Apr 2020

If have been using ifup and ifdown for quite some time now, and as far as i was aware the syntax was pretty simple, however I got really caught out the other day and wanted to share what I discovered (not entirely on my own, with the help of some other, way more clever people).

So, to take down an interface, I would use the following:

ifdown eth1
Device 'eth1' successfully disconnected.


Simple, and to bring that interface back up:

ifup eth1
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

Wow, too easy. I use this normally when I have changed and IP Address and want to invoke that change quickly (without using the systemctl command or a reboot).

What Went Wrong?

Now, this is not going to affect everyone, but in our systems we sometimes have to prove redundancy by taking down an interface, letting a device detect this and watching the system automatically switch. Sounds simple enough, but it seems that using a simple ifdown or ifup does not cause a redundancy switch, you need to do a bit more.

To properly take the interface down, so that it is recognised as being down from a 'link'perspective, you must use the following syntax:

ip link set eth1 down (obviously use your interface name, not mine)

And inversly:

ip link set eth1 up

This will ensure that the interface goes fully down, and that other devices can recognize that the interface is down.

I sincerely hope this saves you some pain :)