ifup / ifdown eth0 / eno1 reports unknown interface when it exists!

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try use :

sudo ifconfig eth0 up/down 

reseaon:

sudo ifdown eno1 or sudo ifup eno1, will not work because eno1 is not explicitly defined inside /etc/network/interfaces file.

So ifup | ifdown | ifquery family of functions is pretty unaware of what is inside the system unless this is written inside /etc/network/interfaces, but they could be since they all depend internally on ip¹ command.

It is more secure to use the sudo ifconfig eno1 down or sudo ifconfig eno1 up.

Note: There are couple things sudo ifconfig eno1 down or sudo ifconfig eno1 up will not do comparing to sudo ifdown eno1 and sudo ifup eno1. ifup and ifdown will keep the state of interfaces inside /run/network/ifstate so ifstate command can tell you the state of the interface. And one another thing, if you ifup or ifdown the interface, the scripts inside /etc/network/if-*.d will run (if-down.d if-post-down.d if-pre-up.d if-up.d)

来源： https://askubuntu.com/questions/870447/ifdown-interface-reports-unknown-interface-when-it-exists