Handle non-127.0.0.1 IPv4 loopback addresses
In legacy/trac#17901 (moved), we identified some FreeBSD jails and OpenVZ VMs as having no 127.0.0.1. legacy/trac#17901 (moved) 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 (moved)) and be accessible locally.
One possible implementation is:
- find all addresses on all loopback interfaces (legacy/trac#17949 (moved))
- as a fallback, resolve localhost (legacy/trac#17953 (moved)), 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.
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