0
# DNS Implementation
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DNS protocol implementation with client resolvers, server functionality, and caching capabilities for domain name resolution. Twisted Names provides comprehensive DNS support for both client and server applications.
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## Capabilities
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### DNS Client Resolution
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DNS client functionality for resolving domain names to IP addresses and other record types.
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```python { .api }
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class client.Resolver:
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    """
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    DNS resolver for performing domain name lookups.
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    Attributes:
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    - servers: List of DNS server addresses
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    - timeout: Query timeout in seconds
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    - tries: Number of retry attempts
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    """
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    servers = None
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    timeout = 10
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    tries = 3
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    def __init__(self, resolv=None, servers=None):
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        """
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        Args:
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            resolv (str): Path to resolv.conf file
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            servers (list): List of DNS server addresses
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        """
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    def lookupAddress(self, name, timeout=None):
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        """
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        Look up A records for hostname.
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        Args:
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            name (str): Hostname to resolve
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of A record objects
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        """
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    def lookupIPV6Address(self, name, timeout=None):
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        """
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        Look up AAAA records for hostname.
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        Args:
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            name (str): Hostname to resolve
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of AAAA record objects
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        """
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    def lookupMailExchange(self, name, timeout=None):
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        """
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        Look up MX records for domain.
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        Args:
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            name (str): Domain name
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of MX record objects
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        """
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    def lookupNameservers(self, name, timeout=None):
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        """
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        Look up NS records for domain.
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        Args:
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            name (str): Domain name
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of NS record objects
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        """
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    def lookupCanonicalName(self, name, timeout=None):
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        """
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        Look up CNAME records for hostname.
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        Args:
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            name (str): Hostname
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of CNAME record objects
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        """
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    def lookupPointer(self, name, timeout=None):
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        """
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        Look up PTR records for reverse DNS.
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        Args:
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            name (str): IP address or reverse domain
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of PTR record objects
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        """
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    def lookupText(self, name, timeout=None):
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        """
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        Look up TXT records for domain.
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        Args:
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            name (str): Domain name
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of TXT record objects
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        """
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    def lookupService(self, name, timeout=None):
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        """
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        Look up SRV records for service.
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        Args:
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            name (str): Service name (_service._proto.domain)
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[list]: List of SRV record objects
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        """
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    def queryUDP(self, queries, timeout=None):
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        """
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        Perform DNS query over UDP.
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        Args:
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            queries (list): List of Query objects
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[Message]: DNS response message
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        """
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    def queryTCP(self, queries, timeout=None):
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        """
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        Perform DNS query over TCP.
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        Args:
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            queries (list): List of Query objects
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            timeout (int): Query timeout override
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        Returns:
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            Deferred[Message]: DNS response message
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        """
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class client.ThreadedResolver:
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    """
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    DNS resolver that uses threads for blocking DNS calls.
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    """
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    def __init__(self, reactor=None, getHostByName=None):
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        """
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        Args:
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            reactor: Reactor to use
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            getHostByName: Function for host resolution
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        """
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    def getHostByName(self, name, timeout=None):
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        """
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        Resolve hostname to IP address.
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        Args:
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            name (str): Hostname
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            timeout (int): Resolution timeout
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        Returns:
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            Deferred[str]: IP address
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        """
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```
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**DNS Client Usage Example**:
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```python
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from twisted.names import client
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from twisted.internet import reactor, defer
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# Create resolver
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resolver = client.Resolver()
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@defer.inlineCallbacks
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def resolve_domains():
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    try:
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        # Look up A records
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        a_records = yield resolver.lookupAddress('example.com')
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        for record in a_records[0]:
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            print(f"A: {record.payload.dottedQuad()}")
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        # Look up MX records
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        mx_records = yield resolver.lookupMailExchange('example.com')
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        for record in mx_records[0]:
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            print(f"MX: {record.payload.exchange} (priority {record.payload.preference})")
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        # Look up TXT records
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        txt_records = yield resolver.lookupText('example.com')
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        for record in txt_records[0]:
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            print(f"TXT: {record.payload.data}")
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        # Reverse DNS lookup
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        ptr_records = yield resolver.lookupPointer('8.8.8.8.in-addr.arpa')
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        for record in ptr_records[0]:
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            print(f"PTR: {record.payload.name}")
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    except Exception as e:
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        print(f"DNS lookup failed: {e}")
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    reactor.stop()
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resolve_domains()
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reactor.run()
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```
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### DNS Protocol and Messages
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Core DNS protocol implementation and message parsing.
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```python { .api }
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class dns.DNSDatagramProtocol:
