plugin/rewrite: add rcode as a rewrite option (#6204)

* plugin/forward add ignore_server_failure for masking upstream server faults

Signed-off-by: schou <pschou@users.noreply.github.com>

* Switch from a ignore_server_fail to a rewrite rcode type.

Signed-off-by: schou <pschou@users.noreply.github.com>

* trim down the tests

Signed-off-by: schou <pschou@users.noreply.github.com>

* fixing readme TTL and using map for rcode

Signed-off-by: schou <pschou@users.noreply.github.com>

* add newline

Signed-off-by: schou <pschou@users.noreply.github.com>

---------

Signed-off-by: schou <pschou@users.noreply.github.com>

plugin/rewrite/rcode.go

@@ -0,0 +1,178 @@
+package rewrite
+
+import (
+	"context"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+type rcodeResponseRule struct {
+	old int
+	new int
+}
+
+func (r *rcodeResponseRule) RewriteResponse(res *dns.Msg, rr dns.RR) {
+	if r.old == res.MsgHdr.Rcode {
+		res.MsgHdr.Rcode = r.new
+	}
+}
+
+type rcodeRuleBase struct {
+	nextAction string
+	response   rcodeResponseRule
+}
+
+func newRCodeRuleBase(nextAction string, old, new int) rcodeRuleBase {
+	return rcodeRuleBase{
+		nextAction: nextAction,
+		response:   rcodeResponseRule{old: old, new: new},
+	}
+}
+
+func (rule *rcodeRuleBase) responseRule(match bool) (ResponseRules, Result) {
+	if match {
+		return ResponseRules{&rule.response}, RewriteDone
+	}
+	return nil, RewriteIgnored
+}
+
+// Mode returns the processing nextAction
+func (rule *rcodeRuleBase) Mode() string { return rule.nextAction }
+
+type exactRCodeRule struct {
+	rcodeRuleBase
+	From string
+}
+
+type prefixRCodeRule struct {
+	rcodeRuleBase
+	Prefix string
+}
+
+type suffixRCodeRule struct {
+	rcodeRuleBase
+	Suffix string
+}
+
+type substringRCodeRule struct {
+	rcodeRuleBase
+	Substring string
+}
+
+type regexRCodeRule struct {
+	rcodeRuleBase
+	Pattern *regexp.Regexp
+}
+
+// Rewrite rewrites the current request based upon exact match of the name
+// in the question section of the request.
+func (rule *exactRCodeRule) Rewrite(ctx context.Context, state request.Request) (ResponseRules, Result) {
+	return rule.responseRule(rule.From == state.Name())
+}
+
+// Rewrite rewrites the current request when the name begins with the matching string.
+func (rule *prefixRCodeRule) Rewrite(ctx context.Context, state request.Request) (ResponseRules, Result) {
+	return rule.responseRule(strings.HasPrefix(state.Name(), rule.Prefix))
+}
+
+// Rewrite rewrites the current request when the name ends with the matching string.
+func (rule *suffixRCodeRule) Rewrite(ctx context.Context, state request.Request) (ResponseRules, Result) {
+	return rule.responseRule(strings.HasSuffix(state.Name(), rule.Suffix))
+}
+
+// Rewrite rewrites the current request based upon partial match of the
+// name in the question section of the request.
+func (rule *substringRCodeRule) Rewrite(ctx context.Context, state request.Request) (ResponseRules, Result) {
+	return rule.responseRule(strings.Contains(state.Name(), rule.Substring))
+}
+
+// Rewrite rewrites the current request when the name in the question
+// section of the request matches a regular expression.
+func (rule *regexRCodeRule) Rewrite(ctx context.Context, state request.Request) (ResponseRules, Result) {
+	return rule.responseRule(len(rule.Pattern.FindStringSubmatch(state.Name())) != 0)
+}
+
+// newRCodeRule creates a name matching rule based on exact, partial, or regex match
+func newRCodeRule(nextAction string, args ...string) (Rule, error) {
+	if len(args) < 3 {
+		return nil, fmt.Errorf("too few (%d) arguments for a rcode rule", len(args))
+	}
+	var oldStr, newStr string
+	if len(args) == 3 {
+		oldStr, newStr = args[1], args[2]
+	}
+	if len(args) == 4 {
+		oldStr, newStr = args[2], args[3]
+	}
+	old, valid := isValidRCode(oldStr)
+	if !valid {
+		return nil, fmt.Errorf("invalid matching RCODE '%s' for a rcode rule", oldStr)
+	}
+	new, valid := isValidRCode(newStr)
+	if !valid {
+		return nil, fmt.Errorf("invalid replacement RCODE '%s' for a rcode rule", newStr)
+	}
+	if len(args) == 4 {
+		switch strings.ToLower(args[0]) {
+		case ExactMatch:
+			return &exactRCodeRule{
+				newRCodeRuleBase(nextAction, old, new),
+				plugin.Name(args[1]).Normalize(),
+			}, nil
+		case PrefixMatch:
+			return &prefixRCodeRule{
+				newRCodeRuleBase(nextAction, old, new),
+				plugin.Name(args[1]).Normalize(),
+			}, nil
+		case SuffixMatch:
+			return &suffixRCodeRule{
+				newRCodeRuleBase(nextAction, old, new),
+				plugin.Name(args[1]).Normalize(),
+			}, nil
+		case SubstringMatch:
+			return &substringRCodeRule{
+				newRCodeRuleBase(nextAction, old, new),
+				plugin.Name(args[1]).Normalize(),
+			}, nil
+		case RegexMatch:
+			regexPattern, err := regexp.Compile(args[1])
+			if err != nil {
+				return nil, fmt.Errorf("invalid regex pattern in a rcode rule: %s", args[1])
+			}
+			return &regexRCodeRule{
+				newRCodeRuleBase(nextAction, old, new),
+				regexPattern,
+			}, nil
+		default:
+			return nil, fmt.Errorf("rcode rule supports only exact, prefix, suffix, substring, and regex name matching")
+		}
+	}
+	if len(args) > 4 {
+		return nil, fmt.Errorf("many few arguments for a rcode rule")
+	}
+	return &exactRCodeRule{
+		newRCodeRuleBase(nextAction, old, new),
+		plugin.Name(args[0]).Normalize(),
+	}, nil
+}
+
+// validRCode returns true if v is valid RCode value.
+func isValidRCode(v string) (int, bool) {
+	i, err := strconv.ParseUint(v, 10, 32)
+	// try parsing integer based rcode
+	if err == nil && i <= 23 {
+		return int(i), true
+	}
+
+	if RCodeInt, ok := dns.StringToRcode[strings.ToUpper(v)]; ok {
+		return RCodeInt, true
+	}
+	return 0, false
+}