all: implement policy failover rcodes

While at it, ensure that config is validated, and fixing a bug related
to reuse ctx between multiple upstreams resolving.

internal/dnsrcode/rcode.go

@@ -0,0 +1,39 @@
+package dnsrcode
+
+import "strings"
+
+// https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6
+var dnsRcode = map[string]int{
+	"NOERROR":   0,  // NoError   - No Error
+	"FORMERR":   1,  // FormErr   - Format Error
+	"SERVFAIL":  2,  // ServFail  - Server Failure
+	"NXDOMAIN":  3,  // NXDomain  - Non-Existent Domain
+	"NOTIMP":    4,  // NotImp    - Not Implemented
+	"REFUSED":   5,  // Refused   - Query Refused
+	"YXDOMAIN":  6,  // YXDomain  - Name Exists when it should not
+	"YXRRSET":   7,  // YXRRSet   - RR Set Exists when it should not
+	"NXRRSET":   8,  // NXRRSet   - RR Set that should exist does not
+	"NOTAUTH":   9,  // NotAuth   - Server Not Authoritative for zone
+	"NOTZONE":   10, // NotZone   - Name not contained in zone
+	"BADSIG":    16, // BADSIG    - TSIG Signature Failure
+	"BADVERS":   16, // BADVERS   - Bad OPT Version
+	"BADKEY":    17, // BADKEY    - Key not recognized
+	"BADTIME":   18, // BADTIME   - Signature out of time window
+	"BADMODE":   19, // BADMODE   - Bad TKEY Mode
+	"BADNAME":   20, // BADNAME   - Duplicate key name
+	"BADALG":    21, // BADALG    - Algorithm not supported
+	"BADTRUNC":  22, // BADTRUNC  - Bad Truncation
+	"BADCOOKIE": 23, // BADCOOKIE - Bad/missing Server Cookie
+}
+
+// FromString returns the DNS Rcode number from given DNS Rcode string.
+// The string value is treated as case-insensitive. If the input string
+// is an invalid DNS Rcode, -1 is returned.
+func FromString(rcode string) int {
+	rcode = strings.ToUpper(rcode)
+	val, ok := dnsRcode[rcode]
+	if !ok {
+		return -1
+	}
+	return val
+}

internal/dnsrcode/rcode_test.go

@@ -0,0 +1,29 @@
+package dnsrcode
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFromString(t *testing.T) {
+	tests := []struct {
+		name          string
+		rcode         string
+		expectedRcode int
+	}{
+		{"valid", "NoError", 0},
+		{"upper", "NOERROR", 0},
+		{"lower", "noerror", 0},
+		{"mix", "nOeRrOr", 0},
+		{"invalid", "foo", -1},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tc.expectedRcode, FromString(tc.rcode))
+		})
+	}
+}