--- name: "if_bridge" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/bridge/4" --- # if\_bridge(4) network bridge device Part of [bridge(4)](/bridge/4.md). ## NAME **if\_bridge** – network bridge device ## SYNOPSIS To compile this driver into the kernel, place the following line in your kernel configuration file: **device if\_bridge** Alternatively, to load the driver as a module at boot time, place the following lines in [loader.conf(5)](/loader.conf/5.md): ``` if_bridge_load="YES" bridgestp_load="YES" ``` ## DESCRIPTION The **if\_bridge** driver creates a logical link between two or more IEEE 802 networks that use the same (or “similar enough”) framing format. For example, it is possible to bridge Ethernet and 802.11 networks together, but it is not possible to bridge Ethernet and Token Ring together. Each **if\_bridge** interface is created at runtime using interface cloning. This is most easily done with the [ifconfig(8)](/ifconfig/8.md) **create** command or using the *cloned\_interfaces* variable in [rc.conf(5)](/rc.conf/5.md). When it is created, the **if\_bridge** interface gets assigned a link (MAC) address in the range of universally administered addresses reserved for the FreeBSD Foundation by hashing the host UUID, jail name, and the interface name. If this fails, a random, locally administered address is generated instead. This address is guaranteed to be unique *only* across all **if\_bridge** interfaces on the local machine. Thus you can theoretically have two bridges on different machines with the same link addresses. The address can be changed by assigning the desired link address using [ifconfig(8)](/ifconfig/8.md). If [sysctl(8)](/sysctl/8.md) node *net.link.bridge.inherit\_mac* has a non-zero value, the newly created bridge will inherit the MAC address from its first member instead of choosing a random link-level address. This will provide more predictable bridge MAC addresses without any additional configuration, but currently this feature is known to break some L2 protocols, for example PPPoE that is provided by [ng\_pppoe(4)](/ng_pppoe/4.md) and [ppp(8)](/ppp/8.md). Currently this feature is considered as experimental and is turned off by default. A bridge can be used to provide several services, such as a simple 802.11-to-Ethernet bridge for wireless hosts, or traffic isolation. A bridge works like a switch, forwarding traffic from one interface to another. Multicast and broadcast packets are always forwarded to all interfaces that are part of the bridge. For unicast traffic, the bridge learns which MAC addresses are associated with which interfaces and will forward the traffic selectively. By default the bridge logs MAC address port flapping to [syslog(3)](/syslog/3.md). This behavior can be disabled by setting the [sysctl(8)](/sysctl/8.md) variable *net.link.bridge.log\_mac\_flap* to 0. All the bridged member interfaces need to be up in order to pass network traffic. These can be enabled using [ifconfig(8)](/ifconfig/8.md) or *ifconfig\_*⟨*interface*⟩="up" in [rc.conf(5)](/rc.conf/5.md). The MTU of the first member interface to be added is used as the bridge MTU. All additional members will have their MTU changed to match. If the MTU of a bridge is changed after its creation, the MTU of all member interfaces is also changed to match. The TOE, TSO, TXCSUM and TXCSUM6 capabilities on all interfaces added to the bridge are disabled if any of the interfaces do not support/enable them. The LRO capability is always disabled. All the capabilities are restored when the interface is removed from the bridge. Changing capabilities at run-time may cause NIC reinit and a link flap. The bridge supports “monitor mode”, where the packets are discarded after [bpf(4)](/bpf/4.md) processing, and are not processed or forwarded further. This can be used to multiplex the input of two or more interfaces into a single [bpf(4)](/bpf/4.md) stream. This is useful for reconstructing the traffic for network taps that transmit the RX/TX signals out through two separate interfaces. To allow the host to communicate with bridge members, IP addresses should be assigned to the **if\_bridge** interface itself, not to the bridge's member interfaces. Attempting to assign an IP address to a