cli: add tailscale dns query (#13368)

Updates tailscale/tailscale#13326

Adds a CLI subcommand to perform DNS queries using the internal DNS forwarder and observe its internals (namely, which upstream resolvers are being used).

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

types/dnstype/messagetypes-string.go

@@ -0,0 +1,84 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package dnstype
+
+import (
+	"errors"
+	"strings"
+
+	"golang.org/x/net/dns/dnsmessage"
+)
+
+// StringForType returns the string representation of a dnsmessage.Type.
+// For example, StringForType(dnsmessage.TypeA) returns "A".
+func StringForDNSMessageType(t dnsmessage.Type) string {
+	switch t {
+	case dnsmessage.TypeAAAA:
+		return "AAAA"
+	case dnsmessage.TypeALL:
+		return "ALL"
+	case dnsmessage.TypeA:
+		return "A"
+	case dnsmessage.TypeCNAME:
+		return "CNAME"
+	case dnsmessage.TypeHINFO:
+		return "HINFO"
+	case dnsmessage.TypeMINFO:
+		return "MINFO"
+	case dnsmessage.TypeMX:
+		return "MX"
+	case dnsmessage.TypeNS:
+		return "NS"
+	case dnsmessage.TypeOPT:
+		return "OPT"
+	case dnsmessage.TypePTR:
+		return "PTR"
+	case dnsmessage.TypeSOA:
+		return "SOA"
+	case dnsmessage.TypeSRV:
+		return "SRV"
+	case dnsmessage.TypeTXT:
+		return "TXT"
+	case dnsmessage.TypeWKS:
+		return "WKS"
+	}
+	return "UNKNOWN"
+}
+
+// DNSMessageTypeForString returns the dnsmessage.Type for the given string.
+// For example, DNSMessageTypeForString("A") returns dnsmessage.TypeA.
+func DNSMessageTypeForString(s string) (t dnsmessage.Type, err error) {
+	s = strings.TrimSpace(strings.ToUpper(s))
+	switch s {
+	case "AAAA":
+		return dnsmessage.TypeAAAA, nil
+	case "ALL":
+		return dnsmessage.TypeALL, nil
+	case "A":
+		return dnsmessage.TypeA, nil
+	case "CNAME":
+		return dnsmessage.TypeCNAME, nil
+	case "HINFO":
+		return dnsmessage.TypeHINFO, nil
+	case "MINFO":
+		return dnsmessage.TypeMINFO, nil
+	case "MX":
+		return dnsmessage.TypeMX, nil
+	case "NS":
+		return dnsmessage.TypeNS, nil
+	case "OPT":
+		return dnsmessage.TypeOPT, nil
+	case "PTR":
+		return dnsmessage.TypePTR, nil
+	case "SOA":
+		return dnsmessage.TypeSOA, nil
+	case "SRV":
+		return dnsmessage.TypeSRV, nil
+	case "TXT":
+		return dnsmessage.TypeTXT, nil
+	case "WKS":
+		return dnsmessage.TypeWKS, nil
+	}
+	return 0, errors.New("unknown DNS message type: " + s)
+}