NetworkManager with nmcli, the hostname and name resolution

Devices and connections

NetworkManager is the service that configures networking on RHEL. It separates two ideas. A device is a network interface, like enp1s0. A connection is a saved set of settings (address, gateway, DNS, and so on) that can be applied to a device. A device can have several saved connections, but only one is active at a time. Changes made with ip addr add are temporary; NetworkManager connections are saved and survive reboots.

$ nmcli device status
DEVICE  TYPE      STATE      CONNECTION
enp1s0  ethernet  connected  enp1s0
lo      loopback  unmanaged  --
$ nmcli connection show
NAME    UUID                                  TYPE      DEVICE
enp1s0  8a1d5f6e-3c2b-4e9a-9f10-2b7c1d0e4a55  ethernet  enp1s0
$ nmcli connection show enp1s0 | grep ipv4
ipv4.method:                            auto
ipv4.addresses:                         --
...

A static address

Servers usually need a fixed (static) address so others can find them. Either change the existing connection or add a new one. Use the property names exactly: ipv4.addresses, ipv4.gateway, ipv4.dns, ipv4.method.

# nmcli connection add con-name static-lan ifname enp1s0 type ethernet \
    ipv4.method manual ipv4.addresses 192.168.122.50/24 \
    ipv4.gateway 192.168.122.1 ipv4.dns "192.168.122.1 1.1.1.1" \
    connection.autoconnect yes
Connection 'static-lan' (2f6c...) successfully added.
# nmcli connection up static-lan
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/4)
# nmcli connection modify static-lan +ipv4.dns 8.8.8.8      # + adds, - removes
# nmcli connection modify static-lan ipv4.addresses 192.168.122.51/24
# nmcli connection up static-lan                            # re-apply after modify!

nmcli connection modify only saves the change. The device keeps the old settings until you run nmcli connection up again (or reboot). Also set the old DHCP connection to not autoconnect (nmcli con mod enp1s0 connection.autoconnect no), or the two may fight at boot.

On RHEL 9, connections are stored as keyfiles in /etc/NetworkManager/system-connections/NAME.nmconnection. The older ifcfg files in /etc/sysconfig/network-scripts are deprecated. nmtui is a simple text menu for the same settings, handy when you forget the property names.

The hostname

# hostnamectl hostname web01.example.com      # (set-hostname also works)
$ hostnamectl
 Static hostname: web01.example.com
       Icon name: computer-vm
Operating System: Red Hat Enterprise Linux 9.4 (Plow)
          Kernel: Linux 5.14.0-427.13.1.el9_4.x86_64
$ cat /etc/hostname
web01.example.com

How a name becomes an address

When a program looks up a name, the system reads /etc/nsswitch.conf. On RHEL the hosts line normally says files first, then dns. So /etc/hosts is checked first, and only if the name is not there, the DNS servers in /etc/resolv.conf are asked. NetworkManager writes /etc/resolv.conf from the connection's ipv4.dns settings, so do not edit resolv.conf by hand; your edit will be overwritten.

# /etc/hosts
127.0.0.1   localhost localhost.localdomain
::1         localhost localhost.localdomain
192.168.122.50  web01.example.com web01
192.168.122.60  db01.example.com  db01

$ grep ^hosts /etc/nsswitch.conf
hosts:      files dns myhostname
$ getent hosts db01           # uses the same order programs use
192.168.122.60  db01.example.com db01
$ cat /etc/resolv.conf
# Generated by NetworkManager
search example.com
nameserver 192.168.122.1

Common mistakes

• Typing ipv4.address (no s) or ipv4.gateway with a prefix. Use Tab completion in nmcli; it completes property names.
• Forgetting nmcli connection up after modify.
• Setting ipv4.addresses but leaving ipv4.method auto. Set method manual for a static address.
• Using dig to test a name that is only in /etc/hosts. dig asks DNS directly and ignores /etc/hosts; getent follows nsswitch.conf.