SRP Client Registration with TXT Records

[mmb-davidsmith]

Hi,
I'm currently trying to build a device which registers itself with an SRP server when it connects to the network. The registration is working, but when I try to add TXT records to my registration, things start to behave in ways that I do not expect.

I start by creating my otSrpClientBuffersServiceEntry * variable and initializing it with otSrpClientBuffersAllocateService. Reading the documentation, this indicates that it should be configured to point to the single mTxtEntry option.

At this point, I'm looking to add the string pv=1 to my service entry. When I start to dig into the documentation, it seems like I should have two options to do this.

1. If I set mKey, then I should be able to set my value to the string "1" as follows

uint16_t size;
uint8_t *txtBuffer = otSrpClientBuffersGetServiceEntryTxtBuffer(serviceEntry, &size);
assert(NULL != txtBuffer);
assert(0 != size);
serviceEntry->mTxtEntry.mKey = "pv";
serviceEntry->mTxtEntry.mValueLength = snprintf((char *)txtBuffer, size, "1");

When I do this, the relevant line when calling srp server service on my server reads TXT: [pv=31], so it seems like the ascii is getting re-encoded, which the documentation doesn't mention anywhere.

2. Alternatively, the documentation suggests that if I set mKey to NULL, then I can encode my string according to RFC6763 section 6.3. I believe the following should also work as a result.

uint16_t size;
uint8_t *txtBuffer = otSrpClientBuffersGetServiceEntryTxtBuffer(serviceEntry, &size);
assert(NULL != txtBuffer);
assert(0 != size);
serviceEntry->mTxtEntry.mKey = NULL;
serviceEntry->mTxtEntry.mValueLength = snprintf((char *)txtBuffer, size, "\4pv=1"); // format is <len><string>

When I do this, the relevant line when calling srp server service on my server reads TXT: [pv=31], so it seems like even "already encoded TXT-DATA" is getting adjusted somewhere.

Am I mis-interpreting the documentation here, or is there a delta between what's documented and what's actually happening which should be fixed?

[mmb-davidsmith]

It turns out the TXT data, when decoded is printed as HEX instead of ASCII characters. Would it be worth updating https://github.com/openthread/openthread/blob/main/src/cli/README_SRP_SERVER.md#service to indicate this is the case, particularly because it's printing the key in ASCII but not touching the value.

[abtink]

Glad you find it.

The decoded TXT data value can be any sequence of bytes (including \0) so we cannot treat it as a C string therefore we are outputting it as hex bytes.

Here is the method that decodes and outputs the TXT data in CLI:

openthread/src/cli/cli_output.cpp

Lines 229 to 272 in 87284f2

	void Output::OutputDnsTxtData(const uint8_t *aTxtData, uint16_t aTxtDataLength)
	{
	otDnsTxtEntry entry;
	otDnsTxtEntryIterator iterator;
	bool isFirst = true;
	otDnsInitTxtEntryIterator(&iterator, aTxtData, aTxtDataLength);
	OutputFormat("[");
	while (otDnsGetNextTxtEntry(&iterator, &entry) == OT_ERROR_NONE)
	{
	if (!isFirst)
	{
	OutputFormat(", ");
	}
	if (entry.mKey == nullptr)
	{
	// A null `mKey` indicates that the key in the entry is
	// longer than the recommended max key length, so the entry
	// could not be parsed. In this case, the whole entry is
	// returned encoded in `mValue`.
	OutputFormat("[");
	OutputBytes(entry.mValue, entry.mValueLength);
	OutputFormat("]");
	}
	else
	{
	OutputFormat("%s", entry.mKey);
	if (entry.mValue != nullptr)
	{
	OutputFormat("=");
	OutputBytes(entry.mValue, entry.mValueLength);
	}
	}
	isFirst = false;
	}
	OutputFormat("]");
	}

Yes. Make sense to update the CLI README_SRP_SERVER documentation for this. Thanks.

[jwhui]

@mmb-davidsmith , would you like to submit a PR updating the README.md?

[mmb-davidsmith]

@jwhui - I'll try to get something drafted over the next few days.

[jwhui]

@mmb-davidsmith , sounds good. Thanks!