LO(4) AerieBSD 1.0 Refernce Manual LO(4)


lo — software loopback network interface


.Cd "pseudo-device loop"


The loop interface is a software loopback mechanism which may be used for performance analysis, software testing, and/or local communication.

A loop interface can be created at runtime using the ifconfiglo command or by setting up a hostname.if(5) configuration file for netstart(8). The lo0 interface will always exist and cannot be destroyed using ifconfig(8).

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 or changed with the SIOCSIFADDR ioctl(2). 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.

Configuring a loopback interface for inet(4) with the link1 flag set will make the interface answer to the whole set of addresses identified as being in super-net which is specified by the address and netmask. Obviously you should not set the link1 flag on interface lo0, but instead use another interface like lo1.


# ifconfig lo1 create
# ifconfig lo1 inet netmask link1

is equivalent to:

# ifconfig lo1 create
# awk 'BEGIN {for(i=1;i<255;i++) \e
	print "ifconfig lo1 inet 192.168.1."i" netmask alias"}'| \e


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


inet(4), inet6(4), netintro(4), hostname.if(5), ifconfig(8), netstart(8)


The lo device appeared in 4.2BSD.

The wildcard functionality first appeared in OpenBSD 2.3.


Previous versions of the system enabled the loopback interface automatically, using a non-standard Internet address (127.1). Use of that address is now discouraged; a reserved host address for the local network should be used instead.

Care should be taken when using NAT with interfaces that have the link1 flag set, because it may believe the packets are coming from a loopback address.

AerieBSD 1.0 Reference Manual August 26 2008 LO(4)