Add knob to limit hostname length

On Linux systems the length limit for hostnames is hardcoded
at 64 in MAXHOSTNAMELEN

Solaris, for example, allows 255 characters, and DNS allows the
total length to be up to 255 (with each label < 64).

Add a knob to allow configuring the maximum hostname length (FQDN)

The same validators are used between hosts and DNS to apply
the knob only when dealing with a FQDN as a hostname.

The maxlen option is included so installers can limit the length
of allowed hostnames when the --hostname option is used.

https://pagure.io/freeipa/issue/2018

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipalib/util.py

@@ -434,14 +434,28 @@ def validate_zonemgr_str(zonemgr):
     zonemgr = DNSName(zonemgr)
     return validate_zonemgr(zonemgr)
 
-def validate_hostname(hostname, check_fqdn=True, allow_underscore=False, allow_slash=False):
+
+def validate_hostname(hostname, check_fqdn=True, allow_underscore=False,
+                      allow_slash=False, maxlen=255):
     """ See RFC 952, 1123
 
+    Length limit of 64 imposed by MAXHOSTNAMELEN on Linux.
+
+    DNS and other operating systems has a max length of 255. Default to
+    the theoretical max unless explicitly told to limit. The cases
+    where a limit would be set might include:
+     * *-install --hostname
+     * ipa host-add
+
+    The *-install commands by definition are executed on Linux hosts so
+    the maximum length needs to be limited.
+
     :param hostname Checked value
     :param check_fqdn Check if hostname is fully qualified
     """
-    if len(hostname) > 255:
-        raise ValueError(_('cannot be longer that 255 characters'))
+    if len(hostname) > maxlen:
+        raise ValueError(_('cannot be longer that {} characters'.format(
+                         maxlen)))
 
     if hostname.endswith('.'):
         hostname = hostname[:-1]
@@ -1029,9 +1043,16 @@ def normalize_hostname(hostname):
     return hostname
 
 
-def hostname_validator(ugettext, value):
+def hostname_validator(ugettext, value, maxlen=255):
+    """Validator used by plugins to ensure hostname compliance.
+
+       In Linux the maximum hostname length is 64. In DNS and
+       other operaring systems (Solaris) it is 255. If not explicitly
+       checking a Linux hostname (e.g. the server) use the DNS
+       default.
+    """
     try:
-        validate_hostname(value)
+        validate_hostname(value, maxlen=maxlen)
     except ValueError as e:
         return _('invalid domain-name: %s') % unicode(e)