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SYSTEMD.NETWORK(5) systemd.network SYSTEMD.NETWORK(5)

NAME
systemd.network - Network configuration

SYNOPSIS
network.network

DESCRIPTION
A plain ini-style text file that encodes network configuration for
matching network interfaces, used by systemd-networkd(8). See
systemd.syntax(7) for a general description of the syntax.

The main network file must have the extension .network; other
extensions are ignored. Networks are applied to links whenever the
links appear.

Note that not all settings and configurations can be made with .network
files, and that it may be necessary to use systemd.link(5)) or
systemd.netdev(5)) files in conjuction with .network files when working
with physical and virtual network devices respectively.

The .network files are read from the files located in the system
network directories /lib/systemd/network and
/usr/local/lib/systemd/network, the volatile runtime network directory
/run/systemd/network and the local administration network directory
/etc/systemd/network. All configuration files are collectively sorted
and processed in alphanumeric order, regardless of the directories in
which they live. However, files with identical filenames replace each
other. It is recommended that each filename is prefixed with a number
(e.g. 10-eth0.network). Otherwise, the default .network files or those
generated by systemd-network-generator.service(8) may take precedence
over user configured files. Files in /etc/ have the highest priority,
files in /run/ take precedence over files with the same name under
/usr/. This can be used to override a system-supplied configuration
file with a local file if needed. As a special case, an empty file
(file size 0) or symlink with the same name pointing to /dev/null
disables the configuration file entirely (it is "masked").

Along with the network file foo.network, a "drop-in" directory
foo.network.d/ may exist. All files with the suffix ".conf" from this
directory will be merged in the alphanumeric order and parsed after the
main file itself has been parsed. This is useful to alter or add
configuration settings, without having to modify the main configuration
file. Each drop-in file must have appropriate section headers.

In addition to /etc/systemd/network, drop-in ".d" directories can be
placed in /lib/systemd/network or /run/systemd/network directories.
Drop-in files in /etc/ take precedence over those in /run/ which in
turn take precedence over those in /lib/. Drop-in files under any of
these directories take precedence over the main network file wherever
located.

[MATCH] SECTION OPTIONS
The network file contains a [Match] section, which determines if a
given network file may be applied to a given interface; and a [Network]
section specifying how the interface should be configured. The first
(in alphanumeric order) of the network files that matches a given
interface is applied, all later files are ignored, even if they match
as well.

A network file is said to match a network interface if all matches
specified by the [Match] section are satisfied. When a network file
does not contain valid settings in [Match] section, then the file will
match all interfaces and systemd-networkd warns about that. Hint: to
avoid the warning and to make it clear that all interfaces shall be
matched, add the following:

Name=*

The following keys are accepted:

MACAddress=
A whitespace-separated list of hardware addresses. The acceptable
formats are:

colon-delimited hexadecimal
Each field must be one byte. E.g. "12:34:56:78:90:ab" or
"AA:BB:CC:DD:EE:FF".

hyphen-delimited hexadecimal
Each field must be one byte. E.g. "12-34-56-78-90-ab" or
"AA-BB-CC-DD-EE-FF".

dot-delimited hexadecimal
Each field must be two bytes. E.g. "1234.5678.90ab" or
"AABB.CCDD.EEFF".

IPv4 address format
E.g. "127.0.0.1" or "192.168.0.1".

IPv6 address format
E.g. "2001:0db8:85a3::8a2e:0370:7334" or "::1".

The total length of each MAC address must be 4 (for IPv4 tunnel), 6
(for Ethernet), 16 (for IPv6 tunnel), or 20 (for InfiniBand). This
option may appear more than once, in which case the lists are
merged. If the empty string is assigned to this option, the list of
hardware addresses defined prior to this is reset. Defaults to
unset.

PermanentMACAddress=
A whitespace-separated list of hardware's permanent addresses.
While MACAddress= matches the device's current MAC address, this
matches the device's permanent MAC address, which may be different
from the current one. Use full colon-, hyphen- or dot-delimited
hexadecimal, or IPv4 or IPv6 address format. This option may appear
more than once, in which case the lists are merged. If the empty
string is assigned to this option, the list of hardware addresses
defined prior to this is reset. Defaults to unset.

Path=
A whitespace-separated list of shell-style globs matching the
persistent path, as exposed by the udev property ID_PATH.

Driver=
A whitespace-separated list of shell-style globs matching the
driver currently bound to the device, as exposed by the udev
property ID_NET_DRIVER of its parent device, or if that is not set,
the driver as exposed by ethtool -i of the device itself. If the
list is prefixed with a "!", the test is inverted.

Type=
A whitespace-separated list of shell-style globs matching the
device type, as exposed by networkctl list. If the list is prefixed
with a "!", the test is inverted. Some valid values are "ether",
"loopback", "wlan", "wwan". Valid types are named either from the
udev "DEVTYPE" attribute, or "ARPHRD_" macros in linux/if_arp.h, so
this is not comprehensive.

Kind=
A whitespace-separated list of shell-style globs matching the
device kind, as exposed by networkctl status INTERFACE or ip -d
link show INTERFACE. If the list is prefixed with a "!", the test
is inverted. Some valid values are "bond", "bridge", "gre", "tun",
"veth". Valid kinds are given by netlink's "IFLA_INFO_KIND"
attribute, so this is not comprehensive.

Property=
A whitespace-separated list of udev property names with their
values after equals sign ("="). If multiple properties are
specified, the test results are ANDed. If the list is prefixed with
a "!", the test is inverted. If a value contains white spaces, then
please quote whole key and value pair. If a value contains
quotation, then please escape the quotation with "\".

Example: if a .link file has the following:

Property=ID_MODEL_ID=9999 "ID_VENDOR_FROM_DATABASE=vendor name" "KEY=with \"quotation\""

then, the .link file matches only when an interface has all the
above three properties.

Name=
A whitespace-separated list of shell-style globs matching the
device name, as exposed by the udev property "INTERFACE", or
device's alternative names. If the list is prefixed with a "!", the
test is inverted.

WLANInterfaceType=
A whitespace-separated list of wireless network type. Supported
values are "ad-hoc", "station", "ap", "ap-vlan", "wds", "monitor",
"mesh-point", "p2p-client", "p2p-go", "p2p-device", "ocb", and
"nan". If the list is prefixed with a "!", the test is inverted.

SSID=
A whitespace-separated list of shell-style globs matching the SSID
of the currently connected wireless LAN. If the list is prefixed
with a "!", the test is inverted.

BSSID=
A whitespace-separated list of hardware address of the currently
connected wireless LAN. Use full colon-, hyphen- or dot-delimited
hexadecimal. See the example in MACAddress=. This option may appear
more than once, in which case the lists are merged. If the empty
string is assigned to this option, the list is reset.

Host=
Matches against the hostname or machine ID of the host. See
ConditionHost= in systemd.unit(5) for details. When prefixed with
an exclamation mark ("!"), the result is negated. If an empty
string is assigned, the previously assigned value is cleared.

Virtualization=
Checks whether the system is executed in a virtualized environment
and optionally test whether it is a specific implementation. See
ConditionVirtualization= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, the previously assigned value is
cleared.

KernelCommandLine=
Checks whether a specific kernel command line option is set. See
ConditionKernelCommandLine= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, the previously assigned value is
cleared.

KernelVersion=
Checks whether the kernel version (as reported by uname -r) matches
a certain expression. See ConditionKernelVersion= in
systemd.unit(5) for details. When prefixed with an exclamation mark
("!"), the result is negated. If an empty string is assigned, the
previously assigned value is cleared.

Credential=
Checks whether the specified credential was passed to the
systemd-networkd.service service. See System and Service
Credentials[1] for details. When prefixed with an exclamation mark
("!"), the result is negated. If an empty string is assigned, the
previously assigned value is cleared.

Architecture=
Checks whether the system is running on a specific architecture.
See ConditionArchitecture= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, the previously assigned value is
cleared.

Firmware=
Checks whether the system is running on a machine with the
specified firmware. See ConditionFirmware= in systemd.unit(5) for
details. When prefixed with an exclamation mark ("!"), the result
is negated. If an empty string is assigned, the previously assigned
value is cleared.

[LINK] SECTION OPTIONS
The [Link] section accepts the following keys:

MACAddress=
The hardware address to set for the device.

MTUBytes=
The maximum transmission unit in bytes to set for the device. The
usual suffixes K, M, G, are supported and are understood to the
base of 1024.

Note that if IPv6 is enabled on the interface, and the MTU is
chosen below 1280 (the minimum MTU for IPv6) it will automatically
be increased to this value.

ARP=
Takes a boolean. If set to true, the ARP (low-level Address
Resolution Protocol) for this interface is enabled. When unset, the
kernel's default will be used.

For example, disabling ARP is useful when creating multiple MACVLAN
or VLAN virtual interfaces atop a single lower-level physical
interface, which will then only serve as a link/"bridge" device
aggregating traffic to the same physical link and not participate
in the network otherwise. Defaults to unset.

Multicast=
Takes a boolean. If set to true, the multicast flag on the device
is enabled. Defaults to unset.

AllMulticast=
Takes a boolean. If set to true, the driver retrieves all multicast
packets from the network. This happens when multicast routing is
enabled. Defaults to unset.

Promiscuous=
Takes a boolean. If set to true, promiscuous mode of the interface
is enabled. Defaults to unset.

If this is set to false for the underlying link of a "passthru"
mode MACVLAN/MACVTAP, the virtual interface will be created with
the "nopromisc" flag set.

Unmanaged=
Takes a boolean. When "yes", no attempts are made to bring up or
configure matching links, equivalent to when there are no matching
network files. Defaults to "no".

This is useful for preventing later matching network files from
interfering with certain interfaces that are fully controlled by
other applications.

Group=
Link groups are similar to port ranges found in managed switches.
When network interfaces are added to a numbered group, operations
on all the interfaces from that group can be performed at once.
Takes an unsigned integer in the range 0...2147483647. Defaults to
unset.

RequiredForOnline=
Takes a boolean or a minimum operational state and an optional
maximum operational state. Please see networkctl(1) for possible
operational states. When "yes", the network is deemed required when
determining whether the system is online (including when running
systemd-networkd-wait-online). When "no", the network is ignored
when determining the online state. When a minimum operational state
and an optional maximum operational state are set, "yes" is
implied, and this controls the minimum and maximum operational
state required for the network interface to be considered online.

Defaults to "yes" when ActivationPolicy= is not set, or set to
"up", "always-up", or "bound". Defaults to "no" when
ActivationPolicy= is set to "manual" or "down". This is forced to
"no" when ActivationPolicy= is set to "always-down".

The network will be brought up normally (as configured by
ActivationPolicy=), but in the event that there is no address being
assigned by DHCP or the cable is not plugged in, the link will
simply remain offline and be skipped automatically by
systemd-networkd-wait-online if "RequiredForOnline=no".

RequiredFamilyForOnline=
Takes an address family. When specified, an IP address in the given
family is deemed required when determining whether the link is
online (including when running systemd-networkd-wait-online). Takes
one of "ipv4", "ipv6", "both", or "any". Defaults to "any". Note
that this option has no effect if "RequiredForOnline=no", or if
"RequiredForOnline=" specifies a minimum operational state below
"degraded".

ActivationPolicy=
Specifies the policy for systemd-networkd managing the link
administrative state. Specifically, this controls how
systemd-networkd changes the network device's "IFF_UP" flag, which
is sometimes controlled by system administrators by running e.g.,
ip link set dev eth0 up or ip link set dev eth0 down, and can also
be changed with networkctl up eth0 or networkctl down eth0.

Takes one of "up", "always-up", "manual", "always-down", "down", or
"bound". When "manual", systemd-networkd will not change the link's
admin state automatically; the system administrator must bring the
interface up or down manually, as desired. When "up" (the default)
or "always-up", or "down" or "always-down", systemd-networkd will
set the link up or down, respectively, when the interface is
(re)configured. When "always-up" or "always-down", systemd-networkd
will set the link up or down, respectively, any time
systemd-networkd detects a change in the administrative state. When
BindCarrier= is also set, this is automatically set to "bound" and
any other value is ignored.

