plugin/header: Introduce header plugin (#4752)

* Add header plugin

Signed-off-by: MQasimSarfraz <syed.qasim.sarfraz@gmail.com>

* fix import format

* improve README.md

* Add codeowners for header plugin

plugin/header/header.go

@@ -0,0 +1,92 @@
+package header
+
+import (
+	"fmt"
+	"strings"
+
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+
+	"github.com/miekg/dns"
+)
+
+// Supported flags
+const (
+	authoritative      = "aa"
+	recursionAvailable = "ra"
+	recursionDesired   = "rd"
+)
+
+var log = clog.NewWithPlugin("header")
+
+// ResponseHeaderWriter is a response writer that allows modifying dns.MsgHdr
+type ResponseHeaderWriter struct {
+	dns.ResponseWriter
+	Rules []Rule
+}
+
+// WriteMsg implements the dns.ResponseWriter interface.
+func (r *ResponseHeaderWriter) WriteMsg(res *dns.Msg) error {
+	// handle all supported flags
+	for _, rule := range r.Rules {
+		switch rule.Flag {
+		case authoritative:
+			res.Authoritative = rule.State
+		case recursionAvailable:
+			res.RecursionAvailable = rule.State
+		case recursionDesired:
+			res.RecursionDesired = rule.State
+		}
+	}
+
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+// Write implements the dns.ResponseWriter interface.
+func (r *ResponseHeaderWriter) Write(buf []byte) (int, error) {
+	log.Warning("ResponseHeaderWriter called with Write: not ensuring headers")
+	n, err := r.ResponseWriter.Write(buf)
+	return n, err
+}
+
+// Rule is used to set/clear Flag in dns.MsgHdr
+type Rule struct {
+	Flag  string
+	State bool
+}
+
+func newRules(key string, args []string) ([]Rule, error) {
+	if key == "" {
+		return nil, fmt.Errorf("no flag action provided")
+	}
+
+	if len(args) < 1 {
+		return nil, fmt.Errorf("invalid length for flags, at least one should be provided")
+	}
+
+	var state bool
+	action := strings.ToLower(key)
+	switch action {
+	case "set":
+		state = true
+	case "clear":
+		state = false
+	default:
+		return nil, fmt.Errorf("unknown flag action=%s, should be set or clear", action)
+	}
+
+	var rules []Rule
+	for _, arg := range args {
+		flag := strings.ToLower(arg)
+		switch flag {
+		case authoritative:
+		case recursionAvailable:
+		case recursionDesired:
+		default:
+			return nil, fmt.Errorf("unknown/unsupported flag=%s", flag)
+		}
+		rule := Rule{Flag: flag, State: state}
+		rules = append(rules, rule)
+	}
+
+	return rules, nil
+}