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I'm sure this question has been asked again and again elsewhere (I did not find anything specific to CentOS vs RHEL in SE), but I would still like to ask and confirm a few specific points.

I am well aware that CentOS removes all RH trademarks, logos, etc. and is based on the same codes with packages built by the community.

  • Are the packages built for CentOS exactly the same? Will the contents of the packages and the behavior of the programs be identical to those found on RHEL?
  • What is RHN other than a medium for license registration? What is it to CentOS?

I'm an Ubuntu desktop user. Attended a RH299 course which did not really touch anything about the support aspect (i.e. RHN). Other than that I've no professional Linux knowledge or experience.

EDIT

I did read the CentOS 6.2 release notes, but I found the details unsatisfactory. The release notes mentions packages modified, removed or added to upstream. But it neither explains nor links to any document detailing what exactly is different in the modified packages. Granted the branding packages are self-explanatory, but it mentions packages like kernel, ntp, anaconda, etc. which have nothing to do with branding as far as I'm aware.

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This was just a guess on my side - derived from your question. I deleted that comment. So diff the source? Or rpm -q --changelog on the CentOS-package? Or ask Karanbir directly. –  Nils Dec 27 '11 at 21:34
    
A bad guess, since I've mentioned my Linux experience. Who exactly is Karanbir? –  Oxwivi Dec 28 '11 at 4:25
    
Karanbir is the guy building and maintaining the x86_64 and i386 line of CentOS. The changelogs of the CentOS-specific packages are full of entries from him. –  Nils Dec 28 '11 at 21:38
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6 Answers

up vote 17 down vote accepted

CentOS is very close to being RHEL without the branding and support. In particular, the library versions are the same, so binaries that work on one will work on the other. The administration tools are the same and configured in similar ways. However, there are a few differences, as the two distributions sometimes apply different minor patches. For example, in this question, it was apparent that RHEL 5 and CentOS 5 apply different rules to identify files under /etc/cron.d.

In other words, at the level of your course, you can treat CentOS and RHEL as interchangeable. But if you needed to look up the precise behavior of a program in a corner of the man page, you may encounter differences.

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Is there anywhere I can find what those differences are? I read the CentOS 6.2 release notes, but other than modified packages (which I think are related to branding) and packages added or missing, no possible discrepancies with upstream is mentioned. –  Oxwivi Dec 21 '11 at 17:53
    
And could you explain the RHN matter as well, please? –  Oxwivi Dec 21 '11 at 17:55
    
CentOS explicitly aims for complete binary compatibility with RHEL. The only real difference is that they replace the branding-related packages from RHEL and recompile everything else from the sources provided by Red Hat. –  vonbrand Jan 23 '13 at 13:37
    
@vonbrand CentOS ships the same library versions (I believe that's what they mean by “complete binary compatibility”). Evidently they do more than rebrand and recompile, some programs are configured differently (e.g. cron). –  Gilles Jan 23 '13 at 13:41
    
@Gilles, no, it goes further than that. They try to set up a build environment as closely similar to the one used by RHEL to rebuild the sources, and rebuild everything (except for the branding bits). That isn't so easy, as you start using version N to build N + 1, and mix with pieces of the new N + 1 to build the rest. Details aren't exactly published by Red Hat. This has been (part of) the reason for delays in the release of some CentOS major versions. –  vonbrand Jan 23 '13 at 14:20
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For all intents and purposes, CentOS is RedHat. CentOS, however is more flexible in what they can do.

From the CentOS website:

CentOS is developed by a small but growing team of core developers.  In turn the core developers are supported by an active user community including system administrators, network administrators, enterprise users, managers, core Linux contributors and Linux enthusiasts from around the world.

CentOS has numerous advantages over some of the other clone projects including:  an active and growing user community, quickly rebuilt, tested, and QA'ed errata packages, an extensive mirror network, developers who are contactable and responsive, multiple free support avenues including IRC Chat, Mailing Lists, Forums, a dynamic FAQ.

In a nutshell, CentOS is a community version of RedHat. You use Ubuntu, so you may understand this analogy a bit better: CentOS is like the Linux Mint of RedHat.

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This may answer your question. Here's an excerpt from the page.

CentOS is a free operating system distribution based upon the Linux kernel. It is derived entirely from the Red Hat Enterprise Linux (RHEL) distribution. CentOS exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream source, Red Hat.[2] CentOS stands for Community ENTerprise Operating System. CentOS is the most popular Linux distribution for web servers with almost 30% of all Linux web servers using it

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Look at the release-notes on the CentOS-pages. There is a list of rpms that are added/different from RH. These are about branding or about the update mechanism (which requires a license in RH).

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I did (please refer to my comments on @Gilles answer). The release notes only says the list of different packages, not what is different about them. And RHN, I'm not sure how it works, and how it is replaced in CentOS - that's what the reason I mentioned it in the question. –  Oxwivi Dec 25 '11 at 6:05
    
Ok - so at least I can give you the hint that anaconda seems to be about kickstart - wich works very well with CentOS, using the same (kickstart) syntax. For updates there is yum. For your question what is RedHat to CentOS? It is the "upstream" source-code-provider. –  Nils Dec 25 '11 at 20:20
    
What is RH**N** to CentOS, not RedHat. –  Oxwivi Dec 26 '11 at 6:11
    
RedHatNetwork is the means to communicate with RH support - since there is no direct support in CentOS they do track bugs with reference to RHN (like: Yes - this is a bug - see RH bug-id XYZ). The solution has to come from RH. Only then it will come to CentOS. So if you need real fast reactive support you need RH. –  Nils Dec 26 '11 at 21:26
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I did read the CentOS 6.2 release notes, but I found the details unsatisfactory. The release notes mentions packages modified, removed or added to upstream. But it neither explains nor links to any document detailing what exactly is different in the modified packages. Granted the branding packages are self-explanatory, but it mentions packages like kernel, ntp, anaconda, etc. which have nothing to do with branding as far as I'm aware.

If you are curious about the differences between the RHEL and CentOS versions of a particular package (ntp, for example), you should compare the source RPMs:

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As commented in the comment on @Gilles's answer, compile-time configuration options. Can make a difference too. Plus, CentOS is supposed to be an exact replica, so I don't see why they would modify the source code. –  Oxwivi Mar 15 '13 at 18:11
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Indeed, CentOS's FAQ states they don't: "Does CentOS change the upstream Source RPMs? No." –  ignis Oct 24 '13 at 22:45
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With RHEL you pay RedHat for support.
With CentOS you do not get commercial support from RedHat.

Most updates are first available for RedHat, then for CentOS.

Other than that they are the same. See this for further reference: CentOS vs RHEL.

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