bridge member interface, or add a member interface with an assigned IP address to a bridge, will return an EINVAL “(Invalid argument”) error. For compatibility with older releases where this was permitted, setting the [sysctl(8)](/sysctl/8.md) variable *net.link.bridge.member\_ifaddrs* to 1 will permit this configuration. This sysctl variable will be removed in FreeBSD 16.0. ## IPV6 SUPPORT **if\_bridge** supports the AF\_INET6 address family on bridge interfaces. The following [rc.conf(5)](/rc.conf/5.md) variable configures an IPv6 link-local address on bridge0 interface: ``` ifconfig_bridge0_ipv6="inet6 auto_linklocal" ``` However, the AF\_INET6 address family has a concept of scope zone. Bridging multiple interfaces changes the zone configuration because multiple links are merged to each other and form a new single link while the member interfaces still work individually. This means each member interface still has a separate link-local scope zone and the **if\_bridge** interface has another single, aggregated link-local scope zone at the same time. This situation is clearly against the description "zones of the same scope cannot overlap" in Section 5, RFC 4007. Although it works in most cases, it can cause some counterintuitive or undesirable behavior in some edge cases when both, the **if\_bridge** interface and one of the member interfaces, have an IPv6 address and applications use both of them. To prevent this situation, **if\_bridge** checks whether a link-local scoped IPv6 address is configured on a member interface to be added and the **if\_bridge** interface. When the **if\_bridge** interface has IPv6 addresses, IPv6 addresses on the member interface will be automatically removed before the interface is added. This behavior can be disabled by setting [sysctl(8)](/sysctl/8.md) variable *net.link.bridge.allow\_llz\_overlap* to 1. Note that ACCEPT\_RTADV and AUTO\_LINKLOCAL interface flags are not enabled by default on **if\_bridge** interfaces even when *net.inet6.ip6.accept\_rtadv* and/or *net.inet6.ip6.auto\_linklocal* is set to 1. ## SPANNING TREE The **if\_bridge** driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with backwards compatibility with the legacy Spanning Tree Protocol (STP). Spanning Tree is used to detect and remove loops in a network topology. RSTP provides faster spanning tree convergence than legacy STP, the protocol will exchange information with neighbouring switches to quickly transition to forwarding without creating loops. The code will default to RSTP mode but will downgrade any port connected to a legacy STP network so is fully backward compatible. A bridge can be forced to operate in STP mode without rapid state transitions via the *proto* command in [ifconfig(8)](/ifconfig/8.md). The bridge can log STP port changes to [syslog(3)](/syslog/3.md) by setting the *net.link.bridge.log\_stp* node using [sysctl(8)](/sysctl/8.md). ## VLAN SUPPORT Virtual LANs (VLANs), defined in the IEEE 802.1Q standard, allow traffic on a bridge to be segregated into separate logical networks which cannot communicate with each other. For example, two interfaces in VLAN 10 would be able to communicate with each other, but not with another interface in VLAN 20. Each VLAN is identified by a number between 1 and 4094 inclusive. By default, all traffic on the bridge is assigned to "VLAN 0", a pseudo-VLAN used for historical compatibility. When VLANs are in use on a bridge, it is recommended to explicitly assign all traffic to a VLAN rather than using VLAN 0. The bridge implements Independent VLAN Learning (IVL), meaning that host addresses are learned separately for each VLAN, and the same host address may exist on several different ports in different VLANs. If a [vlan(4)](/vlan/4.md) interface is configured on an interface which is also an **if\_bridge** member interface, all tagged frames will be processed by the [vlan(4)](/vlan/4.md) interface and will not be visible to the bridge. This configuration is not recommended and may be unsupported in a future release. ### Tagged and untagged traffic Incoming frames on a member interface may be either tagged or untagged. Tagged frames contain an 802.1Q header indicating which VLAN the frame belongs to, while untagged frames do