On Wednesday, May 23, 2012 09:22:34 PM Kaushal Shriyan wrote:
> Thanks for the explanation. Please help me understand when i install CentOS
> 5.8 on a fresh server, what are the available repositories by default 

yum repolist enabled

That command works on both CentOS 6.2 and CentOS 5.8.  Use that command after 
install and you have your answer; use that command on any server you have and 
you can see what repos were used to get the software set installed (the 
yum-utils package includes other tools that can help you visualize the 
dependency tree).

> and
> if i need any packages which are not there in the default repos, Do i need
> to enable third party repositories.

I will put forth the goal of enabling the fewest possible repos.

So you'd want to carefully think about which repo or repos to use if your 
desired package is found in more than one, as well as the differences between 
the way these packages are built.  I have two examples. 

First, if you have a machine with an Intel GMA integrated video devices (say a 
915, 945, or 965 chipset or even newer) you may need the ELrepo 
xorg-x11-drv-intel instead of the provided one in CentOS.  

Second, if you need the xrdp package (RDP remote desktop connections to a Linux 
desktop), EPEL and repoforge have two different versions:

# repoquery --repoid=rpmforge xrdp
xrdp-0:0.4.0-1.el6.rf.i686
# repoquery --repoid=epel xrdp
xrdp-0:0.5.0-0.13.el6.i686

The 0.4.0 xrdp is some different from the 0.5.0 version, and those differences 
by be significant for a particular use.  The repoquery tool is found in the 
yum-utils package.

Now, having RPMforge and EPEL enabled on the same machine can be an adventure.  
As an example, suppose I have a server (an upstream EL6 server in this case) 
which is serving remote desktop connections, being used for digital forensics 
with the scalpel file carver, and I'm wanting to make it my small consultancy's 
e-mail server and use the crm114 system to help with anti-spam.

A yum install of the latest xrdp will pull from EPEL.  A yum install of the 
latest scalpel will pull from EPEL.  So far so good.  Now:
[root@www ~]# yum install crm114
Loaded plugins: product-id, refresh-packagekit, rhnplugin, subscription-manager
Updating certificate-based repositories.
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package crm114.i686 0:20100106-3.el6.rf will be installed
--> Processing Dependency: libtre.so.5 for package: 
crm114-20100106-3.el6.rf.i686
--> Running transaction check
---> Package tre.i686 0:0.7.6-2.el6 will be updated
--> Processing Dependency: libtre.so.4 for package: scalpel-2.0-1.el6.i686
---> Package tre.i686 0:0.8.0-1.el6.rf will be an update
--> Finished Dependency Resolution
Error: Package: scalpel-2.0-1.el6.i686 (@epel)
           Requires: libtre.so.4
           Removing: tre-0.7.6-2.el6.i686 (@epel)
               libtre.so.4
           Updated By: tre-0.8.0-1.el6.rf.i686 (rpmforge)
               Not found
 You could try using --skip-broken to work around the problem
 You could try running: rpm -Va --nofiles --nodigest
[root@www ~]#

Hmmm, looks like I need some more hardware or a VM to run the e-mail (as this 
is a multiprocessor pre-em64t Xeon with loads of RAM and large disk, i686 is a 
reasonable option, but KVM isn't by default available and no em64t Xeons are 
available for the chipset and sockets on this particular SuperMicro 
motherboard....and it's not fully depreciated yet, either, and newer hardware 
is not in this year's budget, but I am open to donations....:-) ).  Or I need 
to rebuild scalpel myself to be compatible with the RPMforge tre package.  Or I 
can wait on EPEL to catch up on tre. Or I can use the RPMforge scalpel (a lower 
version scalpel, but a higher version tre in this case):

[root@www ~]# repoquery --repoid=epel scalpel tre
scalpel-0:2.0-1.el6.i686
tre-0:0.7.6-2.el6.i686
[root@www ~]# repoquery --repoid=rpmforge scalpel tre
scalpel-0:1.60-1.el6.rf.i686
tre-0:0.8.0-1.el6.rf.i686
[root@www ~]#

Oh, but:
[root@www ~]# repoquery --repoid=forensics scalpel tre
scalpel-0:2.0-1.el6.i386
[root@www ~]#

(That's the CERT Forensics repo; I haven't checked to see which tre its scalpel 
is built against.)

This is not a contrived example; this is my own box, and I wanted to do this 
very thing not too long ago.  I haven't solved that problem yet, and haven't 
installed crm114 either, due to time constraints.  It would depend entirely on 
whether I really have to have the features in scalpel 2.0, or if scalpel 1.60 
is good enough (the scalpel website lists: "As for v2.0, Scalpel supports 
regular expressions for headers and footers, minimum carve sizes, 
multithreading and asynchronous I/O, and beta-level support for GPU-accelerated 
file carving. ")  Hmmm, GPU accelerated file carving sounds interesting, but my 
system doesn't have a GPU capable of helping much.  Multithreading and async 
I/O, plus regexp for headers and footers.... while I haven't needed those 
features yet, I might.

I think I've illustrated the process at this point.  I have a tradeoff between 
not having crm114 at all and not having certain features that I may or may not 
use in a program that I use once in a blue moon (but blue moons happen, and I 
have used scalpel a few times in the past).... I'll probably end up doing a yum 
remove of scalpel, excluding the EPEL and the CERT Forensics versions of 
scalpel in the respective repo files in /etc/yum.repos.d, and then reinstalling 
scalpel, which should pull the RPMforge one, and make the crm114 install 
possible.
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