diff --git a/neutron_lib/api/definitions/__init__.py b/neutron_lib/api/definitions/__init__.py
index 247c5bfa0..141bd20ae 100644
--- a/neutron_lib/api/definitions/__init__.py
+++ b/neutron_lib/api/definitions/__init__.py
@@ -13,6 +13,7 @@
 from neutron_lib.api.definitions import bgpvpn
 from neutron_lib.api.definitions import bgpvpn_routes_control
 from neutron_lib.api.definitions import data_plane_status
+from neutron_lib.api.definitions import dns
 from neutron_lib.api.definitions import extra_dhcp_opt
 from neutron_lib.api.definitions import fip64
 from neutron_lib.api.definitions import firewall
@@ -36,6 +37,7 @@ _ALL_API_DEFINITIONS = {
     bgpvpn,
     bgpvpn_routes_control,
     data_plane_status,
+    dns,
     extra_dhcp_opt,
     fip64,
     firewall,
diff --git a/neutron_lib/api/definitions/dns.py b/neutron_lib/api/definitions/dns.py
new file mode 100644
index 000000000..170e0815b
--- /dev/null
+++ b/neutron_lib/api/definitions/dns.py
@@ -0,0 +1,116 @@
+# All rights reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api import converters as convert
+from neutron_lib.api.definitions import l3
+from neutron_lib.api.definitions import network
+from neutron_lib.api.definitions import port
+from neutron_lib.api import validators
+from neutron_lib.api.validators import dns as dns_validator
+from neutron_lib.db import constants
+
+# The alias of the extension.
+ALIAS = 'dns-integration'
+
+# Whether or not this extension is simply signaling behavior to the user
+# or it actively modifies the attribute map (mandatory).
+IS_SHIM_EXTENSION = False
+
+# Whether the extension is marking the adoption of standardattr model for
+# legacy resources, or introducing new standardattr attributes. False or
+# None if the standardattr model is adopted since the introduction of
+# resource extension (mandatory).
+# If this is True, the alias for the extension should be prefixed with
+# 'standard-attr-'.
+IS_STANDARD_ATTR_EXTENSION = False
+
+# The name of the extension (mandatory).
+NAME = 'DNS Integration'
+
+# A prefix for API resources. An empty prefix means that the API is going
+# to be exposed at the v2/ level as any other core resource (mandatory).
+API_PREFIX = ''
+
+# The description of the extension (mandatory).
+DESCRIPTION = "Provides integration with DNS."
+
+# A timestamp of when the extension was introduced (mandatory).
+UPDATED_TIMESTAMP = "2015-08-15T18:00:00-00:00"
+
+DNSNAME = 'dns_name'
+DNSDOMAIN = 'dns_domain'
+DNSASSIGNMENT = 'dns_assignment'
+
+validators.add_validator('dns_host_name', dns_validator.validate_dns_name)
+validators.add_validator('fip_dns_host_name',
+                         dns_validator.validate_fip_dns_name)
+validators.add_validator('dns_domain_name',
+                         dns_validator.validate_dns_domain)
+
+# The resource attribute map for the extension. It is effectively the
+# bulk of the API contract alongside ACTION_MAP (mandatory).
+RESOURCE_ATTRIBUTE_MAP = {
+    port.COLLECTION_NAME: {
+        DNSNAME: {'allow_post': True, 'allow_put': True,
+                  'default': '',
+                  'convert_to': convert.convert_string_to_case_insensitive,
+                  'validate': {'type:dns_host_name':
+                               constants.FQDN_FIELD_SIZE},
+                  'is_visible': True},
+        DNSASSIGNMENT: {'allow_post': False, 'allow_put': False,
+                        'is_visible': True},
+    },
+    l3.FLOATINGIPS: {
+        DNSNAME: {'allow_post': True, 'allow_put': False,
+                  'default': '',
+                  'convert_to': convert.convert_string_to_case_insensitive,
+                  'validate': {'type:fip_dns_host_name':
+                               constants.FQDN_FIELD_SIZE},
+                  'is_visible': True},
+        DNSDOMAIN: {'allow_post': True, 'allow_put': False,
+                    'default': '',
+                    'convert_to': convert.convert_string_to_case_insensitive,
+                    'validate': {'type:dns_domain_name':
+                                 constants.FQDN_FIELD_SIZE},
+                    'is_visible': True},
+    },
+    network.COLLECTION_NAME: {
+        DNSDOMAIN: {'allow_post': True, 'allow_put': True,
+                    'default': '',
+                    'convert_to': convert.convert_string_to_case_insensitive,
+                    'validate': {'type:dns_domain_name':
+                                 constants.FQDN_FIELD_SIZE},
+                    'is_visible': True},
+    },
+}
+
+# The subresource attribute map for the extension. It adds child resources
+# to main extension's resource. The subresource map must have a parent and
+# a parameters entry. If an extension does not need such a map, None can
+# be specified (mandatory). For example:
+SUB_RESOURCE_ATTRIBUTE_MAP = {}
+
+# The action map: it associates verbs with methods to be performed on
+# the API resource (mandatory).
