[avahi] PTR query responses

[Christiano F. Haesbaert]

On 12 March 2012 14:28, Markus Rathgeb <maggu2810 at googlemail.com> wrote:
> 2012/3/12 Markus Rathgeb <maggu2810 at googlemail.com>:
>> 2012/3/12 Christiano F. Haesbaert <haesbaert at haesbaert.org>:
>>> On 12 March 2012 13:08, Markus Rathgeb <maggu2810 at googlemail.com> wrote:
>>>> Hello!
>>>> I have here multiple machines with the same setup.
>>>> Normally if a multicast question for a PTR "_srv._proto.local" is
>>>> made, the machines reply with a multicast response containing four
>>>> records, the types PTR, TXT, SRV, A.
>>>> This morning I see (wireshark) a machine that reply contains only the
>>>> PTR record -- missing the TXT, SRV and A type.
>>>> If a multicast question for the A record of the machine's name is
>>>> requested, the machine answered with a multicast respone given the A
>>>> record.
>>>> After a restart of the avahi daemon of this machine, the machine's
>>>> reply to the PTR question contains all four records (PTR, TXT, SRV, A)
>>>> again.
>>>> What's going on?
>>>> Could someone explain me, how I could run in that situation and
>>>> whether it is correct or not (why)?
>>>> Thanks in advance,
>>>> Markus
>>>
>>> There is a feature called something like "Known Answer Suppression",
>>> the querierer includes the known answers in the query packet, under
>>> the AN section, so the the one getting queried won't include the known
>>> answers in the reply.
>>>
>>> Not sure if I made myself clear, try using avahi-browse, if the
>>> service ain't there, it's wrong.
>>> _______________________________________________
>>> avahi mailing list
>>> avahi at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/avahi
>>
>> Thank you for the reply.
>>
>> avahi-browse shows the service.
>> avahi-resolve-hostname could resolve the A record.
>>
>> The windows command line tool dns-sd seems also to be working (-B and
>> -G v4 hostname).
>>
>> I got the error with some tools written by colleagues.
>> So I do not say it is an error of avahi (it could be an error in the
>> implementation of the tools).
>>
>> The mutlicast PTR question for the special service is done to get a
>> list of all machines that are offering that service (no "known answer"
>> included).
>> So it is possible to request the A record for every machine that sends
>> an answer and no A record in the answer was included.
>>
>> The question is: Why are always the four record types in the answer
>> and now one machine replies with only one record type in the answer (I
>> never seen this before and after restart of avahi I see four record
>> types again for this machine)?
>> The restart of avahi was done after about four hours.
>
> I have had a look at the sending a response packet logic:
>
> ===
> [reponse-sched.c] static void
> send_response_packet(AvahiResponseScheduler *s, AvahiResponseJob *rj)
> calls
> [reponse-sched.c] static int
> packet_add_response_job(AvahiResponseScheduler *s, AvahiDnsPacket *p,
> AvahiResponseJob *rj)
>
> [reponse-sched.c] static int
> packet_add_response_job(AvahiResponseScheduler *s, AvahiDnsPacket *p,
> AvahiResponseJob *rj)
> calls
> [server.c] void avahi_server_enumerate_aux_records(AvahiServer *s,
> AvahiInterface *i, AvahiRecord *r, void (*callback)(AvahiServer *s,
> AvahiRecord *r, int flush_cache, void* userdata), void* userdata) {
>
> For class AVAHI_DNS_CLASS_IN and type AVAHI_DNS_TYPE_PTR
> the "existing" AVAHI_DNS_TYPE_SRV and AVAHI_DNS_TYPE_SRV entries for
> the name should be added.
>
> For class AVAHI_DNS_CLASS_IN and type AVAHI_DNS_TYPE_SRV
> the "existing" AVAHI_DNS_TYPE_A and AVAHI_DNS_TYPE_AAAA entries for
> the name should be added
>
> ...
> ===
>
> So, why could a PTR response packet be sent without the SRV, TXT and A
> entries and after a retry the entries are added.
> No service files or avahi configuration files are changed on the machine.
>

Because the PTR response is what takes part in the enumeration phase,
when you get the SRV/TXT/A records, you actually resolving the
services.

In a normal cenario, you don't want every SRV/TXT/A (you don't want to
resolve them all), you want to enumerate all possible services, and
you (the user) picks one, you resolve it (ask for the SRV/TXT/A).

It's been over a year since I implemented mdns/dns-sd and my mind is a bit fusy.

> The function
> [server.c] static void enum_aux_records(AvahiServer *s, AvahiInterface
> *i, const char *name, uint16_t type, void (*callback)(AvahiServer *s,
> AvahiRecord *r, int flush_cache, void* userdata), void* userdata)
> iterate in the else branch over the entries in a hash map.
> Could there be an error if that hash map is filled?