[avahi] Multiple DHCP connected interfaces, avahi-daemon only "listens" on one

[Jason Heeris]

I've got an Ubuntu 18.04 (avahi-daemon 0.7-3.1ubuntu1) machine with 
three interfaces: enp1s0f0 (PCI eth), wlp2s0 (PCI wifi), 
wlx485d601f9b83 (USB wifi). Everything's managed via Netplan (no 
Network Manager or ifupdown). The machine is used to connect an 
eth-only device to the local wifi network by masquerading over 
wlx485d601f9b83. At the moment enp1s0f0 has a static config, and both 
wifi interfaces are DHCP configured. Due to some Netplan's limitations, 
I have some duplicate routes that look like:

$ ip route show
default via 192.168.1.1 dev wlx485d601f9b83 proto dhcp src 192.168.1.3 
metric 600
default via 192.168.1.1 dev wlp2s0 proto dhcp src 192.168.1.100 metric 
600
192.168.1.0/24 dev wlx485d601f9b83 proto kernel scope link src 
192.168.1.3
192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.100
192.168.1.1 dev wlx485d601f9b83 proto dhcp scope link src 192.168.1.3 
metric 600
192.168.1.1 dev wlp2s0 proto dhcp scope link src 192.168.1.100 metric 
600

My problem is this: I want Avahi to only use the PCI wifi interface 
(wlp2s0). But if I put "allow-interfaces=wlp2s0" in 
/etc/avahi/avahi-daemon.conf, suddenly I can't resolve any .local 
addresses.

Before changing (ie. with no settings for either "allow-interfaces" or 
"deny-interfaces"):

$ avahi-resolve --name -4 han.local
han.local	192.168.1.101

After "allow-interfaces=wlp2s0" and a restart of Avahi:

$ avahi-resolve --name -4 han.local
Failed to resolve host name 'han.local': Timeout reached

It still seems to be broadcasting though. Even after leaving it like 
this for hours, I can still resolve *back* to the machine itself from 
other machines, to the correct (wlp2s0) interface.

If I remove the "allow-interfaces" line, it's not a huge problem. 
Issues are simply that (a) I want to keep non-masquerade traffic off 
the interface and (b) for a brief moment during boot or restarting 
certain services, the machine's broadcast name resolves to the address 
of the wlx485d601f9b83 interface instead of the wlp2s0 interface.

But I'd really like to know what I'm doing wrong here. It seems like a 
simple config change, but it's definitely not having the effect I 
expected.

Cheers,
Jason