CentOS 8 ifup "unknown connection" - Add Second IP

"Error: Unknown Connection" when running ifup on new interface or alias.

Network scripts are deprecated in CentOS 8 and needs to be installed, with NetworkManager set to not be used for the interface.

Example Interface
enp35s0:1   (Alias :1)

DEVICE=enp35s0:1
ONBOOT=yes
BOOTPROTO=none
IPADDR=10.1.69.69
NM_CONTROLLED=no (Add this line to your new interface file)

Install the network-scripts package, which does not ship with CentOS 8.

dnf install network-scripts

Now you can run "ifup enp35s0:1" to bring it up.

As this will be removed in future RHEL releases you can also use ifconfig to configure additional IPs instead. You can do this like so:

ifconfig enp35s0:1 10.1.69.69 netmask 255.255.255.192 up

This will do the same as above, you can then run ifconfig to confirm it's being used.

It is recommended to use nmcli instead, as it's likely this won't work well in future releases. You can add additional IPv4 addresses to your interface like this.

Run the following command to check your interfaces:
nmcli connection show

In this example the interface will be "eno1". Run the following command to add a second IPv4, in this example 10.50.50.51 will be the secondary IP
nmcli con mod eno1 +ipv4.address "10.50.50.51/24"

This assumes a netmask of /24 (255.255.255.0). Reboot your server after adding and additional IPv4.

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