EPAIR(4)

NAME
epair - A pair of virtual back-to-back connected Ethernet interfaces

SYNOPSIS
To compile this driver into the kernel, place the following line in your kernel configuration file:

device epair

Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):

    if_epair_load="YES"

DESCRIPTION
The epair is a pair of Ethernet-like software interfaces, which are connected back-to-back with a virtual cross-over cable.

Each epair interface pair is created at runtime using interface cloning. This is most easily done with the ifconfig(8) create command or using the cloned_interfaces variable in rc.conf(5). While for cloning you only give either epair or epair<n> the epair pair will be named like epair<n>[ab]. This means the names of the first epair interfaces will be epair0a and epair0b.

Like any other Ethernet interface, an epair needs to have a network address. If the tunable net.link.epair.ether_gen_addr =0, each epair will be assigned a random locally administered address, that is only guaranteed to be unique within one network stack. The tunable net.link.epair.ether_gen_addr =1 will generate a stable MAC address with FreeBSD OUI using ether_gen_addr(9). This tunable defaults to 1 in FreeBSD 15.0 and might be removed in FreeBSD 16.0. To change the default addresses one may use the SIOCSIFADDR ioctl(2) or ifconfig(8) utility.

The basic intent is to provide connectivity between two virtual network stack instances. When connected to an if_bridge(4), one end of the interface pair can also be part of another (virtual) LAN. As with any other Ethernet interface, epair can have a vlan(4) configured on top of it.

The epair has RXCSUM and RXCSUM6 enabled because it may receive a packet where the checksum has already been validated by a physical interface.

The epair supports TXCSUM and TXCSUM6 for TCP and UDP, but only by forwarding the order to compute the checksum. Thus, when using an epair interface, a TCP or UDP sender can offload checksum computation to a physical interface. Note that, in case the packet does not leave the host, the checksum is unnecessary and will be ignored if offloaded. Such packets contain an incorrect checksum, since it is not computed yet. TXCSUM and TXCSUM6 are synchronized between the epair interface pair (i.e., enabling/disabling the capability on one end enables/disables it on the other end). In case one end is in a bridge and the bridge disabled TXCSUM or TXCSUM6, this avoids a sender to send packets with checksum offloading into the bridge by using the other end.

The epair supports VLAN_HWTAGGING without actually adding a VLAN tag. The sending epair end just forwards the offloading information to the other end. The receiving epair end leaves the offloading information set to pretend that there was a VLAN tag in the Ethernet header, which has been removed already. To avoid a situation where the receiving epair end has VLAN_HWTAGGING disabled, this capability is synchronized between the epair interface pair (i.e., enabling/disabling the capability on one end enables/disables it on the other end).

SEE ALSO
ioctl(2), altq(4), bpf(4), if_bridge(4), vlan(4), loader.conf(5), rc.conf(5), ifconfig(8)

HISTORY
The epair interface first appeared in FreeBSD 8.0.

AUTHORS
The epair interface was written by Bjoern A. Zeeb, CK Software GmbH, under sponsorship from the FreeBSD Foundation.