ipa-replica-prepare: Wait for the DNS entry to be resolvable

It takes some time after the DNS record is added until it propagates
to Bind. In automated installations, it might happen that
replica-install is attempted before the hostname is resolvable;
in that case the connection check would fail.

Wait for the name to be resolvable at the end of replica-prepare.
Mention that this can be interrupted (Ctrl+C).
Provide an option to skip the wait.

In case DNS is not managed by IPA, this reminds the admin of the necessary
configuration and checks their work, but it's possible to skip (either by
interrupting it interactively, or by the option).

https://fedorahosted.org/freeipa/ticket/4551

Reviewed-By: Petr Spacek <pspacek@redhat.com>

ipaserver/install/ipa_replica_prepare.py

@@ -21,9 +21,12 @@
 import os
 import shutil
 import tempfile
+import time
 from optparse import OptionGroup
 from ConfigParser import SafeConfigParser
 
+import dns.resolver
+
 from ipaserver.install import certs, installutils, bindinstance, dsinstance
 from ipaserver.install.replication import enable_replication_version_checking
 from ipaserver.plugins.ldap2 import ldap2
@@ -64,6 +67,9 @@ class ReplicaPrepare(admintool.AdminTool):
         parser.add_option("--ca", dest="ca_file", default=paths.CACERT_P12,
             metavar="FILE",
             help="location of CA PKCS#12 file, default /root/cacert.p12")
+        parser.add_option('--no-wait-for-dns', dest='wait_for_dns',
+            action='store_false', default=True,
+            help="do not wait until the replica is resolvable in DNS")
 
         group = OptionGroup(parser, "SSL certificate options",
             "Only used if the server was installed using custom SSL certificates")
@@ -290,6 +296,9 @@ class ReplicaPrepare(admintool.AdminTool):
         if options.ip_address:
             self.add_dns_records()
 
+        if options.wait_for_dns:
+            self.wait_for_dns()
+
     def copy_ds_certificate(self):
         options = self.options
 
@@ -452,6 +461,50 @@ class ReplicaPrepare(admintool.AdminTool):
                 raise admintool.ScriptError(
                     "Could not add reverse DNS record for the replica: %s" % e)
 
+    def check_dns(self, replica_fqdn):
+        """Return true if the replica hostname is resolvable"""
+        resolver = dns.resolver.Resolver()
+        exceptions = (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer,
+                      dns.resolver.Timeout, dns.resolver.NoNameservers)
+
+        try:
+            dns_answer = resolver.query(replica_fqdn, 'A', 'IN')
+        except exceptions:
+            try:
+                dns_answer = resolver.query(replica_fqdn, 'AAAA', 'IN')
+            except exceptions:
+                return False
+        except Exception as e:
+            self.log.warn('Exception while waiting for DNS record: %s: %s',
+                          type(e).__name__, e)
+
+        return True
+
+    def wait_for_dns(self):
+        options = self.options
+
+        # Make sure replica_fqdn has a trailing dot, so the
+        # 'search' directive in /etc/resolv.conf doesn't apply
+        replica_fqdn = self.replica_fqdn
+        if not replica_fqdn.endswith('.'):
+            replica_fqdn += '.'
+
+        if self.check_dns(replica_fqdn):
+            self.log.debug('%s A/AAAA record resolvable', replica_fqdn)
+            return
+
+        self.log.info('Waiting for %s A or AAAA record to be resolvable',
+                      replica_fqdn)
+        print 'This can be safely interrupted (Ctrl+C)'
+
+        try:
+            while not self.check_dns(replica_fqdn):
+                time.sleep(1)
+        except KeyboardInterrupt:
+            self.log.info('Interrupted')
+        else:
+            self.log.debug('%s A/AAAA record resolvable', replica_fqdn)
+
     def copy_info_file(self, source, dest):
         """Copy a file into the info directory