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net: doc: Add information of DNS service discovery

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Add documentation how to do DNS service discovery queries.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
This commit is contained in:
Jukka Rissanen 2025-06-16 18:16:03 +03:00 • committed by Anas Nashif
commit 88671dbbba
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107
doc/connectivity/networking/api/dns_resolve.rst
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@ -32,6 +32,113 @@ 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
`IETF RFC6763 <https://datatracker.ietf.org/doc/html/rfc6763>`_
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 == AF_INET) {
net_addr_ntop(AF_INET,
&net_sin(&info->ai_addr)->sin_addr,
str, NET_IPV4_ADDR_LEN);
} else if (info->ai_family == AF_INET6) {
net_addr_ntop(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
************