not. When a tagged frame is received, the frame is automatically assigned to the VLAN in the tag (subject to any configured VLAN access list), while untagged frames are assigned to the interface's configured Port VLAN ID (PVID), or to VLAN 0 if no PVID is configured. ### Assigning interfaces to VLANs An interface's PVID may be configured using the [ifconfig(8)](/ifconfig/8.md) **ifuntagged** command: ``` ifconfig bridge0 ifuntagged ix0 10 ``` Or by using the **untagged** option to **addm**: ``` ifconfig bridge0 addm ix0 untagged 10 ``` This will assign all untagged traffic received on the interface to the specified VLAN, and any traffic transmitted on the interface in this VLAN will have its VLAN tag (if present) removed. Conversely, any traffic transmitted on the interface in a different VLAN will have a tag added, to allow the remote system to assign the traffic to the appropriate VLAN. ### Host communication in a VLAN Sometimes it is useful to allow the host itself to communicate in a VLAN, for example to provide routing to other hosts in the VLAN. To do this, create a [vlan(4)](/vlan/4.md) interface on top of the **if\_bridge** interface with the appropriate VLAN tag. For example, to allow the host to communicate in VLAN 10: ``` ifconfig bridge0.10 create inet6 2001:db8::1/64 ``` ### Configuring the VLAN access list (VLAN filtering) For historical reasons, the default **if\_bridge** configuration allows all interfaces to send tagged traffic for any VLAN, meaning that VLANs do not provide security separation. To restrict which interfaces may communicate in which VLANs, enable VLAN filtering on the bridge: ``` ifconfig bridge0 vlanfilter ``` This has the following effects on bridge members: - No untagged frames will be accepted from a member interface unless the interface has a PVID configured. - No tagged frames will be accepted from a member interface unless the VLAN identifier is present in the interface's VLAN access list. - Frames with stacked tags (Q-in-Q) will not be accepted from a member interface unless the **qinq** option (see below) has been configured for that member. To configure the VLAN access list, use the [ifconfig(8)](/ifconfig/8.md) **iftagged**, **+iftagged** or **-iftagged** commands. For example, to allow an interface to communicate in VLANs 10, 20, and any VLAN from 100 to 199: ``` ifconfig bridge0 iftagged ix0 10,20,100-199 ``` ### IEEE 802.1ad (Q-in-Q) configuration IEEE 802.1ad, also called Q-in-Q or “tag stacking”, allows a single Ethernet frame to contain multiple tags. This allows one Ethernet network to transport traffic between endpoints using its own VLAN tags without interfering with any pre-existing tags, and is often used in service provider networks to provide “virtual wire” Ethernet services. When VLAN filtering is enabled, **if\_bridge** does not permit member interfaces to send Q-in-Q frames, because in certain configuration this allows “VLAN-hopping” attacks on the bridge. For example, consider a bridge with port ix0 configured as a tagged port in VLAN 10, and port ix1 configured as untagged in VLAN 10 and tagged in VLAN 20. If ix0 is allowed to send Q-in-Q frames, then it can send a frame with two tags: one for VLAN 10, followed by one for VLAN 20. When the bridge forwards the frame to ix1, it will strip the VLAN tag for VLAN 10, then forward the frame to ix1 with the tag for VLAN 20 intact, effectively allowing ix1 to send traffic on VLAN 20 even though the bridge configuration should not permit that. To permit an interface to send Q-in-Q frames, set the [ifconfig(8)](/ifconfig/8.md) **qinq** flag on the interface. This is only required on the interface which will send Q-in-Q frames, not the interface receiving the frames. Alternatively, set the **defqinq** flag on the bridge itself to enable Q-in-Q for all newly-added interfaces by default. ## PACKET FILTERING Packet filtering can be used with any firewall package that hooks in via the [pfil(9)](/pfil/9.md) framework. When filtering is enabled, bridged packets will pass through the filter inbound on the originating interface, on the bridge interface and outbound on the appropriate interfaces. Either stage can be disabled. The filtering behavior can be controlled using [sysctl(8)](/sysctl/8.md): - *net.link.bridge.pfil\_onlyip* Controls the handling of non-IP packets which are not passed to [pfil(9)](/pfil/9.md). Set to 1 to only allow IP packets to pass (subject to firewall rules), set to 0 to unconditionally pass all non-IP Ethernet frames. - *net.link.bridge.pfil\_member* Set to 1 to enable filtering on the incoming and outgoing member interfaces, set to 0 to disable it. - *net.link.bridge.pfil\_bridge* Set to 1 to enable filtering on the bridge interface, set to 0 to disable it. - *net.link.bridge.pfil\_local\_phys* Set to 1 to additionally filter on the physical interface for locally destined packets. Set to 0 to disable this feature. - *net.link.bridge.ipfw* Set to 1 to enable layer2 filtering with [ipfirewall(4)](/ipfirewall/4.md), set to 0 to disable it. This needs to be enabled for [dummynet(4)](/dummynet/4.md) support. When *ipfw* is enabled, *pfil\_bridge* and *pfil\_member* will be disabled so that IPFW is not run twice; these can be re-enabled if desired. - *net.link.bridge.ipfw\_arp* Set to 1 to enable layer2 ARP filtering with [ipfirewall(4)](/ipfirewall/4.md), set to 0 to disable it. Requires *ipfw* to be enabled. ARP and REVARP packets are forwarded without being filtered and others that are not IP nor IPv6 packets are not forwarded when *pfil\_onlyip* is enabled. IPFW can filter Ethernet types using **mac-type** so all packets are passed to the filter for processing. The packets originating from the bridging host will be seen by the filter on the interface that is looked up in the routing table. The packets destined to the bridging host will be seen by the filter on the interface with the MAC address equal to the packet's destination MAC. There are situations when some of the bridge members are sharing the same MAC address (for example the [vlan(4)](/vlan/4.md) interfaces: they are currently sharing the MAC address of the parent physical interface). It is not possible to distinguish between these interfaces using their MAC address, excluding the case when the packet's destination MAC address is equal to the MAC address of the interface on which the packet was entered to the system. In this case the filter will see the incoming packet on this interface. In all other cases the interface seen by the packet filter is chosen from the list of bridge members with the same MAC address and the result strongly depends on the member addition sequence and the actual implementation of **if\_bridge**. It is not recommended to rely on the order chosen by the current **if\_bridge** implementation since it may change in the future. The previous paragraph is best illustrated with the following pictures. Let - the MAC address of the incoming packet's destination is **nn:nn:nn:nn:nn:nn**, - the interface on which packet entered the system is **ifX**, - **ifX** MAC address is **xx:xx:xx:xx:xx:xx**, - there are possibly other bridge members with the same MAC address **xx:xx:xx:xx:xx:xx**, - the bridge has more than one interface that are sharing the same MAC address **yy:yy:yy:yy:yy:yy**; we will call them **vlanY1**, **vlanY2**, etc. If the MAC address **nn:nn:nn:nn:nn:nn** is equal to **xx:xx:xx:xx:xx:xx** the filter will see the packet on interface **ifX** no matter if there are any other bridge members carrying the same MAC address. But if the MAC address **nn:nn:nn:nn:nn:nn** is equal to **yy:yy:yy:yy:yy:yy** then the interface that will be seen by the filter is one of the **vlanYn**. It is not possible to predict the name of the actual interface without the knowledge of the system state and the **if\_bridge** implementation details. This problem arises for any bridge members that are sharing the same MAC address, not only to the [vlan(4)](/vlan/4.md) ones: they were taken just as an example of such a situation. So if one wants to filter the locally destined packets based on their interface name, one should be aware of this implication. The described situation will appear at least on the filtering bridges that are doing IP-forwarding; in some of such cases it is better to assign the IP address only to the **if\_bridge** interface and not to the bridge members. Enabling *net.link.bridge.pfil\_local\_phys* will let you do the additional filtering on the physical interface. ## NETMAP [netmap(4)](/netmap/4.md) applications may open a bridge interface in emulated mode. The netmap application will receive all packets which arrive from member interfaces. In particular, packets which would otherwise be forwarded to another member interface will be received by the netmap application. When the [netmap(4)](/netmap/4.md) application transmits a packet to the host stack via the bridge interface, **if\_bridge** receive it and attempts to determine its ‘source’ interface by looking up the source MAC address in the interface's learning tables. Packets for which no matching source interface is found are dropped and the input error counter is incremented. If a matching source interface is found, **if\_bridge** treats the packet as though it was received from the corresponding interface and handles it normally without passing the packet back to [netmap(4)](/netmap/4.md). ## EXAMPLES The following when placed in the file */etc/rc.conf* will cause a bridge called “bridge0” to be created, and will add the interfaces “wlan0” and “fxp0” to the bridge, and then enable packet forwarding. Such a configuration could be used to implement a simple 802.11-to-Ethernet bridge (assuming the 802.11 interface is in ad-hoc mode). ``` cloned_interfaces="bridge0" ifconfig_bridge0="addm wlan0 addm fxp0 up" ``` For the bridge to forward packets, all member interfaces and the bridge need to be up. The above example would also require: ``` create_args_wlan0="wlanmode hostap" ifconfig_wlan0="up ssid my_ap mode 11g" ifconfig_fxp0="up" ``` The following will cause a bridge to be created with two VLANs, 10 and 20, where the “em” interfaces can only communicate in their assigned VLANs, while “ix0” is a trunk port which can communicate in either VLAN: ``` cloned_interfaces="bridge0" ifconfig_bridge0="vlanfilter \ addm em0 untagged 10 \ addm em1 untagged 10 \ addm em2 untagged 20 \ addm em3 untagged 20 \ addm ix0 tagged 10,20" ifconfig_em0="up" ifconfig_em1="up" ifconfig_em2="up" ifconfig_em3="up" ifconfig_ix0="up" ``` The previous example could be extended to allow the host to communicate in VLANs 10 and 20: ``` vlans_bridge0="10 20" ifconfig_bridge0_10_ipv6="inet6 2001:db8:0:10::1/64" ifconfig_bridge0_20_ipv6="inet6 2001:db8:0:20::1/64" ``` Consider a system with two 4-port Ethernet boards. The following will cause a bridge consisting of all 8 ports with Rapid Spanning Tree enabled to be created: ``` ifconfig bridge0 create ifconfig bridge0 \ addm fxp0 stp fxp0 \ addm fxp1 stp fxp1 \ addm fxp2 stp fxp2 \ addm fxp3 stp fxp3 \ addm fxp4 stp fxp4 \ addm fxp5 stp fxp5 \ addm fxp6 stp fxp6 \ addm fxp7 stp fxp7 \ up ``` The bridge can be used as a regular host interface at the same time as bridging between its member ports. In this example, the bridge connects em0 and em1, and will receive its IP address through DHCP: ``` cloned_interfaces="bridge0" ifconfig_bridge0="addm em0 addm em1 DHCP" ifconfig_em0="up" ifconfig_em1="up" ``` The bridge can tunnel Ethernet across an IP internet using the EtherIP protocol. This can be combined with [ipsec(4)](/ipsec/4.md) to provide an encrypted connection. Create a [gif(4)](/gif/4.md) interface and set the local and remote IP addresses for the tunnel, these are reversed on the remote bridge. ``` ifconfig gif0 create ifconfig gif0 tunnel 1.2.3.4 5.6.7.8 up ifconfig bridge0 create ifconfig bridge0 addm fxp0 addm gif0 up ``` ## SEE ALSO [gif(4)](/gif/4.md), [ipf(4)](/ipf/4.md), [ipfw(4)](/ipfw/4.md), [netmap(4)](/netmap/4.md), [pf(4)](/pf/4.md), [vlan(4)](/vlan/4.md), [ifconfig(8)](/ifconfig/8.md) ## HISTORY The **if\_bridge** driver first appeared in FreeBSD 6.0. ## AUTHORS The **bridge** driver was originally written by Jason L. Wright ⟨*jason@thought.net*⟩ as part of an undergraduate independent study at the University of North Carolina at Greensboro. This version of the **if\_bridge** driver has been heavily modified from the original version by Jason R. Thorpe ⟨*thorpej@wasabisystems.com*⟩. Rapid Spanning Tree Protocol (RSTP) support was added by Andrew Thompson ⟨*thompsa@FreeBSD.org*⟩. ## BUGS The **if\_bridge** driver currently supports only Ethernet and Ethernet-like (e.g., 802.11) network devices, which can be configured with the same MTU size as the bridge device.