When the policy is set to "down" or "manual", the default value of
RequiredForOnline= is "no". When the policy is set to
"always-down", the value of RequiredForOnline= forced to "no".

The administrative state is not the same as the carrier state, so
using "always-up" does not mean the link will never lose carrier.
The link carrier depends on both the administrative state as well
as the network device's physical connection. However, to avoid
reconfiguration failures, when using "always-up",
IgnoreCarrierLoss= is forced to true.

[SR-IOV] SECTION OPTIONS
SR-IOV provides the ability to partition a single physical PCI resource
into virtual PCI functions which can then be e.g. injected into a VM.
In the case of network VFs, SR-IOV reduces latency and CPU utilisation
for north-south network traffic (that is, traffic with endpoints
outside the host machine), by allowing traffic to bypass the host
machine's network stack.

The presence of an [SR-IOV] section in a .link file will cause the
creation and configuration of the specified virtual function. Within a
.network file, the specified virtual function will be configured, but
must already exist. Specify several [SR-IOV] sections to configure
several SR-IOVs.

The [SR-IOV] section accepts the following keys.

VirtualFunction=
Specifies a Virtual Function (VF), lightweight PCIe function
designed solely to move data in and out. Takes an integer in the
range 0...2147483646. This option is compulsory.

VLANId=
Specifies VLAN ID of the virtual function. Takes an integer in the
range 1...4095.

QualityOfService=
Specifies quality of service of the virtual function. Takes an
integer in the range 1...4294967294.

VLANProtocol=
Specifies VLAN protocol of the virtual function. Takes "802.1Q" or
"802.1ad".

MACSpoofCheck=
Takes a boolean. Controls the MAC spoof checking. When unset, the
kernel's default will be used.

QueryReceiveSideScaling=
Takes a boolean. Toggle the ability of querying the receive side
scaling (RSS) configuration of the virtual function (VF). The VF
RSS information like RSS hash key may be considered sensitive on
some devices where this information is shared between VF and the
physical function (PF). When unset, the kernel's default will be
used.

Trust=
Takes a boolean. Allows one to set trust mode of the virtual
function (VF). When set, VF users can set a specific feature which
may impact security and/or performance. When unset, the kernel's
default will be used.

LinkState=
Allows one to set the link state of the virtual function (VF).
Takes a boolean or a special value "auto". Setting to "auto" means
a reflection of the physical function (PF) link state, "yes" lets
the VF to communicate with other VFs on this host even if the PF
link state is down, "no" causes the hardware to drop any packets
sent by the VF. When unset, the kernel's default will be used.

MACAddress=
Specifies the MAC address for the virtual function.

[NETWORK] SECTION OPTIONS
The [Network] section accepts the following keys:

Description=
A description of the device. This is only used for presentation
purposes.

DHCP=
Enables DHCPv4 and/or DHCPv6 client support. Accepts "yes", "no",
"ipv4", or "ipv6". Defaults to "no".

Note that DHCPv6 will by default be triggered by Router
Advertisements, if reception is enabled, regardless of this
parameter. By explicitly enabling DHCPv6 support here, the DHCPv6
client will be started in the mode specified by the WithoutRA=
setting in the [DHCPv6] section, regardless of the presence of
routers on the link, or what flags the routers pass. See
IPv6AcceptRA=.

Furthermore, note that by default the domain name specified through
DHCP is not used for name resolution. See option UseDomains= below.

See the [DHCPv4] or [DHCPv6] sections below for further
configuration options for the DHCP client support.

DHCPServer=
Takes a boolean. If set to "yes", DHCPv4 server will be started.
Defaults to "no". Further settings for the DHCP server may be set
in the [DHCPServer] section described below.

LinkLocalAddressing=
Enables link-local address autoconfiguration. Accepts a boolean,
ipv4, and ipv6. An IPv6 link-local address is configured when yes
or ipv6. An IPv4 link-local address is configured when yes or ipv4
and when DHCPv4 autoconfiguration has been unsuccessful for some
time. (IPv4 link-local address autoconfiguration will usually
happen in parallel with repeated attempts to acquire a DHCPv4
lease).

Defaults to no when KeepMaster= or Bridge= is set or when the
specified MACVLAN=/MACVTAP= has Mode=passthru, or ipv6 otherwise.

IPv6LinkLocalAddressGenerationMode=
Specifies how IPv6 link-local address is generated. Takes one of
"eui64", "none", "stable-privacy" and "random". When unset,
"stable-privacy" is used if IPv6StableSecretAddress= is specified,
and if not, "eui64" is used. Note that if LinkLocalAddressing= is
"no" or "ipv4", then IPv6LinkLocalAddressGenerationMode= will be
ignored. Also, even if LinkLocalAddressing= is "yes" or "ipv6",
setting IPv6LinkLocalAddressGenerationMode=none disables to
configure an IPv6 link-local address.

IPv6StableSecretAddress=
Takes an IPv6 address. The specified address will be used as a
stable secret for generating IPv6 link-local address. If this
setting is specified, and IPv6LinkLocalAddressGenerationMode= is
unset, then IPv6LinkLocalAddressGenerationMode=stable-privacy is
implied. If this setting is not specified, and "stable-privacy" is
set to IPv6LinkLocalAddressGenerationMode=, then a stable secret
address will be generated from the local machine ID and the
interface name.

IPv4LLStartAddress=
Specifies the first IPv4 link-local address to try. Takes an IPv4
address for example 169.254.1.2, from the link-local address range:
169.254.0.0/16 except for 169.254.0.0/24 and 169.254.255.0/24. This
setting may be useful if the device should always have the same
address as long as there is no address conflict. When unset, a
random address will be automatically selected. Defaults to unset.

IPv4LLRoute=
Takes a boolean. If set to true, sets up the route needed for
non-IPv4LL hosts to communicate with IPv4LL-only hosts. Defaults to
false.

DefaultRouteOnDevice=
Takes a boolean. If set to true, sets up the IPv4 default route
bound to the interface. Defaults to false. This is useful when
creating routes on point-to-point interfaces. This is equivalent to
e.g. the following,

ip route add default dev veth99

or,

[Route]
Gateway=0.0.0.0

Currently, there are no way to specify e.g., the table for the
route configured by this setting. To configure the default route
with such an additional property, please use the following instead:

[Route]
Gateway=0.0.0.0
Table=1234

If you'd like to create an IPv6 default route bound to the
interface, please use the following:

[Route]
Gateway=::
Table=1234

LLMNR=
Takes a boolean or "resolve". When true, enables Link-Local
Multicast Name Resolution[2] on the link. When set to "resolve",
only resolution is enabled, but not host registration and
announcement. Defaults to true. This setting is read by systemd-
resolved.service(8).

MulticastDNS=
Takes a boolean or "resolve". When true, enables Multicast DNS[3]
support on the link. When set to "resolve", only resolution is
enabled, but not host or service registration and announcement.
Defaults to false. This setting is read by systemd-
resolved.service(8).

DNSOverTLS=
Takes a boolean or "opportunistic". When true, enables
DNS-over-TLS[4] support on the link. When set to "opportunistic",
compatibility with non-DNS-over-TLS servers is increased, by
automatically turning off DNS-over-TLS servers in this case. This
option defines a per-interface setting for resolved.conf(5)'s
global DNSOverTLS= option. Defaults to unset, and the global
setting will be used. This setting is read by systemd-
resolved.service(8).

DNSSEC=
Takes a boolean or "allow-downgrade". When true, enables DNSSEC[5]
DNS validation support on the link. When set to "allow-downgrade",
compatibility with non-DNSSEC capable networks is increased, by
automatically turning off DNSSEC in this case. This option defines
a per-interface setting for resolved.conf(5)'s global DNSSEC=
option. Defaults to unset, and the global setting will be used.
This setting is read by systemd-resolved.service(8).

DNSSECNegativeTrustAnchors=
A space-separated list of DNSSEC negative trust anchor domains. If
specified and DNSSEC is enabled, look-ups done via the interface's
DNS server will be subject to the list of negative trust anchors,
and not require authentication for the specified domains, or
anything below it. Use this to disable DNSSEC authentication for
specific private domains, that cannot be proven valid using the
Internet DNS hierarchy. Defaults to the empty list. This setting is
read by systemd-resolved.service(8).

LLDP=
Controls support for Ethernet LLDP packet reception. LLDP is a
link-layer protocol commonly implemented on professional routers
and bridges which announces which physical port a system is
connected to, as well as other related data. Accepts a boolean or
the special value "routers-only". When true, incoming LLDP packets
are accepted and a database of all LLDP neighbors maintained. If
"routers-only" is set only LLDP data of various types of routers is
collected and LLDP data about other types of devices ignored (such
as stations, telephones and others). If false, LLDP reception is
disabled. Defaults to "routers-only". Use networkctl(1) to query
the collected neighbor data. LLDP is only available on Ethernet
links. See EmitLLDP= below for enabling LLDP packet emission from
the local system.

EmitLLDP=
Controls support for Ethernet LLDP packet emission. Accepts a
boolean parameter or the special values "nearest-bridge",
"non-tpmr-bridge" and "customer-bridge". Defaults to false, which
turns off LLDP packet emission. If not false, a short LLDP packet
with information about the local system is sent out in regular
intervals on the link. The LLDP packet will contain information
about the local hostname, the local machine ID (as stored in
machine-id(5)) and the local interface name, as well as the pretty
hostname of the system (as set in machine-info(5)). LLDP emission
is only available on Ethernet links. Note that this setting passes
data suitable for identification of host to the network and should
thus not be enabled on untrusted networks, where such
identification data should not be made available. Use this option
to permit other systems to identify on which interfaces they are
connected to this system. The three special values control
propagation of the LLDP packets. The "nearest-bridge" setting
permits propagation only to the nearest connected bridge,
"non-tpmr-bridge" permits propagation across Two-Port MAC Relays,
but not any other bridges, and "customer-bridge" permits
propagation until a customer bridge is reached. For details about
these concepts, see IEEE 802.1AB-2016[6]. Note that configuring
this setting to true is equivalent to "nearest-bridge", the
recommended and most restricted level of propagation. See LLDP=
above for an option to enable LLDP reception.

BindCarrier=
A link name or a list of link names. When set, controls the
behavior of the current link. When all links in the list are in an
operational down state, the current link is brought down. When at
least one link has carrier, the current interface is brought up.

This forces ActivationPolicy= to be set to "bound".

Address=
A static IPv4 or IPv6 address and its prefix length, separated by a
"/" character. Specify this key more than once to configure several
addresses. The format of the address must be as described in
inet_pton(3). This is a short-hand for an [Address] section only
containing an Address key (see below). This option may be specified
more than once.

If the specified address is "0.0.0.0" (for IPv4) or "::" (for
IPv6), a new address range of the requested size is automatically
allocated from a system-wide pool of unused ranges. Note that the
prefix length must be equal or larger than 8 for IPv4, and 64 for
IPv6. The allocated range is checked against all current network
interfaces and all known network configuration files to avoid
address range conflicts. The default system-wide pool consists of
192.168.0.0/16, 172.16.0.0/12 and 10.0.0.0/8 for IPv4, and fd00::/8
for IPv6. This functionality is useful to manage a large number of
dynamically created network interfaces with the same network
configuration and automatic address range assignment.

Gateway=
The gateway address, which must be in the format described in
inet_pton(3). This is a short-hand for a [Route] section only
containing a Gateway= key. This option may be specified more than
once.

