A private torrent suddently becomes a public torrent

[yegle]

qBittorrent & operating system versions

qBittorrent: 4.6.4 x64
OS: Synology DSM 7 (run qBittorrent in Docker)
Qt: 6.6.2
Libtorrent: 2.0.10.0

What is the problem?

Sorry if the title is a bit confusing.

I use qBittorrent to download content from private trackers exclusively, all my torrent files should be private torrents.

I've seen at least 3 times when a torrent finishes 99% and suddenly it stuck at "Fetching Metadata" (or something similar, sorry I forgot the exact words). For some PT site, I even got a tracker error message "Banned client".

I've been confused by this behavior for a long time and finally noticed one of the PT site showed the user agent I used was curl/7.81.0, which led me to https://github.com/arvidn/libtorrent/blob/8a7aa8311387b8d530a2ec2a3eb5974fec5edb64/src/http_tracker_connection.cpp#L247-L249.

Reading the libtorrent code around it, it seems like the curl/7.81.0 user-agent is only used when anonymous mode is enabled (I did enable it), and the torrent is a public torrent (this should NOT be the case!). And when I check the torrent, sure enough I don't see DHT/PeX/LSD disabled with a "this torrent is private" message!

Since I was able to download the torrent to 99% w/o get banned message, the torrent must initially a private torrent when added to qBittorrent, and suddenly becomes a public torrent at a later time.

I grabbed the $HASH.fastresume file and it indeed show disable_{dht,lsd,pex}: 0:

{
   "active_time": 218803,
   "added_time": 1713467502,
   "allocation": "allocate",
   "apply_ip_filter": 1,
   "auto_managed": 1,
   "banned_peers": "<hex>xxxx</hex>",
   "banned_peers6": "<hex>xxxx</hex>",
   "completed_time": 1713511697,
   "disable_dht": 0,
   "disable_lsd": 0,
   "disable_pex": 0,
   "download_rate_limit": -1,
   "file-format": "libtorrent resume file",
   "file-version": 1,
   "file_priority": [
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1,
      1
   ],
   "finished_time": 171117,
   "httpseeds": [],
   "i2p": 0,
   "info-hash": "<hex>xxxx</hex>",
   "info-hash2": "\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000",
   "last_download": 1713511697,
   "last_seen_complete": 1713680617,
   "last_upload": 1713680616,
   "libtorrent-version": "2.0.10.0",
   "max_connections": 100,
   "max_uploads": 10,
   "name": "xxxx",
   "num_complete": 6,
   "num_downloaded": 16777215,
   "num_incomplete": 1,
   "paused": 0,
   "peers": "<hex>xxxx</hex>",
   "peers6": "",
   "pieces": "",
   "qBt-category": "cccc",
   "qBt-contentLayout": "Original",
   "qBt-firstLastPiecePriority": 0,
   "qBt-inactiveSeedingTimeLimit": -2,
   "qBt-name": "",
   "qBt-ratioLimit": -2000,
   "qBt-seedStatus": 1,
   "qBt-seedingTimeLimit": -2,
   "qBt-stopCondition": "None",
   "qBt-tags": [],
   "save_path": "xxxx",
   "seed_mode": 0,
   "seeding_time": 171117,
   "sequential_download": 0,
   "share_mode": 0,
   "stop_when_ready": 0,
   "super_seeding": 0,
   "total_downloaded": 9323709715,
   "total_uploaded": 5644609077,
   "trackers": [
      [
         "https://xxxx",
         "https://xxxx",
         "https://xxxx"
      ]
   ],
   "upload_mode": 0,
   "upload_rate_limit": -1,
   "url-list": []
}

Steps to reproduce

I don't know how to reproduce this issue, this seem to happen spontaneously when a torrent is almost finished download.

Additional context

No response

Log(s) & preferences file(s)

No response

[stalkerok]

Are you sure you are describing the issue correctly? One and the same torrent cannot be private and public, only if it is two different torrents (with different hash).

[yegle]

The workaround I used was to download the torrent file again and add it to qBittorent.

Despite getting an error in the log:

Detected an attempt to add a duplicate torrent. Merging of trackers is disabled. Torrent: xxx

The torrent magically becomes private torrent again (DHT/PeX/LSD disabled with a "this torrent is private" message).

[yegle]

Are you sure you are describing the issue correctly? One and the same torrent cannot be private and public, only if it is two different torrents (with different hash).

It suddenly became public from what I can tell (using the DHT/PeX/LSD message). Unfortunately due to #14771 I can't be 100% sure.

And the libtorrent code doesn't lie, it indeed uses the special curl/7.81.0 user agent when anonymous mode is enabled and the torrent is considered "public".

[yegle]

And given the workaround I tried (used the workaround twice and fairly certain it does work), the "public" torrent in the queue and the private torrent file I downloaded again should have the same hash (or I won't see the log line).

[stalkerok]

Perhaps the torrent is hybrid and that plays a role too.

[yegle]

Oh there's another symptom that I suspect is relevant.

When this happened to one of my torrents today, I was trying to confirm if the torrent file contains the "private" bit. But looking at the BT_backup folder, I can only find $HASH.fastresume file (the bencode decoded content is in the first comment) and there was no $HASH.torrent file.

After I tried the workaround of downloading and importing the torrent file, I now can find the $HASH.torrent file in BT_backup folder.

This is potentially relevant because tracker_req().private_torrent is set to false around https://github.com/arvidn/libtorrent/blob/8a7aa8311387b8d530a2ec2a3eb5974fec5edb64/src/http_tracker_connection.cpp#L247-L249, and the .private_torrent field seem to be set from m_torrent_file.priv() (e.g. https://github.com/arvidn/libtorrent/blob/8a7aa8311387b8d530a2ec2a3eb5974fec5edb64/src/torrent.cpp#L3011).

[stalkerok]

A .torrent file cannot disappear by itself. In any case, need steps to reproduce the issue.