DNS discovery

DNS-Based Service Discovery within KNX IoT

The KNX-IoT features Multicast DNS based Service Discovery. KNX servers advertise their KNX Serial Number, KNX Individual Address and KNX Installation IDs using PTR Resource records, which can be discovered using Multicast DNS. This enables KNX-IoT devices to be discoverable on the local network with essentially zero configuration.

Dependencies

On Ubuntu & Debian systems, you must install avahi-utils package before DNS-SD will work within the KNX-IoT stack.

On Windows, you will need to install Bonjour:

via Chocolatey package manager: choco install bonjour

Note: needs to be installed as administrator.
via apple download: search for “Download Bonjour Print Services for Windows” and download the installer & install the print services.

The print services installs the needed services on the windows PC.

Hostname

The KNX-IoT specification mandates that the hostname of a KNX device must be in the format {serialnumber}.knx.local. However, modifying the hostname of a computer from within a C program is highly intrusive and it requires superuser privileges, so the KNX IoT stack does not do this by default.

In order to be KNX IoT compliant, products must ensure that their hostname obeys the {serialnumber}.knx.local format. On Ubuntu & Debian based systems, this can be done by overwriting the contents of the /etc/hostname - the change will take place upon the next reboot.

Discovery Example

Devices in Programming Mode (as defined in the KNX-IoT Point API specification section 2.5.3.9) advertise the _ia0 subtype, which can be discovered by avahi-browse as folows:

$ avahi-browse -rt _ia0._sub._knx._udp
+ enp6s0 IPv6 00FA10010700                                        _knx._udp            local
+ enp6s0 IPv4 00FA10010700                                        _knx._udp            local
= enp6s0 IPv6 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [2a00:23a8:89b:da00:ba27:ebff:fed5:c652]
   port = [5683]
   txt = []
= enp6s0 IPv4 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [192.168.202.161]
   port = [5683]
   txt = []

Devices can also be discovered based on their serial number. Note that the advertised service for the serial number is prefixed with an underscore.

$ avahi-browse -rt _00FA10010700._sub._knx._udp
+ enp6s0 IPv6 00FA10010700                                        _knx._udp            local
+ enp6s0 IPv4 00FA10010700                                        _knx._udp            local
= enp6s0 IPv6 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [2a00:23a8:89b:da00:ba27:ebff:fed5:c652]
   port = [5683]
   txt = []
= enp6s0 IPv4 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [192.168.202.161]
   port = [5683]
   txt = []

After programming, devices can be discover based on their IA (Individual Address) & IID (Installation ID). The KNX Point API Specification defines the subtype for programmed devices as follows: _ia{installationid}-{ia}._sub._knx._udp

In the example below, the device lives on Installation ID 5, with an individual address of 1.

$ avahi-browse -rt _ia5-1._sub._knx._udp
+ enp6s0 IPv6 00FA10010700                                        _knx._udp            local
+ enp6s0 IPv4 00FA10010700                                        _knx._udp            local
= enp6s0 IPv6 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [2a00:23a8:89b:da00:ba27:ebff:fed5:c652]
   port = [5683]
   txt = []
= enp6s0 IPv4 00FA10010700                                        _knx._udp            local
   hostname = [00FA10010700.knx.local]
   address = [192.168.202.161]
   port = [5683]
   txt = []