KNOWN BUGS:

20110413
- dane for ipv6 always gives (2001:4178:2:1269::12): (-9, 'Address family for hostname not supported') even on systems with proper ipv6 support.

20100914
- sshfp -k ~/.ssh/known_hosts -a does not work as expected, because of
  internal confusion about -k requiring an argument or not (-k vs -s)

20061101
- IPv6 untested and might not work at all.

20061016
- Scanning in-addr.arpa for PTR records to add sshfp records is missing.

20061004
- Using opts.append(x) does not work, as x is never appended to the current
  loop. This means the whole option processing needs to become re-entrant
  after fixing an argument. yuck.

20060927
- sshfp -a xelerance.com @ns1.xelerance.com does not work as expected.
  
- Running sshfp against the same nameserver twice using -a @ns0 gives
  different output all the time. Looks like ssh-keyscan is sensitive
  to failures.

20060921:
- If a zone contains non-working glue A records, then ssh-keyscan aborts.
  Bug reported upstream http://bugzilla.mindrot.org/show_bug.cgi?id=1213

FEATURE reqeust:
- Some mode (-V) that verifies all keys in knownhosts file and compares
  them with a scan
