NFTables Addon - Failover

This failover addon enables you to have one NFTables variable that will always point to a server that is online.

Why would you want that?

It's an alternative to using a load-balancer/reverse-proxy to perform a failover between nodes.

Many times a central endpoint is a better solution - but if it is a high-traffic or low-latency application it might be smarter to send the traffic directly to the target node.

Currently only TCP port-checks are supported.

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Other Addons

• DNS
• IPLists

NFTables documentation: docs.o-x-l.com

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Install

• Create directories:

  mkdir -p /var/local/lib/nftables_addons /etc/nftables.d/addons/

• Add the script-files:

  • util.py
  • iplist.py

• Add the config file:

  /etc/nftables.d/addons/failover.json

• Optional: Create a service user

  • Add sudoers privileges
  • Allow to read lib-dir
  • Allow to write to addons-config-dir

• Add cron or systemd-timer to execute the script on a schedule: python3 /var/local/lib/nftables_addons/failover.py

• Test it and verify it's working as expected

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Result

cat /etc/nftables.d/addons/failover.nft 
> # Auto-Generated config - DO NOT EDIT MANUALLY!
> 
> define endpoint_filer_v4 = 192.168.87.100
> define endpoint_filer_v6 = 2001:DB8:2:2
> define endpoint_print_v4 = 192.168.93.52  # if first one is offline
> define endpoint_print_v6 = ::
> define mark_proxy_v4 = 200
> define mark_proxy_v6 = ::  # unused

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How does it work?

1. A configuration file needs to be created:

   /etc/nftables.d/addons/failover.json

   {
     "failover": {
       "endpoint_filer": {
         "ip4": ["192.168.87.100", "192.168.87.101"],
         "ip6": ["2001:DB8:2:2", "2001:DB8:2:3"],
         "port": 443
       },
       "endpoint_print": {
         "ip4": ["192.168.93.51", "192.168.93.52"],
         "port": 631
       },
       "mark_proxy": {  // set fwmark for policy routing
         "ip4": ["192.168.132.2", "192.168.132.3"],
         "port": 3129,
         "values": [200, 201]
       }
     }
   }

   Config options:

   • port: required

     TCP Port to check for online-status

   • values: optional; default = using IPs

     1-to-1 mapping to ip-lists. Lists must be of the same length

2. The script is executed

   python3 /var/local/lib/nftables_addons/failover.py

• It will load the configuration

• Run port-checks for all configured variables - use first host that is online

• Map hosts to values if supplied

• If no host is online - will use first host/value

• The new addon-config is written to /tmp/nftables_failover.nft

• Its md5-hash is compared to the existing config to check if it changed

• If it has changed:

  • Config validation is done:

    • An include-file is written to /tmp/nftables_main.nft:

      include /tmp/nftables_failover.nft
      # including all other adoon configs
      include /etc/nftables.d/addons/other_addon1.nft
      include /etc/nftables.d/addons/other_addon2.nft
      # include other main configs
      include /etc/nftables.d/*.nft
      

    • This include-file is validated:

      sudo nft -cf /tmp/nftables_main.nft

  • The new config is written to /etc/nftables.d/addons/failover.nft

  • The actual config is validated: sudo nft -cf /etc/nftables.conf

  • NFTables is reloaded: sudo systemctl reload nftables.service

3. You will have to include the addon-config in your main-config file /etc/nftables.conf:

   ...
   include /etc/nftables.d/addons/*.nft
   ...
   

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Privileges

If the script should be run as non-root user - you will need to add a sudoers.d file to add the needed privileges:

Cmnd_Alias NFTABLES_ADDON = \
  /usr/bin/systemctl reload nftables.service,
  /usr/sbin/nft -cf *

service_user ALL=(ALL) NOPASSWD: NFTABLES_ADDON

You may not change the owner of the addon-files as the script will not be able to overwrite them.

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Safety

As explained above - there is a config-validation process to ensure the addon will not supply a bad config and lead to a failed nftables reload/restart.

If you want to be even safer - you can add a config-validation inside the nftables.service:

# /etc/systemd/system/nftables.service.d/override.conf
[Service]
# catch errors at start
ExecStartPre=/usr/sbin/nft -cf /etc/nftables.conf

# catch errors at reload
ExecReload=
ExecReload=/usr/sbin/nft -cf /etc/nftables.conf
ExecReload=/usr/sbin/nft -f /etc/nftables.conf

# catch errors at restart
ExecStop=
ExecStop=/usr/sbin/nft -cf /etc/nftables.conf
ExecStop=/usr/sbin/nft flush ruleset

Restart=on-failure
RestartSec=5s

This will catch and log config-errors before doing a reload/restart.

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Scheduling

You can either:

• Add a Systemd Timer: example
• Add a cron job

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Ansible

Here you can find an Ansible Role to manage NFTables Addons:

• ansibleguy.addons_nftables
• examples

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License

MIT