.. _file-analysis-framework: ======================= File Analysis Framework ======================= .. TODO: integrate BoZ revisions .. rst-class:: opening In the past, writing Zeek scripts with the intent of analyzing file content could be cumbersome because of the fact that the content would be presented in different ways, via events, at the script-layer depending on which network protocol was involved in the file transfer. Scripts written to analyze files over one protocol would have to be copied and modified to fit other protocols. The file analysis framework (FAF) instead provides a generalized presentation of file-related information. The information regarding the protocol involved in transporting a file over the network is still available, but it no longer has to dictate how one organizes their scripting logic to handle it. A goal of the FAF is to provide analysis specifically for files that is analogous to the analysis Zeek provides for network connections. Supported Protocols =================== Zeek ships with file analysis for the following protocols: :ref:`FTP `, :ref:`HTTP `, :ref:`IRC `, :ref:`Kerberos `, :ref:`MIME `, :ref:`RDP `, :ref:`SMTP `, and :ref:`SSL/TLS/DTLS `. Protocol analyzers are regular :ref:`Zeek plugins `, so users are welcome to provide additional ones in separate Zeek packages. File Lifecycle Events ===================== The key events that may occur during the lifetime of a file are: :zeek:see:`file_new`, :zeek:see:`file_over_new_connection`, :zeek:see:`file_sniff`, :zeek:see:`file_timeout`, :zeek:see:`file_gap`, and :zeek:see:`file_state_remove`. Handling any of these events provides some information about the file such as which network :zeek:see:`connection` and protocol are transporting the file, how many bytes have been transferred so far, and its MIME type. Here's a simple example: .. literalinclude:: file_analysis_01.zeek :caption: :language: zeek :linenos: :tab-width: 4 .. code-block:: console $ zeek -r http/get.trace file_analysis_01.zeek file_state_remove FakNcS1Jfe01uljb3 CHhAvVGS1DHFjwGM9 [orig_h=141.142.228.5, orig_p=59856/tcp, resp_h=192.150.187.43, resp_p=80/tcp] HTTP connection_state_remove CHhAvVGS1DHFjwGM9 [orig_h=141.142.228.5, orig_p=59856/tcp, resp_h=192.150.187.43, resp_p=80/tcp] HTTP This doesn't perform any interesting analysis yet, but does highlight the similarity between analysis of connections and files. Connections are identified by the usual 5-tuple or a convenient UID string while files are identified just by a string of the same format as the connection UID. So there's unique ways to identify both files and connections and files hold references to a connection (or connections) that transported it. File Type Identification ======================== Zeek ships with its own library of content signatures to determine the type of a file, conveyed as MIME types in the :zeek:see:`file_sniff` event. You can find those signatures in the Zeek distribution's ``scripts/base/frameworks/files/magic/`` directory. (Despite the name, Zeek does *not* rely on libmagic for content analysis.) Adding Analysis =============== Zeek supports customized file analysis via *file analyzers* that users can attach to observed files. You can attach analyzers selectively to individual files, or register them for auto-attachment under certain conditions. Once attached, file analyzers start receiving the contents of files as Zeek parses them from ongoing network connections. Zeek comes with the following built-in analyzers: * :ref:`plugin-zeek-filedataevent` to access file content via events (as data streams or content chunks), * :ref:`plugin-zeek-fileentropy` to compute various entropy for a file, * :ref:`plugin-zeek-fileextract` to extract files to disk, * :ref:`plugin-zeek-filehash` to produce common hash values for files, * :ref:`plugin-zeek-pe` to parse executables in PE format, and * :ref:`plugin-zeek-x509` to extract information about x509 certificates. Like protocol parsers, file analyzers are regular :ref:`Zeek plugins `. Users are free to contribute additional ones via Zeek packages. Per-file analyzer registration ------------------------------ To attach an analyzer to a specific file, call :zeek:see:`Files::add_analyzer` with the analyzer's component tag (such as :zeek:see:`Files::ANALYZER_MD5`; consult the above analyzers for details). Some file analyzers support parameters that you can provide to this function via a :zeek:see:`Files::AnalyzerArgs` record, while others introduce additional event types and tunable script-layer settings. You can add multiple analyzers to a file, and add the same analyzer type multiple times, assuming you use varying :zeek:see:`Files::AnalyzerArgs` parameterization. You may remove these selectively from files via calls to :zeek:see:`Files::remove_analyzer`. You may also enable and disable file analyzers globally by calling :zeek:see:`Files::enable_analyzer` and :zeek:see:`Files::disable_analyzer`, respectively. Generic analyzer registration ----------------------------- The framework provides mechanisms for automatically attaching analyzers to files. For example, the :zeek:see:`Files::register_for_mime_types` function