DNS=
A DNS server address, which must be in the format described in
inet_pton(3). This option may be specified more than once. Each
address can optionally take a port number separated with ":", a
network interface name or index separated with "%", and a Server
Name Indication (SNI) separated with "#". When IPv6 address is
specified with a port number, then the address must be in the
square brackets. That is, the acceptable full formats are
"111.222.333.444:9953%ifname#example.com" for IPv4 and
"[1111:2222::3333]:9953%ifname#example.com" for IPv6. If an empty
string is assigned, then the all previous assignments are cleared.
This setting is read by systemd-resolved.service(8).

Domains=
A whitespace-separated list of domains which should be resolved
using the DNS servers on this link. Each item in the list should be
a domain name, optionally prefixed with a tilde ("~"). The domains
with the prefix are called "routing-only domains". The domains
without the prefix are called "search domains" and are first used
as search suffixes for extending single-label hostnames (hostnames
containing no dots) to become fully qualified domain names (FQDNs).
If a single-label hostname is resolved on this interface, each of
the specified search domains are appended to it in turn, converting
it into a fully qualified domain name, until one of them may be
successfully resolved.

Both "search" and "routing-only" domains are used for routing of
DNS queries: look-ups for hostnames ending in those domains (hence
also single label names, if any "search domains" are listed), are
routed to the DNS servers configured for this interface. The domain
routing logic is particularly useful on multi-homed hosts with DNS
servers serving particular private DNS zones on each interface.

The "routing-only" domain "~." (the tilde indicating definition of
a routing domain, the dot referring to the DNS root domain which is
the implied suffix of all valid DNS names) has special effect. It
causes all DNS traffic which does not match another configured
domain routing entry to be routed to DNS servers specified for this
interface. This setting is useful to prefer a certain set of DNS
servers if a link on which they are connected is available.

This setting is read by systemd-resolved.service(8). "Search
domains" correspond to the domain and search entries in
resolv.conf(5). Domain name routing has no equivalent in the
traditional glibc API, which has no concept of domain name servers
limited to a specific link.

DNSDefaultRoute=
Takes a boolean argument. If true, this link's configured DNS
servers are used for resolving domain names that do not match any
link's configured Domains= setting. If false, this link's
configured DNS servers are never used for such domains, and are
exclusively used for resolving names that match at least one of the
domains configured on this link. If not specified defaults to an
automatic mode: queries not matching any link's configured domains
will be routed to this link if it has no routing-only domains
configured.

NTP=
An NTP server address (either an IP address, or a hostname). This
option may be specified more than once. This setting is read by
systemd-timesyncd.service(8).

IPForward=
Configures IP packet forwarding for the system. If enabled,
incoming packets on any network interface will be forwarded to any
other interfaces according to the routing table. Takes a boolean,
or the values "ipv4" or "ipv6", which only enable IP packet
forwarding for the specified address family. This controls the
net.ipv4.ip_forward and net.ipv6.conf.all.forwarding sysctl options
of the network interface (see IP Sysctl[7] for details about sysctl
options). Defaults to "no".

Note: this setting controls a global kernel option, and does so one
way only: if a network that has this setting enabled is set up the
global setting is turned on. However, it is never turned off again,
even after all networks with this setting enabled are shut down
again.

To allow IP packet forwarding only between specific network
interfaces use a firewall.

IPMasquerade=
Configures IP masquerading for the network interface. If enabled,
packets forwarded from the network interface will be appear as
coming from the local host. Takes one of "ipv4", "ipv6", "both", or
"no". Defaults to "no". If enabled, this automatically sets
IPForward= to one of "ipv4", "ipv6" or "yes".

Note. Any positive boolean values such as "yes" or "true" are now
deprecated. Please use one of the values in the above.

IPv6PrivacyExtensions=
Configures use of stateless temporary addresses that change over
time (see RFC 4941[8], Privacy Extensions for Stateless Address
Autoconfiguration in IPv6). Takes a boolean or the special values
"prefer-public" and "kernel". When true, enables the privacy
extensions and prefers temporary addresses over public addresses.
When "prefer-public", enables the privacy extensions, but prefers
public addresses over temporary addresses. When false, the privacy
extensions remain disabled. When "kernel", the kernel's default
setting will be left in place. Defaults to "no".

IPv6AcceptRA=
Takes a boolean. Controls IPv6 Router Advertisement (RA) reception
support for the interface. If true, RAs are accepted; if false, RAs
are ignored. When RAs are accepted, they may trigger the start of
the DHCPv6 client if the relevant flags are set in the RA data, or
if no routers are found on the link. The default is to disable RA
reception for bridge devices or when IP forwarding is enabled, and
to enable it otherwise. Cannot be enabled on bond devices and when
link-local addressing is disabled.

Further settings for the IPv6 RA support may be configured in the
[IPv6AcceptRA] section, see below.

Also see IP Sysctl[7] in the kernel documentation regarding
"accept_ra", but note that systemd's setting of 1 (i.e. true)
corresponds to kernel's setting of 2.

Note that kernel's implementation of the IPv6 RA protocol is always
disabled, regardless of this setting. If this option is enabled, a
userspace implementation of the IPv6 RA protocol is used, and the
kernel's own implementation remains disabled, since
systemd-networkd needs to know all details supplied in the
advertisements, and these are not available from the kernel if the
kernel's own implementation is used.

IPv6DuplicateAddressDetection=
Configures the amount of IPv6 Duplicate Address Detection (DAD)
probes to send. When unset, the kernel's default will be used.

IPv6HopLimit=
Configures IPv6 Hop Limit. For each router that forwards the
packet, the hop limit is decremented by 1. When the hop limit field
reaches zero, the packet is discarded. When unset, the kernel's
default will be used.

IPv4AcceptLocal=
Takes a boolean. Accept packets with local source addresses. In
combination with suitable routing, this can be used to direct
packets between two local interfaces over the wire and have them
accepted properly. When unset, the kernel's default will be used.

IPv4RouteLocalnet=
Takes a boolean. When true, the kernel does not consider loopback
addresses as martian source or destination while routing. This
enables the use of 127.0.0.0/8 for local routing purposes. When
unset, the kernel's default will be used.

IPv4ProxyARP=
Takes a boolean. Configures proxy ARP for IPv4. Proxy ARP is the
technique in which one host, usually a router, answers ARP requests
intended for another machine. By "faking" its identity, the router
accepts responsibility for routing packets to the "real"
destination. See RFC 1027[9]. When unset, the kernel's default will
be used.

IPv6ProxyNDP=
Takes a boolean. Configures proxy NDP for IPv6. Proxy NDP (Neighbor
Discovery Protocol) is a technique for IPv6 to allow routing of
addresses to a different destination when peers expect them to be
present on a certain physical link. In this case a router answers
Neighbour Advertisement messages intended for another machine by
offering its own MAC address as destination. Unlike proxy ARP for
IPv4, it is not enabled globally, but will only send Neighbour
Advertisement messages for addresses in the IPv6 neighbor proxy
table, which can also be shown by ip -6 neighbour show proxy.
systemd-networkd will control the per-interface `proxy_ndp` switch
for each configured interface depending on this option. When unset,
the kernel's default will be used.

IPv6ProxyNDPAddress=
An IPv6 address, for which Neighbour Advertisement messages will be
proxied. This option may be specified more than once.
systemd-networkd will add the IPv6ProxyNDPAddress= entries to the
kernel's IPv6 neighbor proxy table. This setting implies
IPv6ProxyNDP=yes but has no effect if IPv6ProxyNDP= has been set to
false. When unset, the kernel's default will be used.

IPv6SendRA=
Whether to enable or disable Router Advertisement sending on a
link. Takes a boolean value. When enabled, prefixes configured in
[IPv6Prefix] sections and routes configured in the
[IPv6RoutePrefix] sections are distributed as defined in the
[IPv6SendRA] section. If DHCPPrefixDelegation= is enabled, then the
delegated prefixes are also distributed. See DHCPPrefixDelegation=
setting and the [IPv6SendRA], [IPv6Prefix], [IPv6RoutePrefix], and
[DHCPPrefixDelegation] sections for more configuration options.

DHCPPrefixDelegation=
Takes a boolean value. When enabled, requests subnet prefixes on
another link via the DHCPv6 protocol or via the 6RD option in the
DHCPv4 protocol. An address within each delegated prefix will be
assigned, and the prefixes will be announced through IPv6 Router
Advertisement if IPv6SendRA= is enabled. This behaviour can be
configured in the [DHCPPrefixDelegation] section. Defaults to
disabled.

IPv6MTUBytes=
Configures IPv6 maximum transmission unit (MTU). An integer greater
than or equal to 1280 bytes. When unset, the kernel's default will
be used.

KeepMaster=
Takes a boolean value. When enabled, the current master interface
index will not be changed, and BatmanAdvanced=, Bond=, Bridge=, and
VRF= settings are ignored. This may be useful when a netdev with a
master interface is created by another program, e.g. systemd-
nspawn(1). Defaults to false.

BatmanAdvanced=, Bond=, Bridge=, VRF=
The name of the B.A.T.M.A.N. Advanced, bond, bridge, or VRF
interface to add the link to. See systemd.netdev(5).

IPoIB=, IPVLAN=, IPVTAP=, MACsec=, MACVLAN=, MACVTAP=, Tunnel=, VLAN=,
VXLAN=, Xfrm=
The name of an IPoIB, IPVLAN, IPVTAP, MACsec, MACVLAN, MACVTAP,
tunnel, VLAN, VXLAN, or Xfrm to be created on the link. See
systemd.netdev(5). This option may be specified more than once.

ActiveSlave=
Takes a boolean. Specifies the new active slave. The "ActiveSlave="
option is only valid for following modes: "active-backup",
"balance-alb", and "balance-tlb". Defaults to false.

PrimarySlave=
Takes a boolean. Specifies which slave is the primary device. The
specified device will always be the active slave while it is
available. Only when the primary is off-line will alternate devices
be used. This is useful when one slave is preferred over another,
e.g. when one slave has higher throughput than another. The
"PrimarySlave=" option is only valid for following modes:
"active-backup", "balance-alb", and "balance-tlb". Defaults to
false.

ConfigureWithoutCarrier=
Takes a boolean. Allows networkd to configure a specific link even
if it has no carrier. Defaults to false. If enabled, and the
IgnoreCarrierLoss= setting is not explicitly set, then it is
enabled as well.

IgnoreCarrierLoss=
Takes a boolean or a timespan. When true, systemd-networkd retains
both the static and dynamic configuration of the interface even if
its carrier is lost. When false, systemd-networkd drops both the
static and dynamic configuration of the interface. When a timespan
is specified, systemd-networkd waits for the specified timespan,
and ignores the carrier loss if the link regain its carrier within
the timespan. Setting 0 seconds is equivalent to "no", and
"infinite" is equivalent to "yes".

Setting a finite timespan may be useful when e.g. in the following
cases:

o A wireless interface connecting to a network which has multiple
access points with the same SSID.

o Enslaving a wireless interface to a bond interface, which may
disconnect from the connected access point and causes its
carrier to be lost.

o The driver of the interface resets when the MTU is changed.

When Bond= is specified to a wireless interface, defaults to 3
seconds. When the DHCPv4 client is enabled and UseMTU= in the
[DHCPv4] section enabled, defaults to 5 seconds. Otherwise,
defaults to the value specified with ConfigureWithoutCarrier=. When
ActivationPolicy= is set to "always-up", this is forced to "yes",
and ignored any user specified values.

KeepConfiguration=
Takes a boolean or one of "static", "dhcp-on-stop", "dhcp". When
"static", systemd-networkd will not drop static addresses and
routes on starting up process. When set to "dhcp-on-stop",
systemd-networkd will not drop addresses and routes on stopping the
daemon. When "dhcp", the addresses and routes provided by a DHCP
server will never be dropped even if the DHCP lease expires. This
is contrary to the DHCP specification, but may be the best choice
if, e.g., the root filesystem relies on this connection. The
setting "dhcp" implies "dhcp-on-stop", and "yes" implies "dhcp" and
"static". Defaults to "dhcp-on-stop" when systemd-networkd is
running in initrd, "yes" when the root filesystem is a network
filesystem, and "no" otherwise.

