1.\"- 2.\" Copyright (c) 2008 The FreeBSD Foundation 3.\" 4.\" This software was developed by CK Software GmbH under sponsorship 5.\" from the FreeBSD Foundation. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd September 4, 2025 29.Dt EPAIR 4 30.Os 31.Sh NAME 32.Nm epair 33.Nd A pair of virtual back-to-back connected Ethernet interfaces 34.Sh SYNOPSIS 35To compile this driver into the kernel, 36place the following line in your 37kernel configuration file: 38.Bd -ragged -offset indent 39.Cd "device epair" 40.Ed 41.Pp 42Alternatively, to load the driver as a 43module at boot time, place the following line in 44.Xr loader.conf 5 : 45.Bd -literal -offset indent 46if_epair_load="YES" 47.Ed 48.Sh DESCRIPTION 49The 50.Nm 51is a pair of Ethernet-like software interfaces, 52which are connected back-to-back with a virtual cross-over cable. 53.Pp 54Each 55.Nm 56interface pair is created at runtime using interface cloning. 57This is most easily done with the 58.Xr ifconfig 8 59.Cm create 60command or using the 61.Va cloned_interfaces 62variable in 63.Xr rc.conf 5 . 64While for cloning you only give either 65.Pa epair 66or 67.Pa epair<n> 68the 69.Nm 70pair will be named like 71.Pa epair<n>[ab] . 72This means the names of the first 73.Nm 74interfaces will be 75.Pa epair0a 76and 77.Pa epair0b . 78.Pp 79Like any other Ethernet interface, an 80.Nm 81needs to have a network address. 82If the tunable 83.Va net.link.epair.ether_gen_addr Ns 84=0, each 85.Nm 86will be assigned a random locally administered address, 87that is only guaranteed to be unique within one network stack. 88The tunable 89.Va net.link.epair.ether_gen_addr Ns 90=1 will generate a stable MAC address with 91.Fx 92OUI using 93.Xr ether_gen_addr 9 . 94This tunable defaults to 1 in 95.Fx 15.0 and might be removed in 96.Fx 16.0 . 97To change the default addresses one may use the SIOCSIFADDR 98.Xr ioctl 2 or 99.Xr ifconfig 8 utility. 100.Pp 101The basic intent is to provide connectivity between two virtual 102network stack instances. 103When connected to an 104.Xr if_bridge 4 , 105one end of the interface pair can also be part of another (virtual) LAN. 106As with any other Ethernet interface, 107.Nm epair 108can have a 109.Xr vlan 4 110configured on top of it. 111.Pp 112The 113.Nm 114has RXCSUM and RXCSUM6 enabled because it may receive a packet where the 115checksum has already been validated by a physical interface. 116The 117.Nm 118supports TXCSUM and TXCSUM6 for TCP and UDP, but only by forwarding the order 119to compute the checksum. 120Thus, when using an 121.Nm 122interface, a TCP or UDP sender can offload checksum computation 123to a physical interface. 124Note that, in case the packet does not leave the host, the checksum is 125unnecessary and will be ignored if offloaded. 126Such packets contain an incorrect checksum, since it is not computed yet. 127TXCSUM and TXCSUM6 are synchronized between the 128.Nm 129interface pair (i.e., enabling/disabling the capability on one end 130enables/disables it on the other end). 131In case one end is in a bridge and the bridge disabled TXCSUM or TXCSUM6, 132this avoids a sender to send packets with checksum offloading into the 133bridge by using the other end. 134.Sh SEE ALSO 135.Xr ioctl 2 , 136.Xr altq 4 , 137.Xr bpf 4 , 138.Xr if_bridge 4 , 139.Xr vlan 4 , 140.Xr loader.conf 5 , 141.Xr rc.conf 5 , 142.Xr ifconfig 8 143.Sh HISTORY 144The 145.Nm 146interface first appeared in 147.Fx 8.0 . 148.Sh AUTHORS 149The 150.Nm 151interface was written by 152.An Bjoern A. Zeeb, CK Software GmbH, 153under sponsorship from the FreeBSD Foundation. 154