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>

ipaserver/plugins/config.py

@@ -21,6 +21,7 @@
 from ipalib import api
 from ipalib import Bool, Int, Str, IA5Str, StrEnum, DNParam
 from ipalib import errors
+from ipalib.constants import MAXHOSTNAMELEN
 from ipalib.plugable import Registry
 from ipalib.util import validate_domain_name
 from .baseldap import (
@@ -59,6 +60,12 @@ Password plug-in features: currently defines additional hashes that the
 When setting the order list for mapping SELinux users you may need to
 quote the value so it isn't interpreted by the shell.
 
+The maximum length of a hostname in Linux is controlled by
+MAXHOSTNAMELEN in the kernel and defaults to 64. Some other operating
+systems, Solaris for example, allows hostnames up to 255 characters.
+This option will allow flexibility in length but by default limiting
+to the Linux maximum length.
+
 EXAMPLES:
 
  Show basic server configuration:
@@ -70,6 +77,9 @@ EXAMPLES:
  Change maximum username length to 99 characters:
    ipa config-mod --maxusername=99
 
+ Change maximum host name length to 255 characters:
+   ipa config-mod --maxhostname=255
+
  Increase default time and size limits for maximum IPA server search:
    ipa config-mod --searchtimelimit=10 --searchrecordslimit=2000
 
@@ -110,7 +120,7 @@ class config(LDAPObject):
         'ipamigrationenabled', 'ipacertificatesubjectbase',
         'ipapwdexpadvnotify', 'ipaselinuxusermaporder',
         'ipaselinuxusermapdefault', 'ipaconfigstring', 'ipakrbauthzdata',
-        'ipauserauthtype', 'ipadomainresolutionorder'
+        'ipauserauthtype', 'ipadomainresolutionorder', 'ipamaxhostnamelength',
     ]
     container_dn = DN(('cn', 'ipaconfig'), ('cn', 'etc'))
     permission_filter_objectclasses = ['ipaguiconfig']
@@ -132,6 +142,7 @@ class config(LDAPObject):
                 'ipasearchrecordslimit', 'ipasearchtimelimit',
                 'ipauserauthtype', 'ipauserobjectclasses',
                 'ipausersearchfields', 'ipacustomfields',
+                'ipamaxhostnamelength',
             },
         },
     }
@@ -146,6 +157,11 @@ class config(LDAPObject):
             minvalue=1,
             maxvalue=255,
         ),
+        Int('ipamaxhostnamelength',
+            cli_name='maxhostname',
+            label=_('Maximum hostname length'),
+            minvalue=MAXHOSTNAMELEN,
+            maxvalue=255,),
         IA5Str('ipahomesrootdir',
             cli_name='homedirectory',
             label=_('Home directory base'),