[ADDRESS] SECTION OPTIONS
An [Address] section accepts the following keys. Specify several
[Address] sections to configure several addresses.

Address=
As in the [Network] section. This setting is mandatory. Each
[Address] section can contain one Address= setting.

Peer=
The peer address in a point-to-point connection. Accepts the same
format as the Address= setting.

Broadcast=
Takes an IPv4 address or boolean value. The address must be in the
format described in inet_pton(3). If set to true, then the IPv4
broadcast address will be derived from the Address= setting. If set
to false, then the broadcast address will not be set. Defaults to
true, except for wireguard interfaces, where it default to false.

Label=
Specifies the label for the IPv4 address. The label must be a 7-bit
ASCII string with a length of 1...15 characters. Defaults to unset.

PreferredLifetime=
Allows the default "preferred lifetime" of the address to be
overridden. Only three settings are accepted: "forever",
"infinity", which is the default and means that the address never
expires, and "0", which means that the address is considered
immediately "expired" and will not be used, unless explicitly
requested. A setting of PreferredLifetime=0 is useful for addresses
which are added to be used only by a specific application, which is
then configured to use them explicitly.

Scope=
The scope of the address, which can be "global" (valid everywhere
on the network, even through a gateway), "link" (only valid on this
device, will not traverse a gateway) or "host" (only valid within
the device itself, e.g. 127.0.0.1) or an integer in the range
0...255. Defaults to "global".

RouteMetric=
The metric of the prefix route, which is pointing to the subnet of
the configured IP address, taking the configured prefix length into
account. Takes an unsigned integer in the range 0...4294967295.
When unset or set to 0, the kernel's default value is used. This
setting will be ignored when AddPrefixRoute= is false.

HomeAddress=
Takes a boolean. Designates this address the "home address" as
defined in RFC 6275[10]. Supported only on IPv6. Defaults to false.

DuplicateAddressDetection=
Takes one of "ipv4", "ipv6", "both", or "none". When "ipv4",
performs IPv4 Address Conflict Detection. See RFC 5227[11]. When
"ipv6", performs IPv6 Duplicate Address Detection. See RFC
4862[12]. Defaults to "ipv4" for IPv4 link-local addresses, "ipv6"
for IPv6 addresses, and "none" otherwise.

ManageTemporaryAddress=
Takes a boolean. If true the kernel manage temporary addresses
created from this one as template on behalf of Privacy Extensions
RFC 3041[13]. For this to become active, the use_tempaddr sysctl
setting has to be set to a value greater than zero. The given
address needs to have a prefix length of 64. This flag allows using
privacy extensions in a manually configured network, just like if
stateless auto-configuration was active. Defaults to false.

AddPrefixRoute=
Takes a boolean. When true, the prefix route for the address is
automatically added. Defaults to true.

AutoJoin=
Takes a boolean. Joining multicast group on ethernet level via ip
maddr command would not work if we have an Ethernet switch that
does IGMP snooping since the switch would not replicate multicast
packets on ports that did not have IGMP reports for the multicast
addresses. Linux vxlan interfaces created via ip link add vxlan or
networkd's netdev kind vxlan have the group option that enables
them to do the required join. By extending ip address command with
option "autojoin" we can get similar functionality for openvswitch
(OVS) vxlan interfaces as well as other tunneling mechanisms that
need to receive multicast traffic. Defaults to "no".

NetLabel=label
This setting provides a method for integrating static and dynamic
network configuration into Linux NetLabel[14] subsystem rules, used
by Linux Security Modules (LSMs)[15] for network access control.
The label, with suitable LSM rules, can be used to control
connectivity of (for example) a service with peers in the local
network. At least with SELinux, only the ingress can be controlled
but not egress. The benefit of using this setting is that it may be
possible to apply interface independent part of NetLabel
configuration at very early stage of system boot sequence, at the
time when the network interfaces are not available yet, with
netlabelctl(8), and the per-interface configuration with
systemd-networkd once the interfaces appear later. Currently this
feature is only implemented for SELinux.

The option expects a single NetLabel label. The label must conform
to lexical restrictions of LSM labels. When an interface is
configured with IP addresses, the addresses and subnetwork masks
will be appended to the NetLabel Fallback Peer Labeling[16] rules.
They will be removed when the interface is deconfigured. Failures
to manage the labels will be ignored.

Warning: Once labeling is enabled for network traffic, a lot of LSM
access control points in Linux networking stack go from dormant to
active. Care should be taken to avoid getting into a situation
where for example remote connectivity is broken, when the security
policy hasn't been updated to consider LSM per-packet access
controls and no rules would allow any network traffic. Also note
that additional configuration with netlabelctl(8) is needed.

Example:

[Address]
NetLabel=system_u:object_r:localnet_peer_t:s0

With the example rules applying for interface "eth0", when the
interface is configured with an IPv4 address of 10.0.0.123/8,
systemd-networkd performs the equivalent of netlabelctl operation

netlabelctl unlbl add interface eth0 address:10.0.0.0/8 label:system_u:object_r:localnet_peer_t:s0

and the reverse operation when the IPv4 address is deconfigured.
The configuration can be used with LSM rules; in case of SELinux to
allow a SELinux domain to receive data from objects of SELinux
"peer" class. For example:

type localnet_peer_t;
allow my_server_t localnet_peer_t:peer recv;

The effect of the above configuration and rules (in absence of
other rules as may be the case) is to only allow "my_server_t" (and
nothing else) to receive data from local subnet 10.0.0.0/8 of
interface "eth0".

[NEIGHBOR] SECTION OPTIONS
A [Neighbor] section accepts the following keys. The neighbor section
adds a permanent, static entry to the neighbor table (IPv6) or ARP
table (IPv4) for the given hardware address on the links matched for
the network. Specify several [Neighbor] sections to configure several
static neighbors.

Address=
The IP address of the neighbor.

LinkLayerAddress=
The link layer address (MAC address or IP address) of the neighbor.

[IPV6ADDRESSLABEL] SECTION OPTIONS
An [IPv6AddressLabel] section accepts the following keys. Specify
several [IPv6AddressLabel] sections to configure several address
labels. IPv6 address labels are used for address selection. See RFC
3484[17]. Precedence is managed by userspace, and only the label itself
is stored in the kernel.

Label=
The label for the prefix, an unsigned integer in the range
0...4294967294. 0xffffffff is reserved. This setting is mandatory.

Prefix=
IPv6 prefix is an address with a prefix length, separated by a
slash "/" character. This setting is mandatory.

[ROUTINGPOLICYRULE] SECTION OPTIONS
An [RoutingPolicyRule] section accepts the following settings. Specify
several [RoutingPolicyRule] sections to configure several rules.

TypeOfService=
Takes a number between 0 and 255 that specifies the type of service
to match.

From=
Specifies the source address prefix to match. Possibly followed by
a slash and the prefix length.

To=
Specifies the destination address prefix to match. Possibly
followed by a slash and the prefix length.

FirewallMark=
Specifies the iptables firewall mark value to match (a number in
the range 1...4294967295). Optionally, the firewall mask (also a
number between 1...4294967295) can be suffixed with a slash ("/"),
e.g., "7/255".

Table=
Specifies the routing table identifier to lookup if the rule
selector matches. Takes one of predefined names "default", "main",
and "local", and names defined in RouteTable= in networkd.conf(5),
or a number between 1 and 4294967295. Defaults to "main".

Priority=
Specifies the priority of this rule. Priority= is an integer in
the range 0...4294967295. Higher number means lower priority, and
rules get processed in order of increasing number. Defaults to
unset, and the kernel will pick a value dynamically.

IncomingInterface=
Specifies incoming device to match. If the interface is loopback,
the rule only matches packets originating from this host.

OutgoingInterface=
Specifies the outgoing device to match. The outgoing interface is
only available for packets originating from local sockets that are
bound to a device.

SourcePort=
Specifies the source IP port or IP port range match in forwarding
information base (FIB) rules. A port range is specified by the
lower and upper port separated by a dash. Defaults to unset.

DestinationPort=
Specifies the destination IP port or IP port range match in
forwarding information base (FIB) rules. A port range is specified
by the lower and upper port separated by a dash. Defaults to unset.

IPProtocol=
Specifies the IP protocol to match in forwarding information base
(FIB) rules. Takes IP protocol name such as "tcp", "udp" or "sctp",
or IP protocol number such as "6" for "tcp" or "17" for "udp".
Defaults to unset.

InvertRule=
A boolean. Specifies whether the rule is to be inverted. Defaults
to false.

Family=
Takes a special value "ipv4", "ipv6", or "both". By default, the
address family is determined by the address specified in To= or
From=. If neither To= nor From= are specified, then defaults to
"ipv4".

User=
Takes a username, a user ID, or a range of user IDs separated by a
dash. Defaults to unset.

SuppressPrefixLength=
Takes a number N in the range 0...128 and rejects routing decisions
that have a prefix length of N or less. Defaults to unset.

SuppressInterfaceGroup=
Takes an integer in the range 0...2147483647 and rejects routing
decisions that have an interface with the same group id. It has the
same meaning as suppress_ifgroup in ip rule. Defaults to unset.

Type=
Specifies Routing Policy Database (RPDB) rule type. Takes one of
"blackhole", "unreachable" or "prohibit".

[NEXTHOP] SECTION OPTIONS
The [NextHop] section is used to manipulate entries in the kernel's
"nexthop" tables. The [NextHop] section accepts the following settings.
Specify several [NextHop] sections to configure several hops.

Id=
The id of the next hop. Takes an integer in the range
1...4294967295. If unspecified, then automatically chosen by
kernel.

Gateway=
As in the [Network] section.

Family=
Takes one of the special values "ipv4" or "ipv6". By default, the
family is determined by the address specified in Gateway=. If
Gateway= is not specified, then defaults to "ipv4".

OnLink=
Takes a boolean. If set to true, the kernel does not have to check
if the gateway is reachable directly by the current machine (i.e.,
attached to the local network), so that we can insert the nexthop
in the kernel table without it being complained about. Defaults to
"no".

Blackhole=
Takes a boolean. If enabled, packets to the corresponding routes
are discarded silently, and Gateway= cannot be specified. Defaults
to "no".

Group=
Takes a whitespace separated list of nexthop IDs. Each ID must be
in the range 1...4294967295. Optionally, each nexthop ID can take a
weight after a colon ("id[:weight]"). The weight must be in the
range 1...255. If the weight is not specified, then it is assumed
that the weight is 1. This setting cannot be specified with
Gateway=, Family=, Blackhole=. This setting can be specified
multiple times. If an empty string is assigned, then the all
previous assignments are cleared. Defaults to unset.

[ROUTE] SECTION OPTIONS
The [Route] section accepts the following settings. Specify several
[Route] sections to configure several routes.

Gateway=
Takes the gateway address or the special values "_dhcp4" and
"_ipv6ra". If "_dhcp4" or "_ipv6ra" is set, then the gateway
address provided by DHCPv4 or IPv6 RA is used.

GatewayOnLink=
Takes a boolean. If set to true, the kernel does not have to check
if the gateway is reachable directly by the current machine (i.e.,
attached to the local network), so that we can insert the route in
the kernel table without it being complained about. Defaults to
"no".

Destination=
The destination prefix of the route. Possibly followed by a slash
and the prefix length. If omitted, a full-length host route is
assumed.

Source=
The source prefix of the route. Possibly followed by a slash and
the prefix length. If omitted, a full-length host route is assumed.

Metric=
The metric of the route. Takes an unsigned integer in the range
0...4294967295. Defaults to unset, and the kernel's default will be
used.

IPv6Preference=
Specifies the route preference as defined in RFC 4191[18] for
Router Discovery messages. Which can be one of "low" the route has
a lowest priority, "medium" the route has a default priority or
"high" the route has a highest priority.

