On my system (both in python2 and python3) I get the following error for invalid IP Adresses:
```python
======================================================================
ERROR: testIgnoreIPNOK (fail2ban.tests.filtertestcase.IgnoreIP)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./fail2ban/tests/filtertestcase.py", line 179, in testIgnoreIPNOK
    self.assertFalse(self.filter.inIgnoreIPList(ip))
  File "./fail2ban/server/filter.py", line 304, in inIgnoreIPList
    ips = DNSUtils.dnsToIp(i)
  File "./fail2ban/server/filter.py", line 632, in dnsToIp
    return socket.gethostbyname_ex(dns)[2]
error: [Errno 11] Resource temporarily unavailable

----------------------------------------------------------------------
```