17. File Extraction

17.1. Architecture

The file extraction code works on top of selected protocol parsers (see supported protocols below). The application layer parsers run on top of the stream reassembly engine and the UDP flow tracking.

In case of HTTP, the parser takes care of dechunking and unzipping the request and/or response data if necessary.

This means that settings in the stream engine, reassembly engine and the application layer parsers all affect the workings of the file extraction.

What files are actually extracted and stored to disk is controlled by the rule language.

Supported protocols are:

  • HTTP
  • SMTP
  • FTP
  • NFS
  • SMB

17.2. Settings

stream.checksum_validation controls whether or not the stream engine rejects packets with invalid checksums. A good idea normally, but the network interface performs checksum offloading a lot of packets may seem to be broken. This setting is enabled by default, and can be disabled by setting to “no”. Note that the checksum handling can be controlled per interface, see “checksum_checks” in example configuration.

file-store.stream-depth controls how far into a stream reassembly is done. Beyond this value no reassembly will be done. This means that after this value the HTTP session will no longer be tracked. By default a setting of 1 Megabyte is used. 0 sets it to unlimited. If set to no, it is disabled and stream.reassembly.depth is considered. Non-zero values must be greater than stream.stream-depth to be used.

libhtp.default-config.request-body-limit / libhtp.server-config.<config>.request-body-limit controls how much of the HTTP request body is tracked for inspection by the http_client_body keyword, but also used to limit file inspection. A value of 0 means unlimited.

libhtp.default-config.response-body-limit / libhtp.server-config.<config>.response-body-limit is like the request body limit, only it applies to the HTTP response body.

17.3. Output

17.3.1. File-Store and Eve Fileinfo

There are two output modules for logging information about files extracted. The first is eve.files which is an eve sub-logger that logs fileinfo records. These fileinfo records provide metadata about the file, but not the actual file contents.

This must be enabled in the eve output:

- outputs:
    - eve-log:
      types:
          - files:
              force-magic: no
              force-hash: [md5,sha256]

See Eve (Extensible Event Format) for more details on working with the eve output.

The other output module, file-store stores the actual files to disk.

The file-store module uses its own log directory (default: filestore in the default logging directory) and logs files using the SHA256 of the contents as the filename. Each file is then placed in a directory named 00 to ff where the directory shares the first 2 characters of the filename. For example, if the SHA256 hex string of an extracted file starts with “f9bc6d…” the file we be placed in the directory filestore/f9.

The size of a file that can be stored depends on file-store.stream-depth, if this value is reached a file can be truncated and might not be stored completely. If not enabled, stream.reassembly.depth will be considered.

Setting file-store.stream-depth to 0 permits store of the entire file; here, 0 means “unlimited.”

file-store.stream-depth will always override stream.reassembly.depth when filestore keyword is used. However, it is not possible to set file-store.stream-depth to a value less than stream.reassembly.depth. Values less than this amount are ignored and a warning message will be displayed.

A protocol parser, like modbus, could permit to set a different store-depth value and use it rather than file-store.stream-depth.

Using the SHA256 for file names allows for automatic de-duplication of extracted files. However, the timestamp of a pre-existing file will be updated if the same files is extracted again, similar to the touch command.

Optionally a fileinfo record can be written to its own file sharing the same SHA256 as the file it references. To handle recording the metadata of each occurrence of an extracted file, these filenames include some extra fields to ensure uniqueness. Currently the format is:

<SHA256>.<SECONDS>.<ID>.json

where <SECONDS> is the seconds from the packet that triggered the stored file to be closed and <ID> is a unique ID for the runtime of the Suricata instance. These values should not be depended on, and are simply used to ensure uniqueness.

These fileinfo records are identical to the fileinfo records logged to the eve output.

See File-store (File Extraction) for more information on configuring the file-store output.

Note

This section documents version 2 of the file-store. Version 1 of the file-store has been removed as of Suricata version 6.

17.4. Rules

Without rules in place no extraction will happen. The simplest rule would be:

alert http any any -> any any (msg:"FILE store all"; filestore; sid:1; rev:1;)

This will simply store all files to disk.

Want to store all files with a pdf extension?

alert http any any -> any any (msg:"FILE PDF file claimed"; fileext:"pdf"; filestore; sid:2; rev:1;)

Or rather all actual pdf files?

alert http any any -> any any (msg:"FILE pdf detected"; filemagic:"PDF document"; filestore; sid:3; rev:1;)

Or rather only store files from black list checksum md5 ?

alert http any any -> any any (msg:"Black list checksum match and extract MD5"; filemd5:fileextraction-chksum.list; filestore; sid:4; rev:1;)

Or only store files from black list checksum sha1 ?

alert http any any -> any any (msg:"Black list checksum match and extract SHA1"; filesha1:fileextraction-chksum.list; filestore; sid:5; rev:1;)

Or finally store files from black list checksum sha256 ?

alert http any any -> any any (msg:"Black list checksum match and extract SHA256"; filesha256:fileextraction-chksum.list; filestore; sid:6; rev:1;)

Bundled with the Suricata download is a file with more example rules. In the archive, go to the rules/ directory and check the files.rules file.

17.5. MD5

Suricata can calculate MD5 checksums of files on the fly and log them. See Storing MD5s checksums for an explanation on how to enable this.

17.6. Updating Filestore Configuration