Scope=
The scope of the IPv4 route, which can be "global", "site", "link",
"host", or "nowhere":

o "global" means the route can reach hosts more than one hop
away.

o "site" means an interior route in the local autonomous system.

o "link" means the route can only reach hosts on the local
network (one hop away).

o "host" means the route will not leave the local machine (used
for internal addresses like 127.0.0.1).

o "nowhere" means the destination doesn't exist.

For IPv4 route, defaults to "host" if Type= is "local" or "nat",
and "link" if Type= is "broadcast", "multicast", "anycast", or
"unicast". In other cases, defaults to "global". The value is not
used for IPv6.

PreferredSource=
The preferred source address of the route. The address must be in
the format described in inet_pton(3).

Table=
The table identifier for the route. Takes one of predefined names
"default", "main", and "local", and names defined in RouteTable= in
networkd.conf(5), or a number between 1 and 4294967295. The table
can be retrieved using ip route show table num. If unset and Type=
is "local", "broadcast", "anycast", or "nat", then "local" is used.
In other cases, defaults to "main".

Protocol=
The protocol identifier for the route. Takes a number between 0 and
255 or the special values "kernel", "boot", "static", "ra" and
"dhcp". Defaults to "static".

Type=
Specifies the type for the route. Takes one of "unicast", "local",
"broadcast", "anycast", "multicast", "blackhole", "unreachable",
"prohibit", "throw", "nat", and "xresolve". If "unicast", a regular
route is defined, i.e. a route indicating the path to take to a
destination network address. If "blackhole", packets to the defined
route are discarded silently. If "unreachable", packets to the
defined route are discarded and the ICMP message "Host Unreachable"
is generated. If "prohibit", packets to the defined route are
discarded and the ICMP message "Communication Administratively
Prohibited" is generated. If "throw", route lookup in the current
routing table will fail and the route selection process will return
to Routing Policy Database (RPDB). Defaults to "unicast".

InitialCongestionWindow=
The TCP initial congestion window is used during the start of a TCP
connection. During the start of a TCP session, when a client
requests a resource, the server's initial congestion window
determines how many packets will be sent during the initial burst
of data without waiting for acknowledgement. Takes a number between
1 and 1023. Note that 100 is considered an extremely large value
for this option. When unset, the kernel's default (typically 10)
will be used.

InitialAdvertisedReceiveWindow=
The TCP initial advertised receive window is the amount of receive
data (in bytes) that can initially be buffered at one time on a
connection. The sending host can send only that amount of data
before waiting for an acknowledgment and window update from the
receiving host. Takes a number between 1 and 1023. Note that 100 is
considered an extremely large value for this option. When unset,
the kernel's default will be used.

QuickAck=
Takes a boolean. When true enables TCP quick ack mode for the
route. When unset, the kernel's default will be used.

FastOpenNoCookie=
Takes a boolean. When true enables TCP fastopen without a cookie on
a per-route basis. When unset, the kernel's default will be used.

TTLPropagate=
Takes a boolean. When true enables TTL propagation at Label
Switched Path (LSP) egress. When unset, the kernel's default will
be used.

MTUBytes=
The maximum transmission unit in bytes to set for the route. The
usual suffixes K, M, G, are supported and are understood to the
base of 1024.

TCPAdvertisedMaximumSegmentSize=
Specifies the Path MSS (in bytes) hints given on TCP layer. The
usual suffixes K, M, G, are supported and are understood to the
base of 1024. An unsigned integer in the range 1...4294967294. When
unset, the kernel's default will be used.

TCPCongestionControlAlgorithm=
Specifies the TCP congestion control algorithm for the route. Takes
a name of the algorithm, e.g. "bbr", "dctcp", or "vegas". When
unset, the kernel's default will be used.

MultiPathRoute=address[@name] [weight]
Configures multipath route. Multipath routing is the technique of
using multiple alternative paths through a network. Takes gateway
address. Optionally, takes a network interface name or index
separated with "@", and a weight in 1..256 for this multipath route
separated with whitespace. This setting can be specified multiple
times. If an empty string is assigned, then the all previous
assignments are cleared.

NextHop=
Specifies the nexthop id. Takes an unsigned integer in the range
1...4294967295. If set, the corresponding [NextHop] section must be
configured. Defaults to unset.

[DHCPV4] SECTION OPTIONS
The [DHCPv4] section configures the DHCPv4 client, if it is enabled
with the DHCP= setting described above:

SendHostname=
When true (the default), the machine's hostname (or the value
specified with Hostname=, described below) will be sent to the DHCP
server. Note that the hostname must consist only of 7-bit ASCII
lower-case characters and no spaces or dots, and be formatted as a
valid DNS domain name. Otherwise, the hostname is not sent even if
this option is true.

Hostname=
Use this value for the hostname which is sent to the DHCP server,
instead of machine's hostname. Note that the specified hostname
must consist only of 7-bit ASCII lower-case characters and no
spaces or dots, and be formatted as a valid DNS domain name.

MUDURL=
When configured, the specified Manufacturer Usage Description (MUD)
URL will be sent to the DHCPv4 server. Takes a URL of length up to
255 characters. A superficial verification that the string is a
valid URL will be performed. DHCPv4 clients are intended to have at
most one MUD URL associated with them. See RFC 8520[19].

MUD is an embedded software standard defined by the IETF that
allows IoT device makers to advertise device specifications,
including the intended communication patterns for their device when
it connects to the network. The network can then use this to author
a context-specific access policy, so the device functions only
within those parameters.

ClientIdentifier=
The DHCPv4 client identifier to use. Takes one of mac, duid or
duid-only. If set to mac, the MAC address of the link is used. If
set to duid, an RFC4361-compliant Client ID, which is the
combination of IAID and DUID (see below), is used. If set to
duid-only, only DUID is used, this may not be RFC compliant, but
some setups may require to use this. Defaults to duid.

VendorClassIdentifier=
The vendor class identifier used to identify vendor type and
configuration.

UserClass=
A DHCPv4 client can use UserClass option to identify the type or
category of user or applications it represents. The information
contained in this option is a string that represents the user class
of which the client is a member. Each class sets an identifying
string of information to be used by the DHCP service to classify
clients. Takes a whitespace-separated list of strings.

DUIDType=
Override the global DUIDType= setting for this network. See
networkd.conf(5) for a description of possible values.

DUIDRawData=
Override the global DUIDRawData= setting for this network. See
networkd.conf(5) for a description of possible values.

IAID=
The DHCP Identity Association Identifier (IAID) for the interface,
a 32-bit unsigned integer.

Anonymize=
Takes a boolean. When true, the options sent to the DHCP server
will follow the RFC 7844[20] (Anonymity Profiles for DHCP Clients)
to minimize disclosure of identifying information. Defaults to
false.

This option should only be set to true when MACAddressPolicy= is
set to random (see systemd.link(5)).

When true, ClientIdentifier=mac, SendHostname=no, Use6RD=no,
UseCaptivePortal=no, UseMTU=no, UseNTP=no, UseSIP=no, and
UseTimezone=no are implied and these settings in the .network file
are silently ignored. Also, Hostname=, MUDURL=, RequestOptions=,
SendOption=, SendVendorOption=, UserClass=, and
VendorClassIdentifier= are silently ignored.

With this option enabled DHCP requests will mimic those generated
by Microsoft Windows, in order to reduce the ability to fingerprint
and recognize installations. This means DHCP request sizes will
grow and lease data will be more comprehensive than normally,
though most of the requested data is not actually used.

RequestOptions=
Sets request options to be sent to the server in the DHCPv4 request
options list. A whitespace-separated list of integers in the range
1...254. Defaults to unset.

SendOption=
Send an arbitrary raw option in the DHCPv4 request. Takes a DHCP
option number, data type and data separated with a colon
("option:type:value"). The option number must be an integer in the
range 1...254. The type takes one of "uint8", "uint16", "uint32",
"ipv4address", or "string". Special characters in the data string
may be escaped using C-style escapes[21]. This setting can be
specified multiple times. If an empty string is specified, then all
options specified earlier are cleared. Defaults to unset.

SendVendorOption=
Send an arbitrary vendor option in the DHCPv4 request. Takes a DHCP
option number, data type and data separated with a colon
("option:type:value"). The option number must be an integer in the
range 1...254. The type takes one of "uint8", "uint16", "uint32",
"ipv4address", or "string". Special characters in the data string
may be escaped using C-style escapes[21]. This setting can be
specified multiple times. If an empty string is specified, then all
options specified earlier are cleared. Defaults to unset.

IPServiceType=
Takes one of the special values "none", "CS6", or "CS4". When
"none" no IP service type is set to the packet sent from the DHCPv4
client. When "CS6" (network control) or "CS4" (realtime), the
corresponding service type will be set. Defaults to "CS6".

Label=
Specifies the label for the IPv4 address received from the DHCP
server. The label must be a 7-bit ASCII string with a length of
1...15 characters. Defaults to unset.

UseDNS=
When true (the default), the DNS servers received from the DHCP
server will be used.

This corresponds to the nameserver option in resolv.conf(5).

RoutesToDNS=
When true, the routes to the DNS servers received from the DHCP
server will be configured. When UseDNS= is disabled, this setting
is ignored. Defaults to true.

UseNTP=
When true (the default), the NTP servers received from the DHCP
server will be used by systemd-timesyncd.service.

RoutesToNTP=
When true, the routes to the NTP servers received from the DHCP
server will be configured. When UseNTP= is disabled, this setting
is ignored. Defaults to true.

UseSIP=
When true (the default), the SIP servers received from the DHCP
server will be collected and made available to client programs.

UseMTU=
When true, the interface maximum transmission unit from the DHCP
server will be used on the current link. If MTUBytes= is set, then
this setting is ignored. Defaults to false.

Note, some drivers will reset the interfaces if the MTU is changed.
For such interfaces, please try to use IgnoreCarrierLoss= with a
short timespan, e.g. "3 seconds".

UseHostname=
When true (the default), the hostname received from the DHCP server
will be set as the transient hostname of the system.

UseDomains=
Takes a boolean, or the special value route. When true, the domain
name received from the DHCP server will be used as DNS search
domain over this link, similarly to the effect of the Domains=
setting. If set to route, the domain name received from the DHCP
server will be used for routing DNS queries only, but not for
searching, similarly to the effect of the Domains= setting when the
argument is prefixed with "~". Defaults to false.

It is recommended to enable this option only on trusted networks,
as setting this affects resolution of all hostnames, in particular
of single-label names. It is generally safer to use the supplied
domain only as routing domain, rather than as search domain, in
order to not have it affect local resolution of single-label names.

When set to true, this setting corresponds to the domain option in
resolv.conf(5).

UseRoutes=
When true (the default), the static routes will be requested from
the DHCP server and added to the routing table with a metric of
1024, and a scope of global, link or host, depending on the route's
destination and gateway. If the destination is on the local host,
e.g., 127.x.x.x, or the same as the link's own address, the scope
will be set to host. Otherwise if the gateway is null (a direct
route), a link scope will be used. For anything else, scope
defaults to global.

RouteMetric=
Set the routing metric for routes specified by the DHCP server
(including the prefix route added for the specified prefix). Takes
an unsigned integer in the range 0...4294967295. Defaults to 1024.

RouteTable=num
The table identifier for DHCP routes. Takes one of predefined names
"default", "main", and "local", and names defined in RouteTable= in
networkd.conf(5), or a number between 1...4294967295.

When used in combination with VRF=, the VRF's routing table is used
when this parameter is not specified.

RouteMTUBytes=
Specifies the MTU for the DHCP routes. Please see the [Route]
section for further details.

UseGateway=
When true, the gateway will be requested from the DHCP server and
added to the routing table with a metric of 1024, and a scope of
link. When unset, the value specified with UseRoutes= is used.

UseTimezone=
When true, the timezone received from the DHCP server will be set
as timezone of the local system. Defaults to false.

Use6RD=
When true, subnets of the received IPv6 prefix are assigned to
downstream interfaces which enables DHCPPrefixDelegation=. See also
DHCPPrefixDelegation= in the [Network] section, the
[DHCPPrefixDelegation] section, and RFC 5969[22]. Defaults to
false.

