[avahi] received conflicting record

[Trent Lloyd]

Hi Folkert,

> On 8 Dec 2021, at 4:56 am, folkert <folkert at vanheusden.com> wrote:
> 
>> I made a program which sends an mDNS announcement every 121 seconds.
>> The TTL is 120 seconds.
>> 
>> Yet every time avahi (with --debug) complains about:
>> 
>> dec 07 10:27:54 lappiemctopface avahi-daemon[937]: Received conflicting
>> record [_apple-midi._udp.local        IN        PTR
>> mymdnstest._apple-midi._udp.local ; ttl=120]. Resetting our record.
>> 
>> I've uploaded a pcap:
>> https://vps001.vanheusden.com/~folkert/mdns.pcap
>> (text dump: https://vps001.vanheusden.com/~folkert/mdns.txt )
>> 
>> Anyone an idea what is going wrong?
> 
> I just verified that I get this error already on the first packet.
> 
> So I start avahi-daemon, let the other program send the mdns message
> once and immediately avahi complains about a conflicting record.

What is sending these other packets? I think there are two main problems. The first one leading to the immediate conflict is that mDNS records can be either “Unique” (owned by a single host) or “Shared” (a record publish by multiple hosts, or, a list of records published in aggregate by multiple hosts).

If you refer to RFC6762 Section 10.2 (https://datatracker.ietf.org/doc/html/rfc6762#section-10.2)

   The semantics of the cache-flush bit are as follows: normally when a
   resource record appears in a Resource Record Section of the DNS
   response it means, "This is an assertion that this information is
   true".  When a resource record appears in a Resource Record Section
   of the DNS response with the cache-flush bit set, it means, "This is
   an assertion that this information is the truth and the whole truth,
   and anything you may have heard more than a second ago regarding
   records of this name/rrtype/rrclass is no longer true”.


Unique records such as your hostname ‘mydnstest.local’ are expected to have the cache-flush bit to be set. But for shared resources such as the list of services (_apple-midi._udp.local PTR) we expect a “Shared” record so the cache-flush bit is false. This is because the “_apple-midi._udp.local” record is a shared list of services where multiple services all contribute to the list. 

Additionally the SRV record looks wrong.. from your text output it says:
        _apple-midi._udp.local: type SRV, class IN, cache flush, priority 0, weight 0, port 5004, target mymdnstest.local

The name of this record should be ‘mydnstest._apple-midi._udp.local’ instead, the same name as the target for the PTR record.


The hierarchy of announcements is:
(1) PTR from the shared service name (_http._tcp) with a target of your unique service instance (test._http._tcp.local)

_http._tcp.local PTR test._http._tcp.local (Shared record, cache-flush=false)

(2) SRV record from your unique service name with the port number and target hostname
test._http._tcp.local SRV port 80 target primary.local (Non-shared record, cache-flush=true)

There may also be a TXT record of the same name with any properties

(3) A/AAAA record for the IP address of the target hostname referenced in the SRV (the hostname is usually shared by all services from the same host)
primary.local A 192.168.1.1 (Typically non-shared with cache-flush=true)

Here is an example Wireshark capture I took just now of publishing a service using avahi (avahi-publish-service test _http._tcp 80). I’d suggest reviewing and comparing your announcement to that:

        test._http._tcp.local: type TXT, class IN, cache flush
            Name: test._http._tcp.local
            Type: TXT (Text strings) (16)
            .000 0000 0000 0001 = Class: IN (0x0001)
            1... .... .... .... = Cache flush: True
            Time to live: 4500 (1 hour, 15 minutes)
            Data length: 1
            TXT Length: 0
            TXT: 
        _http._tcp.local: type PTR, class IN, test._http._tcp.local
            Name: _http._tcp.local
            Type: PTR (domain name PoinTeR) (12)
            .000 0000 0000 0001 = Class: IN (0x0001)
            0... .... .... .... = Cache flush: False
            Time to live: 4500 (1 hour, 15 minutes)
            Data length: 2
            Domain Name: test._http._tcp.local
        test._http._tcp.local: type SRV, class IN, cache flush, priority 0, weight 0, port 80, target primary.local
            Service: test
            Protocol: _http
            Name: _tcp.local
            Type: SRV (Server Selection) (33)
            .000 0000 0000 0001 = Class: IN (0x0001)
            1... .... .... .... = Cache flush: True
            Time to live: 120 (2 minutes)
            Data length: 16
            Priority: 0
            Weight: 0
            Port: 80
            Target: primary.local
        primary.local: type A, class IN, cache flush, addr 192.168.1.1
            Name: primary.local
            Type: A (Host Address) (1)
            .000 0000 0000 0001 = Class: IN (0x0001)
            1... .... .... .... = Cache flush: True
            Time to live: 120 (2 minutes)
            Data length: 4
            Address: 192.168.1.1

(The below record is optional, it makes it so you can browse what types of services exist on the local network)
        _services._dns-sd._udp.local: type PTR, class IN, _http._tcp.local
            Name: _services._dns-sd._udp.local
            Type: PTR (domain name PoinTeR) (12)
            .000 0000 0000 0001 = Class: IN (0x0001)
            0... .... .... .... = Cache flush: False
            Time to live: 4500 (1 hour, 15 minutes)
            Data length: 2
            Domain Name: _http._tcp.local

Hope that helps?

Cheers,
Trent (@lathiat)