[avahi] Bonjour/Avahi broadcasting with host names

[Till Kamppeter]

Hi,

tl;dr: Local IPv6 IP address has an alternative host name assigned but 
Avahi broadcast uses the system's default host name.

I am working on the ippusbxd project:

https://github.com/tillkamppeter/ippusbxd/

ippusbxd makes an IPP-over-USB printer appear on the system like a 
network printer. This way one can use the same software and discovery 
method as for IPP network printers, as CUPS' IPP backend, cups-browsed, 
avahi-daemon, ...). Especially newer IPP (and IPP-over-USB) printers 
using the IPP 2.0 protocol and understanding PWG Raster or Apple Raster 
can be set up fully automatically without any printer driver.

The printer needs only be available to the local machine and should not 
pollute other machines in the local network, as a normal USB printer. To 
share it one would share the CUPS queue as usual.

So the most intuitive is to mirror it to localhost (127.0.0.1) on a port 
other than 631 (631 is already used by CUPS). I use 60000. Now for CUPS 
and cups-browsed to discover the printer, ippusbxd has to do 
Bonjour/Avahi broadcasts, as a real network printer does. Unfortunately 
Avahi only supports multicast-capable network interfaces and as the 
loopback interface ("lo") is not multicast-capable, localhost cannot be 
used.

So I set up a dummy interface via

sudo ip link add ippusbxd type dummy
sudo ip link set ippusbxd up
sudo ip link set ippusbxd multicast on
sudo ip -6 addr add 'fd00:1:1::1/64' dev ippusbxd

I use IPv6-only as there are enough IPv6 IP spaces so the chance to 
clash with some other local service is practically zero. For testing I 
have selected a fixed address now, but for production a random prefix in 
the appropriate space (ULAs) has to be used and for each printer a 
suffix derived from the identity of the printer (to make it easier to 
get predictable URIs for the printers). The link-local address which the 
interface automatically gets is very awkward to use as one has to 
specify the interface when accessing the printer.

I am able now to broadcast the printer with this dummy interface named 
"ippusbxd" and cups-browsed correctly discovered it, being able to 
auto-create a working CUPS queue for the printer.

The problem is that all this only works based on IP adresses and there 
are problems with host names.

After having manually run the four lines to create the interface  shown 
above, ippusbxd is called to mirror the printer to the "ippusbxd" 
interface and then ippusbxd resgisters the printer to Avahi with the 
following call:

error =
     avahi_entry_group_add_service_strlst(bonjour_data->ipp_ref,
                 (int)if_nametoindex("ippusbxd"),
         AVAHI_PROTO_UNSPEC, 0,
         dnssd_name,
         "_ipp._tcp", NULL, NULL, 60000,
         ipp_txt);

This makes the printer being broadcasted, but with the host name of the 
system. The "Address:" entry in the Bonjour record visible in 
avahi-discover (Interface: ippusbxd -> local -> Internet Printer) reads:

Address: till-x1carbon.local/fd00:1:1::1:60000

With the host name being from the system. This would mean that printing 
via the URI ipp://till-x1carbon.local:60000/ipp/print or accessing the 
printer's web admin interface via http://till-x1carbon.local:60000/ does 
not work.

So as a first approach I added the line

fd00:1:1::1    ippusbxd-printers

to /etc/hosts. This makes Firefox now access the web interface via

http://ippusbxd-printers:60000/

but the Avahi broadcasts are still wrong, having still

Address: till-x1carbon.local/fd00:1:1::1:60000

in the avahi-discover output.

I already tried to register with

error =
     avahi_entry_group_add_service_strlst(bonjour_data->ipp_ref,
                 (int)if_nametoindex("ippusbxd"),
         AVAHI_PROTO_UNSPEC, 0,
         dnssd_name,
         "_ipp._tcp", NULL, "ippusbxd-printers", 60000,
         ipp_txt);

But with this the registration failed.

How do I get the correct broadcasts with

Address: ippusbxd-printers/fd00:1:1::1:60000

in the avahi-discover output?

Should work somehow, as the local 127.0.0.1 also gets localhost as name 
and not the system's host name.

And also how can I make ippusbxd zeroconf-register the ippusbxd-printers 
-> fd00:1:1::1 name assignment with Avahi, so that I do not need to edit 
/etc/hosts?

Thanks in advance for any kind of help.

    Till