ensures that Zeek automatically attaches a given analyzer to all files of a given MIME type. For fully customized auto-attachment logic take a look at :zeek:see:`Files::register_analyzer_add_callback`, and refer to :doc:`base/frameworks/files/main.zeek ` for additional APIs and data structures. Regardless of which file analyzers end up acting on a file, general information about the file (e.g. size, time of last data transferred, MIME type, etc.) is logged in :file:`files.log`. Protocol-specific state ----------------------- Some protocol analyzers redefine the ``fa_file`` record to add additional state. For example, ``base/protocols/http/entities.zeek``, which Zeek loads by default as part of the HTTP analyzer, makes the transaction's :zeek:see:`HTTP::Info` record available via ``f$http`` to provide HTTP context. As always, make sure to test the presence of optional fields via the ``a?$b`` :ref:`record field operator ` before accessing them. Examples -------- File hashing ^^^^^^^^^^^^ The following script uses the MD5 file analyzer to calculate the hashes of plain text files: .. literalinclude:: file_analysis_02.zeek :caption: :language: zeek :tab-width: 4 .. code-block:: console $ zeek -r http/get.trace file_analysis_02.zeek new file, FakNcS1Jfe01uljb3 file_hash, FakNcS1Jfe01uljb3, md5, 397168fd09991a0e712254df7bc639ac File extraction ^^^^^^^^^^^^^^^ The following example sets up extraction of observed files to disk: .. code-block:: zeek global idx: count = 0; event file_new(f: fa_file) { Files::add_analyzer(f, Files::ANALYZER_EXTRACT, [$extract_filename=fmt("file-%04d", ++idx)]); } The file extraction analyzer now writes the content of each observed file to a separate file on disk. The output file name results from concatenating the :zeek:see:`FileExtract::prefix` (normally ``./extract_files/``) and the enumerated ``file-NNNN`` strings. In a production setting you'll likely want to include additional information in the output, for example from state attached to the provided file record. The Zeek distribution ships with a starting point for such approaches: the ``policy/frameworks/files/extract-all-files.zeek`` script. For additional configurability, take a look at the `file-extraction `_ Zeek package. Script-level content analysis ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The ``FileDataEvent`` analyzer provides script-layer access to file content for customized analysis. Since observed files can be very large, Zeek cannot buffer these files and provide their entire content to the script layer once complete. Instead, the ``FileDataEvent`` analyzer reflects the incremental nature of file content as Zeek observes it, and supports two types of events to allow you to process it: user-provided *stream events* receive new file content as supplied by connection-oriented protocols, while *chunk events* receive observed data as provided by protocols that do not feature stream semantics. The following example manually computes the SHA256 hash of each observed file by building up hash state and feeding streamed file content into the hash computation. When Zeek removes a file's state (because it has fully observed it, or perhaps because its state is timing out), it prints the resulting hash to the console: .. code-block:: zeek global hashstate: table[string] of opaque of sha256; event file_stream(f: fa_file, data: string) { if ( f$id !in hashstate ) hashstate[f$id] = sha256_hash_init(); sha256_hash_update(hashstate[f$id], data); } event file_new(f: fa_file) { Files::add_analyzer(f, Files::ANALYZER_DATA_EVENT, [$stream_event=file_stream]); } event file_state_remove(f: fa_file) { if ( f$id in hashstate ) { print(sha256_hash_finish(hashstate[f$id])); delete hashstate[f$id]; } } Be careful with this approach, as it can quickly prove expensive to route all file content through the script layer. Make sure to add the analyzer only for relevant files, and consider removing it via :zeek:see:`Files::remove_analyzer` when you no longer require content analysis. For performance-critical applications a new file analyzer plugin could be a better approach. Input Framework Integration =========================== The FAF comes with a simple way to integrate with the :doc:`Input Framework `, so that Zeek can analyze files from external sources in the same way it analyzes files that it sees coming over traffic from a network interface it's monitoring. It only requires a call to :zeek:see:`Input::add_analysis`: .. literalinclude:: file_analysis_03.zeek :caption: :language: zeek :linenos: :tab-width: 4 Note that the "source" field of :zeek:see:`fa_file` corresponds to the "name" field of :zeek:see:`Input::AnalysisDescription` since that is what the input framework uses to uniquely identify an input stream. Example output of the above script may be: .. code-block:: console $ echo "Hello world" > myfile $ zeek file_analysis_03.zeek new file, FZedLu4Ajcvge02jA8 file_hash, FZedLu4Ajcvge02jA8, md5, f0ef7081e1539ac00ef5b761b4fb01b3 file_state_remove Nothing that special, but it at least verifies the MD5 file analyzer saw all the bytes of the input file and calculated the checksum correctly!