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    """
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    DNS protocol over UDP.
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    Attributes:
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    - controller: DNS controller object
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    """
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    controller = None
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    def __init__(self, controller):
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        """
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        Args:
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            controller: DNS controller
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        """
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    def query(self, address, queries, timeout=10, id=None):
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        """
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        Send DNS query.
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        Args:
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            address: Server address tuple (host, port)
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            queries (list): List of Query objects
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            timeout (int): Query timeout
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            id (int): Query ID
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        Returns:
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            Deferred[Message]: DNS response
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        """
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class dns.DNSProtocol:
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    """
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    DNS protocol over TCP.
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    """
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    def __init__(self, controller):
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        """
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        Args:
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            controller: DNS controller
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        """
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    def query(self, queries, timeout=60):
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        """
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        Send DNS query over TCP.
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        Args:
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            queries (list): List of Query objects
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            timeout (int): Query timeout
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        Returns:
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            Deferred[Message]: DNS response
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        """
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class dns.Message:
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    """
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    DNS message representation.
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    Attributes:
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    - id: Message ID
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    - answer: Answer flag
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    - opCode: Operation code
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    - auth: Authoritative answer flag
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    - trunc: Truncated flag
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    - recDes: Recursion desired flag
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    - recAv: Recursion available flag
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    - rCode: Response code
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    - queries: List of queries
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    - answers: List of answer records
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    - authority: List of authority records
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    - additional: List of additional records
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    """
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    id = 0
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    answer = 0
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    opCode = 0
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    auth = 0
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    trunc = 0
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    recDes = 0
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    recAv = 0
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    rCode = 0
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    queries = None
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    answers = None
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    authority = None
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    additional = None
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    def __init__(self, id=0, answer=0, opCode=0, recDes=0, recAv=0, auth=0, rCode=0, trunc=0, maxSize=512):
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        """
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        Args:
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            id (int): Message ID
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            answer (int): Answer flag
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            opCode (int): Operation code
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            recDes (int): Recursion desired
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            recAv (int): Recursion available
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            auth (int): Authoritative answer
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            rCode (int): Response code
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            trunc (int): Truncated flag
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            maxSize (int): Maximum message size
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        """
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    def addQuery(self, name, type=A, cls=IN):
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        """
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        Add query to message.
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        Args:
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            name (str): Query name
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            type (int): Record type
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            cls (int): Record class
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        """
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    def encode(self, strio):
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        """
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        Encode message to wire format.
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        Args:
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            strio: String buffer to write to
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        """
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    def decode(self, strio):
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        """
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        Decode message from wire format.
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        Args:
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            strio: String buffer to read from
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        """
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class dns.Query:
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    """
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    DNS query representation.
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    Attributes:
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    - name: Query name
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    - type: Record type
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    - cls: Record class
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    """
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    name = None
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    type = None
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    cls = None
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    def __init__(self, name='', type=A, cls=IN):
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        """
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        Args:
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            name (str): Query name
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            type (int): Record type (A, AAAA, MX, etc.)
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            cls (int): Record class (IN, etc.)
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        """
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class dns.RRHeader:
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    """
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    DNS resource record header.
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    Attributes:
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    - name: Record name
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    - type: Record type
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    - cls: Record class
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    - ttl: Time to live
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    - payload: Record payload
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    """
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    name = None
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    type = None
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    cls = None
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    ttl = None
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    payload = None
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    def __init__(self, name='', type=A, cls=IN, ttl=0, payload=None, auth=False):
382
        """
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        Args:
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            name (str): Record name
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            type (int): Record type
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            cls (int): Record class
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            ttl (int): Time to live in seconds
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            payload: Record data
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            auth (bool): Authoritative flag
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        """
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```
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### DNS Record Types
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Implementations of various DNS record types.
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```python { .api }
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class dns.Record_A:
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    """
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    A record (IPv4 address).
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    Attributes:
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    - address: IPv4 address (4 bytes)
404
    """
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    address = None
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407
    def __init__(self, address='0.0.0.0', ttl=None):
408
        """
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        Args:
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            address (str): IPv4 address
411
            ttl (int): Time to live
412
        """
413
    
