How to get the border router ip address from an end device

[10andy0]

Hi there,

Is there an API (CLI) to get list of border routers in the network form an end device?
I saw some articles where DNS-SD and SRP service discovery protocols are suggested.

With DNS-SD could not find any examples on how to query for _meshcop._udp service from an end device (FTD or MTD),
only examples on how to do it from Linux side.

With SRP I had to know an border router IP address in order to get it working (a chicken and egg situation)

On the border router I added coap-srv service, as follows:

> srp client host name my-br
Done
> srp client host address auto
Done
> srp client service add coap-srv _ipps._udp 12345
Done
> srp client autostart enable
Done
> srp server host
my-br.default.service.arpa.
    deleted: false
    addresses: [xxxxx]

Then I had to run

ipaddr mleid
yyyyy

to get the IP address to use it on an MTD node, as follows:

> dns config yyyyy
Done
> dns service coap-srv _ipps._udp.default.service.arpa.
DNS service resolution response for coap-srv for service _ipps._udp.default.service.arpa.
Port:12345, Priority:0, Weight:0, TTL:6494
Host:my-br.default.service.arpa.
HostAddress:xxxxx TTL:6494
TXT:[] TTL:6494

It seems to me that I am doing something wrong here. Any suggestions are welcome.

Cheers,
Andy

[wgtdkp]

I think you can use > dns service _meshcop._udp.default.service.arpa.. The host address should be the address of the infrastructure interface on the Border Router, but can still reach to that address from a Thread end device

[10andy0]

Thank you for your reply, but this doesn't work:

> dns service _meshcop._udp.default.service.arpa.
Error 7: InvalidArgs

[10andy0]

Let me add some practical example here. Let's say there is some service running on the border router, say CoAP server and I would like to connect to the server from an end device in the Thread network. How can I connect from the device to the server without IP address hardcoding? Reference to any examples, articles, documentation, etc. is appreciated.

[wgtdkp]

Sorry, you should use

dns browse _meshcop._udp.default.service.arpa.

to discover available BRs

then

dns service <service-instance-name> _meshcop._udp.default.service.arpa.

"service-instance-name" should be the name returned in the "dns browse" command

Let me add some practical example here. Let's say there is some service running on the border router, say CoAP server and I would like to connect to the server from an end device in the Thread network. How can I connect from the device to the server without IP address hardcoding? Reference to any examples, articles, documentation, etc. is appreciated.

Even with the above "_mshcop._udp" works, I think the idea of registering a specific "coap-srv" of your own type is the best approach - so that you can add your own service configuration info in the TXT data

[10andy0]

That's ok.

> dns browse _meshcop._udp.default.service.arpa.
DNS browse response for _meshcop._udp.default.service.arpa.
Error 28: ResponseTimeout

it works only when prior to that > dns config yyy was run. Where yyy is a boarder router ip address.

[abtink]

With SRP I had to know an border router IP address in order to get it working (a chicken and egg situation)

The AutoEnable APIs in SRP client will handle this:

openthread/include/openthread/srp_client.h

Lines 271 to 310 in c73829f

	/**
	* Enables the auto-start mode.
	*
	* This is only available when auto-start feature `OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_API_ENABLE` is enabled.
	*
	* Config option `OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_DEFAULT_MODE` specifies the default auto-start mode (whether
	* it is enabled or disabled at the start of OT stack).
	*
	* When auto-start is enabled, the SRP client will monitor the Thread Network Data to discover SRP servers and select
	* the preferred server and automatically start and stop the client when an SRP server is detected.
	*
	* There are three categories of Network Data entries indicating presence of SRP sever. They are preferred in the
	* following order:
	*
	* 1) Preferred unicast entries where server address is included in the service data. If there are multiple options,
	* the one with numerically lowest IPv6 address is preferred.
	*
	* 2) Anycast entries each having a seq number. A larger sequence number in the sense specified by Serial Number
	* Arithmetic logic in RFC-1982 is considered more recent and therefore preferred. The largest seq number using
	* serial number arithmetic is preferred if it is well-defined (i.e., the seq number is larger than all other
	* seq numbers). If it is not well-defined, then the numerically largest seq number is preferred.
	*
	* 3) Unicast entries where the server address info is included in server data. If there are multiple options, the
	* one with numerically lowest IPv6 address is preferred.
	*
	* When there is a change in the Network Data entries, client will check that the currently selected server is still
	* present in the Network Data and is still the preferred one. Otherwise the client will switch to the new preferred
	* server or stop if there is none.
	*
	* When the SRP client is explicitly started through a successful call to `otSrpClientStart()`, the given SRP server
	* address in `otSrpClientStart()` will continue to be used regardless of the state of auto-start mode and whether the
	* same SRP server address is discovered or not in the Thread Network Data. In this case, only an explicit
	* `otSrpClientStop()` call will stop the client.
	*
	* @param[in] aInstance A pointer to the OpenThread instance.
	* @param[in] aCallback A callback to notify when client is auto-started/stopped. Can be NULL if not needed.
	* @param[in] aContext A context to be passed when invoking @p aCallback.
	*
	*/
	void otSrpClientEnableAutoStartMode(otInstance *aInstance, otSrpClientAutoStartCallback aCallback, void *aContext);

You can also get the address of the currently selectd server from client:

openthread/include/openthread/srp_client.h

Lines 245 to 256 in c73829f

	/**
	* Gets the socket address (IPv6 address and port number) of the SRP server which is being used by SRP
	* client.
	*
	* If the client is not running, the address is unspecified (all zero) with zero port number.
	*
	* @param[in] aInstance A pointer to the OpenThread instance.
	*
	* @returns A pointer to the SRP server's socket address (is always non-NULL).
	*
	*/
	const otSockAddr *otSrpClientGetServerAddress(otInstance *aInstance);

BRs that provide SRP server (or DNS resolver/Adv-proxy) function will publish specific "SRP/DNS service entries" in Network Data. This is parsed and monirtoed by SRP client and/or DNS client.

---

A related config for how DNS client will pick the resolver/server address:

openthread/src/core/config/dns_client.h

Lines 93 to 107 in c73829f

	/**
	* @def OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_ADDRESS_AUTO_SET_ENABLE
	*
	* Set to 1 for DNS client to automatically set and update the server IPv6 address in the default config (when it is
	* not explicitly set by user).
	*
	* This feature requires SRP client and its auto-start feature to be also enabled. SRP client will then monitor the
	* Thread Network Data for DNS/SRP Service entries to select an SRP server. The selected SRP server address is also set
	* as the DNS server address in the default config.
	*
	*/
	#ifndef OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_ADDRESS_AUTO_SET_ENABLE
	#define OPENTHREAD_CONFIG_DNS_CLIENT_DEFAULT_SERVER_ADDRESS_AUTO_SET_ENABLE \
	(OPENTHREAD_CONFIG_SRP_CLIENT_ENABLE && OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_API_ENABLE)
	#endif

[10andy0]

Thank you Abtin! This is exactly what I was looking for!

[10andy0]

Resolved!