FAQ-General - CentOS

Frequently Asked Questions about CentOS in general
Questions
Why does CentOS exist?
How long after Red Hat publishes a fix does it take for CentOS to publish a fix?
How do I get Updates?
Many RPMs still contain the name redhat, rhel, or rh. Shouldn‘t these be changed?
Does CentOS change the upstream Source RPMs?
Is there a Contrib area?
What is CentOS‘s relationship with Red Hat®, Inc. or RHEL?
Where can I get package XyZ.rpm for CentOS?
What architectures are supported?
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?
The upstream provider offers Enterprise Linux in several flavors, AS, ES, WS, PWS, etc. Which one is CentOS like?
How can I tell what release of CentOS I am running?
How can I easily compare the what is in CentOS 2, CentOS 3 and CentOS 4?
What are all the CentOS repositories (directories) and what is each one for?
What is the versioning/release scheme of CentOS and how does it compare to the upstream vendor?
How do I download and burn the CentOS isos?
Can I add X to my server after install? or I installed from the Server ISO and it didn‘t install X, how do install X?
What are the Maximum number CPUs, Maximum filesystem size, Minimum / Maximum Memory and other specifications associated with the different CentOS versions?
There is no mp3 support in CentOS! Or is there? How about other multimedia formats?
Answers
Why does CentOS exist?
CentOS exists to provide a free enterprise class computing platform to anyone who wishes to use it. CentOS 2, 3, and 4 are built from publically available open source SRPMS provided by a prominent North American Enterprise Linux vendor. 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 theCentOS 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.
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 thisFAQ) 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. This will normally be within 2 weeks of the Update Set release.
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 seethis 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. CentOS has made a decision to NOT automatically load our RPM-GPG-KEY for CentOS-3, but to allow users to first verify the key and then install it. This will prevent people from modifying our key and installing modified software. In CentOS 4, yum will import the key, but only after the user sees the key and can verify it is authentic.
There is a separateFAQ question concerning how to import the CentOS RPM signature key.
Note on using yum (with CentOS-2 and CentOS-3), the first time you run yum, you may download a large number of header files (100‘s). These are generally very small files. Yum uses the header files to determine dependancies, packages available for install, etc. Do not be alarmed when this initial download occurs. yum is NOT downloading the packages. After the first update, you will only download new headers. (Note, with CentOS-4 we use a newer version of yum, so there is a new metadata system that has the hdr files compressed in a single file ...)
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 byJohnnyHughes:
The upstream vendor is using open source (mostlyGPL) 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 intoRPM 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 theGPL 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 publically 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 publically available sources. You can read about ithere. We support the upstream vendors Trademark rights and strive to be in full compliance with those guide lines.
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 theupstream 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.
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 aWiki page about the various CentOS repositories and their purposes.
Is there a Contrib area?
Yes. Please see the Wiki page onContributing to the CentOS project
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. SeeAbout CentOS if you have any questions.
Where can I get package XyZ.rpm for CentOS?
The Wiki has a page aboutthe CentOS and other ‘‘friendly‘‘ repositories. Chances are good that one of those repositories has the package you are looking for.
What architectures are supported?
The following architectures are supported by each version of CentOS:
CentOS 2 only supports x86.
CentOS 3 currently supports x86, x86_64 (AMD64 and Intel EM64T), s390, s390x, ia64 (Intel Itanium2).
CentOS 4 currently supports x86, x86_64, ia64. ppc (PowerPC) and alpha (DEC Alpha) are released in beta for CentOS 4. s390, s390x, and sparc are being developed, but not yet released for CentOS 4.
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 theCentOS 3 and theCentOS 4 sections of theFAQ.
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 publically 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?
How can I easily compare what major package versions are in CentOS 2, CentOS 3 and CentOS 4?
On theCentOS Distro Page atDistroWatch.com you can compare Major packages (currently 54) and All tracked packages (currently 177).DistroWatch is an great resource for comparing Linux and BSD distributions.
What are all the CentOS repositories (directories) and what is each one for?
addonsContains 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.
centosplusPackages 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.
contribPackages 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.
docsContains manuals and release notes for CentOS
extrasPackages 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.
isosContains 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).
osContains the base OS tree that is on the Main ISO files.
updatesContains updates released for the CentOS distro.
What is the versioning/release scheme of CentOS and how does it compare to the upstream vendor?
The upstream vendor has released 3 versions of enterprise Linux that CentOS rebuilds the freely available SRPMS for (seeAbout CentOS for the details). So, the major CentOS releases are CentOS 2, CentOS 3, and CentOS 4. 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 3 and Version 4 of their product (Currently EL 3 update 5 and EL 4 update 1) 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 ... generally within 2 weeks. CentOS follows these conventions as well, so CentOS 3.3 correlates with EL 3 update 3 and CentOS 3.5 correlates with EL 3 update 5, CentOS 4.1 correlates to EL 4 update 1, etc.
One thing some people have problems understanding is that if you have any CentOS-3 product and update it, you will be updated to the latest CentOS-3.x version.
The same is true for CentOS-4. If you update any CentOS-4 product, you will be updated to the latest CentOS-4.x version. This is exactly the same behavior as the upstream product. Let‘s assume that the latest EL3 product is update 5. If you install the upstream original EL3 CDs (the ones before any update set) and upgrade via their up2date, you will have latest update set installed (EL3 update 5 in our example). Since all updates within a major release (CentOS 2, CentOS 3, CentOS 4) 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 theCentOS mirrors.
There is aCentOS 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.
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 of the ISO file(s) that you downloaded against the published md5sum list in the ISO directory. If the md5sum 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. Here is a reference on how to do an md5sum check of an ISO file:http://www.linuxiso.org/viewdoc.php/verifyiso.html
Once you have verified the md5sum 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. If you need to burn the images with another program (cdrecord on Linux, Nero on Windows, etc.) here is a good reference for burning ISO images:http://www.linuxiso.org/viewdoc.php/howtoburn.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 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 MD5 sums 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 ourofficial 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
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"`
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
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 athird 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.
from;http://wiki.centos.org/FAQ/General