getdns reference

getdns contexts

This section describes the getdns Context object, as well as its as its methods and attributes.

class getdns.Context([set_from_os])

Creates a context, an opaque object which describes the environment within which a DNS query executes. This includes namespaces, root servers, resolution types, and so on. These are accessed programmatically through the attributes described below.

Context() takes one optional constructor argument. set_from_os is an integer and may take the value either 0 or 1. If 1, which most developers will want, getdns will populate the context with default values for the platform on which it’s running.

The Context class has the following public read/write attributes:


Specifies whether to append a suffix to the query string before the API starts resolving a name. Its value must be one of getdns.APPEND_NAME_ALWAYS, getdns.APPEND_NAME_ONLY_TO_SINGLE_LABEL_AFTER_FAILURE, getdns.APPEND_NAME_ONLY_TO_MULTIPLE_LABEL_NAME_AFTER_FAILURE, or getdns.APPEND_NAME_NEVER. This controls whether or not to append the suffix given by suffix.


The value of dns_root_servers is a list of dictionaries containing addresses to be used for looking up top-level domains. Each dict in the list contains two key-value pairs:

  • address_data: a string representation of an IPv4 or IPv6 address
  • address_type: either the string “IPv4” or “IPv6”

For example, the addresses list could look like

>>> addrs = [ { 'address_data': '2001:7b8:206:1::4:53',
    'address_type': 'IPv6' },
...         { 'address_data': '',
    'address_type': 'IPv4' } ]
>>> mycontext.dns_root_servers = addrs

An ordered list of transport options to be used for DNS lookups, ordered by preference (first choice as list element 0, second as list element 1, and so on). The possible values are getdns.TRANSPORT_UDP, getdns.TRANSPORT_TCP, and getdns.TRANSPORT_TLS.


Its value is the number of seconds of skew that is allowed in either direction when checking an RRSIG’s Expiration and Inception fields. The default is 0.


Its value is a list of DNSSEC trust anchors, expressed as RDATAs from DNSKEY resource records.


May be set to 0 or 1. When 1, requests upstreams not to reveal query’s originating network.


Its value must be an integer valued either 0 or 1. The default is 0.


Its value must be an integer between 0 and 255, inclusive. The default is 0.


Its value must be an integer between 512 and 65535, inclusive. The default is 512.


Its value must be an integer between 0 and 255, inclusive. The default is 0.


Specifies whether or not DNS queries follow redirects. The value must be one of getdns.REDIRECTS_FOLLOW for normal following of redirects though CNAME and DNAME; or getdns.REDIRECTS_DO_NOT_FOLLOW to cause any lookups that would have gone through CNAME and DNAME to return the CNAME or DNAME, not the eventual target.


The idle timeout for TCP connections.


A string describing the implementation of the underlying getdns library, retrieved from libgetdns. Currently “


Specifies limit (an integer value) on the number of outstanding DNS queries. The API will block itself from sending more queries if it is about to exceed this value, and instead keep those queries in an internal queue. The a value of 0 indicates that the number of outstanding DNS queries is unlimited.


The namespaces attribute takes an ordered list of namespaces that will be queried. (Important: this context setting is ignored for the getdns.general() function; it is used for the other functions.) The allowed values are getdns.NAMESPACE_DNS, getdns.NAMESPACE_LOCALNAMES, getdns.NAMESPACE_NETBIOS, getdns.NAMESPACE_MDNS, and getdns.NAMESPACE_NIS. When a normal lookup is done, the API does the lookups in the order given and stops when it gets the first result; a different method with the same result would be to run the queries in parallel and return when it gets the first result. Because lookups might be done over different mechanisms because of the different namespaces, there can be information leakage that is similar to that seen with POSIX getaddrinfo(). The default is determined by the OS.


Specifies whether DNS queries are performed with nonrecursive lookups or as a stub resolver. The value is either getdns.RESOLUTION_RECURSING or getdns.RESOLUTION_STUB.

