Unify access to FQDN

FreeIPA's Python and C code used different approaches to get the FQDN of
the host. Some places assumed that gethostname() returns a FQDN. Other
code paths used glibc's resolver to resolve the current node name to a
FQDN.

Python code now uses the ipalib.constants.FQDN where a fully qualified
domain name is expected. The variable is initialized only once and avoids
potential DNS lookups.

C code uses a new helper function ipa_gethostfqdn() in util package. The
function implements similar logic as gethostfqdn() except it uses more
modern getaddrinfo(). The result is cached as well.

Fixes: https://pagure.io/freeipa/issue/8501
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>

ipapython/fqdn.py

@@ -0,0 +1,30 @@
+#
+# Copyright (C) 2020  FreeIPA Contributors see COPYING for license
+#
+"""Get host's FQDN
+"""
+import socket
+
+
+def gethostfqdn():
+    hostname = socket.gethostname()
+
+    # optional optimization, consider hostname with dot as FQDN
+    if "." in hostname:
+        return hostname
+
+    # this call can never fail except for misconfigured nsswitch.conf
+    # without nss-myhostname provider. The myhostname provider translates
+    # gethostname() to local interfaces.
+    gai = socket.getaddrinfo(
+        hostname,
+        None,  # service/port is irrelevant
+        family=socket.AF_UNSPEC,  # IPv4 or IPv6
+        type=socket.SOCK_DGRAM,  # optimization, TCP/RAW gives same result
+        # include canonical name in first addrinfo struct
+        # only use address family when at least one non-local interface
+        # is configured with that address family
+        flags=socket.AI_CANONNAME | socket.AI_ADDRCONFIG
+    )
+    # first addrinfo struct, fourth field is canonical name
+    return gai[0][3]