DNS Resolve

Overview

The DNS resolver implements a basic DNS resolver according to IETF RFC1035 on Domain Implementation and Specification. Supported DNS answers are IPv4/IPv6 addresses and CNAME.

If a CNAME is received, the DNS resolver will create another DNS query. The number of additional queries is controlled by the CONFIG_DNS_RESOLVER_ADDITIONAL_QUERIES Kconfig variable.

The multicast DNS (mDNS) client resolver support can be enabled by setting CONFIG_MDNS_RESOLVER Kconfig option. See IETF RFC6762 for more details about mDNS.

The link-local multicast name resolution (LLMNR) client resolver support can be enabled by setting the CONFIG_LLMNR_RESOLVER Kconfig variable. See IETF RFC4795 for more details about LLMNR.

For more information about DNS configuration variables, see: subsys/net/lib/dns/Kconfig. The DNS resolver API can be found at include/net/dns_resolve.h.

API Reference

group dns_resolve

DNS resolving library.

Defines

DNS_MAX_NAME_SIZE

Max size of the resolved name.

Typedefs

typedef dns_resolve_cb_t

DNS resolve callback.

The DNS resolve callback is called after a successful DNS resolving. The resolver can call this callback multiple times, one for each resolved address.

Parameters
  • status: The status of the query: DNS_EAI_INPROGRESS returned for each resolved address DNS_EAI_ALLDONE mark end of the resolving, info is set to NULL in this case DNS_EAI_CANCELED if the query was canceled manually or timeout happened DNS_EAI_FAIL if the name cannot be resolved by the server DNS_EAI_NODATA if there is no such name other values means that an error happened.
  • info: Query results are stored here.
  • user_data: The user data given in dns_resolve_name() call.

Enums

enum dns_query_type

DNS query type enum

Values:

DNS_QUERY_TYPE_A = 1

IPv4 query

DNS_QUERY_TYPE_AAAA = 28

IPv6 query

enum dns_resolve_status

Status values for the callback.

Values:

DNS_EAI_BADFLAGS = -1

Invalid value for `ai_flags’ field

DNS_EAI_NONAME = -2

NAME or SERVICE is unknown

DNS_EAI_AGAIN = -3

Temporary failure in name resolution

DNS_EAI_FAIL = -4

Non-recoverable failure in name res

DNS_EAI_NODATA = -5

No address associated with NAME

DNS_EAI_FAMILY = -6

`ai_family’ not supported

DNS_EAI_SOCKTYPE = -7

`ai_socktype’ not supported

DNS_EAI_SERVICE = -8

SRV not supported for `ai_socktype’

DNS_EAI_ADDRFAMILY = -9

Address family for NAME not supported

DNS_EAI_MEMORY = -10

Memory allocation failure

DNS_EAI_SYSTEM = -11

System error returned in `errno’

DNS_EAI_OVERFLOW = -12

Argument buffer overflow

DNS_EAI_INPROGRESS = -100

Processing request in progress

DNS_EAI_CANCELED = -101

Request canceled

DNS_EAI_NOTCANCELED = -102

Request not canceled

DNS_EAI_ALLDONE = -103

All requests done

DNS_EAI_IDN_ENCODE = -105

IDN encoding failed

Functions

int dns_resolve_init(struct dns_resolve_context *ctx, const char *dns_servers_str[], const struct sockaddr *dns_servers_sa[])

Init DNS resolving context.

This function sets the DNS server address and initializes the DNS context that is used by the actual resolver. DNS server addresses can be specified either in textual form, or as struct sockaddr (or both). Note that the recommended way to resolve DNS names is to use the dns_get_addr_info() API. In that case user does not need to call dns_resolve_init() as the DNS servers are already setup by the system.

