[ih] History of 127/8 as localhost/loopback addresses?

[Carsten Bormann]

On 2021-01-02, at 14:40, Craig Partridge via Internet-history <internet-history at elists.isoc.org> wrote:
> 
> loopback

I think it is an unfortunate artifact of naming the localhost address “loopback” that it is easily confused with actual loopback functions.  The latter are meant to test a line or path by having the remote end loop back the traffic, and I’m sure that some form of this was used in diagnosing most reasonably operating networks.  Information takes an actual loop (which happens to coincide with the “local loop” in many cases) from one end of a line/path to another end and back.

The localhost interfaces have no physical loop.  Their packets are sent up the IP stack right from the interface, but that is not different from e.g. the way multicast sometimes behaves.  Note that, in another unrelated usage, the term “loopback address” is also used for a routable IP address given to a router; loopback interfaces are then the virtual interfaces carrying those addresses (i.e., not 127/8).

(It is of course too late to fix this terminological conflation, but the second best approach is to be aware of it and make people aware of it.)

IIRC, localhost (“lo0”) was part of 4.2BSD (and I’m sure the various 4.1x BSDs), so I think we can date it to before 1983, probably even 1981.  Source code for those distributions should be available, so a more precise statement can certainly be made after some digging.

Grüße, Carsten