This assumes you have installed HAProxy.

Let's say you have two web servers, www1.example.com and www2.example.com, and these web servers both serve the www.example.com web pages. HAProxy can be configured to load balance requests between both web servers.

Listen Block

One option would be to use the listen block.

listen web
    bind *:80
    mode tcp
    balance roundrobin
    server www1 www1.example.com:80 check
    server www2 www2.example.com:80 check

If you have a firewall, such as iptables or firewalld, you will need to allow the HAProxy bind port, not the ports being used by the "server" directives. In this example, only the http (port 80) service would need to be allowed.

firewall-cmd --add-service=http --permanent
firewall-cmd --add-service=https --permanent
firewall-cmd --reload

Frontend / Backend

Or, the frontend and backend blocks can be used. However, this is more commonly used for more complex situations, such as when there are two web servers serving multiple domains.

hdr is short for HTTP headers. For example, the curl command can be used to see the "host" header.

~]$ curl http://www.freekb.net --verbose
> GET / HTTP/1.1
> Host: www.freekb.net
> User-Agent: curl/7.79.1
> Accept: */*

In this example, acl www hdr(host) -i <HTTP header host> is used to route requests to different backends based on the HTTP host header. The -i flag means case insensitive.

frontend main
    bind *:80
    bind *:443 ssl crt /etc/pki/tls/foo.pem

    acl www hdr(host) -i www.example.com
    use_backend www_example_com if www

    default_backend www_example_com

backend www_example_com
    balance roundrobin
    server www1 www1.example.com:80 check
    server www2 www2.example.com:80 check

Let's say index.html on the www1.example.com web server contains "web server A" and index.html on the other web server www2.example.com contains "web server B". When navigating to http://<HAProxy ip address or hostname>/index.html, the request should first be routed to web server A.

Refreshing the web browser, the next request should go to web server B.