On Tue, 2022-02-01 at 22:38 +0000, Barry wrote:
> I thought that mDNS that Avahi implements only uses multicast on the
> LAN. You could set up multicast across multiple LAN segments.
>
> How does that end up getting answers from the internet?
> Especially when all ISPs block multicast it seems.
It shouldn't (go out on the internet). But what happens when something
doesn't get an answer from within the LAN, or, some part of your LAN
isn't using mDNS? Is *it*, then, going to try a normal DNS query?
*It* being something on your computer, not specifically Avahi, querying
beyond the internal LAN.
On a whim, I've just tried this on my system which doesn't use mDNS:
$ dig router.local
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.8 <<>> router.local
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked
to DNS
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 41071
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;router.local. IN A
;; AUTHORITY SECTION:
. 10800 IN SOA a.root-servers.net.
nstld.verisign-grs.com. 2022020101 1800 900 604800 86400
;; Query time: 86 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Feb 02 13:17:02 ACDT 2022
;; MSG SIZE rcvd: 116
So, the "dig" tool, at least, is prepared to break out of the confines
of my LAN. And, so it would seem, is BIND (I have BIND running on this
machine), and dig queried it. I suppose I should customise BIND to
internally abort .local domain name queries. Though I think I have
turned off mDNS/Avahi, et al, in all the devices in the LAN.
On the other hand:
$ nslookup router.local
Server: 127.0.0.1
Address: 127.0.0.1#53
** server can't find router.local: NXDOMAIN
Doesn't really tell me how far the query went before it got nixed.
--
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