1.\" $OpenBSD: pair.4,v 1.4 2015/10/30 10:48:55 reyk Exp $ 2.\" 3.\" Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org> 4.\" Copyright (c) 2009 Theo de Raadt <deraadt@openbsd.org> 5.\" 6.\" Permission to use, copy, modify, and distribute this software for any 7.\" purpose with or without fee is hereby granted, provided that the above 8.\" copyright notice and this permission notice appear in all copies. 9.\" 10.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 11.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 12.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 13.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 14.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 15.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 16.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 17.\" 18.Dd $Mdocdate: October 30 2015 $ 19.Dt PAIR 4 20.Os 21.Sh NAME 22.Nm pair 23.Nd virtual Ethernet interface pair 24.Sh SYNOPSIS 25.Cd "pseudo-device pair" 26.Sh DESCRIPTION 27The 28.Nm 29interface simulates a normal Ethernet interface by encapsulating 30standard network frames with an Ethernet header, specifically for use 31in a pair of interfaces that are interconnected with each other. 32.Pp 33To use it, the administrator needs to create two 34.Nm 35interfaces and connect them; 36the interfaces are 37.Sq patched , 38as would be done with physical network ports. 39All packets that are sent on the first interface are received on the 40second interface. 41.Sh EXAMPLES 42Set up a pair of interfaces where each of them is a member of a different 43.Xr rdomain 4 : 44.Bd -literal -offset indent 45# ifconfig pair1 rdomain 1 10.1.1.1/24 up 46# ifconfig pair2 rdomain 2 10.1.1.2/24 up 47# ifconfig pair1 patch pair2 48# route -T 1 exec ping 10.1.1.2 49.Ed 50.Pp 51When adding multiple 52.Nm 53to multiple 54.Xr bridge 4 55interfaces, it is possible to create a loop; 56the system load will go up while it is busy sending packets from one 57bridge to another and back. 58By design, the driver does not prevent such loops by itself, but it is 59possible to use the Spanning Tree Protocol (STP) to detect and remove 60loops in the virtual network topology: 61.Bd -literal -offset indent 62# ifconfig pair0 up 63# ifconfig pair1 rdomain 1 patch pair0 up 64# ifconfig pair2 up 65# ifconfig pair3 rdomain 1 patch pair2 up 66# ifconfig bridge0 add pair0 add pair2 stp pair0 stp pair2 up 67# ifconfig bridge1 add pair1 add pair3 stp pair1 stp pair3 up 68.Ed 69.Sh SEE ALSO 70.Xr bridge 4 , 71.Xr inet 4 , 72.Xr inet6 4 , 73.Xr rdomain 4 , 74.Xr vether 4 , 75.Xr hostname.if 5 , 76.Xr ifconfig 8 , 77.Xr netstart 8 78.Sh HISTORY 79The 80.Nm 81interface first appeared in 82.Ox 5.9 . 83.Sh AUTHORS 84The 85.Nm 86driver is based on 87.Xr vether 4 88by 89.An Theo de Raadt Aq Mt deraadt@openbsd.org . 90It has been extended and turned into 91.Nm 92by 93.An Reyk Floeter Aq Mt reyk@openbsd.org . 94.Sh CAVEATS 95Unlike 96.Xr vether 4 , 97the 98.Nm 99interface cannot be used as a stand-alone member in a 100.Xr bridge 4 : 101the link state remains down until it is connected to the second interface. 102Any associated routes will be marked down until it is patched. 103Use 104.Xr vether 4 105as a bridge endpoint for routing purposes instead. 106.Sh BUGS 107Like 108.Xr tun 4 , 109the Ethernet address chosen will be partially random, and may 110occasionally collide with another address. 111