Feature/dns-server (#537)

Adding DNS server for client

Updated the API with new fields

Added custom zone object for peer's DNS resolution

client/internal/dns/local.go

@@ -0,0 +1,56 @@
+package dns
+
+import (
+	"github.com/miekg/dns"
+	nbdns "github.com/netbirdio/netbird/dns"
+	log "github.com/sirupsen/logrus"
+	"sync"
+)
+
+type localResolver struct {
+	registeredMap registrationMap
+	records       sync.Map
+}
+
+// ServeDNS handles a DNS request
+func (d *localResolver) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
+	log.Tracef("received question: %#v\n", r.Question[0])
+	response := d.lookupRecord(r)
+	if response == nil {
+		log.Debugf("got empty response for question: %#v\n", r.Question[0])
+		return
+	}
+
+	replyMessage := &dns.Msg{}
+	replyMessage.SetReply(r)
+	replyMessage.Answer = append(replyMessage.Answer, response)
+
+	err := w.WriteMsg(replyMessage)
+	if err != nil {
+		log.Debugf("got an error while writing the local resolver response, error: %v", err)
+	}
+}
+
+func (d *localResolver) lookupRecord(r *dns.Msg) dns.RR {
+	record, found := d.records.Load(r.Question[0].Name)
+	if !found {
+		return nil
+	}
+
+	return record.(dns.RR)
+}
+
+func (d *localResolver) registerRecord(record nbdns.SimpleRecord) error {
+	fullRecord, err := dns.NewRR(record.String())
+	if err != nil {
+		return err
+	}
+
+	d.records.Store(fullRecord.Header().Name, fullRecord)
+
+	return nil
+}
+
+func (d *localResolver) deleteRecord(recordKey string) {
+	d.records.Delete(dns.Fqdn(recordKey))
+}