No, 127.0.0.1 should never appear on any network, and no network device should ever route it.

The earliest documentation I was able to find is in RFC 1122 [1] from 1989, but according to RFC 6890 [2], the principle dates back to 1981.

[1] https://tools.ietf.org/html/rfc1122#section-3.2.1.3

[2] https://tools.ietf.org/html/rfc6890 (table 4)

Ahhh yeah. But that's how most OS's set things up by default, in order to meet the required specs. (bugs and implementations hiccups aside)

Once the OS is up and running, manipulation of the routing tables at least _used_ to make this possible on Linux and Solaris. Not sure about FreeBSD, but that's just from memory fuzziness on my part. :)

> Traffic sent to 127.0.0.1 is guaranteed not to leave your machine

This is definitely false, without any routing tables. Any unprivileged user can start an SSH tunnel listening on any localhost port above 1024, sending traffic out to wherever.

The implicit threat model here is "no one outside your machine can do something to you to make 127.0.0.1 traffic route elsewhere." It's true that software running on your machine can make copies of things and send them elsewhere, but that's not the point of the sentence you quoted.

Sent to is not the same as sent from

Yeah, but that would rather involve being on the other side of the airtight hatchway (having root).

Good point. :)

At that point... you break it, you buy it.