Bridge network#

As one of the possible network configuration types under LXD, LXD supports creating and managing network bridges.

A network bridge creates a virtual L2 ethernet switch that instance NICs can connect to, making it possible for them to communicate with each other and the host. LXD bridges can leverage underlying native Linux bridges and Open vSwitch.

The bridge network type allows to create an L2 bridge that connects the instances that use it together into a single network L2 segment. Bridges created by LXD are managed, which means that in addition to creating the bridge interface itself, LXD also sets up a local dnsmasq process to provide DHCP, IPv6 route announcements and DNS services to the network. By default, it also performs NAT for the bridge.

See How to configure your firewall for instructions on how to configure your firewall to work with LXD bridge networks.

IPv6 prefix size#

If you’re using IPv6 for your bridge network, you should use a prefix size of 64.

Larger subnets (i.e., using a prefix smaller than 64) should work properly too, but they aren’t typically that useful for SLAAC.

Smaller subnets are in theory possible (when using stateful DHCPv6 for IPv6 allocation), but they aren’t properly supported by dnsmasq and might cause problems. If you must create a smaller subnet, use static allocation or another standalone router advertisement daemon.

Configuration options#

The following configuration key namespaces are currently supported for the bridge network type:

  • bgp (BGP peer configuration)

  • bridge (L2 interface configuration)

  • dns (DNS server and resolution configuration)

  • fan (configuration specific to the Ubuntu FAN overlay)

  • ipv4 (L3 IPv4 configuration)

  • ipv6 (L3 IPv6 configuration)

  • maas (MAAS network identification)

  • security (network ACL configuration)

  • raw (raw configuration file content)

  • tunnel (cross-host tunneling configuration)

  • user (free-form key/value for user metadata)

Note

LXD uses the CIDR notation where network subnet information is required, for example, 192.0.2.0/24 or 2001:db8::/32. This does not apply to cases where a single address is required, for example, local/remote addresses of tunnels, NAT addresses or specific addresses to apply to an instance.

The following configuration options are available for the bridge network type:

Key

Type

Condition

Default

Description

bgp.peers.NAME.address

string

bgp server

-

Peer address (IPv4 or IPv6)

bgp.peers.NAME.asn

integer

bgp server

-

Peer AS number

bgp.peers.NAME.password

string

bgp server

- (no password)

Peer session password (optional)

bgp.ipv4.nexthop

string

bgp server

local address

Override the next-hop for advertised prefixes

bgp.ipv6.nexthop

string

bgp server

local address

Override the next-hop for advertised prefixes

bridge.driver

string

-

native

Bridge driver: native or openvswitch

bridge.external_interfaces

string

-

-

Comma-separated list of unconfigured network interfaces to include in the bridge

bridge.hwaddr

string

-

-

MAC address for the bridge

bridge.mode

string

-

standard

Bridge operation mode: standard or fan

bridge.mtu

integer

-

1500

Bridge MTU (default varies if tunnel or fan setup)

dns.domain

string

-

lxd

Domain to advertise to DHCP clients and use for DNS resolution

dns.mode

string

-

managed

DNS registration mode: none for no DNS record, managed for LXD-generated static records or dynamic for client-generated records

dns.search

string

-

-

Full comma-separated domain search list, defaulting to dns.domain value

dns.zone.forward

string

-

managed

DNS zone name for forward DNS records

dns.zone.reverse.ipv4

string

-

managed

DNS zone name for IPv4 reverse DNS records

dns.zone.reverse.ipv6

string

-

managed

DNS zone name for IPv6 reverse DNS records

fan.overlay_subnet

string

fan mode

240.0.0.0/8

Subnet to use as the overlay for the FAN (CIDR)

fan.type

string

fan mode

vxlan

Tunneling type for the FAN: vxlan or ipip

fan.underlay_subnet

string

fan mode

auto (on create only)

Subnet to use as the underlay for the FAN (use auto to use default gateway subnet) (CIDR)

ipv4.address

string

standard mode

auto (on create only)