If an implementation of this API is only able to act as a recursive resolver, setting resolution_type to getdns.RESOLUTION_STUB will throw an exception.


Its value is a list of strings to be appended based on append_name. The list elements must follow the rules in RFC 4343#section-2.1


Its value must be an integer specifying a timeout for a query, expressed in milliseconds.


The mechanism to be used for authenticating the TLS server when using a TLS transport. May be getdns.AUTHENTICATION_REQUIRED or getdns.AUTHENTICATION_NONE. (getdns.AUTHENTICATION_HOSTNAME remains as an alias for getdns.AUTHENTICATION_REQUIRED but is deprecated and will be removed in a future release)


Optional padding blocksize for queries when using TLS. Used to increase the difficulty for observers to guess traffic content.


A list of dicts defining where a stub resolver will send queries. Each dict in the list contains at least two names: address_type (either “IPv4” or “IPv6”) and address_data (whose value is a string representation of an IP address). It might also contain “port” to specify which port to use to contact these DNS servers; the default is 53. If the stub and a recursive resolver both support TSIG (RFC 2845), the upstream_list entry can also contain tsig_algorithm (a string) that is the name of the TSIG hash algorithm, and tsig_secret (a base64 string) that is the TSIG key.

There is also now support for pinning an upstream’s certificate’s public keys with pinsets (when using TLS for transport). Add an element to the upstream_recursive_server list entry, called ‘tls_pubkey_pinset’, which is a list of public key pins. (See the example code in our examples directory).


The libgetdns version, retrieved from the underlying getdns library.

The Context class includes public methods to execute a DNS query, as well as a method to return the entire set of context attributes as a Python dictionary. Context methods are described below:

general(name, request_type[, extensions][, userarg][, transaction_id][, callback])

Context.general() is used for looking up any type of DNS record. The keyword arguments are:

  • name: a string containing the query term.
  • request_type: a DNS RR type as a getdns constant (listed here)
  • extensions: optional. A dictionary containing attribute/value pairs, as described below
  • userarg: optional. A string containing arbitrary user data; this is opaque to getdns
  • transaction_id: optional. An integer.
  • callback: optional. This is a function name. If it is present the query will be performed asynchronously (described below).
address(name[, extensions][, userarg][, transaction_id][, callback])

There are two critical differences between Context.address() and Context.general() beyond the missing request_type argument:

  • In Context.address(), the name argument can only take a host name.
  • Context.address() always uses all of namespaces from the context (to better emulate getaddrinfo()), while Context.general() only uses the DNS namespace.
hostname(name[, extensions][, userarg][, transaction_id][, callback])

The address is given as a dictionary. The dictionary must have two names:

  • address_type: must be a string matching either “IPv4” or “IPv6”
  • address_data: a string representation of an IPv4 or IPv6 IP address
service(name[, extensions][, userarg][, transaction_id][, callback])

name must be a domain name for an SRV lookup. The call returns the relevant SRV information for the name


Retrieves context information. The information is returned as a Python dictionary with the following keys:

  • version_string
  • implementation_string
  • resolution_type
  • all_context

all_context is a dictionary containing the following keys:

  • append_name
  • dns_transport
  • dnssec_allowed_skew
  • edns_do_bit
  • edns_extended_rcode
  • edns_version
  • follow_redirects
  • limit_outstanding_queries
  • namespaces
  • suffix
  • timeout
  • tls_authentication
  • upstream_recursive_servers

Returns a list of the attributes supported by this Context object.

The getdns module has the following read-only attribute:


Specifies the version string for the getdns python module


Extensions are Python dictionaries, with the keys being the names of the extensions. The definition of each extension describes the values that may be assigned to that extension. For most extensions it is a Boolean, and since the default value is “False” it will most often take the value getdns.EXTENSION_TRUE.

