legacy/trac#17901 and legacy/trac#11360 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 legacy/trac#17901 * 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 legacy/trac#17901 and legacy/trac#11360, 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 legacy/trac#17835, I'll likely do it then if no-one gets to it first.