IPv4 address for the bridge (use none to turn off IPv4 or auto to generate a new random unused subnet) (CIDR)

ipv4.dhcp

boolean

ipv4 address

true

Whether to allocate addresses using DHCP

ipv4.dhcp.expiry

string

ipv4 dhcp

1h

When to expire DHCP leases

ipv4.dhcp.gateway

string

ipv4 dhcp

ipv4.address

Address of the gateway for the subnet

ipv4.dhcp.ranges

string

ipv4 dhcp

all addresses

Comma-separated list of IP ranges to use for DHCP (FIRST-LAST format)

ipv4.firewall

boolean

ipv4 address

true

Whether to generate filtering firewall rules for this network

ipv4.nat

boolean

ipv4 address

false

Whether to NAT (if unset when creating the network, set to true for regular bridges when ipv4.address is generated and always for fan bridges)

ipv4.nat.address

string

ipv4 address

-

The source address used for outbound traffic from the bridge

ipv4.nat.order

string

ipv4 address

before

Whether to add the required NAT rules before or after any pre-existing rules

ipv4.ovn.ranges

string

-

-

Comma-separated list of IPv4 ranges to use for child OVN network routers (FIRST-LAST format)

ipv4.routes

string

ipv4 address

-

Comma-separated list of additional IPv4 CIDR subnets to route to the bridge

ipv4.routing

boolean

ipv4 address

true

Whether to route traffic in and out of the bridge

ipv6.address

string

standard mode

auto (on create only)

IPv6 address for the bridge (use none to turn off IPv6 or auto to generate a new random unused subnet) (CIDR)

ipv6.dhcp

boolean

ipv6 address

true

Whether to provide additional network configuration over DHCP

ipv6.dhcp.expiry

string

ipv6 dhcp

1h

When to expire DHCP leases

ipv6.dhcp.ranges

string

ipv6 stateful dhcp

all addresses

Comma-separated list of IPv6 ranges to use for DHCP (FIRST-LAST format)

ipv6.dhcp.stateful

boolean

ipv6 dhcp

false

Whether to allocate addresses using DHCP

ipv6.firewall

boolean

ipv6 address

true

Whether to generate filtering firewall rules for this network

ipv6.nat

boolean

ipv6 address

false

Whether to NAT (if unset when creating the network, set to true when ipv6.address is generated)

ipv6.nat.address

string

ipv6 address

-

The source address used for outbound traffic from the bridge

ipv6.nat.order

string

ipv6 address

before

Whether to add the required NAT rules before or after any pre-existing rules

ipv6.ovn.ranges

string

-

-

Comma-separated list of IPv6 ranges to use for child OVN network routers (FIRST-LAST format)

ipv6.routes

string

ipv6 address

-

Comma-separated list of additional IPv6 CIDR subnets to route to the bridge

ipv6.routing

boolean

ipv6 address

true

Whether to route traffic in and out of the bridge

maas.subnet.ipv4

string

ipv4 address

-

MAAS IPv4 subnet to register instances in (when using network property on NIC)

maas.subnet.ipv6

string

ipv6 address

-

MAAS IPv6 subnet to register instances in (when using network property on NIC)

raw.dnsmasq

string

-

-

Additional dnsmasq configuration to append to the configuration file

security.acls

string

-

-

Comma-separated list of Network ACLs to apply to NICs connected to this network (see Bridge limitations)

security.acls.default.egress.action

string

security.acls

reject

Action to use for egress traffic that doesn’t match any ACL rule

security.acls.default.egress.logged

boolean

security.acls

false

Whether to log egress traffic that doesn’t match any ACL rule

security.acls.default.ingress.action

string

security.acls

reject

Action to use for ingress traffic that doesn’t match any ACL rule

security.acls.default.ingress.logged

boolean

security.acls

false

Whether to log ingress traffic that doesn’t match any ACL rule

tunnel.NAME.group

string

vxlan

239.0.0.1

Multicast address for vxlan (used if local and remote aren’t set)

tunnel.NAME.id

integer

vxlan

0

Specific tunnel ID to use for the vxlan tunnel

tunnel.NAME.interface

string

vxlan

-

Specific host interface to use for the tunnel

tunnel.NAME.local

string

gre or vxlan

-

Local address for the tunnel (not necessary for multicast vxlan)

tunnel.NAME.port

integer

vxlan

0

Specific port to use for the vxlan tunnel

tunnel.NAME.protocol

string

standard mode

-

Tunneling protocol: vxlan or gre

tunnel.NAME.remote

string

gre or vxlan

-

Remote address for the tunnel (not necessary for multicast vxlan)

tunnel.NAME.ttl

integer

vxlan

1

Specific TTL to use for multicast routing topologies

user.*

string

-

-

User-provided free-form key/value pairs

Supported features#

The following features are supported for the bridge network type: