Add dns64 plugin (#3534)

* Add dns64 plugin

Add external plugin to core in-tree.
* Pull code from upstream: https://github.com/serverwentdown/dns64
* Update docs.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Make dns64 consistent.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup README

Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup minor issues.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Remove proxy method.

Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: big cleanup

* Make the code a bit more idiomatic
* Add tests
* use proper Upstream API

Signed-off-by: Casey Callendrello <c1@caseyc.net>
Signed-off-by: Ben Kochie <superq@gmail.com>

* A little more clenaup

* Fix some docs.
* Use the correct plugin register method.
* Cleanup some review items.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Add metrics counter for DNS64 translations

Add a basic counter of how many DNS64 translations have been completed.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Add DNSSEC bug link

Signed-off-by: Ben Kochie <superq@gmail.com>

* Test cleanup

Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: more test cleanup

Signed-off-by: Casey Callendrello <c1@caseyc.net>

Co-authored-by: Casey Callendrello <c1@caseyc.net>

man/coredns-dns64.7

@@ -0,0 +1,105 @@
+.\" Generated by Mmark Markdown Processer - mmark.miek.nl
+.TH "COREDNS-DNS64" 7 "January 2020" "CoreDNS" "CoreDNS Plugins"
+
+.SH "NAME"
+.PP
+\fIdns64\fP - enables DNS64 IPv6 transition mechanism.
+
+.SH "DESCRIPTION"
+.PP
+From Wikipedia:
+
+.PP
+.RS
+
+.PP
+DNS64 describes a DNS server that when asked for a domain's AAAA records, but only finds
+A records, synthesizes the AAAA records from the A records.
+
+.RE
+
+.PP
+The synthesis in only performed if the query came in via IPv6.
+
+.PP
+See RFC 6147
+\[la]https://tools.ietf.org/html/rfc6147\[ra] for more information.
+
+.SH "SYNTAX"
+.PP
+.RS
+
+.nf
+dns64 [PREFIX] {
+  [translate\\\_all]
+}
+
+.fi
+.RE
+
+.IP \(bu 4
+[PREFIX] defines a custom prefix instead of the default \fB\fC64:ff9b::/96\fR
+.IP \(bu 4
+\fB\fCtranslate_all\fR translates all queries, including respones that have AAAA results.
+
+
+.SH "EXAMPLES"
+.PP
+Translate with the default well known prefix. Applies to all queries
+
+.PP
+.RS
+
+.nf
+dns64
+
+.fi
+.RE
+
+.PP
+Use a custom prefix
+
+.PP
+.RS
+
+.nf
+dns64 64:1337::/96
+dns64 {
+    prefix 64:1337::/96
+}
+
+.fi
+.RE
+
+.PP
+Enable translation even if an existing AAAA record is present
+
+.PP
+.RS
+
+.nf
+dns64 {
+    translate\_all
+}
+
+.fi
+.RE
+
+.IP \(bu 4
+\fB\fCprefix\fR specifies any local IPv6 prefix to use, instead of the well known prefix (64:ff9b::/96)
+
+
+.SH "BUGS"
+.PP
+Not all features required by DNS64 are implemented, only basic AAAA synthesis.
+
+.IP \(bu 4
+Support "mapping of separate IPv4 ranges to separate IPv6 prefixes"
+.IP \(bu 4
+Resolve PTR records
+.IP \(bu 4
+Follow CNAME records
+.IP \(bu 4
+Make resolver DNSSEC aware
+
+