Blacklist 53 unreachable obfs4 bridges.

We cannot establish a TCP connection to any of these obfs4 bridges.
There are two reasons:

  1. They advertise a private IP address, e.g., 10.0.0.1.
  2. Their obfs4 port is unreachable, e.g., because of NAT.

Some of these bridges have contact info and we sent at least one email
to the operator but never heard back.  The other obfs4 bridges don't
have contact info.  After implementing #28655, we no longer hand out the
vanilla line of a bridge that also supports obfs4.  Therefore, bridges
whose obfs4 port is unreachable are effectively useless.