isabella232 / coredns-mdns

CoreDNS plugin that serves .local mDNS info over normal DNS

mDNS

Name

mDNS - CoreDNS plugin that reads mDNS records from the local network and responds to queries based on those records.

Description

Useful for providing mDNS records to non-mDNS-aware applications by making them accessible through a standard DNS server.

Syntax

mdns example.com [minimum SRV records] [filter text] [bind address]

Examples

As a prerequisite to using this plugin, there must be systems on the local network broadcasting mDNS records. Note that the .local domain will be replaced with the configured domain. For example, test.local would become test.example.com using the configuration below.

Specify the domain for the records.

example.com {
	mdns example.com
}

And test with dig:

dig @localhost baremetal-test-extra-1.example.com

;; ANSWER SECTION:
baremetal-test-extra-1.example.com. 60 IN A   12.0.0.24
baremetal-test-extra-1.example.com. 60 IN AAAA fe80::f816:3eff:fe49:19b3

If minimum SRV records is specified in the configuration, the plugin will wait until it has at least that many SRV records before responding with any of them. minimum SRV records defaults to 3.

example.com {
    mdns example.com 2
}

This would mean that at least two SRV records of a given type would need to be present for any SRV records to be returned. If only one record is found, any requests for that type of SRV record would receive no results.

If filter text is specified in the configuration, the plugin will ignore any mDNS records that do not include the specified text in the service name. This allows the plugin to be used in environments where there may be mDNS services advertised that are not intended for use with it. When filter text is not set, all records will be processed.

example.com {
    mdns example.com 3 my-id
}

This configuration would ignore any mDNS records that do not contain the string "my-id" in their service name.

If bind address is specified in the configuration, the plugin will only send mDNS traffic to the associated interface. This prevents sending multicast packets on interfaces where that may not be desirable. To use bind address without setting a filter, set filter text to "".

example.com {
    mdns example.com 3 "" 192.168.1.1
}

This configuration will only send multicast packets to the interface assigned the 192.168.1.1 address. The interface lookup is dynamic each time an mDNS query is sent, so if the address moves to a different interface the plugin will automatically switch to the new one.

Service Discovery

Queries to mDNS are constrained to the local domain, alternative domains that an mDNS server publishes are not supported by this plugin. This does not restrict you to .local TLD, as the address is referenced from the service responses belonging to the local domain.

If you have multiple network interfaces that respond to mDNS for your host(eg on the same system that CoreDNS is running), this can result in the wrong IP address returned for a hostname due to a race condition with multicast. Make sure you configure your mDNS server to whitelist the network interface with the assigned IP address you want to associate the hostname to.

If a query is not responding, check the log for hosts discovered by this plugin reported as mdnsHosts. The plugin will populate mdnsHosts by only discovering mDNS services of the type _workstation._tcp.

Publishing _workstation._tcp service with Avahi

Avahi is commonly installed on Linux systems as the default mDNS server. Your distro may have it configured to publish this service by default, however distros that follow upstream defaults have this feature disabled for security reasons. While it is not required to be enabled to respond to explicit requests, it is required for service discovery over mDNS which this plugin relies on.

You can list the results coredns-mdns will discover with: avahi-browse --resolve --terminate _workstation._tcp

If your hostname is missing but can be resolved with avahi-resolve --name your-hostname-here.local, it needs to be published as a workstation service.

Edit /etc/avahi/avahi-daemon.conf:

[publish]
publish-workstation=yes

If the hostname is defined by avahi-publish --address <hostname> <ip>, /etc/avahi/hosts, or other means like D-Bus, you can publish the workstation service to point to that hostname with:

avahi-publish --service friendly_name _workstation._tcp 9

• The last argument is an associated port for the service, it is not important for this plugin but a value is required to publish.
• You can use --host=<hostname> to choose a value that differs from the default Avahi host-name in /etc/avahi/avahi-daemon.conf. This should be an FQDN value, the TLD should be appended to it (eg, --host=your-hostname-here.local)
• By default it will be published under the local domain which coredns-mdns searches.