mDNS, or multicast DNS, is a way to discover devices on your network at
.local domain without any central DNS configuration (also known as ZeroConf and Bonjour, etc). Fedora Magazine has a good article on setting it up in Fedora, which I won’t repeat here.
If you’re like me, you’re using OpenWRT with multiple VLANs to separate networks. In my case this includes my home automation (HA) network (VLAN 2) from my regular trusted LAN (VLAN 1). Various untrusted home automation products, as well as my own devices, go into the HA network (more on that in a later post).
In my setup, my OpenWRT router acts as my central router, connecting each of my networks and controlling access. My LAN can access everything in my HA network, but generally only establish related TCP traffic is allowed back from HA to LAN. There are some exceptions though, for example my Pi-hole DNS servers which are accessible from all networks, but otherwise that’s the general setup.
With IPv4, mDNS communicates by sending IP multicast
UDP packets to
126.96.36.199 with source and destination ports both using
5353. In order to receive requests and responses, your devices need to be running an mDNS service and also allow incoming UDP traffic on port
As multicast is local only, mDNS doesn’t work natively across routed networks. Therefore, this prevents me from easily talking to my various HA devices from my LAN. In order to support mDNS across routed networks, you need a proxy in the middle to transparently send requests and responses back and forward. There are a few different options for a proxy, such as
igmpproxy, but i prefer to use the standard Avahi server on my OpenWRT router.