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"""A cache for resource records
This module contains a class which implements a cache for DNS resource records,
you still have to do most of the implementation. The module also provides a
class and a function for converting ResourceRecords from and to JSON strings.
It is highly recommended to use these.
"""
import json
import time
from dns.resource import ResourceRecord, RecordData
from dns.types import Type
from dns.classes import Class
class ResourceEncoder(json.JSONEncoder):
""" Conver ResourceRecord to JSON
Usage:
string = json.dumps(records, cls=ResourceEncoder, indent=4)
"""
def default(self, obj):
if isinstance(obj, ResourceRecord):
return {
"name": obj.name,
"type": Type.to_string(obj.type_),
"class": Class.to_string(obj.class_),
"ttl": obj.ttl,
"rdata": obj.rdata.data,
"timestamp": obj.timestamp
}
return json.JSONEncoder.default(self, obj)
def resource_from_json(dct):
""" Convert JSON object to ResourceRecord
Usage:
records = json.loads(string, object_hook=resource_from_json)
"""
name = dct["name"]
type_ = Type.from_string(dct["type"])
class_ = Class.from_string(dct["class"])
ttl = dct["ttl"]
rdata = RecordData.create(type_, dct["rdata"])
timestamp = dct["timestamp"]
return ResourceRecord(name, type_, class_, ttl, rdata, timestamp)
class RecordCache(object):
""" Cache for ResourceRecords """
FILE = '.dns.cache'
def __init__(self, ttl):
""" Initialize the RecordCache
Args:
ttl (int): TTL of cached entries (if > 0)
"""
self.records = []
self.read_cache_file()
self.ttl = ttl
def __del__(self):
self.write_cache_file()
def remove_old(self):
"""Remove entries for which the TTL has expired"""
now = int(time.time())
for record in reversed(self.records):
if min(self.ttl, record.ttl) + record.timestamp < now:
self.records.remove(record)
def lookup(self, dname, type_, class_):
""" Lookup resource records in cache
Lookup for the resource records for a domain name with a specific type
and class.
Args:
dname (str): domain name
type_ (Type): type
class_ (Class): class
"""
self.remove_old()
return [
r for r in self.records
if r.match(name=dname, type_=type_, class_=class_)]
def add_records_from(self, msg):
for record in msg.answers + msg.authorities + msg.additionals:
if record.type_ in [Type.A, Type.AAAA, Type.CNAME, Type.NS] and \
record.class_ == Class.IN:
self.add_record(record)
def add_record(self, record):
""" Add a new Record to the cache
Args:
record (ResourceRecord): the record added to the cache
"""
self.records.append(record)
def read_cache_file(self):
""" Read the cache file from disk """
try:
with open(self.FILE, 'r') as jsonfile:
self.records = json.load(
jsonfile, object_hook=resource_from_json)
except IOError:
pass
def write_cache_file(self):
""" Write the cache file to disk """
self.remove_old()
with open(self.FILE, 'w') as jsonfile:
json.dump(self.records, jsonfile, cls=ResourceEncoder, indent=4)
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