ares_dns_class_t, ares_dns_flags_t, ares_dns_opcode_t, ares_dns_parse, ares_dns_rcode_t, ares_dns_record_create, ares_dns_record_destroy, ares_dns_record_get_flags, ares_dns_record_get_id, ares_dns_record_get_opcode, ares_dns_record_get_rcode, ares_dns_record_query_add, ares_dns_record_query_cnt, ares_dns_record_query_get, ares_dns_rec_type_t, ares_dns_write - DNS Record parsing, writing, creating and destroying functions.


#include <ares.h>
void ares_dns_record_destroy(ares_dns_record_t *dnsrec);
ares_status_t ares_dns_parse(const unsigned char *buf,
                             size_t buf_len, unsigned int flags,
                             ares_dns_record_t **dnsrec);
ares_status_t ares_dns_write(ares_dns_record_t *dnsrec,
                             unsigned char **buf, size_t *buf_len);
ares_status_t ares_dns_record_create(ares_dns_record_t **dnsrec,
                                     unsigned short      id,
                                     unsigned short      flags,
                                     ares_dns_opcode_t   opcode,
                                     ares_dns_rcode_t    rcode);
unsigned short ares_dns_record_get_id(const ares_dns_record_t *dnsrec);
unsigned short ares_dns_record_get_flags(const ares_dns_record_t *dnsrec);
ares_dns_opcode_t ares_dns_record_get_opcode(const ares_dns_record_t *dnsrec);
ares_dns_rcode_t ares_dns_record_get_rcode(const ares_dns_record_t *dnsrec);
ares_status_t ares_dns_record_query_add(ares_dns_record_t  *dnsrec,
                                        const char         *name,
                                        ares_dns_rec_type_t qtype,
                                        ares_dns_class_t    qclass);
size_t ares_dns_record_query_cnt(const ares_dns_record_t *dnsrec);
ares_status_t ares_dns_record_query_get(const ares_dns_record_t *dnsrec,
                                        size_t idx, const char **name,
                                        ares_dns_rec_type_t *qtype,
                                        ares_dns_class_t *qclass);


ares_dns_rec_type_t - DNS Record types handled by c-ares. Some record types may only be valid on requests, and some may only be valid on responses:

ARES_REC_TYPE_A - Host address
ARES_REC_TYPE_NS - Authoritative server
ARES_REC_TYPE_CNAME - Canonical name
ARES_REC_TYPE_SOA - Start of authority zone
ARES_REC_TYPE_PTR - Domain name pointer
ARES_REC_TYPE_HINFO - Host information
ARES_REC_TYPE_MX - Mail routing information
ARES_REC_TYPE_TXT - Text strings
ARES_REC_TYPE_AAAA - RFC 3596. Ip6 Address
ARES_REC_TYPE_SRV - RFC 2782. Server Selection
ARES_REC_TYPE_NAPTR - RFC 3403. Naming Authority Pointer
ARES_REC_TYPE_OPT - RFC 6891. EDNS0 option (meta-RR). Pseudo Record.
ARES_REC_TYPE_TLSA - RFC 6698. DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA
ARES_REC_TYPE_SVCB - RFC 9460. General Purpose Service Binding
ARES_REC_TYPE_HTTPS - - RFC 9460. Service Binding type for use with HTTPS
ARES_REC_TYPE_ANY - Wildcard match. Not response RR
ARES_REC_TYPE_URI - RFC 7553. Uniform Resource Identifier
ARES_REC_TYPE_CAA - RFC 6844. Certification Authority Authorization
ARES_REC_TYPE_RAW_RR - Used as an indicator that the RR record is not parsed, but provided in wire format

ares_dns_class_t - DNS Classes for requests and responses:

ARES_CLASS_IN - Internet
ARES_CLASS_HESOID - Hesoid [Dyer 87]
ARES_CLASS_ANY - Any class (requests only)

ares_dns_opcode_t - DNS Header Opcodes:

ARES_OPCODE_QUERY - Standard query
ARES_OPCODE_IQUERY - Inverse query. Obsolete
ARES_OPCODE_STATUS - Name server status query
ARES_OPCODE_NOTIFY - Zone change notification (RFC 1996)
ARES_OPCODE_UPDATE - Zone update message (RFC 2136)

ares_dns_flags_t - DNS Header Flags:

ARES_FLAG_QR - QR. If set, is a response
ARES_FLAG_AA - Authoritative Answer. If set, is authoritative
ARES_FLAG_TC - Truncation. If set, is truncated response
ARES_FLAG_RD - Recursion Desired. If set, recursion is desired
ARES_FLAG_RA - Recursion Available. If set, server supports recursion
ARES_FLAG_AD - RFC 2065. Authentic Data bit indicates in a response that the data included has been verified by the server providing it
ARES_FLAG_CD - RFC 2065. Checking Disabled bit indicates in a query that non-verified data is acceptable to the resolver sending the query

ares_dns_rcode_t - DNS Response codes from server:

