On 2021-12-07 8:24 p.m., Trent Lloyd wrote:
Hi Folkert,
On 8 Dec 2021, at 4:56 am, folkert <folk...@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)
