Configuring VLANs and Dummy Interfaces in Networkd

VLANs

VLAN functionality in networkd enables Linux hosts to participate in VLANs based on explicit tagging of ethernet frames. VLANs provide a valuable layer of logical configurability on top of physical network infrastructure. VLAN connections are presented in Linux as software-defined network interfaces that can be configured and utilized just like other types of network interfaces.

Creating VLAN Interfaces

Networkd supports various software-defined interface types that are created based on .netdev files in the networkd configuration directories. A .netdev for a VLAN sets Kind=vlan and defines the numeric ID used to tag VLAN traffic on the wire in order to keep it separate from other traffic.

Creating a VLAN interface

/etc/systemd/network/15-vlan25.netdev

[NetDev]
Name=vlan25
Kind=vlan

[VLAN]
# Specify the tag
Id=25

Add VLANs to a Physical Interface

In order to send and receive tagged traffic over a network link, we need to associate the newly created VLAN interfaces with the physical LAN interface. In networkd, this can be done by adding one or more VLAN properties to the physical interface's .network configuration. After doing this, tagged frames will be presented to the operating system as originating from the associated VLAN interface. Likewise, traffic that is sent through a VLAN interface will be tagged with the specified ID before being sent on the network.

Linking a VLAN interface to a physical interface

/etc/systemd/network/20-eth0.network 

[Match]
Name=eth0

[Network]
# Existing Configuration
Address=192.168.0.1/24

# Tagged VLANs Referenced by Interface Name
VLAN=vlan25
VLAN=vlan50

Network Configuration for VLAN Interfaces

Attaching the VLANs to a physical interface enables traffic to be sent and received from the virtual interface via tagged frames on the physical link. At the moment, however, our VLAN interfaces do not have any network-level settings attached to them.

Depending on the task at hand and the configuration of the Linux host, you may configure the new interfaces by any method(s) available, including networkd:

Sample VLAN interface configuration

/etc/systemd/network/25-vlan25.network

``` tab="DHCP" [Match] Name=vlan25

[Network] DHCP=ipv4 


``` tab="Static IP"
[Match]
Name=vlan25

[Network]
Address=172.16.0.1/24
DNS=1.1.1.1

Dummy Interfaces

When we set up labs with multiple services hosted on a single physical node, it can be useful to virtualize additional network connections and interfaces to allow us to extend the logical topology of the system. Dummy interfaces are treated much like standard network interfaces, although they do not require a physical connection and do not go up and down based on link status. Like a standard ethernet interface, a dummy interface can be configured with an IP address and other layer-3 settings. Likewise, we can bind daemons to sockets on our dummy interfaces and respond to network requests received at these addresses.

Creating the Interface

Adding a .netdev definition with Kind=dummy will trigger networkd to add a dummy interface and make it visible to other standard networking tools.

Creating a Dummy Interface

/etc/systemd/network/10-dmz0.netdev 

[NetDev]
Name=dmz0
Kind=dummy

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