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    def dottedQuad(self):
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        """
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        Get dotted quad representation.
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        Returns:
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            str: Dotted quad IPv4 address
420
        """
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class dns.Record_AAAA:
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    """
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    AAAA record (IPv6 address).
425
    
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    Attributes:
427
    - address: IPv6 address (16 bytes)
428
    """
429
    address = None
430
    
431
    def __init__(self, address='::', ttl=None):
432
        """
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        Args:
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            address (str): IPv6 address
435
            ttl (int): Time to live
436
        """
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class dns.Record_CNAME:
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    """
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    CNAME record (canonical name).
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    Attributes:
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    - name: Canonical name
444
    """
445
    name = None
446
    
447
    def __init__(self, name='', ttl=None):
448
        """
449
        Args:
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            name (str): Canonical name
451
            ttl (int): Time to live
452
        """
453

454
class dns.Record_MX:
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    """
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    MX record (mail exchange).
457
    
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    Attributes:
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    - preference: Priority preference
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    - exchange: Mail server name
461
    """
462
    preference = None
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    exchange = None
464
    
465
    def __init__(self, preference=10, exchange='', ttl=None):
466
        """
467
        Args:
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            preference (int): Priority (lower = higher priority)
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            exchange (str): Mail server hostname
470
            ttl (int): Time to live
471
        """
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473
class dns.Record_NS:
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    """
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    NS record (name server).
476
    
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    Attributes:
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    - name: Name server hostname
479
    """
480
    name = None
481
    
482
    def __init__(self, name='', ttl=None):
483
        """
484
        Args:
485
            name (str): Name server hostname
486
            ttl (int): Time to live
487
        """
488

489
class dns.Record_PTR:
490
    """
491
    PTR record (pointer for reverse DNS).
492
    
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    Attributes:
494
    - name: Pointed-to name
495
    """
496
    name = None
497
    
498
    def __init__(self, name='', ttl=None):
499
        """
500
        Args:
501
            name (str): Target hostname
502
            ttl (int): Time to live
503
        """
504

505
class dns.Record_SOA:
506
    """
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    SOA record (start of authority).
508
    
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    Attributes:
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    - mname: Primary name server
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    - rname: Responsible person email
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    - serial: Serial number
513
    - refresh: Refresh interval
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    - retry: Retry interval
515
    - expire: Expiration time
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    - minimum: Minimum TTL
517
    """
518
    mname = None
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    rname = None
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    serial = None
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    refresh = None
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    retry = None
523
    expire = None
524
    minimum = None
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    def __init__(self, mname='', rname='', serial=0, refresh=3600, retry=900, expire=604800, minimum=86400, ttl=None):
527
        """
528
        Args:
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            mname (str): Primary name server
530
            rname (str): Responsible person email
531
            serial (int): Serial number
532
            refresh (int): Refresh interval
533
            retry (int): Retry interval
534
            expire (int): Expiration time
535
            minimum (int): Minimum TTL
536
            ttl (int): Record TTL
537
        """
538

539
class dns.Record_SRV:
540
    """
541
    SRV record (service location).
542
    
543
    Attributes:
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    - priority: Service priority
545
    - weight: Service weight
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    - port: Service port
547
    - target: Service hostname
548
    """
549
    priority = None
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    weight = None
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    port = None
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    target = None
553
    
554
    def __init__(self, priority=0, weight=0, port=0, target='', ttl=None):
555
        """
556
        Args:
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            priority (int): Service priority
558
            weight (int): Service weight
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            port (int): Service port number
560
            target (str): Service hostname
561
            ttl (int): Time to live
562
        """
563

564
class dns.Record_TXT:
565
    """
566
    TXT record (text data).
567
    
568
    Attributes:
569
    - data: Text data (list of strings)
570
    """
571
    data = None
572
    
573
    def __init__(self, *data, **kw):
574
        """
575
        Args:
576
            *data: Text strings
577
            **kw: Additional parameters (ttl, etc.)
578
        """
579
```
580

581
### DNS Constants
582

583
DNS protocol constants and record type definitions.
584

585
```python { .api }
586
# Record types
587
dns.A = 1          # IPv4 address
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dns.NS = 2         # Name server
589
dns.CNAME = 5      # Canonical name
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dns.SOA = 6        # Start of authority
591
dns.PTR = 12       # Pointer
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dns.MX = 15        # Mail exchange
593
dns.TXT = 16       # Text
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dns.AAAA = 28      # IPv6 address
595
dns.SRV = 33       # Service location
596

597
# Record classes
598
dns.IN = 1         # Internet class
599
dns.CS = 2         # CSNET class
600
dns.CH = 3         # CHAOS class
601
dns.HS = 4         # Hesiod class
602

603
# Response codes
604
dns.OK = 0         # No error
605
dns.EFORMAT = 1    # Format error
606
dns.ESERVER = 2    # Server failure
607
dns.ENAME = 3      # Name error
608
dns.ENOTIMP = 4    # Not implemented
609
dns.EREFUSED = 5   # Refused
610

