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What is a DAT File?

A DAT file is a generic data file whose internal structure depends on the application that created it. Some DAT files used on Video CDs contain MPEG program streams, but the extension alone does not identify a format.

How DAT Files work

.dat is a naming convention, not a format: unrelated programs reuse it for their own text or binary payloads. The signature database of the file utility alone attaches the extension to RIFF/CDXA video from Video CDs, the Windows boot log BOOTSTAT.DAT, Inno Setup uninstall logs, and Outlook nickname files, and Outlook’s TNEF email attachments usually arrive as winmail.dat. Because the name carries no structural information, file classifies such inputs by content, running signature tests before falling back to text heuristics. The .DAT entries for a Video CD’s MPEG sequences are a special case: they expose raw MPEG tracks through the disc’s file system instead of behaving like ordinary files, while other .DAT files on the same disc hold access or caption data.

Key facts

  1. No signature is shared across .dat files. The file magic database assigns the extension to formats as different as RIFF/CDXA video, the Windows boot log BOOTSTAT.DAT, Inno Setup uninstall logs, and Outlook nickname files, so a pipeline has to route on detected content. When nothing matches, application/octet-stream, which RFC 2046 defines as arbitrary binary data, is the accurate label.
  2. The .DAT entries that stand for a Video CD’s MPEG sequences, listed under /MPEGAV, are not ordinary files. MPlayer’s documentation describes such an entry as an ISO gateway to raw 2324-byte MPEG sectors: Windows emulates raw track reads through it, while the standard Linux ISO 9660 driver returns unusable data. On Linux, read the track itself, for example with MPlayer’s vcd:// syntax or a ripper such as vcdxrip. Other .DAT files on the disc, such as /EXT/SCANDATA.DAT (playing-time access data) and /EXT/CAPTnn.DAT (closed captions), are not video, so check the disc path and the content before choosing a media tool.
  3. A Video CD’s MPEG .DAT file copied through the Windows file system commonly starts with a RIFF header whose form type is CDXA (the bytes RIFF at offset 0 and CDXA at offset 8); file reports it as wrapped MPEG-1 with the media type video/x-cdxa. The VCDImager manual says this RIFF container is not part of the disc and is presumably inserted by the operating system or file system, whereas a ripper such as vcdxrip extracts the contained MPEG program streams. Renaming the copy to .mpg leaves the RIFF header, sector headers, and padding in place, so a demuxer that expects a bare MPEG program stream may need the wrapper removed first; the same manual documents cdxa2mpeg for stripping it.
  4. Outlook’s TNEF container typically arrives as an email attachment named winmail.dat and can also appear under a generic name such as ATT00008.dat; it wraps the sender’s attachments rather than being one of them. Neither name proves the content: file recognizes TNEF by the little-endian 32-bit signature 0x223E9F78 at the start of the file (the bytes 78 9F 3E 22), so confirm that before unpacking it with a TNEF decoder instead of passing it to a media converter. Per the Library of Congress format description the container packages formatted text, OLE objects, and the regular attachments, and it can embed the sender’s logon name and personal folders path, which a recipient can read by opening the raw file in a binary or text editor.

When DAT Files matter

Identify the producing application or inspect the file signature before selecting a parser or converter. Treating every DAT file alike can cause decoding failures or misinterpret arbitrary binary data.

Sources

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