[Oisf-devel] file extraction -- Re: [COMMIT] OISF branch, master, updated. a556338936ad3cd2b0379a6985fb62084368d99e

Victor Julien victor at inliniac.net
Tue Nov 29 16:54:08 UTC 2011

>From my blog:

File extraction in Suricata

Today I pushed out a new feature in Suricata I’m very excited about. It
has been long in the making and with over 6000 new lines of code it’s a
significant effort. It’s available in the current git master. I’d
consider it alpha quality, so handle with care.

So what is this all about? Simply put, we can now extract files from
HTTP streams in Suricata. Both uploads and downloads. Fully controlled
by the rule language. But thats not all. I’ve added a touch of magic. By
utilizing libmagic (this powers the “file” command), we know the file
type of files as well. Lots of interesting stuff that can be done there.

Rule keywords

Four new rule keywords were added: filename, fileext, filemagic and

Filename and fileext are pretty trivial: match on the full name or file
extension of a file.

    alert http any any -> any any (filename:”secret.xls”;)
    alert http any any -> any any (fileext:”pdf”;)

More interesting is the filemagic keyword. It runs on the magic output
of inspecting the (start of) a file. This value is for example:

    GIF image data, version 89a, 1 x 1
    PE32 executable for MS Windows (GUI) Intel 80386 32-bit
    HTML document text
    Macromedia Flash data (compressed), version 9
    MS Windows icon resource – 2 icons, 16×16, 256-colors
    PNG image data, 70 x 53, 8-bit/color RGBA, non-interlaced
    JPEG image data, JFIF standard 1.01
    PDF document, version 1.6

So how the filemagic keyword allows you to match on this is pretty simple:

    alert http any any -> any any (filemagic:”PDF document”;)
    alert http any any -> any any (filemagic:”PDF document, version 1.6″;)

Pretty cool, eh? You can match both very specifically and loosely. For

    alert http any any -> any any (filemagic:”executable for MS Windows”;)

Will match on (among others) these types:

    PE32 executable for MS Windows (DLL) (GUI) Intel 80386 32-bit
    PE32 executable for MS Windows (GUI) Intel 80386 32-bit
    PE32+ executable for MS Windows (GUI) Mono/.Net assembly

Finally there is the filestore keyword. It is the simplest of all: if
the rule matches, the files will be written to disk.

Naturally you can combine the file keywords with the regular HTTP
keywords, limiting to POST’s for example:

    alert http $EXTERNAL_NET any -> $HOME_NET any (msg:”pdf upload
claimed, but not pdf”; flow:established,to_server; content:”POST”;
http_method; fileext:”pdf”; filemagic:!”PDF document”; filestore; sid:1;

This will alert on and store all files that are uploaded using a POST
request that have a filename extension of pdf, but the actual file is
not pdf.


The storage to disk is handled by a new output module called “file”.
It’s config looks like this:

enabled: yes # set to yes to enable
log-dir: files # directory to store the files
force-magic: no # force logging magic on all stored files

It needs to be enabled for file storing to work.

The files are stored to disk as “file.1″, “file.2″, etc. For each of the
files a meta file is created containing the flow information, file name,
size, etc. Example:

TIME: 01/27/2010-17:41:11.579196
PCAP PKT NUM: 2847035
DST PORT: 56207
MAGIC: PE32+ executable for MS Windows (GUI) Mono/.Net assembly
SIZE: 5204


The file extraction is for HTTP only currently, and works on top of our
HTTP parser. As the HTTP parser runs on top of the stream reassembly
engine, configuration parameters of both these parts of Suricata affect
handling of files.

The stream engine option “stream.reassembly.depth” (default 1 Mb)
controls the depth into a stream in which we look. Set to 0 for no limit.
The libhtp options request-body-limit and response-body-limit control
how far into a HTTP request or response body we look. Again set to 0 for
no limit. This can be controlled per HTTP server.


The file handling is fully streaming, so it’s very efficient.
Nonetheless there will be an overhead for the extra parsing, book
keeping, writing to disk, etc. Memory requirements appear to be limited
as well. Suricata shouldn’t keep more than a few kb per flow in memory.


Lack of limits is a limitation. For file storage no limits have been
implemented yet. So it’s easy to clutter your disk up with files.
Example: 118Gb enterprise pcap storing just JPG’s extracted 400.000
files. Better use a separate partition if you’re on a life link.

Future work

Apart from stabilizing this code and performance optimizing it, the next
step will be SMTP file extraction. Possibly other protocols, although
nothing is set in stone there yet.

Victor Julien
PGP: http://www.inliniac.net/victorjulien.asc

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