FallbackLeaseLifetimeSec=
Allows one to set DHCPv4 lease lifetime when DHCPv4 server does not
send the lease lifetime. Takes one of "forever" or "infinity". If
specified, the acquired address never expires. Defaults to unset.

RequestBroadcast=
Request the server to use broadcast messages before the IP address
has been configured. This is necessary for devices that cannot
receive RAW packets, or that cannot receive packets at all before
an IP address has been configured. On the other hand, this must not
be enabled on networks where broadcasts are filtered out.

MaxAttempts=
Specifies how many times the DHCPv4 client configuration should be
attempted. Takes a number or "infinity". Defaults to "infinity".
Note that the time between retries is increased exponentially, up
to approximately one per minute, so the network will not be
overloaded even if this number is high. The default is suitable in
most circumstances.

ListenPort=
Set the port from which the DHCP client packets originate.

DenyList=
A whitespace-separated list of IPv4 addresses. Each address can
optionally take a prefix length after "/". DHCP offers from servers
in the list are rejected. Note that if AllowList= is configured
then DenyList= is ignored.

AllowList=
A whitespace-separated list of IPv4 addresses. Each address can
optionally take a prefix length after "/". DHCP offers from servers
in the list are accepted.

SendRelease=
When true, the DHCPv4 client sends a DHCP release packet when it
stops. Defaults to true.

SendDecline=
A boolean. When true, systemd-networkd performs IPv4 Duplicate
Address Detection to the acquired address by the DHCPv4 client. If
duplicate is detected, the DHCPv4 client rejects the address by
sending a DHCPDECLINE packet to the DHCP server, and tries to
obtain an IP address again. See RFC 5227[11]. Defaults to false.

NetLabel=
This applies the NetLabel for the addresses received with DHCP,
like NetLabel= in [Address] section applies it to statically
configured addresses. See NetLabel= in [Address] section for more
details.

[DHCPV6] SECTION OPTIONS
The [DHCPv6] section configures the DHCPv6 client, if it is enabled
with the DHCP= setting described above, or invoked by the IPv6 Router
Advertisement:

MUDURL=, IAID=, DUIDType=, DUIDRawData=, RequestOptions=
As in the [DHCPv4] section.

SendOption=
As in the [DHCPv4] section, however because DHCPv6 uses 16-bit
fields to store option numbers, the option number is an integer in
the range 1...65536.

SendVendorOption=
Send an arbitrary vendor option in the DHCPv6 request. Takes an
enterprise identifier, DHCP option number, data type, and data
separated with a colon ("enterprise identifier:option:type:value").
Enterprise identifier is an unsigned integer in the range
1...4294967294. The option number must be an integer in the range
1...254. Data type takes one of "uint8", "uint16", "uint32",
"ipv4address", "ipv6address", or "string". Special characters in
the data string may be escaped using C-style escapes[21]. This
setting can be specified multiple times. If an empty string is
specified, then all options specified earlier are cleared. Defaults
to unset.

UserClass=
A DHCPv6 client can use User Class option to identify the type or
category of user or applications it represents. The information
contained in this option is a string that represents the user class
of which the client is a member. Each class sets an identifying
string of information to be used by the DHCP service to classify
clients. Special characters in the data string may be escaped using
C-style escapes[21]. This setting can be specified multiple times.
If an empty string is specified, then all options specified earlier
are cleared. Takes a whitespace-separated list of strings. Note
that currently NUL bytes are not allowed.

VendorClass=
A DHCPv6 client can use VendorClass option to identify the vendor
that manufactured the hardware on which the client is running. The
information contained in the data area of this option is contained
in one or more opaque fields that identify details of the hardware
configuration. Takes a whitespace-separated list of strings.

PrefixDelegationHint=
Takes an IPv6 address with prefix length in the same format as the
Address= in the [Network] section. The DHCPv6 client will include a
prefix hint in the DHCPv6 solicitation sent to the server. The
prefix length must be in the range 1...128. Defaults to unset.

RapidCommit=
Takes a boolean. The DHCPv6 client can obtain configuration
parameters from a DHCPv6 server through a rapid two-message
exchange (solicit and reply). When the rapid commit option is set
by both the DHCPv6 client and the DHCPv6 server, the two-message
exchange is used. Otherwise, the four-message exchange (solicit,
advertise, request, and reply) is used. The two-message exchange
provides faster client configuration. See RFC 3315[23] for details.
Defaults to true, and the two-message exchange will be used if the
server support it.

UseAddress=
When true (the default), the IP addresses provided by the DHCPv6
server will be assigned.

UseDelegatedPrefix=
When true (the default), the client will request the DHCPv6 server
to delegate prefixes. If the server provides prefixes to be
delegated, then subnets of the prefixes are assigned to the
interfaces that have DHCPPrefixDelegation=yes. See also the
DHCPPrefixDelegation= setting in the [Network] section, settings in
the [DHCPPrefixDelegation] section, and RFC 8415[24].

UseDNS=, UseNTP=, UseHostname=, UseDomains=, NetLabel=
As in the [DHCPv4] section.

WithoutRA=
Allows DHCPv6 client to start without router advertisements's
"managed" or "other configuration" flag. Takes one of "no",
"solicit", or "information-request". If this is not specified,
"solicit" is used when DHCPPrefixDelegation= is enabled and
UplinkInterface=:self is specified in the [DHCPPrefixDelegation]
section. Otherwise, defaults to "no", and the DHCPv6 client will be
started when an RA is received. See also the DHCPv6Client= setting
in the [IPv6AcceptRA] section.

[DHCPPREFIXDELEGATION] SECTION OPTIONS
The [DHCPPrefixDelegation] section configures subnet prefixes of the
delegated prefixes acquired by a DHCPv6 client, or by a DHCPv4 client
through the 6RD option on another interface. The settings in this
section are used only when the DHCPPrefixDelegation= setting in the
[Network] section is enabled.

UplinkInterface=
Specifies the name or the index of the uplink interface, or one of
the special values ":self" and ":auto". When ":self", the interface
itself is considered the uplink interface, and WithoutRA=solicit is
implied if the setting is not explicitly specified. When ":auto",
the first link which acquired prefixes to be delegated from the
DHCPv6 or DHCPv4 server is selected. Defaults to ":auto".

SubnetId=
Configure a specific subnet ID on the interface from a (previously)
received prefix delegation. You can either set "auto" (the default)
or a specific subnet ID (as defined in RFC 4291[25], section
2.5.4), in which case the allowed value is hexadecimal, from 0 to
0x7fffffffffffffff inclusive.

Announce=
Takes a boolean. When enabled, and IPv6SendRA= in [Network] section
is enabled, the delegated prefixes are distributed through the IPv6
Router Advertisement. This setting will be ignored when the
DHCPPrefixDelegation= setting is enabled on the upstream interface.
Defaults to yes.

Assign=
Takes a boolean. Specifies whether to add an address from the
delegated prefixes which are received from the WAN interface by the
DHCPv6 Prefix Delegation. When true (on LAN interfce), the EUI-64
algorithm will be used by default to form an interface identifier
from the delegated prefixes. See also Token= setting below.
Defaults to yes.

Token=
Specifies an optional address generation mode for assigning an
address in each delegated prefix. This accepts the same syntax as
Token= in the [IPv6AcceptRA] section. If Assign= is set to false,
then this setting will be ignored. Defaults to unset, which means
the EUI-64 algorithm will be used.

ManageTemporaryAddress=
As in the [Address] section, but defaults to true.

RouteMetric=
The metric of the route to the delegated prefix subnet. Takes an
unsigned integer in the range 0...4294967295. When set to 0, the
kernel's default value is used. Defaults to 256.

[IPV6ACCEPTRA] SECTION OPTIONS
The [IPv6AcceptRA] section configures the IPv6 Router Advertisement
(RA) client, if it is enabled with the IPv6AcceptRA= setting described
above:

Token=
Specifies an optional address generation mode for the Stateless
Address Autoconfiguration (SLAAC). The following values are
supported:

eui64
The EUI-64 algorithm will be used to generate an address for
that prefix. Only supported by Ethernet or InfiniBand
interfaces.

static:ADDRESS
An IPv6 address must be specified after a colon (":"), and the
lower bits of the supplied address are combined with the upper
bits of a prefix received in a Router Advertisement (RA)
message to form a complete address. Note that if multiple
prefixes are received in an RA message, or in multiple RA
messages, addresses will be formed from each of them using the
supplied address. This mode implements SLAAC but uses a static
interface identifier instead of an identifier generated by
using the EUI-64 algorithm. Because the interface identifier is
static, if Duplicate Address Detection detects that the
computed address is a duplicate (in use by another node on the
link), then this mode will fail to provide an address for that
prefix. If an IPv6 address without mode is specified, then
"static" mode is assumed.

prefixstable[:ADDRESS][,UUID]
The algorithm specified in RFC 7217[26] will be used to
generate interface identifiers. This mode can optionally take
an IPv6 address separated with a colon (":"). If an IPv6
address is specified, then an interface identifier is generated
only when a prefix received in an RA message matches the
supplied address.

This mode can also optionally take a non-null UUID in the
format which sd_id128_from_string() accepts, e.g.
"86b123b969ba4b7eb8b3d8605123525a" or
"86b123b9-69ba-4b7e-b8b3-d8605123525a". If a UUID is specified,
the value is used as the secret key to generate interface
identifiers. If not specified, then an application specific ID
generated with the system's machine-ID will be used as the
secret key. See sd-id128(3), sd_id128_from_string(3), and
sd_id128_get_machine(3).

Note that the "prefixstable" algorithm uses both the interface
name and MAC address as input to the hash to compute the
interface identifier, so if either of those are changed the
resulting interface identifier (and address) will be changed,
even if the prefix received in the RA message has not been
changed.

If no address generation mode is specified (which is the default),
or a received prefix does not match any of the addresses provided
in "prefixstable" mode, then the EUI-64 algorithm will be used for
Ethernet or InfiniBand interfaces, otherwise "prefixstable" will be
used to form an interface identifier for that prefix.

This setting can be specified multiple times. If an empty string is
assigned, then the all previous assignments are cleared.

Examples:

Token=eui64
Token=::1a:2b:3c:4d
Token=static:::1a:2b:3c:4d
Token=prefixstable
Token=prefixstable:2002:da8:1::

UseDNS=
When true (the default), the DNS servers received in the Router
Advertisement will be used.

This corresponds to the nameserver option in resolv.conf(5).

UseDomains=
Takes a boolean, or the special value "route". When true, the
domain name received via IPv6 Router Advertisement (RA) will be
used as DNS search domain over this link, similarly to the effect
of the Domains= setting. If set to "route", the domain name
received via IPv6 RA will be used for routing DNS queries only, but
not for searching, similarly to the effect of the Domains= setting
when the argument is prefixed with "~". Defaults to false.

When set to true, this setting corresponds to the domain option in
resolv.conf(5).

RouteTable=num
The table identifier for the routes received in the Router
Advertisement. Takes one of predefined names "default", "main", and
"local", and names defined in RouteTable= in networkd.conf(5), or a
number between 1...4294967295.

When used in combination with VRF=, the VRF's routing table is used
when this parameter is not specified.

RouteMetric=
Set the routing metric for the routes received in the Router
Advertisement. Takes an unsigned integer in the range
0...4294967295. Defaults to 1024.

UseMTU=
Takes a boolean. When true, the MTU received in the Router
Advertisement will be used. Defaults to true.

UseGateway=
When true (the default), the router address will be configured as
the default gateway.

UseRoutePrefix=
When true (the default), the routes corresponding to the route
prefixes received in the Router Advertisement will be configured.

UseAutonomousPrefix=
When true (the default), the autonomous prefix received in the
Router Advertisement will be used and take precedence over any
statically configured ones.

