libgetdns(3) getdns libgetdns(3)NAMElibgetdns-- an implementation of a modern asynchronous DNS API by and
for application developers
LIBRARY
DNS Resolver library (libgetdns, -lgetdns)
SYNOPSISlibgetdns
This man page describes the getdns library, the general concepts behind
the API and some of the common elements of the public interface to the
library. Each of the public entry points and more complex data types
are captured in separate man pages.
DESCRIPTION
getdns is modern asynchronous DNS API intended to be useful to applica‐
tion developers and operating system distributors as a way of making
all types of DNS information easily available in many types of pro‐
grams. The major features of this new API are:
Full support for event-driven programming
Supports DNSSEC in multiple ways
Mirroring of the resolution in getaddrinfo()
Easily supports all RRtypes, even those yet to be defined
Each of the entry points is offered with both asynchronous and synchro‐
nous signatures. The asynchronous functions rely on event handling and
callback via libevent. Functions are thread safe.
A context structure maintains DNS query and response data and is used
to maintain state during calls to the public entry points.
The project page for this implementation is at
http://getdnsapi.net
The specification is maintained at
http://getdnsapi.net/spec
The git repository for this implementation is at
http://github.com/getdnsapi/getdns
DATA STRUCTURES
The API uses a few data structures to pass data into and return data
from the public entry points.
list an ordered list, the members of the list can be any of the four
data types.
dict a name-value pair. The name is a string literal, and the value
can be any of the four data types. The order of the name-value
pairs in a dict is not important.
int an integer compatible with uint32_t.
bindata
a struct used to hold binary data defined as { size_t size;
uint8_t *binary_stuff; }.
ASYNCHRONOUS USE
The getdns specification emphasizes the asynchronous nature of the API
and allows implementations to define their own approach. This page doc‐
uments this implementation's decisions and facilities provided to the
developer.
This implementation provides asynchronous support via the following
mechanisms:
File Descriptor Polling
Event Loop Integrations:
libevent
libuv
libev
Custom Event Loop Integrations
All functions and types discussed in this page are declared in
getdns_extra.h
Build-in Event loop
The library has an built in event loop that can be used if none of the
extensions for external event loops are used. The library will execute
requests and dispatch callbacks with a call to getdns_context_run().
If an event loop extension is used, this will run the extension's
eventloop.
void getdns_context_run(getdns_context *context)
Run the context's event loop until nothing more to do.
uint32_t getdns_context_get_num_pending_requests(getdns_context* con‐
text, struct timeval* next_timeout)
Get the number of outstanding asynchronous requests for a given con‐
text as well as the the amount of time until the next timeout. The
next_timeout struct can be NULL. If supplied and the number of out‐
standing requests is > 0, then the timeout represents the relative
time until the next timeout.
getdns_return_t getdns_context_process_async(getdns_context* context)
Inform the context to process its outstanding requests. Users
should call this when either a timeout has occurred or the file
descriptor signals that it is ready. User callbacks are fired dur‐
ing this call.
Included Event Loop Integrations
A number of applications achieve asynchronous behavior by leveraging
event loop abstraction libraries. If the build system discovers a sup‐
ported event loop, the event loop extension is built in addition to the
getdns library. Extensions are built as an additional shared library.
The following event loop libraries are supported:
libevent1 and libevent2
The libevent extension allows a context to attach to a event_base. The
event loop is then run like any other application using libevent via
event_base_dispatch or event_base_loop and expect getdns callbacks to
fire.
Note that if both libevent1 and libevent2 reside on system, the exten‐
sion uses libevent2.
Extension library: libgetdns_ext_event.[shared_lib_ext]
Extension header: getdns/getdns_ext_libevent.h
libuv
The libuv extension allows a context to attach to a uv_loop_s. The
event loop can then be run like any other application using libuv via
uv_run and expect getdns callbacks to fire.
Extension library: libgetdns_ext_uv.[shared_lib_ext]
Extension header: getdns_ext_libuv.h
libev
The libev extension allows a context to attach to a ev_loop. The event
loop can then be run like any other application using libev via ev_run
and expect getdns callbacks to fire.
Extension library: libgetdns_ext_ev.[shared_lib_ext]
Extension header: getdns_ext_libev.h
getdns_context event loop extension functions
The following are functions used by the extension entry point to attach
to a particular context.
The application sets an event loop extension on a context. The exten‐
sion_data is optional data that is passed into the extension methods.
If an event loop is already set on a context then it is cleaned up.