611
# Operation codes
612
dns.OP_QUERY = 0   # Standard query
613
dns.OP_IQUERY = 1  # Inverse query
614
dns.OP_STATUS = 2  # Server status
615
```
616

617
### DNS Server
618

619
DNS server implementation for authoritative and caching servers.
620

621
```python { .api }
622
class server.DNSServerFactory:
623
    """
624
    DNS server factory.
625
    
626
    Attributes:
627
    - authorities: List of authority resolvers
628
    - caches: List of cache resolvers
629
    - clients: List of client resolvers
630
    """
631
    authorities = None
632
    caches = None
633
    clients = None
634
    
635
    def __init__(self, authorities=None, caches=None, clients=None, verbose=0):
636
        """
637
        Args:
638
            authorities (list): Authority resolvers
639
            caches (list): Cache resolvers
640
            clients (list): Client resolvers
641
            verbose (int): Verbosity level
642
        """
643
    
644
    def buildProtocol(self, addr):
645
        """
646
        Build DNS server protocol.
647
        
648
        Args:
649
            addr: Connection address
650
            
651
        Returns:
652
            DNSProtocol: DNS server protocol
653
        """
654
```
655

656
### DNS Caching
657

658
DNS caching resolver for improved performance.
659

660
```python { .api }
661
class cache.CacheResolver:
662
    """
663
    DNS caching resolver.
664
    
665
    Attributes:
666
    - cache: Cache storage
667
    - reactor: Reactor for timeouts
668
    """
669
    cache = None
670
    reactor = None
671
    
672
    def __init__(self, _cache=None, reactor=None):
673
        """
674
        Args:
675
            _cache: Cache storage object
676
            reactor: Reactor instance
677
        """
678
    
679
    def query(self, query, timeout=None):
680
        """
681
        Perform cached DNS query.
682
        
683
        Args:
684
            query: DNS query object
685
            timeout (int): Query timeout
686
            
687
        Returns:
688
            Deferred[Message]: DNS response (cached or fresh)
689
        """
690
    
691
    def lookupAddress(self, name, timeout=None):
692
        """
693
        Cached A record lookup.
694
        
695
        Args:
696
            name (str): Hostname
697
            timeout (int): Query timeout
698
            
699
        Returns:
700
            Deferred[list]: A records
701
        """
702
```
703

704
### DNS Authority
705

706
Authoritative DNS server for specific domains.
707

708
```python { .api }
709
class authority.FileAuthority:
710
    """
711
    File-based DNS authority.
712
    
713
    Loads DNS records from zone files.
714
    """
715
    def __init__(self, file):
716
        """
717
        Args:
718
            file (str): Zone file path
719
        """
720
    
721
    def _lookup(self, name, cls, type, timeout=None):
722
        """
723
        Look up records in authority data.
724
        
725
        Args:
726
            name (str): Query name
727
            cls (int): Record class
728
            type (int): Record type
729
            timeout (int): Query timeout
730
            
731
        Returns:
732
            Deferred: Query results
733
        """
734
```
735

736
**DNS Server Example**:
737

738
```python
739
from twisted.names import dns, server, authority, cache, client
740
from twisted.internet import reactor, endpoints
741
from twisted.application import service
742

743
# Create resolvers
744
authority_resolver = authority.FileAuthority('/etc/bind/db.example.com')
745
cache_resolver = cache.CacheResolver()
746
client_resolver = client.Resolver()
747

748
# Create DNS server factory
749
dns_factory = server.DNSServerFactory(
750
    authorities=[authority_resolver],
751
    caches=[cache_resolver], 
752
    clients=[client_resolver]
753
)
754

755
# Start DNS server
756
udp_endpoint = endpoints.UDP4ServerEndpoint(reactor, 53)
757
tcp_endpoint = endpoints.TCP4ServerEndpoint(reactor, 53)
758

759
udp_endpoint.listen(dns.DNSDatagramProtocol(dns_factory))
760
tcp_endpoint.listen(dns_factory)
761

762
print("DNS server listening on port 53")
763
reactor.run()
764
```
765

766
### Hosts File Resolver
767

768
DNS resolver that uses system hosts file.
769

770
```python { .api }
771
class hosts.Resolver:
772
    """
773
    DNS resolver using hosts file.
774
    """
775
    def __init__(self, file='/etc/hosts'):
776
        """
777
        Args:
778
            file (str): Hosts file path
779
        """
780
    
781
    def lookupAddress(self, name, timeout=None):
782
        """
783
        Look up address in hosts file.
784
        
785
        Args:
786
            name (str): Hostname
787
            timeout (int): Query timeout (ignored)
788
            
789
        Returns:
790
            Deferred[list]: A records from hosts file
791
        """
792
```