UseOnLinkPrefix=
When true (the default), the onlink prefix received in the Router
Advertisement will be used and takes precedence over any statically
configured ones.

RouterDenyList=
A whitespace-separated list of IPv6 router addresses. Each address
can optionally take a prefix length after "/". Any information
advertised by the listed router is ignored.

RouterAllowList=
A whitespace-separated list of IPv6 router addresses. Each address
can optionally take a prefix length after "/". Only information
advertised by the listed router is accepted. Note that if
RouterAllowList= is configured then RouterDenyList= is ignored.

PrefixDenyList=
A whitespace-separated list of IPv6 prefixes. Each prefix can
optionally take its prefix length after "/". IPv6 prefixes supplied
via router advertisements in the list are ignored.

PrefixAllowList=
A whitespace-separated list of IPv6 prefixes. Each prefix can
optionally take its prefix length after "/". IPv6 prefixes supplied
via router advertisements in the list are allowed. Note that if
PrefixAllowList= is configured then PrefixDenyList= is ignored.

RouteDenyList=
A whitespace-separated list of IPv6 route prefixes. Each prefix can
optionally take its prefix length after "/". IPv6 route prefixes
supplied via router advertisements in the list are ignored.

RouteAllowList=
A whitespace-separated list of IPv6 route prefixes. Each prefix can
optionally take its prefix length after "/". IPv6 route prefixes
supplied via router advertisements in the list are allowed. Note
that if RouteAllowList= is configured then RouteDenyList= is
ignored.

DHCPv6Client=
Takes a boolean, or the special value "always". When true, the
DHCPv6 client will be started in "solicit" mode if the RA has the
"managed" flag or "information-request" mode if the RA lacks the
"managed" flag but has the "other configuration" flag. If set to
"always", the DHCPv6 client will be started in "solicit" mode when
an RA is received, even if neither the "managed" nor the "other
configuration" flag is set in the RA. This will be ignored when
WithoutRA= in the [DHCPv6] section is enabled, or
UplinkInterface=:self in the [DHCPPrefixDelegation] section is
specified. Defaults to true.

NetLabel=
This applies the NetLabel for the addresses received with RA, like
NetLabel= in [Address] section applies it to statically configured
addresses. See NetLabel= in [Address] section for more details.

[DHCPSERVER] SECTION OPTIONS
The [DHCPServer] section contains settings for the DHCP server, if
enabled via the DHCPServer= option described above:

ServerAddress=
Specifies server address for the DHCP server. Takes an IPv4 address
with prefix length, for example 192.168.0.1/24. This setting may be
useful when the link on which the DHCP server is running has
multiple static addresses. When unset, one of static addresses in
the link will be automatically selected. Defaults to unset.

PoolOffset=, PoolSize=
Configures the pool of addresses to hand out. The pool is a
contiguous sequence of IP addresses in the subnet configured for
the server address, which does not include the subnet nor the
broadcast address. PoolOffset= takes the offset of the pool from
the start of subnet, or zero to use the default value. PoolSize=
takes the number of IP addresses in the pool or zero to use the
default value. By default, the pool starts at the first address
after the subnet address and takes up the rest of the subnet,
excluding the broadcast address. If the pool includes the server
address (the default), this is reserved and not handed out to
clients.

DefaultLeaseTimeSec=, MaxLeaseTimeSec=
Control the default and maximum DHCP lease time to pass to clients.
These settings take time values in seconds or another common time
unit, depending on the suffix. The default lease time is used for
clients that did not ask for a specific lease time. If a client
asks for a lease time longer than the maximum lease time, it is
automatically shortened to the specified time. The default lease
time defaults to 1h, the maximum lease time to 12h. Shorter lease
times are beneficial if the configuration data in DHCP leases
changes frequently and clients shall learn the new settings with
shorter latencies. Longer lease times reduce the generated DHCP
network traffic.

UplinkInterface=
Specifies the name or the index of the uplink interface, or one of
the special values ":none" and ":auto". When emitting DNS, NTP, or
SIP servers is enabled but no servers are specified, the servers
configured in the uplink interface will be emitted. When ":auto",
the link which has a default gateway with the highest priority will
be automatically selected. When ":none", no uplink interface will
be selected. Defaults to ":auto".

EmitDNS=, DNS=
EmitDNS= takes a boolean. Configures whether the DHCP leases handed
out to clients shall contain DNS server information. Defaults to
"yes". The DNS servers to pass to clients may be configured with
the DNS= option, which takes a list of IPv4 addresses, or special
value "_server_address" which will be converted to the address used
by the DHCP server.

If the EmitDNS= option is enabled but no servers configured, the
servers are automatically propagated from an "uplink" interface
that has appropriate servers set. The "uplink" interface is
determined by the default route of the system with the highest
priority. Note that this information is acquired at the time the
lease is handed out, and does not take uplink interfaces into
account that acquire DNS server information at a later point. If no
suitable uplink interface is found the DNS server data from
/etc/resolv.conf is used. Also, note that the leases are not
refreshed if the uplink network configuration changes. To ensure
clients regularly acquire the most current uplink DNS server
information, it is thus advisable to shorten the DHCP lease time
via MaxLeaseTimeSec= described above.

This setting can be specified multiple times. If an empty string is
specified, then all DNS servers specified earlier are cleared.

EmitNTP=, NTP=, EmitSIP=, SIP=, EmitPOP3=, POP3=, EmitSMTP=, SMTP=,
EmitLPR=, LPR=
Similar to the EmitDNS= and DNS= settings described above, these
settings configure whether and what server information for the
indicate protocol shall be emitted as part of the DHCP lease. The
same syntax, propagation semantics and defaults apply as for
EmitDNS= and DNS=.

EmitRouter=, Router=
The EmitRouter= setting takes a boolean value, and configures
whether the DHCP lease should contain the router option. The
Router= setting takes an IPv4 address, and configures the router
address to be emitted. When the Router= setting is not specified,
then the server address will be used for the router option. When
the EmitRouter= setting is disabled, the Router= setting will be
ignored. The EmitRouter= setting defaults to true, and the Router=
setting defaults to unset.

EmitTimezone=, Timezone=
Takes a boolean. Configures whether the DHCP leases handed out to
clients shall contain timezone information. Defaults to "yes". The
Timezone= setting takes a timezone string (such as "Europe/Berlin"
or "UTC") to pass to clients. If no explicit timezone is set, the
system timezone of the local host is propagated, as determined by
the /etc/localtime symlink.

BootServerAddress=
Takes an IPv4 address of the boot server used by e.g. PXE boot
systems. When specified, this address is sent in the siaddr field
of the DHCP message header. See RFC 2131[27] for more details.
Defaults to unset.

BootServerName=
Takes a name of the boot server used by e.g. PXE boot systems. When
specified, this name is sent in the DHCP option 66 ("TFTP server
name"). See RFC 2132[28] for more details. Defaults to unset.

Note that typically setting one of BootServerName= or
BootServerAddress= is sufficient, but both can be set too, if
desired.

BootFilename=
Takes a path or URL to a file loaded by e.g. a PXE boot loader.
When specified, this path is sent in the DHCP option 67 ("Bootfile
name"). See RFC 2132[28] for more details. Defaults to unset.

SendOption=
Send a raw option with value via DHCPv4 server. Takes a DHCP option
number, data type and data ("option:type:value"). The option number
is an integer in the range 1...254. The type takes one of "uint8",
"uint16", "uint32", "ipv4address", "ipv6address", or "string".
Special characters in the data string may be escaped using C-style
escapes[21]. This setting can be specified multiple times. If an
empty string is specified, then all options specified earlier are
cleared. Defaults to unset.

SendVendorOption=
Send a vendor option with value via DHCPv4 server. Takes a DHCP
option number, data type and data ("option:type:value"). The option
number is an integer in the range 1...254. The type takes one of
"uint8", "uint16", "uint32", "ipv4address", or "string". Special
characters in the data string may be escaped using C-style
escapes[21]. This setting can be specified multiple times. If an
empty string is specified, then all options specified earlier are
cleared. Defaults to unset.

BindToInterface=
Takes a boolean value. When "yes", DHCP server socket will be bound
to its network interface and all socket communication will be
restricted to this interface. Defaults to "yes", except if
RelayTarget= is used (see below), in which case it defaults to
"no".

RelayTarget=
Takes an IPv4 address, which must be in the format described in
inet_pton(3). Turns this DHCP server into a DHCP relay agent. See
RFC 1542[29]. The address is the address of DHCP server or another
relay agent to forward DHCP messages to and from.

RelayAgentCircuitId=
Specifies value for Agent Circuit ID suboption of Relay Agent
Information option. Takes a string, which must be in the format
"string:value", where "value" should be replaced with the value of
the suboption. Defaults to unset (means no Agent Circuit ID
suboption is generated). Ignored if RelayTarget= is not specified.

RelayAgentRemoteId=
Specifies value for Agent Remote ID suboption of Relay Agent
Information option. Takes a string, which must be in the format
"string:value", where "value" should be replaced with the value of
the suboption. Defaults to unset (means no Agent Remote ID
suboption is generated). Ignored if RelayTarget= is not specified.

[DHCPSERVERSTATICLEASE] SECTION OPTIONS
The "[DHCPServerStaticLease]" section configures a static DHCP lease to
assign a fixed IPv4 address to a specific device based on its MAC
address. This section can be specified multiple times.

MACAddress=
The hardware address of a device to match. This key is mandatory.

Address=
The IPv4 address that should be assigned to the device that was
matched with MACAddress=. This key is mandatory.

[IPV6SENDRA] SECTION OPTIONS
The [IPv6SendRA] section contains settings for sending IPv6 Router
Advertisements and whether to act as a router, if enabled via the
IPv6SendRA= option described above. IPv6 network prefixes or routes are
defined with one or more [IPv6Prefix] or [IPv6RoutePrefix] sections.

Managed=, OtherInformation=
Takes a boolean. Controls whether a DHCPv6 server is used to
acquire IPv6 addresses on the network link when Managed= is set to
"true" or if only additional network information can be obtained
via DHCPv6 for the network link when OtherInformation= is set to
"true". Both settings default to "false", which means that a DHCPv6
server is not being used.

RouterLifetimeSec=
Takes a timespan. Configures the IPv6 router lifetime in seconds.
The value must be 0 seconds, or between 4 seconds and 9000 seconds.
When set to 0, the host is not acting as a router. Defaults to 1800
seconds (30 minutes).

RouterPreference=
Configures IPv6 router preference if RouterLifetimeSec= is
non-zero. Valid values are "high", "medium" and "low", with
"normal" and "default" added as synonyms for "medium" just to make
configuration easier. See RFC 4191[18] for details. Defaults to
"medium".

UplinkInterface=
Specifies the name or the index of the uplink interface, or one of
the special values ":none" and ":auto". When emitting DNS servers
or search domains is enabled but no servers are specified, the
servers configured in the uplink interface will be emitted. When
":auto", the value specified to the same setting in the
[DHCPPrefixDelegation] section will be used if
DHCPPrefixDelegation= is enabled, otherwise the link which has a
default gateway with the highest priority will be automatically
selected. When ":none", no uplink interface will be selected.
Defaults to ":auto".

EmitDNS=, DNS=
DNS= specifies a list of recursive DNS server IPv6 addresses that
are distributed via Router Advertisement messages when EmitDNS= is
true. DNS= also takes special value "_link_local"; in that case
the IPv6 link-local address is distributed. If DNS= is empty, DNS
servers are read from the [Network] section. If the [Network]
section does not contain any DNS servers either, DNS servers from
the uplink interface specified in UplinkInterface= will be used.
When EmitDNS= is false, no DNS server information is sent in Router
Advertisement messages. EmitDNS= defaults to true.

EmitDomains=, Domains=
A list of DNS search domains distributed via Router Advertisement
messages when EmitDomains= is true. If Domains= is empty, DNS
search domains are read from the [Network] section. If the
[Network] section does not contain any DNS search domains either,
DNS search domains from the uplink interface specified in
UplinkInterface= will be used. When EmitDomains= is false, no DNS
search domain information is sent in Router Advertisement messages.
EmitDomains= defaults to true.

