Network config in Ubuntu 18.04 Bionic is now elsewhere and handled by netplan

Written by Claudio Kuenzler - 0 comments

Published on June 8th 2018 - Listed in Linux

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Whlie creating a new template for Ubuntu 18.04, I stumbled across something I didn't expect. 

At the end of the template creation process, I usually comment the used IP addresses (to avoid human mistake when someone powers up the template with network card enabled). So I wanted to configure /etc/network/interfaces, as always. But:

root@ubuntu1804:~# cat /etc/network/interfaces
root@ubuntu1804:~#

Umm... the config file is empty. Huh? What's going on?

But during the setup I defined a static IP (10.10.4.44)?

root@ubuntu1804:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:8d:5c:45 brd ff:ff:ff:ff:ff:ff
    inet 10.10.4.44/24 brd 194.40.217.255 scope global ens192
       valid_lft forever preferred_lft forever
    inet6 fe80::250:56ff:fe8d:5c45/64 scope link
       valid_lft forever preferred_lft forever

Yep, the static IP is configured. Where the hell is this stored now?

root@ubuntu1804:~# grep -rni 10.10.4.44 /etc/*
/etc/hosts:2:10.10.4.44    ubuntu1804.nzzmg.local    ubuntu1804
/etc/netplan/01-netcfg.yaml:8:      addresses: [ 10.10.4.44/24 ]

/etc/netplan? Never heard of it... 

Let's check out this config file (which is written in YAML format):

root@ubuntu1804:~# cat /etc/netplan/01-netcfg.yaml
# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    ens192:
      addresses: [ 10.10.4.44/24 ]
      gateway4: 10.10.4.44.1
      nameservers:
          search: [ example.com ]
          addresses:
              - "1.1.1.1"
              - "8.8.8.8"

OK, doesn't look that complicated and is quite understandable. Important here is also the "renderer". According to the documentation, this refers to the network daemon reading the netplan config file:

"Netplan reads network configuration from /etc/netplan/*.yaml which are written by administrators, installers, cloud image instantiations, or other OS deployments. During early boot, Netplan generates backend specific configuration files in /run to hand off control of devices to a particular networking daemon."

The Ubuntu default is to hand off control to networkd, which is part of the SystemD jungle:

root@ubuntu1804:~# ps auxf|grep networkd
systemd+   750  0.0  0.1 71816  5244 ?        Ss   13:27   0:00 /lib/systemd/systemd-networkd
root       930  0.0  0.4 170424 17128 ?        Ssl  13:27   0:00 /usr/bin/python3 /usr/bin/networkd-dispatcher

So if I change the IP address to something else, how does one apply the new IP and will it be immediate?

root@ubuntu1804:~# ip address show ens192
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:8d:5d:37 brd ff:ff:ff:ff:ff:ff
    inet 10.10.4.44/25 brd 10.150.2.127 scope global ens192
       valid_lft forever preferred_lft forever
    inet6 fe80::250:56ff:fe8d:5d37/64 scope link
       valid_lft forever preferred_lft forever

root@ubuntu1804:~# sed -i "s/10.10.4.44/10.150.2.98/g" /etc/netplan/01-netcfg.yaml

root@ubuntu1804:~# netplan apply

# [ Note: Here my SSH session got disconnected - so yes, IP change was immediate ]

admck@ubuntu1804:~$ ip address show ens192
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:8d:5d:37 brd ff:ff:ff:ff:ff:ff
    inet 10.150.2.98/25 brd 10.150.2.127 scope global ens192
       valid_lft forever preferred_lft forever
    inet6 fe80::250:56ff:fe8d:5d37/64 scope link
       valid_lft forever preferred_lft forever

Right after "netplan apply", the IP was changed. I lost my SSH connection (of course) and could log back in using the new IP. So yes, IP change was immediate.

I just wonder how often I will still use "vi /etc/network/interfaces" as this has been stuck in my head for more than 10 years. Old habits die hard.

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