CentOS

Protect SSH by fail2ban on centos 7

yum install -y fail2ban
cat >> /etc/fail2ban/jail.d/customisation.local <<EOF
[sshd]
enabled = true
EOF
systemctl enable fail2ban
systemctl start fail2ban
fail2ban-client status sshd

Install docker on Centos 7

Installing

https://docs.docker.com/engine/installation/linux/docker-ce/centos/

sudo yum install -y yum-utils device-mapper-persistent-data lvm2
sudo yum-config-manager \
--add-repo \
https://download.docker.com/linux/centos/docker-ce.repo
sudo yum makecache fast
sudo yum install -y docker-ce
sudo systemctl start docker
sudo systemctl enable docker

Optional

firewall-cmd --permanent --zone=trusted --change-interface=docker0
firewall-cmd --reload

Booting CentOS 5.5 in xen DomU

CentOS 5.5
kernel-xen0-*.*.rpm and kernel-xen-*.*.rpm are built with xenblk as a module, so that an attempt to boot into a DomU with the xen0 kernel dies with a message like:
mount: could not find filesystem '/dev/root'

solution:
sudo mkinitrd --with=xennet --preload=xenblk /boot/$KERNEL-$DOMNAME.img $KERNEL