Return
0 if ok, <0 if error.
Parameters
  • ctx: DNS context. If the context variable is allocated from the stack, then the variable needs to be valid for the whole duration of the resolving. Caller does not need to fill the variable beforehand or edit the context afterwards.
  • dns_servers_str: DNS server addresses using textual strings. The array is NULL terminated. The port number can be given in the string. Syntax for the server addresses with or without port numbers: IPv4 : 10.0.9.1 IPv4 + port : 10.0.9.1:5353 IPv6 : 2001:db8::22:42 IPv6 + port : [2001:db8::22:42]:5353
  • dns_servers_sa: DNS server addresses as struct sockaddr. The array is NULL terminated. Port numbers are optional in struct sockaddr, the default will be used if set to 0.

int dns_resolve_close(struct dns_resolve_context *ctx)

Close DNS resolving context.

This releases DNS resolving context and marks the context unusable. Caller must call the dns_resolve_init() again to make context usable.

Return
0 if ok, <0 if error.
Parameters
  • ctx: DNS context

int dns_resolve_cancel(struct dns_resolve_context *ctx, u16_t dns_id)

Cancel a pending DNS query.

This releases DNS resources used by a pending query.

Return
0 if ok, <0 if error.
Parameters
  • ctx: DNS context
  • dns_id: DNS id of the pending query

int dns_resolve_name(struct dns_resolve_context *ctx, const char *query, enum dns_query_type type, u16_t *dns_id, dns_resolve_cb_t cb, void *user_data, s32_t timeout)

Resolve DNS name.

This function can be used to resolve e.g., IPv4 or IPv6 address. Note that this is asynchronous call, the function will return immediately and system will call the callback after resolving has finished or timeout has occurred. We might send the query to multiple servers (if there are more than one server configured), but we only use the result of the first received response.

Return
0 if resolving was started ok, < 0 otherwise
Parameters
  • ctx: DNS context
  • query: What the caller wants to resolve.
  • type: What kind of data the caller wants to get.
  • dns_id: DNS id is returned to the caller. This is needed if one wishes to cancel the query. This can be set to NULL if there is no need to cancel the query.
  • cb: Callback to call after the resolving has finished or timeout has happened.
  • user_data: The user data.
  • timeout: The timeout value for the query. Possible values: K_FOREVER: the query is tried forever, user needs to cancel it manually if it takes too long time to finish >0: start the query and let the system timeout it after specified ms

struct dns_resolve_context *dns_resolve_get_default(void)

Get default DNS context.

The system level DNS context uses DNS servers that are defined in project config file. If no DNS servers are defined by the user, then resolving DNS names using default DNS context will do nothing. The configuration options are described in subsys/net/lib/dns/Kconfig file.

Return
Default DNS context.

static int dns_get_addr_info(const char *query, enum dns_query_type type, u16_t *dns_id, dns_resolve_cb_t cb, void *user_data, s32_t timeout)

Get IP address info from DNS.

This function can be used to resolve e.g., IPv4 or IPv6 address. Note that this is asynchronous call, the function will return immediately and system will call the callback after resolving has finished or timeout has occurred. We might send the query to multiple servers (if there are more than one server configured), but we only use the result of the first received response. This variant uses system wide DNS servers.

Return
0 if resolving was started ok, < 0 otherwise
Parameters
  • query: What the caller wants to resolve.
  • type: What kind of data the caller wants to get.
  • dns_id: DNS id is returned to the caller. This is needed if one wishes to cancel the query. This can be set to NULL if there is no need to cancel the query.
  • cb: Callback to call after the resolving has finished or timeout has happened.
  • user_data: The user data.
  • timeout: The timeout value for the connection. Possible values: K_FOREVER: the query is tried forever, user needs to cancel it manually if it takes too long time to finish >0: start the query and let the system timeout it after specified ms

static int dns_cancel_addr_info(u16_t dns_id)

Cancel a pending DNS query.

This releases DNS resources used by a pending query.

Return
0 if ok, <0 if error.
Parameters
  • dns_id: DNS id of the pending query

struct dns_addrinfo
#include <dns_resolve.h>

Address info struct is passed to callback that gets all the results.

struct dns_resolve_context
#include <dns_resolve.h>

DNS resolve context structure.