[tor-bugs] #20151 [Core Tor/Tor]: Fix parse_virtual_addr_network minimum network size

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#20151: Fix parse_virtual_addr_network minimum network size
------------------------------+--------------------------
     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.???
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  easy intro
Actual Points:                |  Parent ID:
       Points:  0.5           |   Reviewer:
      Sponsor:                |
------------------------------+--------------------------
 parse_virtual_addr_network does:
 {{{
   const int max_bits = ipv6 ? 40 : 16;
 }}}
 then:
 {{{
   if (bits > max_bits) {
     if (msg)
       tor_asprintf(msg, "VirtualAddressNetwork%s expects a /%d "
                    "network or larger",ipv6?"IPv6":"", max_bits);
     return -1;
   }
 }}}

 Firstly, the log message refers to a minimum ("n or larger" makes n a
 minimum, not a maximum), but the variable is named "max_bits". So we
 should rename it to min_bits.

 Secondly, an IPv6 /40 is terribly restrictive.

 For people to use their local IPv6 allocations, we should allow at least a
 /64.

 If the goal is to have a /16 available, we could allow up to 128 - 16 =
 /112. But IPv6 has more addresses than IPv4, so I suggest that a /104 is a
 sensible minimum. (If someone wants to map more than 2**24 addresses at
 once, they can choose a larger network. We could make the minimum /96, but
 some providers split up /64s into /96s and give them out to end users.)

 These limitations should also be documented in the tor man page.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20151>
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