IFFLOOPBACK

IFFLOOPBACK, more commonly referred to as IFF_LOOPBACK, is a network interface flag used in Unix-like operating systems to identify a loopback network device. A loopback interface is a virtual interface that provides host-local communication, meaning packets sent to it are routed back within the same host rather than being transmitted onto any physical network.

Purpose and behavior

The IFF_LOOPBACK flag marks an interface as a loopback device. Interfaces with this flag do not correspond to physical hardware and are used for internal testing and inter-process communication. When a host sends a packet to a loopback address (such as 127.0.0.1 for IPv4 or ::1 for IPv6), the kernel routes the traffic back to the host’s networking stack rather than onto any external network. Routes and protocols operate as if the traffic remains inside the same machine.

Usage in code and administration

In kernel and network API definitions, IFF_LOOPBACK is a bitmask flag checked with bitwise operations (for example, if (flags & IFF_LOOPBACK)). It is typically defined in headers such as linux/if.h or net/if.h. The flag helps the kernel distinguish loopback devices from physical interfaces, affecting behaviors like ARP handling and link-layer operations.

Common characteristics

The standard loopback interface is usually named lo (or lo0 on some systems) and is configured with loopback addresses (127.0.0.1/8 and ::1/128). It is designed to be always available for internal host communication and does not depend on physical network connectivity. The exact flag value for IFF_LOOPBACK can vary by operating system, but its purpose remains consistent: to designate a loopback interface.

See also: IFF_UP, loopback addresses, lo interface.