Rewrite edns0 (#561)

* Add edns0 code rewrite

* check arg count

* change `new`; set EDNS0 if request doesn't have it set

* change set to replace_or_append

* change to append_or_replace

* return error in new

* update documents

* fixt UT

* return error

* go fmt

* Rework for more general EDNS0 use

Also changed how rules are created and validated. Implements
EDNS0 NSID in addition to local.

* go fmt

* README updates, NSID tests and fixes

* gofmt -s -w

* Fix tests for rewrite syntax change

* Add tests, fix error message

* Review nits

* Missed on nit

* More tests, integration test, fix edns0 parse issue

* Fix README, use RewriteIgnored

* go fmt

middleware/rewrite/edns0.go

@@ -0,0 +1,141 @@
+// Package rewrite is middleware for rewriting requests internally to something different.
+package rewrite
+
+import (
+	"encoding/hex"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+// edns0LocalRule is a rewrite rule for EDNS0_LOCAL options
+type edns0LocalRule struct {
+	action string
+	code   uint16
+	data   []byte
+}
+
+// ends0NsidRule is a rewrite rule for EDNS0_NSID options
+type edns0NsidRule struct {
+	action string
+}
+
+// setupEdns0Opt will retrieve the EDNS0 OPT or create it if it does not exist
+func setupEdns0Opt(r *dns.Msg) *dns.OPT {
+	o := r.IsEdns0()
+	if o == nil {
+		r.SetEdns0(4096, true)
+		o = r.IsEdns0()
+	}
+	return o
+}
+
+// Rewrite will alter the request EDNS0 NSID option
+func (rule *edns0NsidRule) Rewrite(r *dns.Msg) Result {
+	result := RewriteIgnored
+	o := setupEdns0Opt(r)
+	found := false
+	for _, s := range o.Option {
+		switch e := s.(type) {
+		case *dns.EDNS0_NSID:
+			if rule.action == "replace" || rule.action == "set" {
+				e.Nsid = "" // make sure it is empty for request
+				result = RewriteDone
+			}
+			found = true
+			break
+		}
+	}
+
+	// add option if not found
+	if !found && (rule.action == "append" || rule.action == "set") {
+		o.SetDo(true)
+		o.Option = append(o.Option, &dns.EDNS0_NSID{Code: dns.EDNS0NSID, Nsid: ""})
+		result = RewriteDone
+	}
+
+	return result
+}
+
+// Rewrite will alter the request EDNS0 local options
+func (rule *edns0LocalRule) Rewrite(r *dns.Msg) Result {
+	result := RewriteIgnored
+	o := setupEdns0Opt(r)
+	found := false
+	for _, s := range o.Option {
+		switch e := s.(type) {
+		case *dns.EDNS0_LOCAL:
+			if rule.code == e.Code {
+				if rule.action == "replace" || rule.action == "set" {
+					e.Data = rule.data
+					result = RewriteDone
+				}
+				found = true
+				break
+			}
+		}
+	}
+
+	// add option if not found
+	if !found && (rule.action == "append" || rule.action == "set") {
+		o.SetDo(true)
+		var opt dns.EDNS0_LOCAL
+		opt.Code = rule.code
+		opt.Data = rule.data
+		o.Option = append(o.Option, &opt)
+		result = RewriteDone
+	}
+
+	return result
+}
+
+// newEdns0Rule creates an EDNS0 rule of the appropriate type based on the args
+func newEdns0Rule(args ...string) (Rule, error) {
+	if len(args) < 2 {
+		return nil, fmt.Errorf("Too few arguments for an EDNS0 rule")
+	}
+
+	ruleType := strings.ToLower(args[0])
+	action := strings.ToLower(args[1])
+	switch action {
+	case "append":
+	case "replace":
+	case "set":
+	default:
+		return nil, fmt.Errorf("invalid action: %q", action)
+	}
+
+	switch ruleType {
+	case "local":
+		if len(args) != 4 {
+			return nil, fmt.Errorf("EDNS0 local rules require exactly three args")
+		}
+		return newEdns0LocalRule(action, args[2], args[3])
+	case "nsid":
+		if len(args) != 2 {
+			return nil, fmt.Errorf("EDNS0 NSID rules do not accept args")
+		}
+		return &edns0NsidRule{action: action}, nil
+	default:
+		return nil, fmt.Errorf("invalid rule type %q", ruleType)
+	}
+}
+
+func newEdns0LocalRule(action, code, data string) (*edns0LocalRule, error) {
+	c, err := strconv.ParseUint(code, 0, 16)
+	if err != nil {
+		return nil, err
+	}
+
+	decoded := []byte(data)
+	if strings.HasPrefix(data, "0x") {
+		decoded, err = hex.DecodeString(data[2:])
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return &edns0LocalRule{action: action, code: uint16(c), data: decoded}, nil
+}