All outstanding requests are also canceled.
getdns_return_t getdns_extension_set_eventloop(struct
getdns_context* context, getdns_eventloop_extension* extension,
void* extension_data);
The application gets the extension data associated with a context.
void* getdns_context_get_extension_data(struct getdns_context*
context);
When no more work must be done the application detaches an event loop
from a context
getdns_return_t getdns_extension_detach_eventloop(struct
getdns_context* context);
SYNCHRONOUS USE
There are four synchronous functions parallel to the four getdns async
functions, except that there is no callback parameter. When an applica‐
tion calls one of these synchronous functions, the API gathers all the
required information and then returns the result. The value returned is
exactly the same as the response returned in the callback if you had
used the async version of the function.
When you are done with the data in the response, call
getdns_free_sync_request_memory so that the API can free the memory
from its internal pool.
EXTENSIONS
Applications may populate an extension dictionary when making a call to
the public entry points. To use an extension add it to the extension
dictionary prior to making the call to the public entry point and set
the value depending on the behavior you expect. These extensions
include:
"dnssec_return_status" (int)
Set to GETDNS_EXTENSION_TRUE to include the DNSSEC status for each
DNS record in the replies_tree
"dnssec_return_only_secure" (int)
Set to GETDNS_EXTENSION_TRUE to cause only records that the API can
validate as secure withe DNSSEC to be returned in the replies_tree
and replies_full lists
"dnssec_return_validation_chain" (int)
Set to GETDNS_EXTENSION_TRUE to cause the set of additional DNSSEC-
related records needed for validation to be returned in the response
object as the list named additional_dnssec at the top level of the
response object
"return_both_v4_and_v6" (int)
Set to GETDNS_EXTENSION_TRUE to cause the results of both A and AAAA
records for the queried name to be included in the response object.
"add_opt_parameters" (dict)
TBD (complicated)
"add_warning_for_bad_dns"
Set to GETDNS_EXTENSION_TRUE to cause each reply in the replies_tree
to contain an additional name whose data type is a list, bad_dns
which contains zero or more ints that indicate the types of bad DNS
found in the reply.
GETDNS_BAD_DNS_CNAME_IN_TARGET: query type does not allow a CNAME
pointed to a CNAME
GETDNS_BAD_DNS_ALL_NUMERIC_LABEL: one or more labels is all
numeric
GETDNS_BAD_DNS_CNAME_RETURNED_FOR_OTHER_TYPE: query type for
other than CNAME returned a CNAME
"specify_class" (int)
Set to the DNS class number (other than Internet (IN) class desired
in query.
"return_call_reporting" (int)
Set to GETDNS_EXTENSION_TRUE to add the name call_reporting (list)
to the top level of the response object that includes a dict for
each call made to the API. TBD: more detail
This implementation of the getdns API is licensed under the BSD
license.
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. 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. The extension's value (an
int) is set to GETDNS_EXTENSION_TRUE to cause the returned status to
have the name dnssec_status (an int) added to the other names in the
record's dict ("header", "question", and so on). The values for that
name are GETDNS_DNSSEC_SECURE, GETDNS_DNSSEC_BOGUS, GETDNS_DNSSEC_INDE‐
TERMINATE, and GETDNS_DNSSEC_INSECURE. Thus, a reply might look like:
{ # This is the first reply
"dnssec_status": GETDNS_DNSSEC_INDETERMINATE,
"header": { "id": 23456, "qr": 1, "opcode": 0, ... },
. . .
If instead of returning the status, you want to only see secure
results, use the dnssec_return_only_secure extension. The extension's
value (an int) 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_RESPSTA‐
TUS_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. The extension's value (an
int) is set 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. Thus, a reply
might look like:
{ # This is the response object
"validation_chain":
[ { "name": <bindata for .>,
"type": GETDNS_RRTYPE_DNSKEY,
"rdata": { "flags": 256, . . . },
. . .
},
{ "name": <bindata for .>,
"type": GETDNS_RRTYPE_DNSKEY,
"rdata": { "flags": 257, . . . },
. . .
},
{ "name": <bindata for .>,
"type": GETDNS_RRTYPE_RRSIG,
"rdata": { "signers_name": <bindata for .>,
"type_covered": GETDNS_RRTYPE_DNSKEY,
. . .
},
},
{ "name": <bindata for com.>,
"type": GETDNS_RRTYPE_DS,
. . .
},
{ "name": <bindata for com.>,
"type": GETDNS_RRTYPE_RRSIG
"rdata": { "signers_name": <bindata for .>,
"type_covered": GETDNS_RRTYPE_DS,
. . .
},
. . .
},
{ "name": <bindata for com.>,
"type": GETDNS_RRTYPE_DNSKEY
"rdata": { "flags": 256, . . . },
. . .
},
{ "name": <bindata for com.>,
"type": GETDNS_RRTYPE_DNSKEY
"rdata": { "flags": 257, . . . },
. . .
},
{ "name": <bindata for com.>,
"type": GETDNS_RRTYPE_RRSIG
"rdata": { "signers_name": <bindata for com.>,
"type_covered": GETDNS_RRTYPE_DNSKEY,
. . .
