Add bufsize plugin for preparing the DNS Flag Day and avoiding IP fragmentation (#3401)

* add bufsize plugin

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* add docstring and comment

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* delete stdout messages when get an error

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* change to context.Background from TODO

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* define default bufsize as defaultBufSize constant

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* fix some comments

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* function name change: parse

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

* function name change: parse

Signed-off-by: ykhr53 <yukihira.lab@gmail.com>

plugin/bufsize/setup.go

@@ -0,0 +1,52 @@
+package bufsize
+
+import (
+	"strconv"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/caddyserver/caddy"
+)
+
+func init() { plugin.Register("bufsize", setup) }
+
+func setup(c *caddy.Controller) error {
+	bufsize, err := parse(c)
+	if err != nil {
+		return plugin.Error("bufsize", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Bufsize{Next: next, Size: bufsize}
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (int, error) {
+	const defaultBufSize = 512
+	for c.Next() {
+		args := c.RemainingArgs()
+		switch len(args) {
+		case 0:
+			// Nothing specified; use 512 as default
+			return defaultBufSize, nil
+		case 1:
+			// Specified value is needed to verify
+			bufsize, err := strconv.Atoi(args[0])
+			if err != nil {
+				return -1, plugin.Error("bufsize", c.ArgErr())
+			}
+			// Follows RFC 6891
+			if bufsize < 512 || bufsize > 4096 {
+				return -1, plugin.Error("bufsize", c.ArgErr())
+			}
+			return bufsize, nil
+		default:
+			// Only 1 argument is acceptable
+			return -1, plugin.Error("bufsize", c.ArgErr())
+		}
+	}
+	return -1, plugin.Error("bufsize", c.ArgErr())
+}