We will start by creating the first node step by step and then use the cloning feature in Virtualbox to instantiate an identical node. This will reduce the necessary time for the installation as it will only require a slight modification to the hostname and network. Follow these steps to install CentOS 7 in a virtual machine:

Since our nodes will communicate between each other over the network, we will first define our network addresses and configuration. Our rather basic network infrastructure will consist of two CentOS 7 boxes with static IP addresses and host names node01 [192.168.0.2] and node02 [192.168.0.3], and a gateway router called simply gateway [192.168.0.1].

In CentOS, all network interfaces are configured using scripts in the /etc/sysconfig/network-scripts directory. If you followed the steps outlined earlier to create a second network interface, you should have a ifcfg-enp0s3 and ifcfg-enp0s8 file inside that directory. The first one is the configuration file for the network card that we will use to access the Internet and to connect via SSH using an outside client, whereas the second will be used in a later chapter to be a part of a cluster resource. Note that the exact naming of the network interfaces may differ a little, but it is safe to assume that they will follow the ifcfg-enp0sX format, where X is an integer number.

This is the minimum content that is needed in the /etc/sysconfig/network-scripts/ifcfg-enp0s3 directory for our purposes in our first node (when you set up the second node later, just change the IP address (IPADDR) to 192.168.0.3):

HWADDR="08:00:27:C8:C2:BE"
TYPE="Ethernet"
BOOTPROTO="static"
NAME="enp0s3"
ONBOOT="yes"
IPADDR="192.168.0.2"
NETMASK="255.255.255.0"
GATEWAY="192.168.0.1"
PEERDNS="yes"
DNS1="8.8.8.8"
DNS2="8.8.4.4"

Note that the UUID and HWADDR values will be different in your case as they are assigned as part of the underlying hardware. For this reason, it is safe to leave the default values for those settings. In addition to this, beware that cluster machines need to be assigned a static IP address—never leave that up to DHCP! In the configuration file used previously, we are using Google's DNS but if you wish to, feel free to use another DNS.

When you are done making changes, save the file and restart the network service in order to apply them. Since CentOS, beginning with version 7, uses systemd instead of SysVinit for service management, we will use the systemctl command instead of the /etc/init.d scripts to restart the services throughout this book, as follows:

$ systemctl restart network.service # Restart the network service

You can verify that the previous changes have taken effect using the following command:

$ systemctl status network.service # Display the status of the network service

You can verify that the expected changes have been correctly applied with the following command:

$ ip addr | grep 'inet' ''# Display the IP addresses

You can disregard all error messages related to the loopback interface as shown in the preceding screenshot. However, you will need to examine carefully any error messages related to enp0s3, if any, and get them resolved in order to proceed further.

The second interface will be called enp0sX, where X is typically 8, as it is in our case. You can verify this with the following command, as shown in the following screenshot:

$ ip link show

As for the configuration file of enp0s8, you can safely create it copying the contents of ifcfg-enp0s3. Do not forget, however, to change the hardware (MAC) address as returned by the information on the NIC by the ip link show enp0s8 command and leave the IP address field blank now, using the following command:

ip link show enp0s8
cp /etc/sysconfig/network-scripts/ifcfg-enp0s3 /etc/sysconfig/network-scripts/ifcfg-enp0s8

Next, restart the network service as explained earlier.

Note that you will also need to set up at least a basic DNS resolution method. Considering that we will set up a cluster with two nodes only, we will use /etc/hosts in both hosts for this purpose.

Edit /etc/hosts with the following content:

192.168.0.2	node01
192.168.0.3	node02
192.168.0.1	gateway

Once you have set up both nodes as explained in the following sections, at this point and before proceeding further, you can perform a ping as a basic test for connectivity between the two hosts to ensure that they are reachable from each other.

To begin, execute in node01:

$ ping –c 4 node02

Next, do the same in node02:

$ ping –c 4 node01

Each node will need the following software components in order to work as a member of the cluster. These packages are fully supported in CentOS 7 as part of a cluster setup, as opposed to other alternatives that have been deprecated:

To install the three preceding software packages, run the following command:

$ yum update && yum install pacemaker corosync pcs

Yum will update all the installed packages to their most recent version in order to better satisfy dependencies, and it will then proceed with the actual installation.

In addition to installing the preceding packages, we also need to enable iptables, as the default firewall for CentOS 7 is firewalld. We choose iptables over firewalld because its use is far more extended, and there is a chance that you will be familiar with it compared with the relatively new firewalld. We will install the necessary packages here and leave the configuration for the next chapter.

In order to manage iptables via systemd utilities, you will need to install (if it is not already installed) the iptables-services package using the following command:

yum update && yum install iptables-services

Now, you can stop and disable firewalld using the following commands:

systemctl stop firewalld.service
systemctl disable firewalld.service

Next, enable iptables to both initialize on boot and start during the current session:

systemctl enable iptables.service
systemctl start iptables.service

You can refer to the following screenshot for a step-by-step example of this process:

Once the installation of the first node (node01) has been completed successfully, clone the first node following the outline in section 1.13 of Virtualbox manual (Cloning virtual machines). Once you're done cloning the virtual machine, add the following minor changes to the second virtual machine:

First, we will ping node02 and gateway from node01, and we will see the following output:

Then, we will ping node01 and gateway from node02:

If any of the pings do not return the expected result, as shown in the preceding screenshot, check the network interface configuration in both Virtualbox and in the configuration files, as outlined earlier in this chapter.

While not strictly required, we will also set up a public key-based authentication for SSH so that we can access each host from the other without entering the accounts password every time we want to access a different node. This feature will come in handy in case, for some reason, we need to perform some system administration task on one of the nodes. Note that you will need to repeat this operation on both nodes.

In order to increase security, we may also enter a passphrase while creating the RSA key, which is shown in the following screenshot. This step is optional and you can omit it if you want. In fact, I advise you to leave it empty in order to make things easier down the road, but it's up to you.

Run the following command in order to create a RSA key:

$ ssh-keygen -t rsa

To enable passwordless login, we will copy the newly created key to node02, and vice versa, as shown in the next two figures, respectively.

$ cat .ssh/id_rsa.pub | ssh root@node02 'cat' >> .ssh/authorized_keys'

Copy the key from node01 to node02:

Copy the key from node02 to node01:

Next, we need to verify that we can connect from each cluster member to the other without a password but with the passphrase we entered previously:

Finally, if passwordless login is not successful, you may want to ensure that the SSH daemon is running on both hosts:

$ systemctl status sshd

If it is not running, start it using the following command:

$ systemctl start sshd

You may want to check the status of the service again after attempting to restart it. If there have been any errors, the output of systemctl status sshd will give you indications as to what is wrong with the service and why it is refusing to start properly. Following those directions, you will be able to troubleshoot the problem without much hassle.