mdns-filoxy

This is mdns-filoxy, the mDNS filter proxy!

Table of contents

• mdns-filoxy
  • Table of contents
  • Introduction
    • What is it for?
    • But isn't that what avahi-daemon et al are for?!
    • Draw me a picture
    • What's with the name?!
  • Installation
    • Running as a systemd service
  • License

Introduction

Welcome to mdns-filoxy, the mDNS filter proxy.

What is it for?

mdns-filoxy allows you to send configured mDNS service queries out a specified interface, cache the response, and then listen for further queries and answer them on a different interface.

This is particularly useful when you (rather smartly) put your IoT devices in a different network/VLAN to your "main" network. I have my Sonos speakers setup this way, and without some form of proxy, I simply cannot talk to them anymore, because of their odd use of mDNS and SSDP, and 14,000 other convoluted methods to reach their ecosystem.

But isn't that what avahi-daemon et al are for?!

avahi-daemon does have a reflector mode, and there are other packages like mdns-repeater. The issue is that they both listen for queries and then just re-send them out other interfaces. avahi-daemon even lets you filter reflections, but they don't allow you to control the direction of the reflecting.

Furthermore, in the case where you have some other server on the same networks, with two or more interfaces bound to it, systemd can have a rather major meltdown whilst it tries to think for you and choose unique mDNS names. A good example of this is Home Assistant's HAOS. systemd behaves badly with mdns repeaters too.

Draw me a picture

Ok. Lets pretend you have a phone that sits in a VLAN, connected to a firewall interface called eth0 (known as the dest-interface). Lets further pretend your Sonos speaker is in another VLAN, connected to iot0 (known as the --source-interface).

When mdns-filoxy starts, it will bind to the specified interfaces, and on the source interface will query for the chosen services (--mdns-services):

flowchart LR
  Firewall -->|iot0| mc[Multicast to VLAN
  query for mdns-services] --> sonos[Speakers respond]-->|iot0 reply| Firewall

mdns-filoxy caches that response, and then lies in wait, ready to respond to any queries that arrive on eth0.

Once mdns-filoxy is up and listening, here's the sequence of events:

flowchart LR
    phone[Phone
    Sonos app
    launched] --> mc[Multicast to VLAN
    query for _sonos._tcp.local.] -->|eth0| Firewall -->|eth0 reply| phone

Notice that the queries and responses aren't bi-directional, and this is intentional. The software is meant to give minimal assistance to mDNS/IoT devices to get bootstrapped and then on with whatever other evil protocol(s) they use.

What's with the name?!

filoxy is my blend of "filter" and "proxy", as that's basically what this software does.

Installation

The recommended way to install mdns-filoxy is to use pipx.

After getting pipx installed, simply run:

username@host:~$ pipx install mdns-filoxy

You can of course also install it using classic virtualenvs.

Running as a systemd service

mdns-filoxy runs continuously, so systemd is the tidiest way to keep it running. The repository ships a template unit (mdns-filoxy@.service). The instance name is the user to run as, and the pipx-installed binary is resolved from that user's home directory (assuming homes live under /home), so no username is hardcoded.

Edit the ExecStart= line to match your interfaces and services, then:

username@host:~$ sudo install -m 644 mdns-filoxy@.service \
    /etc/systemd/system/mdns-filoxy@.service
username@host:~$ sudo systemctl daemon-reload
username@host:~$ sudo systemctl enable --now mdns-filoxy@$(whoami)

Follow the logs with:

username@host:~$ journalctl -u mdns-filoxy@$(whoami) -f

License

mdns-filoxy is distributed under the terms of the MIT license.