OPNsense Forum

Archive => 19.1 Legacy Series => Topic started by: Stilez on February 10, 2019, 05:20:45 pm

Title: I thought I understood mDNS/Airplay, but now having HP dns_sd - how to handle?
Post by: Stilez on February 10, 2019, 05:20:45 pm

I thought that I understood the way that Apple services, HP printers with Airplay implemented, and other such services worked on a segmented LAN.  The LAN here has a transparent WIFI AP that bridges wireless devices to OPT3, and filter rules on OPT3 allow access to the local resolver (Unbound: it's a small LAN with maybe <12 devices needing a static IP) and the LAN itself.

As I've understood it, devices broadcast requests/discovery announcements to their segment (bridged if needed by mdnsrepeater/avahi), and discover other devices on the LAN that way.  Irritating to have to bridge, but okay, doable.

Just now I hit a completely new thing in the zero-config realm, and I'm trying to understand how to integrate that into my LAN. I tried to use my printer from my mobile phone. The HP print service couldn't find the printer, but I could easily reach its built-in web UI from my phone, so TCP/UDP all work fine between the phone and printer. Intrigued, I ran tcpdump and found the HP Android print server plugin was sending odd DNS requests, which I now know are dns-sd packets, to the local LAN resolver, to try and find presumably statically coded or dynamic discovery records of local services.

The domain's it's looking for(initially at least) are: lb._dns-sd._udp.MY.DOMAIN and b._dns-sd._udp.MY.DOMAIN

This has thrown me on many levels. Please help me work out what to do!

I take a guess that it's piggybacking on (or similar to) Apple Airplay, and HP uses this as a kind of subnet-aware zeroconf autodiscovery, where device details are dumped into DNS TXT strings by local dns-sd aware devices, and DNS can be interrogated for them by service users. That leaves me 3 questions:

• I don't want to have to manually add TXT strings to Unbound any time a new service is offered by a friend's device.  But Unbound is generally read-only on DNS records, once running.  Is there a way to make this dns-sd discovery method work without ditching Unbound as the LAN resolver?  (I've never used Dyn DNS for anything but could use it if it helps)
• The HP print plugin seems to only look for dns-sd records. If it knew the device IPs, would it be able to interrogate them directly, or does it need that middleman DNS service to log them and serve them? Like, is there a way I could tell the HP thing to just add a printer at some IP or another, or is that unlikely to work?
• Should I just use different (non-HP) software? But then, friends with Apple iThings will want to print and share services on the LAN, and print software won't solve it for those. Will a fix for dns-sd combined with some kind of mdnsrepeater, finally fix all common zeroconfig discoveries, or are there others that are popular and I need to handle?

Title: Re: I thought I understood mDNS/Airplay, but now having HP dns_sd - how to handle?
Post by: newsense on February 10, 2019, 08:32:32 pm

Do you have DHCP Static Mapping and TXT Comment Support checked in Unbound - General ?