+ACTION_MAP = {}
+
+# The action status: it associates response statuses with methods to be
+# performed on the API resource (mandatory).
+ACTION_STATUS = {}
+
+# The list of required extensions (mandatory).
+REQUIRED_EXTENSIONS = [l3.ALIAS]
+
+# The list of optional extensions (mandatory).
+OPTIONAL_EXTENSIONS = []
diff --git a/neutron_lib/api/validators/dns.py b/neutron_lib/api/validators/dns.py
new file mode 100644
index 000000000..a2b16fdfc
--- /dev/null
+++ b/neutron_lib/api/validators/dns.py
@@ -0,0 +1,190 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import re
+
+from oslo_config import cfg
+
+from neutron_lib._i18n import _
+from neutron_lib.api import validators
+from neutron_lib import constants
+from neutron_lib.db import constants as db_constants
+
+
+def _validate_dns_format(data, max_len=db_constants.FQDN_FIELD_SIZE):
+    # NOTE: An individual name regex instead of an entire FQDN was used
+    # because its easier to make correct. The logic should validate that the
+    # dns_name matches RFC 1123 (section 2.1) and RFC 952.
+    if not data:
+        return
+    try:
+        # A trailing period is allowed to indicate that a name is fully
+        # qualified per RFC 1034 (page 7).
+        trimmed = data[:-1] if data.endswith('.') else data
+        if len(trimmed) > max_len:
+            raise TypeError(
+                _("'%(trimmed)s' exceeds the %(maxlen)s character FQDN "
+                  "limit") % {'trimmed': trimmed, 'maxlen': max_len})
+        labels = trimmed.split('.')
+        for label in labels:
+            if not label:
+                raise TypeError(_("Encountered an empty component"))
+            if label.endswith('-') or label.startswith('-'):
+                raise TypeError(
+                    _("Name '%s' must not start or end with a hyphen") % label)
+            if not re.match(constants.DNS_LABEL_REGEX, label):
+                raise TypeError(
+                    _("Name '%s' must be 1-63 characters long, each of "
+                      "which can only be alphanumeric or a hyphen") % label)
+        # RFC 1123 hints that a TLD can't be all numeric. last is a TLD if
+        # it's an FQDN.
+        if len(labels) > 1 and re.match("^[0-9]+$", labels[-1]):
+            raise TypeError(
+                _("TLD '%s' must not be all numeric") % labels[-1])
+    except TypeError as e:
+        msg = _("'%(data)s' not a valid PQDN or FQDN. Reason: %(reason)s") % {
+            'data': data, 'reason': e}
+        return msg
+
+
+def _validate_dns_name_with_dns_domain(request_dns_name, dns_domain):
+    # If a PQDN was passed, make sure the FQDN that will be generated is of
+    # legal size
+    higher_labels = dns_domain
+    if dns_domain:
+        higher_labels = '.%s' % dns_domain
+    higher_labels_len = len(higher_labels)
+    dns_name_len = len(request_dns_name)
+    if not request_dns_name.endswith('.'):
+        if dns_name_len + higher_labels_len > db_constants.FQDN_FIELD_SIZE:
+            msg = _("The dns_name passed is a PQDN and its size is "
+                    "'%(dns_name_len)s'. The dns_domain option in "
+                    "neutron.conf is set to %(dns_domain)s, with a "
+                    "length of '%(higher_labels_len)s'. When the two are "
+                    "concatenated to form a FQDN (with a '.' at the end), "
+                    "the resulting length exceeds the maximum size "
+                    "of '%(fqdn_max_len)s'"
+                    ) % {'dns_name_len': dns_name_len,
+                         'dns_domain': cfg.CONF.dns_domain,
+                         'higher_labels_len': higher_labels_len,
+                         'fqdn_max_len': db_constants.FQDN_FIELD_SIZE}
+            return msg
+        return
+
+    # A FQDN was passed
+    if (dns_name_len <= higher_labels_len or not
+            request_dns_name.endswith(higher_labels)):
+        msg = _("The dns_name passed is a FQDN. Its higher level labels "
+                "must be equal to the dns_domain option in neutron.conf, "
+                "that has been set to '%(dns_domain)s'. It must also "
+                "include one or more valid DNS labels to the left "
+                "of '%(dns_domain)s'") % {'dns_domain':
+                                          cfg.CONF.dns_domain}
+        return msg
+
+
+def _get_dns_domain_config():
+    if not cfg.CONF.dns_domain:
+        return ''
+    if cfg.CONF.dns_domain.endswith('.'):
+        return cfg.CONF.dns_domain
+    return '%s.' % cfg.CONF.dns_domain
+
+
+def _get_request_dns_name(dns_name):
+    dns_domain = _get_dns_domain_config()
+    if (dns_domain and dns_domain != constants.DNS_DOMAIN_DEFAULT):
+        # If CONF.dns_domain is the default value 'openstacklocal',
+        # neutron don't let the user to assign dns_name to ports
+        return dns_name
+    return ''
+
+
+def validate_dns_name(data, max_len=db_constants.FQDN_FIELD_SIZE):
+    """Validate DNS name.
