1afe61c15SRodney W. Grimes.\" Copyright (c) 1983, 1991, 1993 23cb73e3dSRobert Watson.\" The Regents of the University of California. 33cb73e3dSRobert Watson.\" Copyright (c) 2009 Robert N. M. Watson 43cb73e3dSRobert Watson.\" All rights reserved. 5afe61c15SRodney W. Grimes.\" 6afe61c15SRodney W. Grimes.\" Redistribution and use in source and binary forms, with or without 7afe61c15SRodney W. Grimes.\" modification, are permitted provided that the following conditions 8afe61c15SRodney W. Grimes.\" are met: 9afe61c15SRodney W. Grimes.\" 1. Redistributions of source code must retain the above copyright 10afe61c15SRodney W. Grimes.\" notice, this list of conditions and the following disclaimer. 11afe61c15SRodney W. Grimes.\" 2. Redistributions in binary form must reproduce the above copyright 12afe61c15SRodney W. Grimes.\" notice, this list of conditions and the following disclaimer in the 13afe61c15SRodney W. Grimes.\" documentation and/or other materials provided with the distribution. 14fbbd9655SWarner Losh.\" 3. Neither the name of the University nor the names of its contributors 15afe61c15SRodney W. Grimes.\" may be used to endorse or promote products derived from this software 16afe61c15SRodney W. Grimes.\" without specific prior written permission. 17afe61c15SRodney W. Grimes.\" 18afe61c15SRodney W. Grimes.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19afe61c15SRodney W. Grimes.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20afe61c15SRodney W. Grimes.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21afe61c15SRodney W. Grimes.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22afe61c15SRodney W. Grimes.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23afe61c15SRodney W. Grimes.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24afe61c15SRodney W. Grimes.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25afe61c15SRodney W. Grimes.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26afe61c15SRodney W. Grimes.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27afe61c15SRodney W. Grimes.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28afe61c15SRodney W. Grimes.\" SUCH DAMAGE. 29afe61c15SRodney W. Grimes.\" 30*60117f9eSMichael Tuexen.Dd June 23, 2024 31afe61c15SRodney W. Grimes.Dt LO 4 323d45e180SRuslan Ermilov.Os 33afe61c15SRodney W. Grimes.Sh NAME 34afe61c15SRodney W. Grimes.Nm lo 35afe61c15SRodney W. Grimes.Nd software loopback network interface 36afe61c15SRodney W. Grimes.Sh SYNOPSIS 37ec09ef4fSJens Schweikhardt.Cd "device loop" 38afe61c15SRodney W. Grimes.Sh DESCRIPTION 39afe61c15SRodney W. GrimesThe 40afe61c15SRodney W. Grimes.Nm loop 41afe61c15SRodney W. Grimesinterface is a software loopback mechanism which may be 42afe61c15SRodney W. Grimesused for performance analysis, software testing, and/or local 43afe61c15SRodney W. Grimescommunication. 44afe61c15SRodney W. GrimesAs with other network interfaces, the loopback interface must have 45afe61c15SRodney W. Grimesnetwork addresses assigned for each address family with which it is to be used. 46afe61c15SRodney W. GrimesThese addresses 47938f0fd5SGleb Smirnoffmay be set with the appropriate 48938f0fd5SGleb Smirnoff.Xr ioctl 2 49938f0fd5SGleb Smirnoffcommands for corresponding address families. 50afe61c15SRodney W. GrimesThe loopback interface should be the last interface configured, 51afe61c15SRodney W. Grimesas protocols may use the order of configuration as an indication of priority. 52afe61c15SRodney W. GrimesThe loopback should 53afe61c15SRodney W. Grimes.Em never 54afe61c15SRodney W. Grimesbe configured first unless no hardware 55afe61c15SRodney W. Grimesinterfaces exist. 563cb73e3dSRobert Watson.Pp 573cb73e3dSRobert WatsonIf the transmit checksum offload capability flag is enabled on a loopback 58*60117f9eSMichael Tuexeninterface, checksums will not be generated by IP, UDP, TCP, or SCTP for packets 593cb73e3dSRobert Watsonsent on the interface. 603cb73e3dSRobert Watson.Pp 613cb73e3dSRobert WatsonIf the receive checksum offload capability flag is enabled on a loopback 62*60117f9eSMichael Tuexeninterface, checksums will not be validated by IP, UDP, TCP, or SCTP for packets 633cb73e3dSRobert Watsonreceived on the interface. 643cb73e3dSRobert Watson.Pp 653cb73e3dSRobert WatsonBy default, both receive and transmit checksum flags will be enabled, in 663cb73e3dSRobert Watsonorder to avoid the overhead of checksumming for local communication where 673cb73e3dSRobert Watsondata corruption is unlikely. 683cb73e3dSRobert WatsonIf transmit checksum generation is disabled, then validation should also be 693cb73e3dSRobert Watsondisabled in order to avoid packets being dropped due to invalid checksums. 70afe61c15SRodney W. Grimes.Sh DIAGNOSTICS 71afe61c15SRodney W. Grimes.Bl -diag 72afe61c15SRodney W. Grimes.It lo%d: can't handle af%d. 73afe61c15SRodney W. GrimesThe interface was handed 74afe61c15SRodney W. Grimesa message with addresses formatted in an unsuitable address 75afe61c15SRodney W. Grimesfamily; the packet was dropped. 76afe61c15SRodney W. Grimes.El 77afe61c15SRodney W. Grimes.Sh SEE ALSO 780b992c1dSWolfram Schneider.Xr inet 4 , 790b992c1dSWolfram Schneider.Xr intro 4 80e40f3f85SMike Pritchard.\" .Xr ns 4 81afe61c15SRodney W. Grimes.Sh HISTORY 82afe61c15SRodney W. GrimesThe 83afe61c15SRodney W. Grimes.Nm 84afe61c15SRodney W. Grimesdevice appeared in 85afe61c15SRodney W. Grimes.Bx 4.2 . 863cb73e3dSRobert WatsonThe current checksum generation and validation avoidance policy appeared in 873cb73e3dSRobert Watson.Fx 8.0 . 88