ARES_RCODE_FORMERR - Format error. The name server was unable to interpret the query
ARES_RCODE_SERVFAIL - Server Failure. The name server was unable to process this query due to a problem with the nameserver
ARES_RCODE_NXDOMAIN - Name Error. Meaningful only for responses from an authoritative name server, this code signifies that the domain name referenced in the query does not exist.
ARES_RCODE_NOTIMP - Not implemented. The name server does not support the requested kind of query
ARES_RCODE_REFUSED - Refused. The name server refuses to perform the specified operation for policy reasons.
ARES_RCODE_YXDOMAIN - RFC 2136. Some name that ought not to exist, does exist
ARES_RCODE_YXRRSET - RFC 2136. Some RRset that ought to not exist, does exist
ARES_RCODE_NXRRSET - RFC 2136. Some RRset that ought to exist, does not exist
ARES_RCODE_NOTAUTH - RFC 2136. The server is not authoritative for the zone named in the Zone section.
ARES_RCODE_NOTZONE - RFC 2136. A name used in the Prerequisite or Update Section is not within the zone denoted by the Zone Section.
ARES_RCODE_DSOTYPEI - RFC 8409. DSO-TYPE Not implemented
ARES_RCODE_BADSIG - RFC 8945. TSIG Signature Failure
ARES_RCODE_BADKEY - RFC 8945. Key not recognized
ARES_RCODE_BADTIME - RFC 8945. Signature out of time window
ARES_RCODE_BADNAME - RFC 2930. Duplicate Key Name
ARES_RCODE_BADALG - RFC 2930. Algorithm not supported
ARES_RCODE_BADTRUNC - RFC 8945. Bad Truncation
ARES_RCODE_BADCOOKIE - RFC 7973. Bad/missing Server Cookie


The ares_dns_record_destroy function destroys the memory associated with the dns record created by either ares_dns_record_create or ares_dns_parse passed in via dnsrec.

The ares_dns_parse function parses the buffer provided in buf with length provided in buf_len. The flags parameter can be one or more ares_dns_parse_flags_t, or zero if no flags are needed. The resulting dns record data structure is stored into the variable pointed to by dnsrec and must be destroyed using ares_dns_record_destroy.

The ares_dns_write function takes a populated DNS record structure in dnsrec and writes a wire-format DNS message into the variable pointed to by buf and writes the length of the buffer into the variable pointed to by buf_len. The buffer must be destroyed using ares_free_string.

The ares_dns_record_create function creates an empty DNS record structure in the variable pointed to by dnsrec. The id parameter is the DNS message id, however if passing to ares_send this identifier will be overwritten, so should typically be 0. The flags parameter is one or more ares_dns_flags_t. The opcode is passed in the opcode parameter and should typically be ARES_OPCODE_QUERY. The response code is meant mostly for responses and is passed in the rcode parameter and is typically ARES_RCODE_NOERROR.

The ares_dns_record_get_id function is used to retrieve the DNS message id from the DNS record provided in the dnsrec parameter.

The ares_dns_record_get_flags function is used to retrieve the DNS message flags from the DNS record provided in the dnsrec parameter.

The ares_dns_record_get_opcode function is used to retrieve the DNS message flags from the DNS record provided in the dnsrec parameter.

The ares_dns_record_get_rcode function is used to retrieve the DNS message response code from the DNS record provided in the dnsrec parameter.

The ares_dns_record_query_add function is used to add a question to the DNS record provided in the dnsrec parameter. The domain name specified for the question is provided in the name parameter, along with the question type in the qtype parameter and the question class (typically ARES_CLASS_IN) in the qclass parameter.

The ares_dns_record_query_cnt function is used to retrieve the number of DNS questions in the DNS record provided in the dnsrec parameter.

The ares_dns_record_query_get function is used to retrieve the details of a single DNS question in the provided dnsrec parameter. The index provided in the idx parameter must be less than the value returned from ares_dns_record_query_cnt. The DNS question name will be returned in the variable pointed to by the name parameter, this may be provided as NULL if the name is not needed. The DNS question type will be returned in the variable pointed to by the qtype parameter, this may be provided as NULL if the type is not needed. The DNS question class will be returned in the variable pointed to by the qclass parameter, this may be provided as NULL if the class is not needed.

Return values

ares_dns_parse, ares_dns_write, ares_dns_record_create, ares_dns_record_query_add, and ares_dns_record_query_get all return an ares_status_t error code. ARES_SUCCESS is returned on success, ARES_ENOMEM is returned on out of memory, ARES_EFORMERR is returned on misuse.

ares_dns_record_get_id, ares_dns_record_get_flags, ares_dns_record_get_opcode, ares_dns_record_get_rcode, and ares_dns_record_query_cnt all returned their prescribed datatype values and in general can't fail except for misuse cases, in which a 0 may be returned, however 0 can also be a valid return value for most of these functions.


These functions were first introduced in c-ares version 1.22.0.

See also

ares_dns_mapping(3), ares_dns_rr(3), ares_free_string(3)


Copyright (C) 2023 The c-ares project and its members.

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