+
+    This method validates dns name and also needs to have dns_domain in config
+    because this may call a method which uses the config.
+
+    :param data: The data to validate.
+    :param max_len: An optional cap on the length of the string.
+    :returns: None if data is valid, otherwise a human readable message
+    indicating why validation failed.
+    """
+    msg = _validate_dns_format(data, max_len)
+    if msg:
+        return msg
+
+    request_dns_name = _get_request_dns_name(data)
+    if request_dns_name:
+        dns_domain = _get_dns_domain_config()
+        msg = _validate_dns_name_with_dns_domain(request_dns_name, dns_domain)
+        if msg:
+            return msg
+
+
+def validate_fip_dns_name(data, max_len=db_constants.FQDN_FIELD_SIZE):
+    """Validate DNS name for floating IP.
+
+    :param data: The data to validate.
+    :param max_len: An optional cap on the length of the string.
+    :returns: None if data is valid, otherwise a human readable message
+    indicating why validation failed.
+    """
+    msg = validators.validate_string(data)
+    if msg:
+        return msg
+    if not data:
+        return
+    if data.endswith('.'):
+        msg = _("'%s' is a FQDN. It should be a relative domain name") % data
+        return msg
+    msg = _validate_dns_format(data, max_len)
+    if msg:
+        return msg
+    length = len(data)
+    if length > max_len - 3:
+        msg = _("'%(data)s' contains %(length)s characters. Adding a "
+                "domain name will cause it to exceed the maximum length "
+                "of a FQDN of '%(max_len)s'") % {"data": data,
+                                                 "length": length,
+                                                 "max_len": max_len}
+        return msg
+
+
+def validate_dns_domain(data, max_len=db_constants.FQDN_FIELD_SIZE):
+    """Validate DNS domain.
+
+    :param data: The data to validate.
+    :param max_len: An optional cap on the length of the string.
+    :returns: None if data is valid, otherwise a human readable message
+    indicating why validation failed.
+    """
+    msg = validators.validate_string(data)
+    if msg:
+        return msg
+    if not data:
+        return
+    if not data.endswith('.'):
+        msg = _("'%s' is not a FQDN") % data
+        return msg
+    msg = _validate_dns_format(data, max_len)
+    if msg:
+        return msg
+    length = len(data)
+    if length > max_len - 2:
+        msg = _("'%(data)s' contains %(length)s characters. Adding a "
+                "sub-domain will cause it to exceed the maximum length of a "
+                "FQDN of '%(max_len)s'") % {"data": data,
+                                            "length": length,
+                                            "max_len": max_len}
+        return msg
diff --git a/neutron_lib/constants.py b/neutron_lib/constants.py
index 2728511a0..f0627ad9b 100644
--- a/neutron_lib/constants.py
+++ b/neutron_lib/constants.py
@@ -305,6 +305,11 @@ GENEVE_ENCAP_MIN_OVERHEAD = 30
 GRE_ENCAP_OVERHEAD = 22
 VXLAN_ENCAP_OVERHEAD = 30
 
+# For DNS extension
+DNS_DOMAIN_DEFAULT = 'openstacklocal.'
