nullprogram.com/blog/2009/10/26/
Your BitTorrent client probably has DRM in it, even if it's Free
software. Torrent files (.torrent) may contain a special,
but unofficial, "private" flag. When it does, clients are supposed to
disable decentralized tracking features, regardless of the user's
desires. This is a direct analog to the copy-prevention flag that PDFs
may have set, which tell PDF viewers to cripple themselves, except
that your Free PDF reader is actually more likely to ignore it.
It's impossible to simply open the torrent file and turn off the
flag. The client has to be modified, fixing the purposeful defect, to
ignore it. Note, simpler clients that don't have these features in the
first place don't have this problem, since they don't have any
features to disable.
The private flag exists because modern BitTorrent trackers can
function without a central tracker. If the central tracker is down, or
if the user doesn't want to use it, the client can fetch a list of
peers in the torrent from a worldwide
distributed hash table. It's one big decentralized BitTorrent
tracker (though any arbitrary data can be inserted into it). Clients
also have the ability to tell each other about peers when they are
doing their normal data exchange. Thanks to this, clients can
transcend central trackers and join the larger global torrent of
peers. It makes for healthier torrents.
Anyone who knows a few peers involved with a torrent can join in,
regardless of their ability to talk to the central tracker. But
private tracker sites don't want their torrents to be available
outside to those outside of their control, so they proposed an
addition to the BitTorrent spec for a "private" flag. Clients with
decentralized capabilities are advised cripple that ability when the
flag is on, so no peer lists will leak outside the private
tracker. This flag was never accepted into the official spec, and I
hope it never is.
Unfortunately the private trackers set an ultimatum: obey the private
flag or find your client banned. The client developers fell in line
and, and as far as I am aware, no publicly available clients will use
decentralized tracking while the flag is on. At one point, the
BitComet client ignored the flag and was banned for some time until it
was "fixed".
The private flag wasn't placed in front with the rest of the metadata
where it belonged. It's intentionally placed at the end of the torrent
file inside of the info section. This means that the flag is part of
the info_hash property of the torrent, which is the global identifier
for the torrent. Unset or remove the private flag and the hash
changes, creating a whole new torrent without any seeds.
This is DRM, an artificial restriction imposed on the user. It's
insulting. Users should be the ones that control what happens with
their computers. The reasonable approach to a private flag is that,
when the private flag is enabled, decentralized tracking is turned off
by default, but can be re-enabled by the user should they desire. That
way the desired behavior is indicated but the user has the final say,
not some unrelated website operator.
I rarely use private trackers, since they are nearly pointless, but I
still find this private flag set on public torrents, probably from
someone simply reposting the torrent file from a private site. It's
annoying to run into. It makes the torrents weaker.
Debian, which is my distribution of choice, is generally good about
removing DRM from the software it distributes. For example, the PDF
readers in the repositories have their DRM disabled (i.e. xpdf). So
why not do the same thing for all the intentionally defective
BitTorrent clients?
I went on the Debian IRC channel and brought up the issue only to find
out that everyone thought a little DRM was reasonable. So then I filed
a bug report on it, which was simply closed citing
that the DRM is a beneficial "feature" and that removing the
intentional defect would make the clients "poorer". They also insisted
that it's part of the spec when it's
not. I'm really disappointed in Debian now.
Now, I could modify a client to ignore the flag, but it's not
useful if I am the only one not running DRM. It takes two to tango. A
client used by many people would have to be fixed before it becomes
beneficial.
So when someone asks for an example of Free Software or Open Source
software with DRM in it, you can point to BitTorrent clients.