Create a CentOS mirror for Kickstarting

Create a CentOS mirror for Kickstarting

June 29, 2017
linux, homelab

This is an update over my earlier post about PXE booting. I’ve learned a few more things and refined some rough edges.

Update: By now I’ve learned even more. See homelab/bootstrap for an updated guide.

My Motivation is similar to the last post: I started building my homelab with virtual machines. Most of them are based on a minimal CentOS 7 installation, and as such I have a lot of very similar systems. Yes, I could probably use containers to great effect. But I prefer the separation/isolation that I get from virtual machines on ESXi.

Since I don’t want to spend my time clicking through the installation wizard each time and repeating all those steps, I use kickstart. And since all those machines need the same rpm packages, I might aswell configure a local mirror for all those updates.

The next logical step up would be a provisioning system like Katello / the Foreman but I haven’t gotten around to properly implementing that yet.

1. Draft #

  • prepare a minimal CentOS 7.3 installation
  • plan your harddrive space! (~25 GB per architecture/version combination)
  • you need to be able to set DHCP options for the network
  • install & configure a tftp and a http server
  • download packages from a mirror
  • create kickstart configuration

2. Setup #

I don’t think the first few steps need any explanation. Make sure you provision enough harddrive space and configure your network, either through a static IP address or through a DHCP reservation. Also make sure you know how to configure DHCP options in your router / DHCP server. I will be showing the settings in LEDE later.

2.1 Install server packages #

We need a TFTP server and some Syslinux packages. Those two enable booting kernels over the network. The CentOS mirrors provide appropriate images, which we will be using later. Furthermore, a simple HTTP server is required to serve our kickstart configuration and all the packages later.

Install all required packages with:

# yum install -y tftp-server syslinux-tftpboot httpd

This creates and populates the directories /var/lib/tftpboot and /var/www/html. The tftp-server serves Syslinux’ files from the former by default and Apache httpd will serve files from the latter.

Hint: take a look in /etc/httpd/conf.d/welcome.conf to disable the default Welcome page.

2.2 Synchronize mirror #

Now would be a good time to select a fast mirror from the [mirrorlist]( List of CentOS mirrors), which also supports the rsync protocol.

After you have run any yum command, which populated the cache, there will be a list of ’timed hosts’, indicating mirrors with good performance. The smaller the number, the better:

$ cat /var/cache/yum/x86_64/7/timedhosts.txt | awk '{print $2" "$1}' | sort

We’ll want to synchronize at least the os and updates trees for a given version.

Since this is a very repetitive task, I have developed this little script to automate the creation of directory structures locally and incrementally synchronize the repositories with rsync:

#!/usr/bin/env bash

# synchronize files
# $1 source, $2 destination, $3 label
sync() {
  printf '\e[1mSynchronizing %s...\e[0m\n' "${3/%/ }";
  printf '\e[1m╭╴src╶─ \e[0m%s\n\e[1m╰╴dst╶→ \e[0m%s\n' "$1" "$2";
  mkdir -p "$2";
  rsync \
    --archive --hard-links --delete \
    --compress --no-motd --progress \
    "$1" "$2";
  printf '\n';

# remote and local mirror

# sync packages
for version in '7.3.1611'; do
  for repo in 'os' 'updates'; do
    sync {"$remote","$mirror"}"/centos/$version/$repo/x86_64/" "$version/$repo";

Replace the mirror="..." assignment with your chosen mirror. You might want to create a simple systemd.timer in order to synchronize the repositories every night. But I’ll leave that as an exercise for you.

Running the script will then create repositories under /var/www/html/mirror/centos/7.3.1611/{os,updates}/x86_64/.

2.3 Configure TFTP server #

Point your DHCP clients to this mirror by specifying (at least) [options]( List of DHCP options) 67 and 150. I am using a router flashed with LEDE, so the options can be configured in a single field under Network > DHCP and DNS > TFTP Settings:

Network boot image:

Use gpxelinux.0 here to be able to use http:// links to your kernel and initrd in your pxelinux configuration and avoids the need to copy the pxeboot images into the TFTP root seperately. Speaking of configuration ..

