DNS Handler

The DNS handler requests a zone transfer (AXFR) from a DNS server. The A and AAAA records are retrieved and used to build an import requisition. This is useful for organizations that use DNS, possibly coupled with an IP management tool, as the database of record for nodes in the network. So, rather than ping sweeping the network or entering the nodes manually into the Horizon Requisition UI, you can manage nodes via one or more DNS servers.

The format of the URL for this protocol handler is: dns://<host>[:port]/<zone>[/<foreign-source>/][?expression=<regex>]

DNS Import Examples:

Simple

dns://my-dns-server/myzone.com

This URL imports all A and AAAA records from the zone myzone.com on the host my-dns-server via port 53 (default DNS port). Since the foreign source is not specified, it will be added to the default requisition.

Use a regular expression filter

dns://my-dns-server/myzone.com/portland/?expression=^por-.*

This URL imports all nodes from the same server and zone, but only imports the nodes in the zone that match the regular expression ^port-.*. The matching nodes will be assigned to a requisition named 'portland'. If your expression requires URL encoding, for example you need to use a ? in the expression, it must be properly encoded.

dns://my-dns-server/myzone.com/portland/?expression=^por[0-9]%3F

Location

You can use the location parameter to assign a single location to all nodes in this requisition. This would apply only if you are using Minions in your network.

dns://my-dns-server/myzone.com/portland/?location=Raleigh

By prefixing with a ~, you can specify a URL-encoded regular expression to determine the location based on the host entries. The following example derives the location based on the subdomain. This example uses a URL-encoded regular expression ^(?:.\.|)(.?)\.hs-fulda\.de\.$ with exactly one capturing group containing the subdomain.

dns://my-dns-server/hs-fulda.de/?location=~%5E%28%3F%3A.%2A%5C.%7C%29%28.%2A%3F%29%5C.hs-fulda%5C.de%5C.%24

This will associate nodes for the host entries foo.e46.hs-fulda.de. and bar.e46.hs-fulda.de. to the location e46 while hosts like aaa.g51.hs-fulda.de and bbb.g51.hs-fulda.de will be assigned location g51.

You can use online tools like https://www.urlencoder.org to encode your parameters.

DNS setup

The DNS import handler performs an AXFR zone transfer request to collect DNS resource records for use in provisioning nodes. Zone transfers are designed for zone replication between DNS servers. You may want to provide additional security mechanisms to protect your DNS infrastructure. At a minimum, you should always restrict AXFR requests to only trusted IP addresses and/or use a hidden, read-only secondary DNS server for Horizon.

One suggestion is to set up a secondary DNS server on your Horizon and configure the rest of the DNS servers to allow transfers to your Horizon server. This allows you to point the DNS import handler to localhost.

There is no encryption or authentication available for AXFR transfer requests. Transaction signatures (TSIG) to prevent IP spoofing attempts are currently not supported by Horizon.

A quick way to test if zone transfers are working is:

dig -t AXFR @<dnsServer> <zone>