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.

Note

Static DHCP assignments depend on the client using its MAC address as the DHCP identifier. This method prevents conflicting leases when copying an instance, and thus makes statically assigned leases work properly.

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	bool	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	bool	IPv4 address	true	Whether to generate filtering firewall rules for this network
ipv4.nat	bool	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	bool	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	bool	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	bool	IPv6 DHCP	false	Whether to allocate addresses using DHCP
ipv6.firewall	bool	IPv6 address	true	Whether to generate filtering firewall rules for this network
ipv6.nat	bool	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	bool	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	bool	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	bool	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:

• How to configure network ACLs

• How to configure network forwards

• How to configure network zones

• How to configure LXD as a BGP server

• How to integrate with systemd-resolved