systemd-networkd
systemd-networkd is the network configuration component of the systemd software suite. It is well integrated into NixOS below systemd.network and should be preferred over networking.interfaces options for most use cases, since it receives far superior maintenance.
Configuration for networkd is split into three sections.
systemd.network.linksreconfigures existing network devices- https://www.freedesktop.org/software/systemd/man/systemd.link.html
- actually implemented by udev, not networkd
systemd.network.netdevscreates virtual network devicessystemd.network.networksconfigures network devices
In most simple scenarios configuring existing network devices is what you want to do.
Basics
When to use
Use systemd-networkd for setups that rely on static configuration, that doesn't change much during its lifetime, that does not require varying profiles for a single interface. Common examples are:
- Servers/Routers
- Always-On VPN Tunnels
Following that logic, it is less suitable to
- Varying WLAN profiles
- Selectively used VPN tunnels
These use cases are better served by NetworkManager and its various frontends, that provides a better integrated user experience for various desktop systems.
Enabling
To be able to use networkd configuration, it needs to be enabled first.
systemd.network.enable = true;
Some guides will mention the networking.useNetworkd option, which
in addition to enabling systemd-networkd, also offers translation of some
networking.interfaces and networking.useDHCP options
into networkd.
If you can write your complete network setup in native networkd configuration, you
should stay away from that option.
Configuring
The configuration of networkd happens through one or multiple configuration files per
interface stored in /etc/systemd/network.
The following declaration in your NixOS configuration
systemd.network.networks."10-lan" = {
matchConfig.Name = "lan";
networkConfig.DHCP = "ipv4";
};
renders the following network configuration:
/etc/systemd/network/10-lan.network
[Match]
Name=lan
[Network]
DHCP=ipv4
Note that we usually prefix the configuration file with a number. This can be important, because networkd collects all available configuration files, then sorts them alphanumerically, and uses the first match for each interface as its configuration. This happens separately for .link, .netdev and .network files, so that you can have one configuration of each type per interface.
Debugging
When things don't work as expected, the journal for systemd-networkd.service should be consulted. Unfortunately, by default the log is not very useful in its default loglevel. Increasing the loglevel can be done using the SYSTEMD_LOG_LEVEL environment variable.
systemd.services."systemd-networkd".environment.SYSTEMD_LOG_LEVEL = "debug";
Limitations
Some limitations might be surprising, so it is probably helpful to get them out of the way early.
systemd.network.links- Executed by udev and only applied on boot
systemd.network.netdevs- Does not modify properties (e.g., MTU, VLAN ID, VXLAN ID, Wireguard Peers) of existing netdevs
network-online.target
While network.target only requires the network management stack
to be up, which means it does not care about network interfaces being configured,
the network-online.target waits until a defined set of network
interfaces are in a state, that by its configuration is considered online.
When networkd is enabled, the network-online.target is implemented
through the systemd-networkd-wait-online.service, which makes
sure interfaces configured through networkd are in their expected operational state.
The current operational state of network interfaces can be learned from networkctl.
❯ networkctl
IDX LINK TYPE OPERATIONAL SETUP
1 lo loopback carrier unmanaged
2 enp10s0 ether routable unmanaged
3 wlp9s0 wlan no-carrier unmanaged
For most network interfaces that will mean that they have routable
network connectivity, but in more complex setups some links may be content with more
simple states like carrier or enslaved. Interfaces that
are managed by networkd, but not always in use, shouldn't be required for
network-online.target
systemd.network.networks."50-enp3s0" = {
matchConfig.Name = "enp3s0";
# acquire a DHCP lease on link up
networkConfig.DHCP = "yes";
# this port is not always connected and not required to be online
linkConfig.RequiredForOnline = "no";
};
Note that the default value for linkConfig.RequiredForOnline is unexpectedly
"yes", which often leads to a failing network-online.target.
Setting individual interfaces to "no" is a perfectly valid choice
and should be considered, before disabling the systemd-networkd-wait-online.service
entirely, because a working network-online.target is required for some
services to properly start without race conditions.
Also consider enabling the systemd.network.wait-online.anyInterface option,
which makes networkd consider the network online when any interface is online,
as opposed to all that have a positive value for linkConfig.RequiredForOnline.
This is useful on portable machines with a wired and a wireless interface, for example.
Recommended documentation:
Examples
Examples should be concise and give proper hints on how to achieve a reliably working network-online.target.
DHCP/RA
Common scenario for dynamic configuration, DHCP for IPv4 and router advertisements for IPv6 connectivity. Make network-online.target wait until addresses and routes are configured.
systemd.network.networks."10-wan" = {
matchConfig.Name = "enp1s0";
networkConfig = {
# start a DHCP Client for IPv4 Addressing/Routing
DHCP = "ipv4";
# accept Router Advertisements for Stateless IPv6 Autoconfiguraton (SLAAC)
IPv6AcceptRA = true;
};
# make routing on this interface a dependency for network-online.target
linkConfig.RequiredForOnline = "routable";
};
Static
Apply a static address and routing configuration onto enp1s0.
