Using yum to install packages, and keep your server updated.

One of the biggest complaints users of RPM based Linux distros have
always had, is the package management. RPM’s are great, but you often
find yourself in what we so kindly refer to as “dependency hell”. In
recent years RedHat implemented their up2date program, which uses their
RedHat Network to install/update packages. After RedHat 9, RedHat
decided to split off their Linux Distro into two projects: RedHat Enterprise & The Fedora Project. In doing so, they decided that RHN would become a pay service, only available to Enterprise customers. Fedora decided to use YUM.

First, a bit of history.

YUM stands for: “Yellow dog Updater, Modifed”. If you are not familiar
with Yellow dog, it is a Linux distribution based on RedHat/Fedora, for
Apple computers. Originally, they created a package management system
called YUP (Yellow dog UPdater). It handled basic dependency
resolution, which made for automation of RPM installation, and
maintenance. The Duke University Physics department began using it for
their internal needs. YUP was rather slow, and lacking features, so a
sysadmin at Duke named Seth Vidal began reworking the code. Eventually, he ended up rewriting it complately, so he renamed it YUM.


So what’s so great about YUM?

Unlike the up2date model, YUM can connect to multiple repositories, and
is not reliant on a single server side solution. A repository (repo) is
simply a directory full of RPM’s on a server accessible via ftp, http,
or even a local filesystem. Each repo has a directory full of headers,
and a packagelist. A repo can have anything from a small group of custom packages, to
a full Linux distribution. This makes it a great way to disrtrubute RPM’s.

Basic configuration:

In addition to Fedora, several other Linux distros use YUM as their package managemnt system. most notably, CentOS.
Since this is a hosting site, I’m going to focus on CentOS rather than
Fedora. Fedora is considered a development OS, and if you are serious
about hosting, you should not be using bleeding edge software. CentOS
is simply RedHat Enterprise stripped of any proprietary code. It gets
updates within a day or two (usually), or RedHat, and is a great
alternative if you do not require the support that RedHat provides.

In it’s most basic configuration, YUM will have two repos; base & updates. These repos are configured in one of two places. Either /etc/yum.conf, or /etc/yum.repos.d/reponame.repo
The actual configuration is the same on either, but the yum.repos.d
method is the new way. Let’s take a look at a couple of examples.

/etc/yum.conf on a CentOS 3.4 system:


[main]
cachedir=/var/cache/yum
debuglevel=2
logfile=/var/log/yum.log
pkgpolicy=newest
distroverpkg=redhat-release
installonlypkgs=kernel kernel-smp kernel-hugemem kernel-enterprise
kernel-debug kernel-unsupported kernel-smp-unsupported
kernel-hugemem-unsupported
tolerant=1
exactarch=1

[base]
name=CentOS-$releasever – Base
baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1

#released updates
[update]
name=CentOS-$releasever – Updates
baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1


/etc/yum.conf on a CentOS 4.1 system:


[main]
cachedir=/var/cache/yum
debuglevel=2
logfile=/var/log/yum.log
pkgpolicy=newest
distroverpkg=centos-release
tolerant=1
exactarch=1
retries=20
obsoletes=1
gpgcheck=1

# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d


/etc/yum.repos.d/CentOS-Base.repo


[base]
name=CentOS-$releasever – Base
baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1

#released updates
[update]
name=CentOS-$releasever – Updates
baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1


What does it all mean?

From the trusty man page (man yum.conf)

[main] options

The [main] section must exist for yum to do anything. It
consists of the following options:

cachedir
directory where yum should store its cache and db files.

debuglevel

debug level. practical range is 0-10. default is 2.

errorlevel
debug level. practical range is 0-10. default is 2.

logfile
full directory and file name for where yum should write its log
file.

assumeyes
1 or 0 – tells yum whether or not to prompt you for
confirmation of actions. Same as -y on the command line. Default to
0.

tolerant
1 or 0 – Tells yum to be tolerant of errors on the command line
with regard to packages. For example: if you request to install
foo, bar and baz and baz is installed; yum won’t error out
complaining that baz is already installed. same as -t on the
command line. Default to 0(not tolerant)

pkgpolicy=[newest|last]
Default: newest. Package sorting order. When a package
is available from multiple servers, newest will install the
most recent version of the package found. last will sort the
servers alphabetically by serverid and install the version
of the package found on the last server in the resulting list. If
you don’t understand the above then you’re best left not including
this option at all and letting the default occur.