+DNS_LABEL_MAX_LEN = 63
+DNS_LABEL_REGEX = "^[a-z0-9-]{1,%d}$" % DNS_LABEL_MAX_LEN
+
 
 class Sentinel(object):
     """A constant object that does not change even when copied."""
diff --git a/neutron_lib/exceptions/dns.py b/neutron_lib/exceptions/dns.py
new file mode 100644
index 000000000..cdcfae438
--- /dev/null
+++ b/neutron_lib/exceptions/dns.py
@@ -0,0 +1,34 @@
+# All rights reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib._i18n import _
+from neutron_lib import exceptions
+
+
+class DNSDomainNotFound(exceptions.NotFound):
+    message = _("Domain %(dns_domain)s not found in the external DNS service")
+
+
+class DuplicateRecordSet(exceptions.Conflict):
+    message = _("Name %(dns_name)s is duplicated in the external DNS service")
+
+
+class ExternalDNSDriverNotFound(exceptions.NotFound):
+    message = _("External DNS driver %(driver)s could not be found.")
+
+
+class InvalidPTRZoneConfiguration(exceptions.Conflict):
+    message = _("Value of %(parameter)s has to be multiple of %(number)s, "
+                "with maximum value of %(maximum)s and minimum value of "
+                "%(minimum)s")
diff --git a/neutron_lib/tests/unit/api/definitions/test_dns.py b/neutron_lib/tests/unit/api/definitions/test_dns.py
new file mode 100644
index 000000000..bb236356c
--- /dev/null
+++ b/neutron_lib/tests/unit/api/definitions/test_dns.py
@@ -0,0 +1,21 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api.definitions import dns
+from neutron_lib.api.definitions import l3
+from neutron_lib.tests.unit.api.definitions import base
+
+
+class DnsDefinitionTestCase(base.DefinitionBaseTestCase):
+    extension_module = dns
+    extension_resources = (l3.FLOATINGIPS,)
+    extension_attributes = (dns.DNSNAME, dns.DNSDOMAIN, dns.DNSASSIGNMENT,)
diff --git a/neutron_lib/tests/unit/api/validators/test_dns.py b/neutron_lib/tests/unit/api/validators/test_dns.py
new file mode 100644
index 000000000..bd0cfb22f
--- /dev/null
+++ b/neutron_lib/tests/unit/api/validators/test_dns.py
@@ -0,0 +1,158 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import mock
+
+from neutron_lib.api.validators import dns
+from neutron_lib.db import constants as db_constants
+from neutron_lib.tests import _base as base
+
+
+class TestDnsValidators(base.BaseTestCase):
+
+    @mock.patch('oslo_config.cfg.CONF')
+    def test_validate_dns_name(self, CONF):
+        CONF.dns_domain = ''
+        msg = dns.validate_dns_name('')
+        self.assertIsNone(msg)
+
+        CONF.dns_domain = 'example.org.'
+        dns_name = 'host'
+        msg = dns.validate_dns_name(dns_name)
+        self.assertIsNone(msg)
+
+        invalid_data = 'A' * 256
+        max_len = 255
+        expected = ("'%(data)s' not a valid PQDN or FQDN. Reason: "
+                    "'%(data)s' exceeds the %(maxlen)s character FQDN "
+                    "limit") % {'data': invalid_data, 'maxlen': max_len}
+        msg = dns.validate_dns_name(invalid_data, max_len)
+        self.assertEqual(expected, msg)
+
+        invalid_data = '.hostname'
+        expected = ("'%(data)s' not a valid PQDN or FQDN. Reason: "
+                    "Encountered an empty component") % {'data': invalid_data}
+        msg = dns.validate_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+
+        invalid_data = 'hostname-'
+        expected = ("'%(data)s' not a valid PQDN or FQDN. Reason: "
+                    "Name '%(data)s' must not start or end with a "
+                    "hyphen") % {'data': invalid_data}
+        msg = dns.validate_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+
+        invalid_data = 'hostname@host'
+        expected = ("'%(data)s' not a valid PQDN or FQDN. Reason: "
+                    "Name '%(data)s' must be 1-63 characters long, each of "
+                    "which can only be alphanumeric or a "
+                    "hyphen") % {'data': invalid_data}
+        msg = dns.validate_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+        invalid_suffix = '1234'
+        invalid_data = 'hostname.' + invalid_suffix
+        expected = ("'%(data)s' not a valid PQDN or FQDN. Reason: "
+                    "TLD '%(suffix)s' must not be all "
+                    "numeric") % {'data': invalid_data,
+                                  'suffix': invalid_suffix}
+        msg = dns.validate_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+
+        # len(dns_name + dns_domain) > 255
+        invalid_domain = 'A' * 250 + '.org.'
