[avahi] avahi-core on Linux without netlink?

Sun Feb 27 17:46:59 UTC 2022

Hello,

I have built libavahi-core.a from source without netlink support on 
Linux (x86 Debian 11).  Why --> Google has disabled binding to a netlink 
socket in api >= 30.  Unfortunately my program is not able to publish or 
browse for services without netlink enabled in avahi-core.  Is this 
correct behaviour?  I noticed iface-none.c is built instead of 
iface-linux.c.

Jon

On 2021-12-07 8:24 p.m., Trent Lloyd wrote:
> 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://linkprotect.cudasvc.com/url?a=https%3a%2f%2fvps001.vanheusden.com%2f~folkert%2fmdns.pcap&c=E,1,qAnUYiH7cFCdxQbDZpGLALJ3CssMNFzEVwiE1YbVHM6xWACOIfv0bXq4NyQO47Se-scGGdgXM8Y-4LPX_zaw-lMTYcZTObE8F7JZtvBCze4WWoAqIg,,&typo=1
>>> (text dump: https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fvps001.vanheusden.com%2f~folkert%2fmdns.txt&c=E,1,j_MXzszdG-MCfApSpw2sf4M8LBrQJcvTlDZUHKUshzPaBaT2VINalQ-vLxTh1PcaTI7hzfBzJ8GIdmcqriVdhknuL4iP35MAIF2fNwH9jQZ_2tNyB-AAdodHqJs,&typo=1 )
>>>
>>> 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://linkprotect.cudasvc.com/url?a=https%3a%2f%2fdatatracker.ietf.org%2fdoc%2fhtml%2frfc6762%23section-10.2&c=E,1,hRneuq7gmccKO3LOVORaP2tWQygWE5bJiKLRQha1JzYmzqty-tJg-1qQYtkUu_e_UjO2kU6ACgPcwFQz4fxni2UFlKtmaHdTPZhuKufxrZxAWjuemctkl_UUwB8,&typo=1)
>
>     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)
>