Nmcli

Network Manager Command Line - is used for controlling NetworkManager and reporting network status.

Check connection status of interfaces

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nmcli dev status

# nmcli dev status

DEVICE      TYPE      STATE         CONNECTION 
docker0     bridge    connected     docker0    
virbr0      bridge    connected     virbr0     
enp0s3      ethernet  connected     enp0s3     
virbr0-nic  ethernet  disconnected  --         
lo          loopback  unmanaged     --   

Turn Connection Up or Down

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nmcli connection down <interface> or nmcli connection up <interface>

# nmcli connection down ens192
Connection 'ens192' successfully deactivated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/1)

# nmcli connection up ens192
Connection 'ens192' successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/2)

Turn Networking Up or Down

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nmcli networking off
nmcli networking on

Turn Autoconnect On/Off

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Turn on:

# nmcli connection modify ens192 connection.autoconnect "yes"

Turn of:

# nmcli connection modify ens192 connection.autoconnect "no"

For changes to take effect, turn the interface down and up (in some cases, this works better than restarting NetworkManager):

# nmcli con down ens192
# nmcli con up ens192

Error: unknown connection

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If you tried adding a secondary NIC and you see it there in ip a command output, but you are getting the error: "Error: unknown connection <interface>", that can be because NetworkManager is unable to recognize or find the specified connection. This can happen due to various reasons such as incorrect configuration files, missing connections, or permission issues.

# nmcli connection up enp0s8
Error: unknown connection 'enp0s8'.

Ensure that the connection file is correctly configured and placed in the appropriate directory. The file should be located in /etc/NetworkManager/system-connections/ and have the correct permissions. Example:

$ sudo cat /etc/NetworkManager/system-connections/ens3.nmconnection
[sudo] password for cisadmin:
[connection]
id=ens3
uuid=761c9d6a-3e4d-47a3-8c0d-f5ce612ee301
type=ethernet
interface-name=ens3 

[ethernet]

[ipv4]
method=auto

[ipv6]
addr-gen-mode=eui64
method=auto

[proxy]

If the interface configuration file for the secondary NIC is not there, then you can add it using the following nmcli command (You would usually have to do this when adding a secondary NIC):

$ nmcli con add con-name <connection name> type ethernet ifname <interface> ipv4.method auto