exclude

list of packages to exclude from updates or installs. This
should be a space separated list. Filename globs *,?,., etc are
allowed

exactarch
1 or 0 – set to 1 to make yum update only update the
architectures of packages that you have installed. IE: with this
enabled yum will not install an i686 package to update an i386
package.

commands
list of functional commands to run if no functional commands
are specified on the command line. ie: commands = update foo bar
baz quux none of the short options (-y, -e, -d, etc) will be taken
in the field.

distroverpkg
package to use to determine the “version” of the distribution –
can be any package you have installed – defaults to redhat-release.

diskspacecheck
set this to 0 to disable the checking for sufficient diskspace
before the rpm transaction is run. default is 1 (to perform the
check)

retries

Set the number of times any attempt to retrieve a file should
retry before returning an error. Setting this to 0 makes it try
forever. Default to 6.

kernelpkgnames
list of package names that are kernels – this is really only
here for the kernel updating portion – this should be removed out
in 2.1 series.

installonlypkgs
list of packages that should only ever be installed, never
updated – kernels in particular fall into this category. Defaults
to kernel, kernel-smp, kernel-bigmem, kernel-enterprise,
kernel-debug, kernel-unsupported.

[server] options

The server section(s) take the following form:

serverid
must be a unique name for each server, one word.

baseurl

must be a url to the directory where the yum repository’s
‘headers’ directory lives. Can be an http://, ftp:// or file:// url. You can specify
multiple urls in one baseurl statement. The best way to do this is
like this:
[serverid]
name=Some name for this server
baseurl=url://server1/path/to/repository/
url://server2/path/to/repository/
url://server3/path/to/repository/
If you list more than one baseurl= statement in a repository you
will find yum will ignore the earlier ones and probably act
bizarrely. Don’t do this, you’ve been warned.

name
a human readable string describing the repository.

gpgcheck
either ‘1’ or ‘0’. This tells yum whether or not it should
perform a gpg signature check on the packages gotten from this
server

failovermethod
can be either ’roundrobin’ or ‘priority’. roundrobin randomly
selects a url out of the list of urls to start with and proceeds
through each of them as it encounters a failure contacting the
host.
priority starts from the first baseurl listed and reads through
them sequentially.
failovermethod defaults to roundrobin if not specified.

exclude
same as the [main] exclude but this is only for this server
variables, listed below, are honored here.

Now Let’s put this to use:

Let’s say we just installed CentOS 4.1. We want to get our system fully
updated, since new errata has been released since the CD images were
created. Since this is (at the time of this writing) a fairly recent
release, there won’t be too many errata released yet. But on an older
release, there will be a lot. to update everything, we simply type:
yum update

Now let’s say we wanted to install Apache. Simply type:
yum install httpd

Don’t remember exactly what the package is called? Apache for example is now called httpd. Try searching for it using:
yum search apache

It’s gonna give you a lot of results, but if you dig through it, you’ll find:

httpd.i386

2.0.52-12.ent.centos4 installed
Matched from:
Apache HTTP Server
Apache is a powerful, full-featured, efficient, and freely-available
Web server. Apache is also the most popular Web server on the
Internet.
http://httpd.apache.org/

3rd Party repositories:

There are tons of 3rd party repos out there. You must be careful from
which you choose to install packages. Not all “maintainers” adhere to
standards, so mixing packages from multiple repos could become a
nightmare. Personally, I stick to the repos under RPMForge:
DAG
Dries
FreshRPMS

Adding repos is as simple as editing your /etc/yum.conf or creating a file under /etc/yum.repos.d/. We’ll create a repo for Dag as an example.

/etc/yum.conf on a CentOS 3.4 system add the following to the bottom of the file:


[Dag]
name=Dag RPM Repository
baseurl=http://apt.sw.be/redhat/el3/en/$basearch/dag
gpgcheck=1

Create a file called /etc/yum.repos.d/Dag.repo with the following:


[dag]
name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
enabled=1

Nightly updates:

The YUM RPM comes with an init script to do nightly updates. To enable this, simply type the following:
chkconfig yum on

Full version upgrades:

It is completely possible to do a full version upgrade (CentOS 3.x –
CentOS 4.1) via YUM. This is not recommended, as there are often major
changes between releases, that wont clean up properly. However, it can
get an old system that you dont have physical access to updated, and if
you know what you are doing, it’s only a little extra work to get
things right.

I make no guarantees that this will work for you! In
fact I recommend you not use it, as it could leave your system in an
unusable state. If you do try this, don’t be a jerk, BACK UP YOUR
SYSTEM! YOU HAVE BEEN WARNED!

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