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VEB(4) Device Drivers Manual VEB(4)

veb, vportVirtual Ethernet Bridge network device

pseudo-device veb

The veb pseudo-device supports the creation of a single layer 2 Ethernet network between multiple ports. Ethernet interfaces are added to the bridge to be used as ports. veb takes over the operation of the interfaces that are added as ports and uses them independently of the host network stack. The network stack can be connected to the Ethernet network managed by veb by creating a vport interface and attaching it as a port to the bridge. From the perspective of the host network stack, a vport interface acts as a normal interface connected to an Ethernet network.

veb is a learning bridge that maintains a table of Ethernet addresses and the port that each address is reachable with. The bridge learns about the reachability of Ethernet addresses by reading the source address on packets received by ports, and then entering the address and port into the table dynamically. Static address entries may also be configured in the table, disabling dynamic learning for that address. Ethernet address learning can be disabled on individual ports.

When forwarding a packet, the address table is searched for the destination Ethernet address and the packet is sent to the associated port in the table entry. If no entry is found in the table, or the packet is addressed to a multicast or broadcast address, the packet is flooded to all other ports on the bridge. Flooding of packets to unknown unicast addresses can be disabled on individual ports.

veb provides multiple mechanisms for filtering packets traversing the bridge. By default it filters IEEE 802.1Q VLAN and SVLAN packets, but can be configured to forward them by setting the link0 flag. veb can filter Ethernet packets entering or leaving ports using bridge rules. Ports can be configured as members of protected domains to restrict communication between them.

pf(4) can be used to filter IP packets as they enter or leave the bridge. By default this filtering is disabled, but can be enabled by setting the link1 flag. The exception to this policy is on vport interfaces, where pf(4) runs as packets enter and leave the network stack regardless of the value of the link1 flag. A consequence of this behaviour is that packets traversing vport interfaces appear to travel in the opposite direction to packets travelling over other ports.

veb supports the addition of span ports to the bridge. Span ports transmit a copy of every packet received by the bridge, allowing for passive monitoring of traffic on a separate host.

ipsec(4), options(4), pf(4), hostname.if(5), ifconfig(8), netstart(8)

The veb driver first appeared in OpenBSD 6.9.

David Gwynne <>

February 23, 2021 OpenBSD-6.9