Android NsdServiceInfo setAttribute update after register

Question

After initially setting up attributes for the NsdServiceInfo and calling registerService, how is it possible to update the attributes?

With iOS, you just call [service setTXTRecordData...]. I don't see any way to do it with Android so far.

Answer 1

I'm not familiar with iOS API, but I don't think there is a direct way to do this with NSD API, according to rfc 6762 section 8.4:

At any time, if the rdata of any of a host's Multicast DNS records changes, the host MUST repeat the Announcing step described above to update neighboring caches. For example, if any of a host's IP addresses change, it MUST re-announce those address records. The host does not need to repeat the Probing step because it has already established unique ownership of that name.

In the case of shared records, a host MUST send a "goodbye" announcement with RR TTL zero (see Section 10.1, "Goodbye Packets") for the old rdata, to cause it to be deleted from peer caches, before announcing the new rdata. In the case of unique records, a host SHOULD omit the "goodbye" announcement, since the cache-flush bit on the newly announced records will cause old rdata to be flushed from peer caches anyway.

Attributes are usually advertised in TXT record, So I don't think iOS has some magic to update the TXT record, it must re-announce or send a "goodbye" announcement. Since Android NSD doesn't provide an API to send a single TXT record, AFAIK, the only way to do this is to call NsdManager.unregisterService() then NsdManager.registerService().

Answer 2

I know everyone hates that type of answers...

But consider trying 3rd party lib for doing that. E.g. JmDNS: https://github.com/jmdns/jmdns

Pros:

• it doesn't have issues on pre-Marshmallow (built-in Android NSD is very buggy on pre-M, e.g. you can't read attributes of the discovered service);
• you can update registered service attributes using the setText() method of ServiceInfo object.

Cons:

• it doesn't work if Android device is used as a hotspot (has no Internet access and provides its own WiFi access point without Internet access).

Answer 3

FWIW the native bonjour API supports updating txt records w/o unregistering the service.

From dns_sd.h

DNSServiceUpdateRecord
 *
 * Update a registered resource record. The record must either be:
 *   - The primary txt record of a service registered via DNSServiceRegister()
 *   - A record added to a registered service via DNSServiceAddRecord()
 *   - An individual record registered by DNSServiceRegisterRecord()