In legacy/trac#17901, we identified some FreeBSD jails and OpenVZ VMs as having no 127.0.0.1. legacy/trac#17901 deals with those systems that have no loopback at all. But some FreeBSD jails block access to 127.0.0.1, and have loopback on a valid yet unexpected address, like 127.0.0.2. Tor could bind to any address in 127/8 (or ::1, see legacy/trac#11360) and be accessible locally. One possible implementation is: * find all addresses on all loopback interfaces (legacy/trac#17949) * as a fallback, resolve localhost (legacy/trac#17953), and check that it's 127.0.0.0/8 or ::1 * choose the address that's closest to 127.0.0.1 * use that address as the bind address * If there is no 127.0.0.0/8 (or ::1) on the server, reject the *Port with a warning that tells the user to use AF_UNIX (if their system supports it), or supply an explicit IP address if they really want their *Port listening on a non-local address. Operators can always specify an explicit bind address in the *Port line, so this isn't a serious usability issue.