Configuring Failover on a Compute Instance

In cloud computing, failover is the concept of rerouting traffic to a backup system should the original system become inaccessible. Linode Compute Instances support failover through the IP Sharing feature. This allows two Compute Instances to share a single IP address, one serving as the primary and one serving as the secondary. If the primary Compute Instance becomes unavailable, the shared IP address is seamlessly routed to the secondary Compute Instance (failover). Once the primary instance is back online, the IP address route is restored (failback).

Why Should I Implement Failover?

There’s always a possibility that your Compute Instance may become inaccessible, perhaps due to a spike in traffic, your own internal configuration issues, a natural disaster, or planned (or unplanned) maintenance. When this happens, any websites or services hosted on that instance would also stop working. Failover provides a mechanism for protecting your services against a single point of failure.

Note

For many production applications, you may want to consider a load balancing tool that goes beyond basic failover. Linode’s NodeBalancers combines load balancing with built-in failover. You can also configure load balancing software, such as HAProxy, on your own Compute Instances and utilize our IP Sharing feature to provide failover.

IP Sharing Availability

Within Linode’s platform, failover is configured by first enabling IP Sharing and then configuring software on both the primary and secondary Compute Instances. IP Sharing availability varies by data center. Review the list below to learn which data centers support IP Sharing and how it can be implemented.

• New IP Sharing Method (BGP): Supports IPv4 failover. This is currently being rolled out across our fleet in conjunction with our planned network infrastructure upgrades. Since it is implemented using BGP routing, customers can configure it on their Compute Instances using the Linode provided lelastic tool or FRR. Follow the instructions within this guide.

• Legacy IP Sharing Method (ARP): Supports IPv4 failover within limited data centers. Since it is arp-based, customers can configure it on their Compute Instances using a service like keepalived. Follow the instructions within the keepalived guide.

Note

IP failover for VLAN IP addresses is supported within every data center where VLANs are available. This feature does not depend on Linode’s IP Sharing feature and is configurable through keepalived.

Configure Failover

The instructions within this guide enable you to configure failover using IP Sharing and the lelastic tool, a Linode provided tool based on GoBGP that automates much of the configuration. If you prefer to manually configure failover software, follow the Configuring IP Failover over BPG using FRR guide or use any BGP client that you wish.

Note

If your data center supports the legacy method (ARP), use the Configuring IP Failover using keepalived guide instead. That guide should also be used when setting up failover for VLAN IP addresses.

To configure failover, complete each section in the order shown:

1. Create and Share the Shared IP Address
2. For each Compute Instance:
   • Add the Shared IP to the Networking Configuration
   • Install and Configure Lelastic
3. Test Failover

Create and Share the Shared IP Address

1. Log in to the Cloud Manager.

2. Determine which two Compute Instances are to be used within your failover setup. They both must be located in the same data center. If you need to, create those Compute Instances now and allow them to fully boot up.

3. Disable Network Helper on both instances. For instructions, see the Network Helper guide.

4. Add an additional IPv4 address to one of the Compute Instances. See the Managing IP Addresses guide for instructions. Make a note of the newly assigned IP address. Each additional IP address costs $1 per month.

5. On the other Compute Instance, add the newly assigned IP address as a Shared IP using Linode’s IP Sharing feature. See Managing IP Addresses for instructions on configuring IP sharing.

Add the Shared IP to the Networking Configuration

Adjust the network configuration file on each Compute Instance, adding the shared IP address and restarting the service.

1. Log in to the Compute Instance using SSH or Lish.

2. Add the shared IP address to the system’s networking configuration file. Within the instructions for your distribution below, open the designated file with a text editor (such as nano or vim) and add the provided lines to the end of that file. Replace [shared-ip] with your own shared IP address.

   Note

   Review the configuration file and verify that the shared IP address does not already appear. If it does, delete associated lines before continuing.

   • Debian and Ubuntu 16.04 (and older): Using ifupdown.

     File: /etc/network/interfaces

     1 2 3 4

     ... # Add Shared IP Address iface lo inet static address [shared-ip]/32

     To apply the changes, reboot the instance or run:

     sudo ifdown lo && sudo ip addr flush lo && sudo ifup lo
     

     If you receive the following output, you can safely ignore it: RTNETLINK answers: Cannot assign requested address.

   • Ubuntu 18.04 LTS and newer: Using netplan. The entire configuration file is shown below, though you only need to copy the lo: directive.

     File: /etc/netplan/01-netcfg.yaml

     1 2 3 4 5 6 7 8 9 10 11

     network: version: 2 renderer: networkd ethernets: eth0: dhcp4: yes lo: match: name: lo addresses: - [shared-ip]/32

     To apply the changes, reboot the instance or run:

     sudo netplan apply
     

Install and Configure Lelastic

Next, we need to configure the failover software on each Compute Instance. For this, the lelastic utility is used.

1. Log in to the Compute Instance using SSH or Lish.

2. Download and install the lelastic utility from GitHub by running the following commands:

   curl -LO https://github.com/linode/lelastic/releases/download/v0.0.3/lelastic.gz
   gunzip lelastic.gz
   chmod 755 lelastic
   sudo mv lelastic /usr/local/bin/
   

3. Next, prepare the command to configure BGP routing through lelastic. Replace [id] with the ID corresponding to your data center in the table above and [role] with either primary or secondary. You do not need to run this command, as it is configured as a service in the following steps.

   lelastic -dcid [id] -[role] &
   

   See Test Failover to learn more about the expected behavior for each role.

4. Create and edit the service file using either nano or vim.

   sudo nano /etc/systemd/system/lelastic.service
   

5. Paste in the following contents and then save and close the file. Replace $command with the lelastic command you prepared in a previous step.

   File: /etc/systemd/system/lelastic.service

   1 2 3 4 5 6 7 8 9 10 11

   [Unit] Description= Lelastic After=network-online.target Wants=network-online.target [Service] Type=simple ExecStart=$command [Install] WantedBy=multi-user.target

6. Apply the correct permissions to the service file.

   sudo chmod 644 /etc/systemd/system/lelastic.service
   

7. Start and enable the lelastic service.

   sudo systemctl start lelastic
   sudo systemctl enable lelastic
   

   You can check the status of the service to make sure it’s running (and to view any errors)

   sudo systemctl status lelastic
   

   If you need to, you can stop and disable the service to stop failover functionality on the particular Compute Instance.

   sudo systemctl stop lelastic
   sudo systemctl disable lelastic
   

Test Failover

Once configured, the shared IP address is routed to the primary Compute Instance. If that instance becomes inaccessible, the shared IP address is automatically routed to the secondary instance (failover). Once the primary instance is back online, the shared IP address is restored to that instance (failback).

If desired, both instances can be configured with the same role (both primary or both secondary). This prevents failback functionality, meaning that the shared IP address is not restored to the original system, even if the original system comes back online.

You can test the failover functionality of the shared IP using the steps below.

1. Power off the primary Compute Instance.

2. Using your local machine, ping the shared IP address.

   ping 192.0.2.1
   

   If failover is successfully configured, the output should be similar to the following (once the primary Compute Instance has fully powered off):

   64 bytes from 192.0.2.1: icmp_seq=3310 ttl=64 time=0.373 ms

   If you are instead receiving output telling you that the host is unreachable, failover likely has not been configured successfully.

   From 192.0.2.1 icmp_seq=3293 Destination Host Unreachable