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flesh out veb(4)/vport description a bit, spell out in more words what
is meant by "independently of the host network stack"

feedback jmc dlg

.\" $OpenBSD: veb.4,v 1.3 2021/10/28 15:06:36 sthen Exp $
.\"
.\" Copyright (c) 2021 David Gwynne <dlg@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\"
.Dd $Mdocdate: October 28 2021 $
.Dt VEB 4
.Os
.Sh NAME
.Nm veb ,
.Nm vport
.Nd Virtual Ethernet Bridge network device
.Sh SYNOPSIS
.Cd "pseudo-device veb"
.Sh DESCRIPTION
The
.Nm veb
pseudo-device supports the creation of a single layer 2 Ethernet
network between multiple ports.
Ethernet interfaces are added to the
.Nm veb
bridge to be used as ports.
Unlike
.Xr bridge 4 ,
.Nm veb
takes over the operation of the interfaces that are added as ports.
They are then independent of the host network stack: the individual
Ethernet ports no longer function as independent devices and cannot
be configured with
.Xr inet 4
or
.Xr inet6 4
addresses or other layer-3 features themselves.
.Pp
The Ethernet network managed by
.Nm veb
can be connected to the network stack as a whole by creating a
.Nm vport
interface and attaching it as a port to the bridge.
From the perspective of the host network stack, a
.Nm vport
interface acts as a normal interface connected to an Ethernet
network and can be configured with addresses.
.Pp
.Nm 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.
.Pp
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.
.Pp
.Nm 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.
.Nm 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.
.\" Individual ports can be configured to only allow relaying of IP
.\" (and ARP/RARP) packets by setting the blocknonip flag.
.Pp
.Xr 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
.Nm vport
interfaces, where
.Xr 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
.Nm vport
interfaces appear to travel in the opposite direction to packets
travelling over other ports.
.\" Packets traversing vport interfaces get their direction relative
.\" to the host network stack, while other ports get their direction
.\" from their relationship to the bridge.
.\" .Pp
.\" Transparent
.\" .Xr ipsec 4
.\" processing can be enabled by setting the link2 flag on the bridge.
.Pp
.Nm 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.
.\" .Pp
.\" .Nm veb
.\" interfaces support the following
.\" .Xr ioctl 2
.\" calls:
.Sh SEE ALSO
.Xr ipsec 4 ,
.Xr options 4 ,
.Xr pf 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Ox 6.9 .
.Sh AUTHORS
.An David Gwynne Aq Mt dlg@openbsd.org