When the gateway is not on the same prefix as the address configured, as is customary on some cloud providers, you usually also need to set GatewayOnLink, to indicate the gateway is directly reachable on the interface.
systemd.network.networks."10-wan" = {
# match the interface by name
matchConfig.Name = "enp1s0";
address = [
# configure addresses including subnet mask
"192.0.2.100/24"
"2001:DB8::2/64"
];
routes = [
# create default routes for both IPv6 and IPv4
{ routeConfig.Gateway = "fe80::1"; }
{ routeConfig.Gateway = "192.0.2.1"; }
# or when the gateway is not on the same network
{
routeConfig = {
Gateway = "172.31.1.1";
GatewayOnLink = true;
};
}
];
# make the routes on this interface a dependency for network-online.target
linkConfig.RequiredForOnline = "routable";
};
VLAN
VLANs can be configured on top of hardlinks as well as virtual links, like bonding interfaces. They provide separate logical networks over physical links.
In this example we tag two VLANs with Ids 10 and 20 on a physical link enp1s0. The VLAN interfaces become available as vlan10 and vlan20 and can receive additional configuration.
systemd.network = {
netdevs = {
"20-vlan10" = {
netdevConfig = {
Kind = "vlan";
Name = "vlan10";
};
vlanConfig.Id = 10;
};
"20-vlan20" = {
netdevConfig = {
Kind = "vlan";
Name = "vlan20";
};
vlanConfig.Id = 20;
};
};
networks = {
"30-enp1s0" = {
matchConfig.Name = "enp1s0";
# tag vlan on this link
vlan = [
"vlan10"
"vlan20"
];
networkConfig.LinkLocalAddressing = "no";
linkConfig.RequiredForOnline = "carrier";
};
"40-vlan10" = {
matchConfig.Name = "vlan10";
# add relevant configuration here
};
"40-vlan20" = {
matchConfig.Name = "vlan20";
# add relevant configuration here
};
};
};
Bridge
Given multiple interfaces, that are connected into a bridge will act like a common switch and forward Ethernet frames between all connected bridge ports. The Linux bridge supports various features, like spanning tree, bridge port isolation or acting as a multicast router.
The configuration on top of the bridge interface depends on the desired functionality, e.g., configuring an IP address would make the bridge host reachable on the Ethernet segment.
Recommended documentation:
systemd.network = {
netdevs = {
# Create the bridge interface
"20-br0" = {
netdevConfig = {
Kind = "bridge";
Name = "br0";
};
};
};
networks = {
# Connect the bridge ports to the bridge
"30-enp1s0" = {
matchConfig.Name = "enp1s0";
networkConfig.Bridge = "br0";
linkConfig.RequiredForOnline = "enslaved";
};
"30-enp2s0" = {
matchConfig.Name = "enp2s0";
networkConfig.Bridge = "br0";
linkConfig.RequiredForOnline = "enslaved";
};
# Configure the bridge for its desired function
"40-br0" = {
matchConfig.Name = "br0";
bridgeConfig = {};
# Disable address autoconfig when no IP configuration is required
#networkConfig.LinkLocalAddressing = "no";
linkConfig = {
# or "routable" with IP addresses configured
RequiredForOnline = "carrier";
};
};
};
};
Bonding
Given two hardlinks enp2s0 and enp3s0 create a virtual bond0 interface using Dynamic LACP (802.3ad), hashing outgoing packets using a packet's layer 3/4 (network/transport layer in the OSI model) information.
systemd.network = {
netdevs = {
"10-bond0" = {
netdevConfig = {
Kind = "bond";
Name = "bond0";
};
bondConfig = {
Mode = "802.3ad";
TransmitHashPolicy = "layer3+4";
};
};
};
networks = {
"30-enp2s0" = {
matchConfig.Name = "enp2s0";
networkConfig.Bond = "bond0";
};
"30-enp3s0" = {
matchConfig.Name = "enp3s0";
networkConfig.Bond = "bond0";
};
"40-bond0" = {
matchConfig.Name = "bond0";
linkConfig = {
RequiredForOnline = "carrier";
};
networkConfig.LinkLocalAddressing = "no";
};
};
};
Router Advertisement
Router advertisements are way to allow clients to achieve stateless autoconfiguration (SLAAC). The most prominent setup is where the router announces a prefix onto a LAN segment, which the receiving client can use to set up an address on that prefix, and configure the sender as its default gateway.
In this example the router will announce a static IPv6 prefix on the lan interface from it's automatically configured link local address on that link. The router does not generally require a unique local or globally reachable address on the link, unless you also want to host services like DNS and NTP on that LAN segment.
Recommended documentation:
systemd.network = {
networks = {
"30-lan" = {
matchConfig.Name = "lan";
address = [ "2001:db8:1122:3344::1/64" ];
networkConfig = {
IPv6SendRA = true;
};
ipv6Prefixes = [
{
# Announce a static prefix
ipv6PrefixConfig.Prefix = "2001:db8:1122:3344::/64";
}
];
ipv6SendRAConfig = {
# Provide a DNS resolver
EmitDNS = true;
DNS = "2001:db8:1122:3344::1";
};
};
};
};
An extended form of this setup uses DHCPv6 prefix delegation to acquire a dynamic prefix over a WAN link, which then gets distributed onto designated LAN segments.
WireGuard
WireGuard can also be set up using systemd.network.netdevs. More details can be found at WireGuard#Setting up WireGuard with systemd-networkd.
User configurations
This section allows references to actual user configurations. They show how individual configuration snippets get integrated as a whole and serve as real world examples.
When adding new links, please describe the primary features of your setup and against which NixOS version it was last updated.