DNSLifetimeSec=
Lifetime in seconds for the DNS server addresses listed in DNS= and
search domains listed in Domains=. Defaults to 3600 seconds (one
hour).

[IPV6PREFIX] SECTION OPTIONS
One or more [IPv6Prefix] sections contain the IPv6 prefixes that are
announced via Router Advertisements. See RFC 4861[30] for further
details.

AddressAutoconfiguration=, OnLink=
Takes a boolean to specify whether IPv6 addresses can be
autoconfigured with this prefix and whether the prefix can be used
for onlink determination. Both settings default to "true" in order
to ease configuration.

Prefix=
The IPv6 prefix that is to be distributed to hosts. Similarly to
configuring static IPv6 addresses, the setting is configured as an
IPv6 prefix and its prefix length, separated by a "/" character.
Use multiple [IPv6Prefix] sections to configure multiple IPv6
prefixes since prefix lifetimes, address autoconfiguration and
onlink status may differ from one prefix to another.

PreferredLifetimeSec=, ValidLifetimeSec=
Preferred and valid lifetimes for the prefix measured in seconds.
PreferredLifetimeSec= defaults to 1800 seconds (30 minutes) and
ValidLifetimeSec= defaults to 3600 seconds (one hour).

Assign=
Takes a boolean. When true, adds an address from the prefix.
Default to false.

Token=
Specifies an optional address generation mode for assigning an
address in each prefix. This accepts the same syntax as Token= in
the [IPv6AcceptRA] section. If Assign= is set to false, then this
setting will be ignored. Defaults to unset, which means the EUI-64
algorithm will be used.

RouteMetric=
The metric of the prefix route. Takes an unsigned integer in the
range 0...4294967295. When unset or set to 0, the kernel's default
value is used. This setting is ignored when Assign= is false.

[IPV6ROUTEPREFIX] SECTION OPTIONS
One or more [IPv6RoutePrefix] sections contain the IPv6 prefix routes
that are announced via Router Advertisements. See RFC 4191[18] for
further details.

Route=
The IPv6 route that is to be distributed to hosts. Similarly to
configuring static IPv6 routes, the setting is configured as an
IPv6 prefix routes and its prefix route length, separated by a "/"
character. Use multiple [IPv6RoutePrefix] sections to configure
multiple IPv6 prefix routes.

LifetimeSec=
Lifetime for the route prefix measured in seconds. LifetimeSec=
defaults to 3600 seconds (one hour).

[BRIDGE] SECTION OPTIONS
The [Bridge] section accepts the following keys:

UnicastFlood=
Takes a boolean. Controls whether the bridge should flood traffic
for which an FDB entry is missing and the destination is unknown
through this port. When unset, the kernel's default will be used.

MulticastFlood=
Takes a boolean. Controls whether the bridge should flood traffic
for which an MDB entry is missing and the destination is unknown
through this port. When unset, the kernel's default will be used.

MulticastToUnicast=
Takes a boolean. Multicast to unicast works on top of the multicast
snooping feature of the bridge. Which means unicast copies are only
delivered to hosts which are interested in it. When unset, the
kernel's default will be used.

NeighborSuppression=
Takes a boolean. Configures whether ARP and ND neighbor suppression
is enabled for this port. When unset, the kernel's default will be
used.

Learning=
Takes a boolean. Configures whether MAC address learning is enabled
for this port. When unset, the kernel's default will be used.

HairPin=
Takes a boolean. Configures whether traffic may be sent back out of
the port on which it was received. When this flag is false, then
the bridge will not forward traffic back out of the receiving port.
When unset, the kernel's default will be used.

Isolated=
Takes a boolean. Configures whether this port is isolated or not.
Within a bridge, isolated ports can only communicate with
non-isolated ports. When set to true, this port can only
communicate with other ports whose Isolated setting is false. When
set to false, this port can communicate with any other ports. When
unset, the kernel's default will be used.

UseBPDU=
Takes a boolean. Configures whether STP Bridge Protocol Data Units
will be processed by the bridge port. When unset, the kernel's
default will be used.

FastLeave=
Takes a boolean. This flag allows the bridge to immediately stop
multicast traffic on a port that receives an IGMP Leave message. It
is only used with IGMP snooping if enabled on the bridge. When
unset, the kernel's default will be used.

AllowPortToBeRoot=
Takes a boolean. Configures whether a given port is allowed to
become a root port. Only used when STP is enabled on the bridge.
When unset, the kernel's default will be used.

ProxyARP=
Takes a boolean. Configures whether proxy ARP to be enabled on this
port. When unset, the kernel's default will be used.

ProxyARPWiFi=
Takes a boolean. Configures whether proxy ARP to be enabled on this
port which meets extended requirements by IEEE 802.11 and Hotspot
2.0 specifications. When unset, the kernel's default will be used.

MulticastRouter=
Configures this port for having multicast routers attached. A port
with a multicast router will receive all multicast traffic. Takes
one of "no" to disable multicast routers on this port, "query" to
let the system detect the presence of routers, "permanent" to
permanently enable multicast traffic forwarding on this port, or
"temporary" to enable multicast routers temporarily on this port,
not depending on incoming queries. When unset, the kernel's default
will be used.

Cost=
Sets the "cost" of sending packets of this interface. Each port in
a bridge may have a different speed and the cost is used to decide
which link to use. Faster interfaces should have lower costs. It is
an integer value between 1 and 65535.

Priority=
Sets the "priority" of sending packets on this interface. Each port
in a bridge may have a different priority which is used to decide
which link to use. Lower value means higher priority. It is an
integer value between 0 to 63. Networkd does not set any default,
meaning the kernel default value of 32 is used.

[BRIDGEFDB] SECTION OPTIONS
The [BridgeFDB] section manages the forwarding database table of a port
and accepts the following keys. Specify several [BridgeFDB] sections to
configure several static MAC table entries.

MACAddress=
As in the [Network] section. This key is mandatory.

Destination=
Takes an IP address of the destination VXLAN tunnel endpoint.

VLANId=
The VLAN ID for the new static MAC table entry. If omitted, no VLAN
ID information is appended to the new static MAC table entry.

VNI=
The VXLAN Network Identifier (or VXLAN Segment ID) to use to
connect to the remote VXLAN tunnel endpoint. Takes a number in the
range 1...16777215. Defaults to unset.

AssociatedWith=
Specifies where the address is associated with. Takes one of "use",
"self", "master" or "router". "use" means the address is in use.
User space can use this option to indicate to the kernel that the
fdb entry is in use. "self" means the address is associated with
the port drivers fdb. Usually hardware. "master" means the address
is associated with master devices fdb. "router" means the
destination address is associated with a router. Note that it's
valid if the referenced device is a VXLAN type device and has route
shortcircuit enabled. Defaults to "self".

OutgoingInterface=
Specifies the name or index of the outgoing interface for the VXLAN
device driver to reach the remote VXLAN tunnel endpoint. Defaults
to unset.

[BRIDGEMDB] SECTION OPTIONS
The [BridgeMDB] section manages the multicast membership entries
forwarding database table of a port and accepts the following keys.
Specify several [BridgeMDB] sections to configure several permanent
multicast membership entries.

MulticastGroupAddress=
Specifies the IPv4 or IPv6 multicast group address to add. This
setting is mandatory.

VLANId=
The VLAN ID for the new entry. Valid ranges are 0 (no VLAN) to
4094. Optional, defaults to 0.

[LLDP] SECTION OPTIONS
The [LLDP] section manages the Link Layer Discovery Protocol (LLDP) and
accepts the following keys:

MUDURL=
When configured, the specified Manufacturer Usage Descriptions
(MUD) URL will be sent in LLDP packets. The syntax and semantics
are the same as for MUDURL= in the [DHCPv4] section described
above.

The MUD URLs received via LLDP packets are saved and can be read
using the sd_lldp_neighbor_get_mud_url() function.

[CAN] SECTION OPTIONS
The [CAN] section manages the Controller Area Network (CAN bus) and
accepts the following keys:

BitRate=
The bitrate of CAN device in bits per second. The usual SI prefixes
(K, M) with the base of 1000 can be used here. Takes a number in
the range 1...4294967295.

SamplePoint=
Optional sample point in percent with one decimal (e.g. "75%",
"87.5%") or permille (e.g. "875"). This will be ignored when
BitRate= is unspecified.

TimeQuantaNSec=, PropagationSegment=, PhaseBufferSegment1=,
PhaseBufferSegment2=, SyncJumpWidth=
Specifies the time quanta, propagation segment, phase buffer
segment 1 and 2, and the synchronization jump width, which allow
one to define the CAN bit-timing in a hardware independent format
as proposed by the Bosch CAN 2.0 Specification. TimeQuantaNSec=
takes a timespan in nanoseconds. PropagationSegment=,
PhaseBufferSegment1=, PhaseBufferSegment2=, and SyncJumpWidth= take
number of time quantum specified in TimeQuantaNSec= and must be an
unsigned integer in the range 0...4294967295. These settings except
for SyncJumpWidth= will be ignored when BitRate= is specified.

DataBitRate=, DataSamplePoint=
The bitrate and sample point for the data phase, if CAN-FD is used.
These settings are analogous to the BitRate= and SamplePoint= keys.

DataTimeQuantaNSec=, DataPropagationSegment=, DataPhaseBufferSegment1=,
DataPhaseBufferSegment2=, DataSyncJumpWidth=
Specifies the time quanta, propagation segment, phase buffer
segment 1 and 2, and the synchronization jump width for the data
phase, if CAN-FD is used. These settings are analogous to the
TimeQuantaNSec= or related settings.

FDMode=
Takes a boolean. When "yes", CAN-FD mode is enabled for the
interface. Note, that a bitrate and optional sample point should
also be set for the CAN-FD data phase using the DataBitRate= and
DataSamplePoint= keys, or DataTimeQuanta= and related settings.

FDNonISO=
Takes a boolean. When "yes", non-ISO CAN-FD mode is enabled for the
interface. When unset, the kernel's default will be used.

RestartSec=
Automatic restart delay time. If set to a non-zero value, a restart
of the CAN controller will be triggered automatically in case of a
bus-off condition after the specified delay time. Subsecond delays
can be specified using decimals (e.g. "0.1s") or a "ms" or "us"
postfix. Using "infinity" or "0" will turn the automatic restart
off. By default automatic restart is disabled.

Termination=
Takes a boolean or a termination resistor value in ohm in the range
0...65535. When "yes", the termination resistor is set to 120 ohm.
When "no" or "0" is set, the termination resistor is disabled. When
unset, the kernel's default will be used.

TripleSampling=
Takes a boolean. When "yes", three samples (instead of one) are
used to determine the value of a received bit by majority rule.
When unset, the kernel's default will be used.

BusErrorReporting=
Takes a boolean. When "yes", reporting of CAN bus errors is
activated (those include single bit, frame format, and bit stuffing
errors, unable to send dominant bit, unable to send recessive bit,
bus overload, active error announcement, error occurred on
transmission). When unset, the kernel's default will be used. Note:
in case of a CAN bus with a single CAN device, sending a CAN frame
may result in a huge number of CAN bus errors.

ListenOnly=
Takes a boolean. When "yes", listen-only mode is enabled. When the
interface is in listen-only mode, the interface neither transmit
CAN frames nor send ACK bit. Listen-only mode is important to debug
CAN networks without interfering with the communication or
acknowledge the CAN frame. When unset, the kernel's default will be
used.

Loopback=
Takes a boolean. When "yes", loopback mode is enabled. When the
loopback mode is enabled, the interface treats messages transmitted
by itself as received messages. The loopback mode is important to
debug CAN networks. When unset, the kernel's defau a boolean. When
true enables TCP fastopen without a cookie on a per-route basis.
When unset, the kernel's default will be used.