refactor(dnsx): prepare for merging with netxlite (#515)

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dnsx/dnsencoder.go

@@ -0,0 +1,52 @@
+package dnsx
+
+import "github.com/miekg/dns"
+
+// The DNSEncoder encodes DNS queries to bytes
+type DNSEncoder interface {
+	Encode(domain string, qtype uint16, padding bool) ([]byte, error)
+}
+
+// DNSEncoderMiekg uses github.com/miekg/dns to implement the Encoder.
+type DNSEncoderMiekg struct{}
+
+const (
+	// dnsPaddingDesiredBlockSize is the size that the padded query should be multiple of
+	dnsPaddingDesiredBlockSize = 128
+
+	// dnsEDNS0MaxResponseSize is the maximum response size for EDNS0
+	dnsEDNS0MaxResponseSize = 4096
+
+	// dnsDNSSECEnabled turns on support for DNSSEC when using EDNS0
+	dnsDNSSECEnabled = true
+)
+
+// Encode implements Encoder.Encode
+func (e *DNSEncoderMiekg) Encode(domain string, qtype uint16, padding bool) ([]byte, error) {
+	question := dns.Question{
+		Name:   dns.Fqdn(domain),
+		Qtype:  qtype,
+		Qclass: dns.ClassINET,
+	}
+	query := new(dns.Msg)
+	query.Id = dns.Id()
+	query.RecursionDesired = true
+	query.Question = make([]dns.Question, 1)
+	query.Question[0] = question
+	if padding {
+		query.SetEdns0(dnsEDNS0MaxResponseSize, dnsDNSSECEnabled)
+		// Clients SHOULD pad queries to the closest multiple of
+		// 128 octets RFC8467#section-4.1. We inflate the query
+		// length by the size of the option (i.e. 4 octets). The
+		// cast to uint is necessary to make the modulus operation
+		// work as intended when the desiredBlockSize is smaller
+		// than (query.Len()+4) ¯\_(ツ)_/¯.
+		remainder := (dnsPaddingDesiredBlockSize - uint(query.Len()+4)) % dnsPaddingDesiredBlockSize
+		opt := new(dns.EDNS0_PADDING)
+		opt.Padding = make([]byte, remainder)
+		query.IsEdns0().Option = append(query.IsEdns0().Option, opt)
+	}
+	return query.Pack()
+}
+
+var _ DNSEncoder = &DNSEncoderMiekg{}