Techniques for Domain Generation Algorithm (DGA) behavior detection are provided. In some embodiments, a system, process, and/or computer program product for DGA behavior detection includes receiving passive Domain Name System (DNS) data that comprises a plurality of DNS responses at a security device; and applying a signature to the passive DNS data to detect DGA behavior, in which applying the signature to the passive DNS data to detect DGA behavior further comprises: parsing each of the plurality of DNS responses to determine whether one or more of the plurality of DNS responses correspond to a non-existent domain (NXDOMAIN) response.