coredns-auto-ipv6-ptr

Some services require that RDNS requests resolve to PTR records. With this CoreDNS plugin, you can generate these PTR records on the fly based on the requested IPv6 address. The plugin translates the requested address and appends a suffix.

Installation

add the following to coredns/plugin.cfg:

autoipv6ptr:github.com/peerau/coredns-auto-ipv6-ptr

Translation process

Let's say the plugin receives a PTR request for 4.4.b.d.b.4.e.f.f.f.0.0.4.5.0.5.2.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa.. If there is a preset found the preset value will be used in the anwer. If there is none the regular translation process starts:

Strip .ip6.arpa.: 4.4.b.d.b.4.e.f.f.f.0.0.4.5.0.5.2.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2
Remove all dots: 44bdb4efff004505200b00308bd01002
Reverse the string: 20010db80300b002505400fffe4bdb44
Append the suffix and return the result: 20010db80300b002505400fffe4bdb44.mydomain.tld

Corefile example

Possible plugin arguments:

Argument	Default value	Description
suffix		The suffix to append when regular translating happens
ttl	900	The TTL value the answer should have in seconds

Let's say your provider allocated 2001:db8:300:b000::/56 to you. You sliced two subnets out of it:

2001:db8:300:b000::/64 => lan.myhost.tld
2001:db8:300:b001::/64 => servers.myhost.tld

You Corefile would look something like this:

0.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa {
    log
    autoipv6ptr {
        suffix lan.mydomain.tld
    }
}

1.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa {
    log
    autoipv6ptr {
        suffix servers.mydomain.tld
        ttl 60
    }
}

Working with known hosts

If you have knows hosts you want to return specific PTR records you can do this via the file or secondary plugin. However there is a catch to this! file and secondary are so called backends which return NXDOMAIN when no record was found. You can find a patch provided by GitHub user @dorchain in the file file-fallthrough.patch. This little patch makes file and secondary falling through if no record was found. Apply it via git patch from your CoreDNS root directory:

git apply plugin/autoipv6ptr/file-fallthrough.patch

And build CoreDNS to your needs. Sample Corefile:

0.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa {
    log
    file your.reverse.zone
    autoipv6ptr {
        suffix lan.mydomain.tld
    }
}

1.0.0.b.0.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa {
    log
    secondary {
        transfer from your.master.dns
    }
    autoipv6ptr {
        suffix servers.mydomain.tld
        ttl 60
    }
}