[tor-bugs] #17949 [Tor]: Make loopback address search more efficient

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#17949: Make loopback address search more efficient
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     Reporter:  teor         |      Owner:  teor
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:  easy
Actual Points:               |  Parent ID:
       Points:               |    Sponsor:
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 #17901 and #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 #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 #17901 and #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 #17835, I'll likely do it then if no-one gets to it
 first.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17949>
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