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ipv6 mDNS PTR querywith avahi
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szatox
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PostPosted: Sat Nov 11, 2017 10:31 pm    Post subject: ipv6 mDNS PTR querywith avahi Reply with quote

I'm trying to get a bunch of machines talk to each other using avahi. Actually it worked just fine until I reconfigured network on my kvm host (up-to-date stable amd64) earlier today, even though changes should not really affect any endpoints (up-to-date ~amd64).
Like in:
- Changed the hierarchy of bridges and VLANs.
- Resized IP pools
- Assigned different IP prefixes
Nothing the guests should actually care about, since they reside on a single bridge anyway. And they do talk to each other, to the host, and to the internet

Somehow, avahi running on guests started prioritizing IPv6 over IPv4. I don't mind this part. However, some services demand reverse DNS information, and (confirmed with wireshark) there are no attempts to translate IPv6 addresses to hostnames using mdns. Those queries go straight to the DNS and - obviously - fail.

Avahi is running with default configs.
Any tips on making IPv6 mDNS PTR work? I'd rather not disable ipov6 completely if there is another option.


Edit: Actually which IP stack is preferred seems to depend on version. Turns out that stable chooses IPv4 and testing picks IPv6.
Still, any tips on directing IPv6 reverse DNS calls to mDNS are welcome.
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Ant P.
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PostPosted: Sun Nov 12, 2017 7:00 pm    Post subject: Reply with quote

If you run avahi-resolve-address it won't normally show up in wireshark; it pulls from avahi-daemon's cache.

You might need to set enable-reflector=yes on your kvm host if the guests and the outside network live on separate IPv6 subnets, even with a bridge.
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szatox
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PostPosted: Tue Nov 14, 2017 11:53 pm    Post subject: Reply with quote

It definitely does not pull anything from avahi's cache, because it fails to return any results for ipv6 PTR queries and everything else results in an exchange of a few mDNS packets.
Maybe the problem is actually in nss-mdns rather than in avahi... Gotta find some way to test it.
Still, it's really elusive, whenever I seem to have found a pattern I find out it doesn't really fit.

One thing that I am actually sure is that "ping" on my host is not capable of using ipv6. There is a separate command, ping6 for that.
On the other hand, testing branch ping I have on guests does accept both, ipv4 and ipv6 addresses. And here the pinging fun begins:
GuestA -> GuestB: ipv6
GuestB-> GuestA: ipv4
GuestA-> host: ipv4
GuestB-> host: ipv6
host-> * : ipv4. Always. Forced by "ping" tool. SSH can go over ipv6 though.

GuestB is the only one with USE="search-domains" enabled. Using short name vs FQDN (local.) doesn't matter. I once noticed behaviour changed when I added trailing dot to the FQDN, but couldn't reproduce it so perhaps I changed something else at that time (like e.g. ping's target host).
I'm gonna create more of those guests and see if the behaviour becomes more consistent. Right now it simply doesn't make any sense. What could I have done wrong? :roll:

Quote:
You might need to set enable-reflector=yes on your kvm host if the guests and the outside network live on separate IPv6 subnets, even with a bridge.
Link-local, at least for now. All machines do see each other, there is no need for a proxy/repeater. Hopefully I'll get something better than link-local later. This is the primary reason for me not to disable ipv6 completely, I want to have some way to move forward. And I'd rather have conflicts in network layer sorted sooner than later, so things don't break when I enable it.
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