doc/nas/transmission-bt.md

# transmission-bt


> Moved from nuc to `nas` on 2026-08-30, together with the media
> dataset ([nas-install.md](nas-install.md) §7). Its WireGuard tunnel is
> entirely in-container, so ks4 needed no change — the peer is still
> `10.8.0.21`. The watch folder moved with it:
> `/export/media/.watchdir` on nas, not `/srv/media/.watchdir` on nuc.

BitTorrent client in an unprivileged Incus container on `nas`, with an
**always-on VPN**: all peer traffic exits via ks4's public IP through a
WireGuard tunnel to the `wireguard` container on ks4. Kill switch by
construction — `eth0` has **no default route**, so with the tunnel down
the container simply has no path to the internet.

- Image: `images:ubuntu/24.04`, IP: `192.168.0.7` (macvlan, static via netplan)
- Web UI: `http://192.168.0.7:9091` — no auth
  (`rpc-authentication-required: false`); access control is the RPC
  whitelist (`192.168.0.*` only)
- Egress: WG peer `10.8.0.21` → `193.70.35.17:51845`, `AllowedIPs 0.0.0.0/0`
  (verified: `curl ifconfig.me` from the container returns ks4's IP)
- Downloads: `/media/downloads` (= `tank/media`, same dataset Jellyfin
  reads); in-progress files in `/media/.incomplete` so Jellyfin never
  scans partials
- Watch folder: `scp` a `.torrent` into `/export/media/.watchdir` on the host
  and it auto-downloads (see "Watch folder" below)
- `transmission-daemon` is `BindsTo=wg-quick@wg0.service` and binds
  peer traffic to `10.8.0.21` — three independent layers against leaks
  (no default route, unit binding, socket binding)

## Anti-leak design

1. netplan gives `eth0` only: LAN `/24` (web UI + DNS via blocky) and a
   `/32` host route to the WG endpoint via the home gateway.
2. `wg-quick` full-tunnel mode adds its fwmark policy routing +
   iptables anti-leak rule (`iptables` package required — its absence
   makes `wg-quick` fail with `iptables-restore: command not found`).
3. LAN traffic keeps working thanks to wg-quick's
   `suppress_prefixlength 0` rule (connected routes win over the
   tunnel's default).