+        CONF.dns_domain = invalid_domain
+        dns_name = 'hostname'
+        expected = ("The dns_name passed is a PQDN and its size is "
+                    "'%(dns_name_len)s'. The dns_domain option in "
+                    "neutron.conf is set to %(dns_domain)s, with a "
+                    "length of '%(higher_labels_len)s'. When the two are "
+                    "concatenated to form a FQDN (with a '.' at the end), "
+                    "the resulting length exceeds the maximum size "
+                    "of '%(fqdn_max_len)s'"
+                    ) % {'dns_name_len': len(dns_name),
+                         'dns_domain': invalid_domain,
+                         'higher_labels_len': len(invalid_domain) + 1,
+                         'fqdn_max_len': db_constants.FQDN_FIELD_SIZE}
+        msg = dns.validate_dns_name(dns_name)
+        self.assertEqual(expected, msg)
+
+        dns_name = 'host.'
+        dns_domain = 'example.com.'
+        CONF.dns_domain = dns_domain
+        expected = ("The dns_name passed is a FQDN. Its higher level labels "
+                    "must be equal to the dns_domain option in neutron.conf, "
+                    "that has been set to '%(dns_domain)s'. It must also "
+                    "include one or more valid DNS labels to the left "
+                    "of '%(dns_domain)s'") % {'dns_domain': dns_domain}
+        msg = dns.validate_dns_name(dns_name)
+        self.assertEqual(expected, msg)
+
+    def test_validate_fip_dns_name(self):
+        # Don't run tests duplicated to validate_dns_name()
+
+        msg = dns.validate_fip_dns_name('')
+        self.assertIsNone(msg)
+
+        msg = dns.validate_fip_dns_name('host')
+        self.assertIsNone(msg)
+
+        invalid_data = 1234
+        expected = "'%s' is not a valid string" % invalid_data
+        msg = dns.validate_fip_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+
+        invalid_data = 'host.'
+        expected = ("'%s' is a FQDN. It should be a relative domain "
+                    "name") % invalid_data
+        msg = dns.validate_fip_dns_name(invalid_data)
+        self.assertEqual(expected, msg)
+
+        length = 10
+        invalid_data = 'a' * length
+        max_len = 12
+        expected = ("'%(data)s' contains %(length)s characters. Adding a "
+                    "domain name will cause it to exceed the maximum length "
+                    "of a FQDN of '%(max_len)s'") % {"data": invalid_data,
+                                                     "length": length,
+                                                     "max_len": max_len}
+        msg = dns.validate_fip_dns_name(invalid_data, max_len)
+        self.assertEqual(expected, msg)
+
+    def test_validate_dns_domain(self):
+        # Don't run tests duplicated to validate_dns_name()
+
+        msg = dns.validate_dns_domain('')
+        self.assertIsNone(msg)
+
+        msg = dns.validate_dns_domain('example.com.')
+        self.assertIsNone(msg)
+
+        invalid_data = 1234
+        expected = "'%s' is not a valid string" % invalid_data
+        msg = dns.validate_dns_domain(invalid_data)
+        self.assertEqual(expected, msg)
+
+        invalid_data = 'example.com'
+        expected = "'%s' is not a FQDN" % invalid_data
+        msg = dns.validate_dns_domain(invalid_data)
+        self.assertEqual(expected, msg)
+
+        length = 9
+        invalid_data = 'a' * length + '.'
+        max_len = 11
+        expected = ("'%(data)s' contains %(length)s characters. Adding a "
+                    "sub-domain will cause it to exceed the maximum length "
+                    "of a FQDN of '%(max_len)s'") % {"data": invalid_data,
+                                                     "length": length + 1,
+                                                     "max_len": max_len}
+        msg = dns.validate_dns_domain(invalid_data, max_len)
+        self.assertEqual(expected, msg)
diff --git a/releasenotes/notes/dns-api-def-bc24a58f56c5fbfb.yaml b/releasenotes/notes/dns-api-def-bc24a58f56c5fbfb.yaml
new file mode 100644
index 000000000..0eea4af28
--- /dev/null
+++ b/releasenotes/notes/dns-api-def-bc24a58f56c5fbfb.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - The ``DNS Integration`` extension API definition has been added as
+    ``neutron_lib.api.definitions.dns``.
+  - The ``validate_dns_name``, ``validate_fip_dns_name``, and
+    ``validate_dns_domain`` are now available as neutron-lib validators.
\ No newline at end of file