Simple High Availability with CARP

Table of contents

1. Common Address Redundancy Protocol
2. Configuration on first server
3. Configuration on second server
4. Starting the virtual interfaces and testing

Common Address Redundancy Protocol

In one of my older blog posts I wrote about Simple load balancing with Apache proxy_balancer. This blog post was about how to increase the availability of a web server by using two Apache servers and it’s proxy_balancer module. But what about if the server with the “Load Balancer” running is going down?

To address this case, two servers with the same configuration are required (Apache + proxy_balancer) and CARP (Common Address Redundancy Protocol). CARP works similar like Cisco’s HSRP/VRRP (Hot Standby Routing Protocol/ Virtual Router Redundancy Protocol) and Debian GNU/Linux provides the package ucarp: user-space replacement to VRRP — automatic IP fail-over. The package can be installed on both servers by using:

    # apt install ucarp

Configuration on first server

The example assumes the first server is at IPn.n.n.11, the second server is at IPn.n.n.12 and the shared, virtual IP of both servers is at n.n.n.10. On the first server I add the following lines to the file /etc/network/interfaces:

    #
    # First server
    #
    # Virtual Server ID
    ucarp-vid       1
    # Virtual Server IP
    ucarp-vip       n.n.n.10

    # Shared Password
    ucarp-password  some_password

    # Advertisement skew
    ucarp-advskew   1

    # Advertisement base interval
    ucarp-advbase   1

    # Master server
    ucarp-master    no

    # Virtual interface
    iface eth0:ucarp inet static
      address n.n.n.10/24
      gateway n.n.n.1

Configuration on second server

On the second server I’m adding the same configuration into the /etc/network/interfaces file:

    #
    # Second Server
    #
    # Virtual Server ID
    ucarp-vid       1

    # Virtual Server IP
    ucarp-vip       n.n.n.10

    # Shared Password
    ucarp-password  some_password

    # Advertisement skew
    ucarp-advskew   1

    # Advertisement base interval
    ucarp-advbase   1

    # Master server
    ucarp-master    no

    # Virtual interface
    iface eth0:ucarp inet static
      address n.n.n.10/24
      gateway n.n.n.1

Starting the virtual interfaces and testing

By using the traditional ifconfig command I’m bringing up on both servers the virtual interface:

    # ifconfig eth0:ucarp up

Note: same can be accomplished by the new ip command.

For testing, I’m constantly pinging the virtual IPn.n.n.10, the IP of the first server n.n.n.11 and the IP of the second server n.n.n.12 from another computer.

    PING n.n.n.10 (n.n.n.10) 56(84) bytes of data.
    64 bytes from n.n.n.10: icmp_seq=1 ttl=64 time=0.256 ms
    ...
    PING n.n.n.11 (n.n.n.11) 56(84) bytes of data.
    64 bytes from n.n.n.11: icmp_seq=1 ttl=64 time=0.358 ms
    ...
    PING n.n.n.12 (n.n.n.12) 56(84) bytes of data.
    64 bytes from n.n.n.12: icmp_seq=1 ttl=64 time=0.252 ms
    ...

By using the XCP-NG management console, I’m logging in and shutting down on the first server the eth0 interface:

    # ifconfig eth0 down

The IP address of the first server stops to respond to the ping command, however the virtual IP address n.n.n.10 is still responding (which is in this case now the second server). By using the command:

    # ifconfig eth0 up

I’m bringing up the eth0 interface again and I repeat this process on the second server to verify that the virtual IP address is also responding if the second server is down.