How to add a blackhole route in Netplan

I will give an example of setting up blackhole routes in Netplan.

For example, I often specify a subnet of white IP addresses for NAT in bird, quagga, cisco, etc. as one blackhole route (null), so as not to assign hundreds of IP addresses to the device.

If you need to temporarily add/remove a blackhole route in Linux, you can run the command:

ip route add blackhole 10.20.30.128/25
ip route del blackhole 10.20.30.128/25

Let’s assume the following standard settings in the Netplan configuration:

network:
  ethernets:
    enp0s3:
      dhcp4: true
  version: 2

Now let’s add the lo interface settings and blackhole route:

network:
  ethernets:
    enp0s3:
      dhcp4: true
    lo:
      match:
        name: lo
      addresses: [ 127.0.0.2/32 ]
      routes:
# ixnfo.com
        - to: 10.20.30.128/25
          via: 0.0.0.0
          metric: 240
          type: blackhole
  version: 2

Let’s apply the changes:

netplan try
netplan apply

Let’s look at the current routes:

ip route
netstat -rn
ip route | grep 'blackhole'

blackhole 10.20.30.128/25 proto static metric 240

You can add an unreachable route in a similar way:

      routes:
        - to: 10.10.10.0/24
          via: 0.0.0.0
          metric: 240
          type: unreachable

See also my articles:
Configuring the Network in Linux
How to configure networking with Netplan
Installing and configuring BIRD (BGP)