The extensions currently supported by getdns are:

  • dnssec_return_status
  • dnssec_return_only_secure
  • dnssec_return_validation_chain
  • return_both_v4_and_v6
  • add_opt_parameters
  • add_warning_for_bad_dns
  • specify_class
  • return_call_reporting

Extensions for DNSSEC

If an application wants the API to do DNSSEC validation for a request, it must set one or more DNSSEC-related extensions. Note that the default is for none of these extensions to be set and the API will not perform DNSSEC validation. Note that getting DNSSEC results can take longer in a few circumstances.

To return the DNSSEC status for each DNS record in the replies_tree list, use the dnssec_return_status extension. Set the extension’s value to getdns.EXTENSION_TRUE to cause the returned status to have the name dnssec_status added to the other names in the record’s dictionary (“header”, “question”, and so on). The potential values for that name are getdns.DNSSEC_SECURE, getdns.DNSSEC_BOGUS, getdns.DNSSEC_INDETERMINATE, and getdns.DNSSEC_INSECURE.

If instead of returning the status, you want to only see secure results, use the dnssec_return_only_secure extension. The extension’s value is set to getdns.EXTENSION_TRUE to cause only records that the API can validate as secure with DNSSEC to be returned in the replies_tree and replies_full lists. No additional names are added to the dict of the record; the change is that some records might not appear in the results. When this context option is set, if the API receives DNS replies but none are determined to be secure, the error code at the top level of the response object is getdns.RESPSTATUS_NO_SECURE_ANSWERS.

Applications that want to do their own validation will want to have the DNSSEC-related records for a particular response. Use the dnssec_return_validation_chain extension. Set the extension’s value to getdns.EXTENSION_TRUE to cause a set of additional DNSSEC-related records needed for validation to be returned in the response object. This set comes as validation_chain (a list) at the top level of the response object. This list includes all resource record dicts for all the resource records (DS, DNSKEY and their RRSIGs) that are needed to perform the validation from the root up.

If a request is using a context in which stub resolution is set, and that request also has any of the dnssec_return_status, dnssec_return_only_secure, or dnssec_return_validation_chain extensions specified, the API will not perform the request and will instead return an error of getdns.RETURN_DNSSEC_WITH_STUB_DISALLOWED.

Returning both IPv4 and IPv6 responses

Many applications want to get both IPv4 and IPv6 addresses in a single call so that the results can be processed together. The address() method is able to do this automatically. If you are using the general() method, you can enable this with the return_both_v4_and_v6 extension. The extension’s value must be set to getdns.EXTENSION_TRUE to cause the results to be the lookup of either A or AAAA records to include any A and AAAA records for the queried name (otherwise, the extension does nothing). These results are expected to be usable with Happy Eyeballs systems that will find the best socket for an application.

Setting up OPT resource records

For lookups that need an OPT resource record in the Additional Data section, use the add_opt_parameters extension. The extension’s value (a dict) contains the parameters; these are described in more detail in RFC 2671. They are:

  • maximum_udp_payload_size: an integer between 512 and 65535 inclusive. If not specified it defaults to the value in the getdns context.
  • extended_rcode: an integer between 0 and 255 inclusive. If not specified it defaults to the value in the getdns context.
  • version: an integer betwen 0 and 255 inclusive. If not specified it defaults to 0.
  • do_bit: must be either 0 or 1. If not specified it defaults to the value in the getdns context.
  • options: a list containing dictionaries for each option to be specified. Each dictionary contains two keys: option_code (an integer) and option_data (in the form appropriate for that option code).

It is very important to note that the OPT resource record specified in the add_opt_parameters extension might not be the same the one that the API sends in the query. For example, if the application also includes any of the DNSSEC extensions, the API will make sure that the OPT resource record sets the resource record appropriately, making the needed changes to the settings from the add_opt_parameters extension.

The program in our example directory shows how to pack and send an OPT record.

