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#17901 (moved) and #11360 (moved) search local interfaces for loopback addresses.

We could make this more efficient by using the flags returned with each address:

• getifaddrs returns struct ifaddrs with ifa_flags which has IFF_LOOPBACK
• ioctl(.,SIOCGIFCONF,.) returns struct ifreq with ifr_flags which has IFF_LOOPBACK
• GetAdaptersAddresses (Win32) returns IP_ADAPTER_ADDRESSES with IfType which has IF_TYPE_SOFTWARE_LOOPBACK
  • Also pass GAA_FLAG_SKIP_FRIENDLY_NAME as we never use it
• tor_getsockname/get_interface_address6_via_udp_socket_hack connects to a public address. When passed the localhost flag, it could connect to a loopback address, but this seems rather tautologous, and it's hard to know which loopback address to pick in 127/8. Alternately, we could lookup localhost using the resolver, and check the returned address is 127/8 or ::1 (otherwise, broken DNS configs could lead to security issues).

A design for this could be:

• pass a flag to get_interface_address* indicating whether we want loopback addresses
• pass that flag down to get_interface_addresses_raw
• pass that flag to the API-specific functions
• when converting the address to a smartlist, include/exclude addresses matching the specified types
  • always exclude tor_addr_is_null()
  • always exclude addresses matching tor_addr_is_multicast
• for extra points, create a flags argument for:
  • loopback
    • this will require disabling the checks for tor_addr_is_loopback() in get_interface_address6_list() and get_interface_address6_via_udp_socket_hack(), see #17901 (moved)
  • listen any (0.0.0.0 and [::], needs tor_addr_is_listen_any())
  • link-local (needs tor_addr_is_link_local())
  • other internal/private
  • public
  • with #defines:
    • _INTERNAL: everything except public
    • _INTERNAL_FOR_LISTENING: everything except public and listen any

After doing this, refactor the changes in #17901 (moved) and #11360 (moved), and all existing code, to use the new flags argument.

This will also resolve issues where systems have addresses other than 127.0.0.0/8 or ::1 as the loopback address (like some BSD jails and OpenVZ). But we're not too worried about this, they're non-standards-compliant, so the operator can configure the exact address correctly, as long as we fail with an informative message.

I need this to do #17835 (moved), I'll likely do it then if no-one gets to it first.