## Install script

Run as root on the Incus host. Requires the peer added on ks4 (below).

```bash
#!/usr/bin/env bash
set -euxo pipefail

CNAME="${CNAME:-transmission-bt}"
IMAGE="${IMAGE:-images:ubuntu/24.04}"

incus launch "$IMAGE" "$CNAME"
sleep 8
incus config set "$CNAME" environment.DEBIAN_FRONTEND=noninteractive
incus exec "$CNAME" -- timedatectl set-timezone Europe/Paris

# static LAN config, NO default route (kill switch), /32 to the WG endpoint
incus exec "$CNAME" -- bash -c 'cat > /etc/netplan/10-lxc.yaml <<EOF
network:
  version: 2
  ethernets:
    eth0:
      addresses: [192.168.0.7/24]
      nameservers:
        addresses: [192.168.0.254]
      routes:
        - to: 193.70.35.17/32
          via: 192.168.0.2
EOF
chmod 600 /etc/netplan/10-lxc.yaml
netplan apply'

# packages need a temporary default route (removed right after)
incus exec "$CNAME" -- ip route add default via 192.168.0.2
incus exec "$CNAME" -- apt-get update
incus exec "$CNAME" -- apt-get install -y --no-install-recommends \
    transmission-daemon wireguard-tools iptables curl
incus exec "$CNAME" -- ip route del default via 192.168.0.2

# WireGuard full tunnel (generate key, print pubkey for the ks4 side)
incus exec "$CNAME" -- bash -c 'umask 077
wg genkey > /etc/wireguard/wg0.key
wg pubkey < /etc/wireguard/wg0.key
cat > /etc/wireguard/wg0.conf <<EOF
[Interface]
Address = 10.8.0.21/24
PrivateKey = $(cat /etc/wireguard/wg0.key)

[Peer]
# wireguard container on ks4 — ALL traffic routes through it
PublicKey = TVs6d7bXTvJ0ZluTLb8wR+zIrsLvkH1944pzM+3dZXM=
Endpoint = 193.70.35.17:51845
AllowedIPs = 0.0.0.0/0
PersistentKeepalive = 25
EOF'
incus exec "$CNAME" -- systemctl enable --now wg-quick@wg0

# media share (same dataset as jellyfin-server)
incus config device add "$CNAME" media disk source=/export/media path=/media shift=true
incus exec "$CNAME" -- mkdir -p /media/downloads /media/.incomplete
incus exec "$CNAME" -- chown debian-transmission:debian-transmission \
    /media/downloads /media/.incomplete

# transmission config (edit only while the daemon is stopped)
incus exec "$CNAME" -- systemctl stop transmission-daemon
incus exec "$CNAME" -- bash -c '
cd /var/lib/transmission-daemon/.config/transmission-daemon
sed -i \
  -e "s|\"download-dir\":.*|\"download-dir\": \"/media/downloads\",|" \
  -e "s|\"incomplete-dir\":.*|\"incomplete-dir\": \"/media/.incomplete\",|" \
  -e "s|\"incomplete-dir-enabled\":.*|\"incomplete-dir-enabled\": true,|" \
  -e "s|\"rpc-whitelist\":.*|\"rpc-whitelist\": \"127.0.0.1,::1,192.168.0.*\",|" \
  -e "s|\"rpc-authentication-required\":.*|\"rpc-authentication-required\": false,|" \
  -e "s|\"bind-address-ipv4\":.*|\"bind-address-ipv4\": \"10.8.0.21\",|" \
  settings.json'

# transmission lives and dies with the tunnel
incus exec "$CNAME" -- mkdir -p /etc/systemd/system/transmission-daemon.service.d
incus exec "$CNAME" -- bash -c 'cat > /etc/systemd/system/transmission-daemon.service.d/vpn.conf <<EOF
[Unit]
BindsTo=wg-quick@wg0.service
After=wg-quick@wg0.service
EOF'
incus exec "$CNAME" -- systemctl daemon-reload
incus exec "$CNAME" -- systemctl start transmission-daemon
incus config set "$CNAME" boot.autostart=true
```

On **ks4** (root), authorize the peer with the pubkey printed above:

```sh
incus exec wireguard -- wg set wg0 peer <PUBKEY> allowed-ips 10.8.0.21/32
incus exec wireguard -- wg-quick save wg0
```

## Watch folder (auto-add torrents)

Drop a `.torrent` into `/export/media/.watchdir` on the host and Transmission
auto-adds it and starts downloading — no web UI needed. The folder lives on
the shared `tank/media` dataset (`/media/.watchdir` inside the container).

Edit the **active** config only while the daemon is stopped (it rewrites
`settings.json` on exit). The active file is
`/var/lib/transmission-daemon/info/settings.json` — the daemon runs with
`--config-dir /var/lib/transmission-daemon/info` from
`/etc/default/transmission-daemon`, *not* the `.config` dir.

```bash
incus exec transmission-bt -- install -d -o debian-transmission \
    -g debian-transmission -m 0775 /media/.watchdir
incus exec transmission-bt -- systemctl stop transmission-daemon
incus exec transmission-bt -- python3 - <<'PY'
import json
p = "/var/lib/transmission-daemon/info/settings.json"
c = json.load(open(p))
c.update({
    "watch-dir": "/media/.watchdir",
    "watch-dir-enabled": True,
    "watch-dir-force-generic": True,      # poll (reliable across the shift mount)
    "trash-original-torrent-files": True, # delete the .torrent once added
})
json.dump(c, open(p, "w"), indent=4)
PY
incus exec transmission-bt -- systemctl start transmission-daemon
```

Usage — the `.torrent` is consumed within a few seconds:

```sh
scp some.torrent root@192.168.0.4:/export/media/.watchdir/
```

- `watch-dir-force-generic: true` makes Transmission **poll** the folder
  instead of using inotify, so it reliably sees files written from the host
  side across the shift-mounted share.
- `trash-original-torrent-files: true` self-cleans the folder. A *malformed*
  `.torrent` is not trashed and gets retried each poll — delete it by hand.
- The watchdir is owned by `debian-transmission` (0775): root's `scp` writes
  fine, and Transmission can read the file and remove it after adding.