Getting Warnings for Responses that Violate the DNS Standard

To receive a warning if a particular response violates some parts of the DNS standard, use the add_warning_for_bad_dns extension. The extension’s value is set to getdns.EXTENSION_TRUE to cause each reply in the replies_tree to contain an additional name, bad_dns (a list). The list is zero or more values that indicate types of bad DNS found in that reply. The list of values is:


A DNS query type that does not allow a target to be a CNAME pointed to a CNAME


One or more labels in a returned domain name is all-numeric; this is not legal for a hostname


A DNS query for a type other than CNAME returned a CNAME response

Using other class types

The vast majority of DNS requests are made with the Internet (IN) class. To make a request in a different DNS class, use, the specify_class extension. The extension’s value (an int) contains the class number. Few applications will ever use this extension.

Extensions relating to the API

An application might want to see debugging information for queries, such as the length of time it takes for each query to return to the API. Use the return_call_reporting extension. The extension’s value is set to getdns.EXTENSION_TRUE to add the name call_reporting (a list) to the top level of the response object. Each member of the list is a dict that represents one call made for the call to the API. Each member has the following names:

  • query_name is the name that was sent
  • query_type is the type that was queried for
  • query_to is the address to which the query was sent
  • start_time is the time the query started in milliseconds since the epoch, represented as an integer
  • end_time is the time the query was received in milliseconds since the epoch, represented as an integer
  • entire_reply is the entire response received
  • dnssec_result is the DNSSEC status, or getdns.DNSSEC_NOT_PERFORMED if DNSSEC validation was not performed

Asynchronous queries

The getdns Python bindings support asynchronous queries, in which a query returns immediately and a callback function is invoked when the response data are returned. The query method interfaces are fundamentally the same, with a few differences:

  • The query returns a transaction id. That transaction id may be used to cancel future callbacks
  • The query invocation includes the name of a callback function. For example, if you’d like to call the function “my_callback” when the query returns, an address lookup could look like
>>> c = getdns.Context()
>>> tid = c.address('', callback=my_callback)
  • We’ve introduced a new Context method, called run. When your program is ready to check to see whether or not the query has returned, invoke the run() method on your context. Note that we use the libevent asynchronous event library and an event_base is associated with a context. So, if you have multiple outstanding events associated with a particular context, run will invoke all of those that are waiting and ready.
  • In previous releases the callback argument took the form of a literal string, but as of this release you may pass in the name of any Python runnable, without quotes. The newer form is preferred.

The callback script takes four arguments: type, result, userarg, and transaction_id.  The ``type argument contains the callback type, which may have one of the following values:

  • getdns.CALLBACK_COMPLETE: The query was successful and the results are contained in the result argument
  • getdns.CALLBACK_CANCEL: The callback was cancelled before the results were processed
  • getdns.CALLBACK_TIMEOUT: The query timed out before the results were processed
  • getdns.CALLBACK_ERROR: An unspecified error occurred

The result argument contains a result object, with the query response

The userarg argument contains the optional user argument that was passed to the query at the time it was invoked.

The transaction_id argument contains the transaction_id associated with a particular query; this is the same transaction id that was returned when the query was invoked.

This is an example callback function:

def cbk(type, result, userarg, tid):
    if type == getdns.CALLBACK_COMPLETE:
        status = result.status
        if status == getdns.RESPSTATUS_GOOD:
            for addr in result.just_address_answers:
                addr_type = addr['address_type']
                addr_data = addr['address_data']
                print '{0}: {1} {2}'.format(userarg, addr_type, addr_data)
        elif status == getdns.RESPSTATUS_NO_SECURE_ANSWERS:
            print "{0}: No DNSSEC secured responses found".format(hostname)
            print "{0}: getdns.address() returned error: {1}".format(hostname, status)
    elif type == getdns.CALLBACK_CANCEL:
        print 'Callback cancelled'
    elif type == getdns.CALLBACK_TIMEOUT:
        print 'Query timed out'
        print 'Unknown error'