hplip is in need of a version update. I rejected request 625066 today. The main
problem, apart from formalities, was that the request simply ditched my mDNS
patches. I've been aware for some time that the patches don't apply cleanly to
3.17.11 ff (upstream has added one more mDNS query, for "_uscan._tcp.local").
That sounds trivial, but extending my patches to this case isn't
straighforward.

Unfortunately upstream hasn't put any effort into integrating my mDNS patches.
I do have modified patches for 3.18, but I can't test them as the printer I
used for that testing (an HP Envy 5530) is out of order, and the replacement I
bought (OfficeJet 6950) doesn't use mDNS. To test this properly, you need

 - an HP printer using mDNS for discovery, and with only Wifi network
 - a setup where a host connected to the local network through both wired LAN +
Wifi tries to access the printer
 - a Wifi router that doesn't forward UDP broadcast packets from wired LAN to
Wifi properly (that's quite common for cheap routers, actually)

My patches cause the mDNS broadcast packets from the host to be sent not on the
primary LAN interface, but also on the Wifi interface directly, which fixes
device discovery.

The true solution for mDNS in hplip would be to use avahi rather than a
second-grade custom implementation (avahi sends out packets on all interfaces
unless otherwise configured). But that, also, requires substantial work, using
the avahi API isn't exactly easy. And upstream doesn't do anything beyond
support for new printers any more, it seems.