Pxelinux expects to find its initial configuration in $tftproot/pxelinux.cfg/default. Thus, create the directory /var/lib/tftpboot/pxelinux.cfg and create a configuration in the file default. A very minimalistic file is sufficient for a single kickstart target:

#// general
default     menu.c32
kbdmap      de-latin1.ktl
prompt      0
timeout     600
ontimeout   reboot

#// text
menu title
menu autoboot   Rebooting system in # seconds ...

#// menu entries

label       reboot
menu label  ^Reboot system

  kernel      reboot.c32
  append      --warm

label       kickstart-centos-7.3.1611
menu label  Kickstart: ^CentOS 7.3.1611

  append      ramdisk_size=262144 ks=

Adjust kbdmap and the hostname in menu title, kernel, initrd and append lines to fit your network. If you left out the ks=... assignment in the append, you would boot into a minimal CentOS installer by default. To further automate the process we need to create this kickstart configuration next.

2.4 Kickstart configuration #

Chances are, the Anaconda installer left a kickstart file in /root/anaconda-ks.cfg. If you used that file, the installer would create an identical installation to your mirror server.

I used mine as a template, cleaned it up and consulted the documentation for additional commands and syntax. This is what I came up with:

#// installation mode and source
url --url
repo --name=mirror.base --baseurl= --install --cost=100
repo --name=mirror.updates --baseurl= --install --cost=100

#// language
lang en_GB.UTF-8
keyboard --vckeymap=de --xlayouts=de

#// time and network
%include /tmp/network.kick
timezone Europe/Berlin --utc

#// root authentication
auth --enableshadow --passalgo=sha512
rootpw --iscrypted $6$kXBCG...98MeGIR8AEpLRfc0

#// partitioning
ignoredisk --only-use=sda
bootloader --location=mbr --boot-drive=sda
autopart --type=btrfs
clearpart --none --initlabel

#// installed packages and services
services --enabled=ntpd,rngd

#// pre-installation scripts

echo "network --device=ens160 --ipv6=auto --hostname=kick-$(< /dev/urandom tr -dc '0-9' | head -c4) --activate" > /tmp/network.kick


#// post-installation scripts

# add gpgkey in local repo definitions
tee -a /etc/yum.repos.d/mirror.{base,updates}.repo <<EOF

# root pubkey auth
mkdir -m0700 /root/.ssh/
cat <<EOF >/root/.ssh/authorized_keys
ssh-rsa AAAAB3N...
chmod 0600 /root/.ssh/authorized_keys
restorecon -R /root/.ssh/


This is placed in /var/www/html/kickstart/centos.ks, so anaconda can find it later. A few highlights:

  • the mirrored repositories are used for the installation and in the installed system; i.e. any yum operations will prefer downloading from your local mirror first
  • a pre-installation script creates a configuration for a random hostname like kick-####; if your DHCP server adds dynamic DNS entries, you can reach the installed system by hostname more easily
  • the first disk is auto-partitioned with a BTRFS scheme
  • a random number generator daemon is installed by default to remedy a lack of entropy in virtual machines
  • a public SSH key is configured for root

This answer on describes how you can create your own salted password hash for the rootpw --iscrypted .. line above. This oneliner creates the required SHA512 hash:

python -c'import crypt as c,getpass as p; print(c.crypt(p.getpass(),c.mksalt(c.METHOD_SHA512)))'

2.5 Enable services #

Finally, enable the tftp-server and httpd services and open firewall ports:

systemctl enable --now tftp.socket httpd.service
firewall-cmd --permanent --add-service={tftp,http}
firewall-cmd --reload

2.6 Booting #

You should now be greeted with a Pxelinux menu upon booting a new machine:

And if you select the second option, a fresh copy of CentOS should automatically be installed:

I hope this tutorial was somewhat helpful to you. I have created a repository on GitHub to track some of the relevant files and additional post-installation scripts. Take a look and leave a comment if you like!