DNS Resolve

.. _dns_resolve_interface:

DNS Resolve ###########

.. contents:: :local: :depth: 2

Overview

The DNS resolver implements a basic DNS resolver according to :rfc:1035. 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 :kconfig:option:CONFIG_DNS_RESOLVER_ADDITIONAL_QUERIES Kconfig variable.

The multicast DNS (mDNS) client resolver support can be enabled by setting :kconfig:option:CONFIG_MDNS_RESOLVER Kconfig option. See :rfc:6762 for more details about mDNS.

The link-local multicast name resolution (LLMNR) client resolver support can be enabled by setting the :kconfig:option:CONFIG_LLMNR_RESOLVER Kconfig variable. See :rfc:4795 for more details about LLMNR.

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

DNS-based service discovery queries described in :rfc:6763 can be done by :c:func:dns_resolve_service API. The returned service descriptions are passed to user supplied callback and the API sets the address family to :c:macro:AF_LOCAL to indicate that the value is not an IPv4 or IPv6 address but a service description.

Example:

.. code-block:: c

#include <zephyr/net/dns_resolve.h>

#define MAX_STR_LEN CONFIG_DNS_RESOLVER_MAX_NAME_LEN

static void dns_result_cb(enum dns_resolve_status status, struct dns_addrinfo *info, void user_data) { if (status == DNS_EAI_CANCELED) { / dns: Timeout while resolving name */ return; }

    if (status == DNS_EAI_INPROGRESS && info) {
            char str[MAX_STR_LEN + 1];

            if (info->ai_family == NET_AF_INET) {
                    net_addr_ntop(NET_AF_INET,
                                  &net_sin(&info->ai_addr)->sin_addr,
                                  str, NET_IPV4_ADDR_LEN);
            } else if (info->ai_family == NET_AF_INET6) {
                    net_addr_ntop(NET_AF_INET6,
                                  &net_sin6(&info->ai_addr)->sin6_addr,
                                  str, NET_IPV6_ADDR_LEN);
            } else if (info->ai_family == AF_LOCAL) {
                    /* service discovery */
                    memset(str, 0, MAX_STR_LEN);
                    memcpy(str, info->ai_canonname,
                           MIN(info->ai_addrlen, MAX_STR_LEN));
            } else {
                    strncpy(str, "Invalid proto family", MAX_STR_LEN + 1);
            }

            str[MAX_STR_LEN] = '\0';

            printk("dns: %s\n", str);
            return;
    }

    if (status == DNS_EAI_ALLDONE) {
            printk("dns: All results received\n");
            return;
    }

    if (status == DNS_EAI_FAIL) {
            printk("dns: No such name found.\n");
            return;
    }

    printk("dns: Unhandled status %d received (errno %d)\n", status, errno);

}

#define DNS_TIMEOUT (MSEC_PER_SEC * 5) /* in ms */

static void discover_service(void) { int ret = dns_resolve_service(dns_resolve_get_default(), "_http._tcp.dns-sd.org", NULL, dns_result_cb, NULL, DNS_TIMEOUT); ... }

The above query would return output like this:

.. code-block: console

Query for '_http._tcp.dns-sd.org' sent.
dns: . * cnn, world news._http._tcp.dns-sd.org
dns: .source de télévision, département de langues._http._tcp.dns-sd.org
dns: . * multicast dns._http._tcp.dns-sd.org
dns: . * amazon.com, on-line shopping._http._tcp.dns-sd.org
dns: . * google, searching the web._http._tcp.dns-sd.org
dns: . * ebay, online auctions._http._tcp.dns-sd.org
dns: . * apple, makers of the ipod._http._tcp.dns-sd.org
dns: . * yahoo, maps, weather, and stock quotes._http._tcp.dns-sd.org
dns: .about bonjour in web browsers._http._tcp.dns-sd.org
dns: .π._http._tcp.•bullets•.dns-sd.org
dns: . * dns service discovery._http._tcp.dns-sd.org
dns: . * wired, technology, culture, business, politics._http._tcp.dns-sd.org
dns: . * slashdot, news for nerds, stuff that matters._http._tcp.dns-sd.org
dns: . * bbc, world news._http._tcp.dns-sd.org
dns: .stuart’s printer._http._tcp.dns-sd.org
dns: . * zeroconf._http._tcp.dns-sd.org
dns: All results received

As the service discovery query could return long strings and the packet size could be large, you might need to adjust following Kconfig options:

• :kconfig:option:CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE. This tells the maximum size of the answer, typical value for this option could be 1024. The default size for this option is 512 bytes.

• :kconfig:option:CONFIG_DNS_RESOLVER_MAX_NAME_LEN. This tells the maximum length of the returned name. The value depends on your expected data size, typical value might be 128 bytes.

Sample usage

See :zephyr:code-sample:dns-resolve sample application for details.

API Reference

.. doxygengroup:: dns_resolve