RHEL7: Configure a high available load-balancer.

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Presentation

For this tutorial, you need 2 virtual/physical servers for the load-balancers and 2 virtual/physical servers to load-balance.

In addition to the 4 IP addresses needed by the servers themselves, a fifth virtual IP address (VIP) is necessary. The two load-balancers and the VIP need to be in the same network segment.

Piranha has been replaced in RHEL7 with HAProxy and keepalived. For this reason, HAProxy will be used as load-balancing software, keepalived as high availability solution and apache as software to load-balance.

Here is the addressing schema chosen to write into the /etc/hosts file of each server:

  • 192.168.0.100 vip
  • 192.168.0.101 haproxy1
  • 192.168.0.102 haproxy2
  • 192.168.0.103 httpd1
  • 192.168.0.104 httpd2

HAProxy Installation

On the haproxy1/haproxy2 servers, execute the following instructions:

Install the HAProxy package:

# yum install -y haproxy

Edit the /etc/haproxy/haproxy.cfg file, replace the line “frontend  main *:5000” with “frontend  main *:80” and comment out the line “use_backend static if url_static“.
At the end of the same file, remove the lines starting with “server app” and replace them with the following lines:

server httpd1 192.168.0.103:80 check
server httpd2 192.168.0.104:80 check

Activate at boot and start the HAProxy service:

# systemctl enable haproxy
# systemctl start haproxy

Create the /etc/firewalld/services/haproxy.xml file and paste the following lines:

<?xml version="1.0" encoding="utf-8"?>
<service>
<short>HAProxy</short>
<description>HAProxy load-balancer</description>
<port protocol="tcp" port="80"/>
</service>

Assign correct SELinux context and file permissions to the haproxy.xml file:

# cd /etc/firewalld/services
# restorecon haproxy.xml
# chmod 640 haproxy.xml

Update the firewall configuration:

# firewall-cmd --permanent --add-service=haproxy
# firewall-cmd --reload

Keepalived Installation

On the haproxy1/haproxy2 servers, execute the following instructions:

Install the keepalived package:

# yum install -y keepalived

Create a new /etc/keepalived/keepalived.conf file and paste the following lines:

vrrp_script chk_haproxy {
  script "killall -0 haproxy" # check the haproxy process
  interval 2 # every 2 seconds
  weight 2 # add 2 points if OK
}

vrrp_instance VI_1 {
  interface eth0 # interface to monitor
  state MASTER # MASTER on haproxy1, BACKUP on haproxy2
  virtual_router_id 51
  priority 101 # 101 on haproxy1, 100 on haproxy2
  virtual_ipaddress {
    192.168.0.100 # virtual ip address 
  }
  track_script {
    chk_haproxy
  }
}

Activate at boot and start the keepalived service:

# systemctl enable keepalived
# systemctl start keepalived

Check the presence of the VIP on the haproxy1 server:

# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 52:54:00:f7:2a:a9 brd ff:ff:ff:ff:ff:ff
inet 192.168.0.101/24 brd 192.168.0.255 scope global eth0
valid_lft forever preferred_lft forever
inet 192.168.0.100/32 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::5054:ff:fef7:2aa9/64 scope link
valid_lft forever preferred_lft forever

Apache Installation

On the httpd1/httpd2 servers, follow the instructions for the Apache installation.
Create a file called index.html in the /var/www/html directory on the httpd1 server and paste the following line:

Test httpd1

Do the same operation on the httpd2 server but replace “httpd1” with “httpd2” in the index.html file.

From another server, test the configuration:

# yum install -y elinks
# elinks http://192.168.0.100

HAProxy Intricacies

In some cases, because HaProxy tries to bind to an IP address that doesn’t exist on any interfaces, it doesn’t start and displays an error message like “Starting proxy stats: cannot bind socket“.

The solution is to create a file called /etc/sysctl.d/10-haproxy.conf and paste the following line:

net.ipv4.ip_nonlocal_bind=1

Then, update the configuration:

# sysctl -p

Note1: There is a page dedicated to kernel runtime parameters assignment on this website.
Note2: Additional information about HAProxy boot errors and solutions can be found here.

Additional Resources

RedHat documentation provides a guide called RHEL 7 Load Balancer Administration.
Oracle Linux Administrator’s Guide for Release 7 provides a chapter dedicated to HAProxy configuration.

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2 Comments on "RHEL7: Configure a high available load-balancer."

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hunter86_bg
Member

For HAProxy it’s nice to say that it supports a Active-Backup mode.
For example you can have 1 front-end and 1 back-end (multiple front-, back-ends are possible) listening on specific port and to define 1 (or more servers) as primary and 1 (or more for backup) connection. When all primary servers fall down, HAPRoxy will use the backup servers.
Also HAProxy has a Web statistics page to keep an eye on the usage of every interface.

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