sh_DOS
cat getinfo.sh_DOS | tr -d '\r' > getinfo.sh
chmod +x getinfo.sh

1. Basic info - appropriate for any and all CentOS questions. - ./getinfo.sh basic

2. Disk/filesystem problem - Basic + full disk info - requires the user to become root to run as this really needs "fdisk -l". - ./getinfo.sh disk

3. Hardware/driver/kernel problem - Basic + hardware and kernel info. - ./getinfo.sh driver

4. Network problem - Basic + full network info. - ./getinfo.sh network

5. Package problems with yum, rpm, and repos - Basic + rpm, yum, kernel. - ./getinfo.sh package

6. The kitchen sink - combines all of the above. Run as either root or non-root, but won't get fdisk info unless root. - ./getinfo.sh all

All the scripts will save the information to a temporary file which can be posted in-line, or for longer files saved to a site such as pastebin.centos.org. All can be run as a normal user except getinfo_disk.sh. You should check the contents of the output file to ensure it does not contain any sensitive information such as public IP addresses or hostnames.

If necessary obfuscate such information before posting. For example:

146.xxx.xxx.41 or myhost.mydomain.net

Note that it is not necessary to hide network information for private subnets often assigned by routers in ranges

10.0.0.0 – 10.255.255.255
172.16.0.0 – 172.31.255.255
192.168.0.0 – 192.168.255.255

Other tools for gathering more detailed system information include dmidecode and sos, available in the CentOS core repos. Providing appropriate information will help you get a prompt and accurate answer, and will help other forum members to efficiently answer your questions.

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