HAProxy is very fast and dependable solution that offers high availability, load balancing, and proxying for TCP and HTTP-based applications. It is ideally suited for web sites very high traffic.

As a pre-requirement for the best results, you should have a minimum of two web servers and a server for the load balancer. The web servers need to be running at least the basic web service such as nginx or httpd to test out the load balancing between them.

Note that the firewall on CentOS 7 is restrictive for by default for the setup.

Whether you are using built-in, iptables, csf/apf type of firewall please open port 8181 TCP. We will use firewall-cmd now:

Enter the commands below to permit the required services and reload the firewall.

                     # firewall-cmd --permanent --zone=public --add-service=http
                     # firewall-cmd --permanent --zone=public --add-port=8181/tcp
                     # firewall-cmd --reload

Note: If you don't have firewalld installed you can install it with yum install firewalld then service firewalld restart and proceed with the commands above. Otherwise, use iptables as stated earlier.

2. To check current iptables rules (the output below show no iptables rules set).

                      # iptables -L
                      Chain INPUT (policy ACCEPT)
                      target     prot opt source               destination         

                      Chain FORWARD (policy ACCEPT)
                      target     prot opt source               destination         

                      Chain OUTPUT (policy ACCEPT)
                      target     prot opt source               destination
Deploying your Load balancing server

Configure the load balancer

A HAProxy configuration notify the load balancing system specific type of connections it should be listening for and which servers it should relay the connections to.

Create a configuration file /etc/haproxy/haproxy.cfg which contains the required settings and configurations. 

                      
                        # vi /etc/haproxy/haproxy.cfg

Enter the following into the file:

                      
                        global
                          log /dev/log local0
                          log /dev/log local1 notice
                          chroot /var/lib/haproxy
                          stats socket /run/haproxy/admin.sock mode 660 level admin
                            stats timeout 30s
                            user haproxy
                            group haproxy
                            daemon

                        defaults
                            log global
                            mode http
                            option httplog
                            option dontlognull
                            timeout connect 5000
                            timeout client 50000
                            timeout server 50000

                        frontend http_front
                            bind *:80
                            stats uri /haproxy?stats
                            default_backend http_back

                        backend http_back
                            balance roundrobin
                            server your_server1 private_IP:80 check
                            server your_server2 private_IP:80 check

Ensure to save the file before closing it.

Next, restart Haproxy using the command below:
                      
                        # systemctl restart haproxy
                        # systemctl enable haproxy

If you get any errors or warnings at startup, ensure that there are no typographical errors in configuration and all the required files and folders are created, then restart.