The following Python code fails to loook through all AD DCs advertised in the SRV DNS records:

      netrc = net.Net(creds=self._creds, lp=self._parm)
        finddc_error = None
        result = None
        try:
            result = netrc.finddc(domain=domain, flags=nbt.NBT_SERVER_LDAP | nbt.NBT_SERVER_GC | nbt.NBT_SERVER_CLOSEST)
        except RuntimeError, e:
            try:
                # If search of closest GC failed, attempt to find any one
                result = netrc.finddc(domain=domain, flags=nbt.NBT_SERVER_LDAP | nbt.NBT_SERVER_GC)
            except RuntimeError, e:
                finddc_error = e

The reason why it fails, is because out of, say, 4 addresses, two are IPv6 and IPv6 networking is not fully configured -- IPv6 stack is enabled but not really routed, and the code in finddcs_cldap.c does not handle the failure of creating a socket or sending the CLDAP ping and never getting to the reply.