---
name: "lo"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/lo"
---

# lo(4)

software loopback network interface

## NAME

**lo** – software loopback network interface

## SYNOPSIS

**device loop**

## DESCRIPTION

The **loop** interface is a software loopback mechanism which may be used for performance analysis, software testing, and/or local communication. As with other network interfaces, the loopback interface must have network addresses assigned for each address family with which it is to be used. These addresses may be set with the appropriate [ioctl(2)](/ioctl/2.md) commands for corresponding address families. The loopback interface should be the last interface configured, as protocols may use the order of configuration as an indication of priority. The loopback should *never* be configured first unless no hardware interfaces exist.

If the transmit checksum offload capability is enabled on a loopback interface, checksums will not be generated by IP, UDP, TCP, or SCTP for packets sent on the interface. By default, this capability is enabled to avoid the overhead of checksumming for local communication where data corruption is unlikely.

If the receive checksum offload capability is disabled on a loopback interface, the flags that indicate a valid checksum will be unset if set previously. By default, this capability is enabled to avoid another checksum validation if it has already been validated.

## DIAGNOSTICS

- lo%d: can't handle af%d.

  The interface was handed a message with addresses formatted in an unsuitable address family; the packet was dropped.

## SEE ALSO

[inet(4)](/inet/4.md), [intro(4)](/intro/4.md)

## HISTORY

The **lo** device appeared in 4.2BSD. The current checksum generation and validation avoidance policy appeared in FreeBSD 8.0.