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====== ifup and ifdown ======
<color darkorange>Apr 2020</color>
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==== Overview ====

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If have been using <color #ed1c24>ifup</color> and <color #ed1c24>ifdown</color> 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).
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==== Simple Syntax ====
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So, to take down an interface, I would use the following:
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<color #ed1c24>ifdown eth1</color>
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Device 'eth1' successfully disconnected.
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Simple, and to bring that interface back up:
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<color #ed1c24>ifup eth1</color>
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Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)
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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).
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**What Went Wrong?**
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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 <color #ed1c24>ifdown</color> or <color #ed1c24>ifup</color> does not cause a redundancy switch, you need to do a bit more.
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To properly take the interface down, so that it is recognised as being down from a 'link'perspective, you must use the following syntax:
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<color #ed1c24>ip link set eth1 down</color> (obviously use your interface name, not mine)
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And inversly:
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<color #ed1c24>ip link set eth1 up</color>
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This will ensure that the interface goes fully down, and that other devices can recognize that the interface is down.
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I sincerely hope this saves you some pain :)
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