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>

plugin/dns64/setup.go

@@ -0,0 +1,99 @@
+package dns64
+
+import (
+	"net"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/plugin/pkg/upstream"
+
+	"github.com/caddyserver/caddy"
+)
+
+var log = clog.NewWithPlugin("dns64")
+
+func init() { plugin.Register("dns64", setup) }
+
+func setup(c *caddy.Controller) error {
+	dns64, err := dns64Parse(c)
+	if err != nil {
+		return plugin.Error("dns64", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		dns64.Next = next
+		return dns64
+	})
+
+	// Register all metrics.
+	c.OnStartup(func() error {
+		metrics.MustRegister(c, RequestsTranslatedCount)
+		return nil
+	})
+
+	return nil
+}
+
+func dns64Parse(c *caddy.Controller) (*DNS64, error) {
+	_, defaultPref, _ := net.ParseCIDR("64:ff9b::/96")
+	dns64 := &DNS64{
+		Upstream: upstream.New(),
+		Prefix:   defaultPref,
+	}
+
+	for c.Next() {
+		args := c.RemainingArgs()
+		if len(args) == 1 {
+			pref, err := parsePrefix(c, args[0])
+
+			if err != nil {
+				return nil, err
+			}
+			dns64.Prefix = pref
+			continue
+		}
+		if len(args) > 0 {
+			return nil, c.ArgErr()
+		}
+
+		for c.NextBlock() {
+			switch c.Val() {
+			case "prefix":
+				if !c.NextArg() {
+					return nil, c.ArgErr()
+				}
+				pref, err := parsePrefix(c, c.Val())
+
+				if err != nil {
+					return nil, err
+				}
+				dns64.Prefix = pref
+			case "translate_all":
+				dns64.TranslateAll = true
+			default:
+				return nil, c.Errf("unknown property '%s'", c.Val())
+			}
+		}
+	}
+	return dns64, nil
+}
+
+func parsePrefix(c *caddy.Controller, addr string) (*net.IPNet, error) {
+	_, pref, err := net.ParseCIDR(addr)
+	if err != nil {
+		return nil, err
+	}
+
+	// Test for valid prefix
+	n, total := pref.Mask.Size()
+	if total != 128 {
+		return nil, c.Errf("invalid netmask %d IPv6 address: %q", total, pref)
+	}
+	if n%8 != 0 || n < 32 || n > 96 {
+		return nil, c.Errf("invalid prefix length %q", pref)
+	}
+
+	return pref, nil
+}