pair —
virtual Ethernet interface pair
pseudo-device pair
The
pair interface simulates a normal
Ethernet interface by encapsulating standard network frames with an Ethernet
header, specifically for use in a pair of interfaces that are interconnected
with each other.
To use it, the administrator needs to create two
pair interfaces and connect them; the
interfaces are ‘patched’, as would be done with physical network
ports. All packets that are sent on the first interface are received on the
second interface.
Set up a pair of interfaces where each of them is a member of a different
rdomain(4):
# ifconfig pair1 rdomain 1 10.1.1.1/24 up
# ifconfig pair2 rdomain 2 10.1.1.2/24 up
# ifconfig pair1 patch pair2
# route -T 1 exec ping 10.1.1.2
When adding multiple
pair to multiple
bridge(4) interfaces, it is
possible to create a loop; the system load will go up while it is busy sending
packets from one bridge to another and back. By design, the driver does not
prevent such loops by itself, but it is possible to use the Spanning Tree
Protocol (STP) to detect and remove loops in the virtual network topology:
# ifconfig pair0 up
# ifconfig pair1 rdomain 1 patch pair0 up
# ifconfig pair2 up
# ifconfig pair3 rdomain 1 patch pair2 up
# ifconfig bridge0 add pair0 add pair2 stp pair0 stp pair2 up
# ifconfig bridge1 add pair1 add pair3 stp pair1 stp pair3 up
bridge(4),
inet(4),
inet6(4),
rdomain(4),
vether(4),
hostname.if(5),
ifconfig(8),
netstart(8)
The
pair interface first appeared in
OpenBSD 5.9.
The
pair driver is based on
vether(4) by
Theo de Raadt
<
deraadt@openbsd.org>.
It has been extended and turned into
pair
by
Reyk Floeter
<
reyk@openbsd.org>.
Unlike
vether(4), the
pair interface cannot be used as a
stand-alone member in a
bridge(4): the link state
remains down until it is connected to the second interface. Any associated
routes will be marked down until it is patched. Use
vether(4) as a bridge endpoint
for routing purposes instead.
Like
tun(4), the Ethernet address
chosen will be partially random, and may occasionally collide with another
address.
![[OpenBSD]](/openbsd.gif){kind=small}