Add health-check agent recognition to avoid error logs (#4917)

Health-check connections now send a properly formatted auth message
with a well-known peer ID instead of immediately closing. The server
recognizes this peer ID and handles the connection gracefully with a
debug log instead of error logs.

relay/server/handshake.go

@@ -97,7 +97,7 @@ func (h *handshake) handshakeReceive() (*messages.PeerID, error) {
 		return nil, fmt.Errorf("invalid message type %d from %s", msgType, h.conn.RemoteAddr())
 	}
 	if err != nil {
-		return nil, err
+		return peerID, err
 	}
 	h.peerID = peerID
 	return peerID, nil
@@ -147,7 +147,7 @@ func (h *handshake) handleAuthMsg(buf []byte) (*messages.PeerID, error) {
 	}
 
 	if err := h.validator.Validate(authPayload); err != nil {
-		return nil, fmt.Errorf("validate %s (%s): %w", rawPeerID.String(), h.conn.RemoteAddr(), err)
+		return rawPeerID, fmt.Errorf("validate %s (%s): %w", rawPeerID.String(), h.conn.RemoteAddr(), err)
 	}
 
 	return rawPeerID, nil

relay/server/relay.go

@@ -12,6 +12,7 @@ import (
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/metric"
 
+	"github.com/netbirdio/netbird/relay/healthcheck/peerid"
 	//nolint:staticcheck
 	"github.com/netbirdio/netbird/relay/metrics"
 	"github.com/netbirdio/netbird/relay/server/store"
@@ -123,7 +124,11 @@ func (r *Relay) Accept(conn net.Conn) {
 	}
 	peerID, err := h.handshakeReceive()
 	if err != nil {
-		log.Errorf("failed to handshake: %s", err)
+		if peerid.IsHealthCheck(peerID) {
+			log.Debugf("health check connection from %s", conn.RemoteAddr())
+		} else {
+			log.Errorf("failed to handshake: %s", err)
+		}
 		if cErr := conn.Close(); cErr != nil {
 			log.Errorf("failed to close connection, %s: %s", conn.RemoteAddr(), cErr)
 		}