synthetic

Overview

synthetic is a CoreDNS plugin to synthetically handle DNS records with IP addresses embedded. Named after DNSMASQ's "synth-domain" option.

synthetic aims to provide an easy mechanism for alignment between forward and reverse lookups. This is a common DNS operational and configuration error as noted in RFC1912.

This plugin supports works nicely with the file plugin such that records present in the file will take precedence over this plugin.

Forward Lookups

Forward Lookups are hostname -> IP address. synthetic supports IPs "embedded" in the DNS hostname. For IP addresses embedded in DNS hostnames the general model is ip-<address>.example.com (where "address" can be either IPv6 or IPv4, and "example.com" is a domain of your choosing). In IPv6 the colons are converted to a dash; in IPv4 the dots are converted to a dash.

The following are all considered valid for A or AAAA queries.

ip-2001-0db8-0000-0000-0000-0000-0000-0001.example.com
ip-2001-db8--1.example.com
ip-192-0-2-0.example.com

Reverse Lookups

Reverse Lookups are IP -> hostname, and are known as pointer records (PTR). synthetic will respond to a PTR query and return a result that is also supported by the forward lookup mechanism. Reverse lookups for IPv6 addresses will return a fully compressed IPv6 address (per RFC5952).

Corefile Configuration Examples

Reverse Lookup Example

2001:db8:abcd::/48 {
    synthetic {
        forward example.com
    }
    file d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa
}

Forward Lookup Example

example.com {
    synthetic {
        net 2001:db8:abcd::/64
        net 2001:db8:1234::/64
    }
    file db.example.com

Compiling into CoreDNS

To compile this with CoreDNS you can follow the normal procedure for external plugins. This plugin can be used by adding the following to plugin.cfg:

synthetic:code.fritzlab.net/dns/synthetic

FAQ

Why not use templates?

1- It appears that the template plugin is the recommended pattern for providing the resolution pattern we're after here. However, it's not possible to have the file plugin provide the primary source of data and use a template at the same time. See this GitHub comment. Thus, it's not possible to have a PTR response from a file take priority over a template.

2- Using regex in a template for IPv4 and IPv6 addresses is very challanging with CIDR notation. This plugin provides an easier experience by just providing an IP prefix in CIDR notation.

Development

TODO: Add development instructions

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