Repeating mDNS/Bonjour requests from eth0 through a tunnel (tun0)

To start off, I am quite new at both networking and Unix/Ubuntu/Linux distros. Just a warning, for any setup/code may look a bit ugly.

Basically, my end goal is to successfully AirPlay Mirror to a remote Ubuntu server from my iPhone on a different wifi network or on LTE.

TL;DR: With mdns-repeater/avahi-daemon and OpenVPN, I still cannot pass the mDNS requests from eth0 to tun0.

To start, I knew I needed an AirPlay receiver for Ubuntu/Linux/Unix based OS that supported mirroring (and hopefully open-source). I found a couple, most for Mac OS/Windows, or didn't support mirroring at all. After a little more searching I found Slave in the Magic Mirror[Link 1 below], an open-source Linux AirPlay server/receiver that runs and works (based on my debugging, as I have no physical access to the server I ran it on).

Now, I knew AirPlay only ran over LAN (at the time not understanding how Bonjour only works on the same subnet) so I looked into some VPN options. OpenVPN seemed to be the most flexible and easiest to set up. To speed things up and to guarantee that I don't make any mistakes setting OpenVPN up I used a pre-made script from here[Link 2 below]. Tested and worked flawlessly, VPN connects without DNS leaks and all traffic routes successfully through VPN.

I have my VPN to act as if my device is on the LAN of my server now, and I have Slave in the Magic Mirror (AirPlay server) running successfully. So it should just work now, right? Not surprisingly, it did not, as I did not understand the AirPlay server actually sends out mDNS/Bonjour requests (or probes? The real term is slipping my mind right now..). As a home, conventional user, since these mDNS requests are zeroconf (zero configuration), this is amazing! But as an enterprise or business user, this is difficult to get working across VLAN's.

Through research I came up with the end result that I need some kind of mDNS repeater/proxy/bridge type setup. I ended up with the mDNS repeater. There were two programs I attempted to use.

Avahi-Daemon[Link 3 below] Avahi seemed to be the most talked about and most documented, so I decided to use that. I edited the config file to allow Config location /etc/avahi/avahi-daemon.conf

[reflector]
enable-reflector=yes

and

[server]
allow-point-to-point=yes

As explained here[Link 4 below] and here[Link 5 below].

Running the Avahi Daemon in debug mode (avahi-daemon --debug) seemed to work at first glance but as soon as Slave in the Magic Mirror (running on eth0 interface, OpenVPN running on tun0 interface) is ran it sees the mDNS packets somehow but always outputs a bunch of these:

Received packet from invalid interface.
Received packet from invalid interface.
Received packet from invalid interface.
Received packet from invalid interface.

Forcing Avahi to use only eth0 and tun0, upon many other changes and settings will always output this.

To verify it was not just an output bug I ran

tcpdump -i eth0 udp port 5353 and tcpdump -i tun0 udp port 5353 (port where mDNS requests go through) eth0 successfully receives packets from filter while tun0 receives none. So not an output bug. I even tried it on port 7000 (port that AirPlay server listens on for Mirroring)

With no success with Avahi, I suspected it might just be because it hasn't been updated since 2011.

mdns-repeater[Link 6 below] With no config files or setup needed, this is the next option I took. And it seems this is working correctly. Run mdns-repeater with

mdns-repeater eth0 tun0 -f

Just add the interfaces you want to have the requests repeated over and -f for foreground/debugging. That's it! I ran Slave in the Magic Mirror and mdns-repeater successfully detected and repeated the requests (according to its' logs at least). But sadly, running the same tcpdump commands as seen above, the requests are still not going through the tunnel (tun0).

Now from my debugging I can only conclude it is either the cause of iptables/firewall or OpenVPN filtering the ports or requests somehow. Finding nothing in the config related to filtering in OpenVPN, I moved on to my iptables theory. But running iptables -L brings nothing, literally no rules are in iptables.

Knowing little about iptables, I do not know if this is the cause. For my own debugging I added every different iptables rule I could find related to anything with allowing mDNS / Bonjour / AirPlay. Nothing seems to be of help.

Any and all help is appreciated! I know this was a long read, I did not want any small issue falling through.

TL;DR: With mdns-repeater/avahi-daemon and OpenVPN, I still cannot pass the mDNS requests from eth0 to tun0.

All links to sources here: http://pastebin.com/mVkpZwRY Apologies, I do not have enough reputation for more than 2 links at the moment.

Solution 1:

Don't know the answers but for a start tun interfaces do not support broadcast. If you use tap they do. Tho it appears that tap is used for bridging in the OVPN documentation you can use them in configurations that use tun. They will behave nearly identically but will indicate BROADCAST as supported option when you ifconfig them.