},
. . .
},
{ "name": <bindata for example.com.>,
"type": GETDNS_RRTYPE_DS,
. . .
},
{ "name": <bindata for example.com.>,
"type": GETDNS_RRTYPE_RRSIG
"rdata": { "signers_name": <bindata for com.>,
"type_covered": GETDNS_RRTYPE_DS,
. . .
},
. . .
},
{ "name": <bindata for example.com.>,
"type": GETDNS_RRTYPE_DNSKEY
"rdata": { "flags": 257, ... },
. . .
},
. . .
]
"replies_tree":
[
. . .
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.
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 int) between 512 and 65535; if not speci‐
fied, this defaults to those from the DNS context
extended_rcode (an int) between 0 and 255; if not specified, this
defaults to those from the DNS context
version (an int) between 0 and 255; if not specified, this defaults to
0
do_bit (an int) between 0 and 1; if not specified, this defaults to
those from the DNS context
options (a list) contains dicts for each option to be specified. Each
list time contains two names: option_code (an int) and option_data
(a bindata). The API marshalls the entire set of options into a
properly-formatted RDATA for the resource record.
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 use of this extension can conflict with the values in the DNS con‐
text. For example, the default for an OS might be a maximum payload
size of 65535, but the extension might specify 1550. In such a case,
the API will honor the values stated in the extension, but will honor
the values from the DNS context if values are not given in the exten‐
sion.
RESPONSE DATA
The callback function contains a pointer to a response object. A
response object is always a dict. The response object always contains
at least three names: replies_full (a list) and replies_tree (a list),
and status (an int). replies_full is a list of DNS replies (each is
bindata) as they appear on the wire. replies_tree is a list of DNS
replies (each is a dict) with the various part of the reply parsed out.
status is a status code for the query.
Because the API might be extended in the future, a response object
might also contain names other than replies_full, replies_tree, and
status. Similarly, any of the dicts described here might be extended in
later versions of the API. Thus, an application using the API must not
assume that it knows all possible names in a dict.
The following lists the status codes for response objects. Note that,
if the status is that there are no responses for the query, the lists
in replies_full and replies_tree will have zero length.
GETDNS_RESPSTATUS_GOOD At least one response was returned
GETDNS_RESPSTATUS_NO_NAME Queries for the name yielded all negative
responses
GETDNS_RESPSTATUS_ALL_TIMEOUT All queries for the name timed out
GETDNS_RESPSTATUS_NO_SECURE_ANSWERS The context setting for getting
only secure responses was specified, and at least one DNS response
was received, but no DNS response was determined to be secure
through DNSSEC.
The top level of replies_tree can optionally have the following names:
canonical_name (a bindata), intermediate_aliases (a list),
answer_ipv4_address (a bindata), answer_ipv6_address (a bindata), and
answer_type (an int).
The value of canonical_name is the name that the API used for its
lookup. It is in FQDN presentation format. The values in the interme‐
diate_aliases list are domain names from any CNAME or unsynthesized
DNAME found when resolving the original query. The list might have zero
entries if there were no CNAMEs in the path. These may be useful, for
example, for name comparisons when following the rules in RFC 6125.
The value of answer_ipv4_address and answer_ipv6_address are the
addresses of the server from which the answer was received. The value
of answer_type is the type of name service that generated the response.
The values are:
GETDNS_NAMETYPE_DNS
Normal DNS (RFC 1035)
GETDNS_NAMETYPE_WINS
The WINS name service (some reference needed)
If the call was getdns_address or getdns_address_sync, the top level of
replies_tree has an additional name, just_address_answers (a list). The
value of just_address_answers is a list that contains all of the A and
AAAA records from the answer sections of any of the replies, in the
order they appear in the replies. Each item in the list is a dict with
at least two names: address_type (whose value is a bindata; it is cur‐
rently either "IPv4" or "IPv6") and address_data (whose value is a
bindata). Note that the dnssec_return_only_secure extension affects
what will appear in the just_address_answers list. If the DNS returns
other address types, those types will appear in this list as well.
The API can make service discovery through SRV records easier. If the
call was getdns_service or getdns_service_sync, the top level of
replies_tree has an additional name, srv_addresses (a list). The list
is ordered by priority and weight based on the weighting algorithm in
RFC 2782, lowest priority value first. Each element of the list is dict
has at least two names: port and domain_name. If the API was able to
determine the address of the target domain name (such as from its cache
or from the Additional section of responses), the dict for an element
will also contain address_type (whose value is a bindata; it is cur‐
rently either "IPv4" or "IPv6") and address_data (whose value is a
bindata). Note that the dnssec_return_only_secure extension affects
what will appear in the srv_addresses list.
STRUCTURE OF DNS REPLIES_TREE
The names in each entry in the the replies_tree list for DNS responses
include header (a dict), question (a dict), answer (a list), authority
(a list), and additional (a list), corresponding to the sections in the
DNS message format. The answer, authority, and additional lists each
contain zero or more dicts, with each dict in each list representing a
resource record.
The names in the header dict are all the fields from Section 4.1.1. of
RFC 1035. They are: id, qr, opcode, aa, tc, rd, ra, z, rcode, qdcount,
ancount, nscount, and arcount. All are ints.
The names in the question dict are the three fields from Section 4.1.2.
of RFC 1035: qname (a bindata), qtype (an int), and qclass (an int).
Resource records are a bit different than headers and question sections
in that the RDATA portion often has its own structure. The other names
in the resource record dicts are name (a bindata), type (an int), class
(an int), ttl (an int) and rdata (a dict); there is no name equivalent
to the RDLENGTH field.
The rdata dict has different names for each response type. There is a
complete list of the types defined in the API. For names that end in
"-obsolete" or "-unknown", the bindata is the entire RDATA field. For
example, the rdata for an A record has a name ipv4_address (a bindata);
the rdata for an SRV record has the names priority (an int), weight (an
int), port (an int), and target (a bindata).
Each rdata dict also has a rdata_raw field (a bindata). This is useful
for types not defined in this version of the API. It also might be of
value if a later version of the API allows for additional parsers.
Thus, doing a query for types not known by the API still will return a
result: an rdata with just a rdata_raw.
It is expected that later extensions to the API will give some DNS
types different names. It is also possible that later extensions will
change the names for some of the DNS types listed above.
CALLBACK FUNCTIONS
A call to the async getdns functions typically returns before any net‐
work or file I/O occurs. After the API marshalls all the needed infor‐
mation, it calls the callback function that was passed by the applica‐
tion. The callback function might be called at any time, even before
the calling function has returned. The API guarantees that the callback
will be called exactly once unless the calling function returned an
error, in which case the callback function is never called.
The getdns calling function calls the callback with the parameters
defined as follows:
typedef void (*getdns_callback_t)(
getdns_context_t context,
uint16_t callback_type,
getdns_dict *response,
void *userarg,
getdns_transaction_t transaction_id)
context see getdns_context (3)
callback_type Supplies the reason for the callback.
GETDNS_CALLBACK_COMPLETE The response has the requested data in it
GETDNS_CALLBACK_CANCEL The calling program canceled the callback;
response is NULL
GETDNS_CALLBACK_TIMEOUT The requested action timed out; response is
NULL
GETDNS_CALLBACK_ERROR The requested action had an error; response is
NULL
response A response object with the response data. This is described in
the section titled "RESPONSE DATA" elsewhere in this manual page.
The response object is part of the API's memory space, and will be
freed by the API with the callback returns.
userarg Identical to the userarg passed to the calling function.
transaction_id The transaction identified assigned by the calling func‐
tion, used to associate a DNS response to a specific DNS request.
To cancel an outstanding callback, use the following function.
getdns_return_t
getdns_cancel_callback (getdns_context_t context, getdns_transac‐
tion_t transaction_id)
This causes the API to call the callback with a callback_type of
GETDNS_CALLBACK_CANCEL if the callback for this transaction_id has not
already been called. The callback code for cancellation should clean up
any memory related to the identified call, such as to deallocate the
memory for the userarg. getdns_cancel_callback() may return immedi‐
ately, even before the callback finishes its work and returns. Calling
getdns_cancel_callback() with a transaction_id of a callback that has
already been called or an unknown transaction_id returns
GETDNS_RETURN_UNKNOWN_TRANSACTION; otherwise, getdns_cancel_callback()
returns GETDNS_RETURN_GOOD.
FILESEXAMPLES
TBD
DIAGNOSTICS
TBD
SEE ALSOgetdns_address(3), getdns_bindata(3), getdns_context(3), getdns_con‐
vert(3), getdns_dict(3), getdns_general(3), getdns_hostname(3),
getdns_list(3), getdns_root_trust_anchor(3)getdns_service(3)getdns_validate_dnssec(3)REPORTING PROBLEMS
Bug reports should be sent to the getdns-bugs@getdns.net
AUTHORS
The getdns API was documented by Paul Hoffman. This implementation of
the getdns API was written by:
Craig Despeaux, Verisign Inc.
John Dickinson, Sinodun
Sara Dickinson, Sinodun
Neel Goyal, Verisign Inc.
Shumon Huque, Verisign Labs
Olaf Kolkman, NLnet Labs
Allison Mankin, Verisign Inc. - Verisign Labs.
Melinda Shore, No Mountain Software LLC
Willem Toorop, NLnet Labs
Gowri Visweswaran, Verisign Labs
Wouter Wijngaards, NLnet Labs
Glen Wiley, Verisign Inc.
getdns 0.9.0 December 2015 libgetdns(3)