## Verification

```sh
incus exec transmission-bt -- wg show wg0 latest-handshakes   # non-zero timestamp
incus exec transmission-bt -- curl -s https://ifconfig.me     # must print 193.70.35.17
incus exec transmission-bt -- bash -c "ping -c1 -W2 8.8.8.8 || echo kill-switch OK"  # with wg0 down
# web UI must be tested from a LAN machine — the macvlan quirk means the
# nas host itself cannot reach 192.168.0.7 (macvlan, by design)
```

## Notes

- No inbound peer port is forwarded (would need a proxy device on ks4
  + DNAT through the tunnel); torrents work fine outbound-only, just
  connect to fewer peers.
- Jellyfin sees finished downloads under `/media/downloads` — add it as
  a library folder or move files into the movie/show trees.
- The image server check can make `incus launch` hang on slow WAN —
  launching from the cached image fingerprint (`incus image list`)
  bypasses it.

## Jellyfin library scan on completion (2026-08-31)

Jellyfin cannot notice finished downloads by itself any more. It watches
libraries with **inotify**, but since the media moved to nas the writer
(transmission, here) and the reader (`jellyfin-server` on nuc, over NFS)
are on different machines — an inotify event never crosses that. Before
the move both shared one local dataset on nuc, so it just worked.

So transmission tells Jellyfin explicitly, via
`script-torrent-done`:

```json
"script-torrent-done-enabled": true,
"script-torrent-done-filename": "/usr/local/bin/jellyfin-scan.sh"
```

The hook POSTs to Jellyfin's `/Library/Refresh`:

```sh
#!/bin/sh
KEY_FILE=/etc/jellyfin-scan.key
JF=http://192.168.0.5:8096
NAME="${TR_TORRENT_NAME:-unknown}"
[ -r "$KEY_FILE" ] || { logger -t jellyfin-scan "no readable key file; skipped ($NAME)"; exit 0; }
KEY=$(tr -d " \t\r\n" < "$KEY_FILE")
if curl -fsS -m 15 -X POST -H "X-Emby-Token: $KEY" "$JF/Library/Refresh" >/dev/null 2>&1; then
    logger -t jellyfin-scan "library scan requested after: $NAME"
else
    logger -t jellyfin-scan "library scan request FAILED (nuc off?) after: $NAME"
fi
exit 0
```

Design points, each of which matters:

- **Always `exit 0`, never block.** transmission runs the hook
  synchronously; a hanging or failing hook stalls the daemon. Both paths
  are tested — success and unreadable-key both exit 0.
- **nuc is usually powered off.** The request then fails, logs
  `FAILED (nuc off?)`, and Jellyfin picks the file up on its next
  scheduled scan. Not an error worth alerting on.
- **The key file is `640 root:debian-transmission`** — the hook runs as
  `debian-transmission`, so it must be group-readable, and nothing wider.
- Reachable despite the kill switch: `192.168.0.5` is on the directly
  connected LAN, so it needs no default route.

Verify:

```sh
incus exec transmission-bt -- su -s /bin/sh debian-transmission \
    -c 'TR_TORRENT_NAME=selftest /usr/local/bin/jellyfin-scan.sh'
incus exec transmission-bt -- journalctl -t jellyfin-scan -n 3
```

**Rotating the key**: create a new one in Jellyfin (Dashboard → API Keys),
then

```sh
printf %s '<new-key>' | incus exec transmission-bt -- sh -c \
  'umask 027; cat > /etc/jellyfin-scan.key; chown root:debian-transmission /etc/jellyfin-scan.key'
```

⚠️ Edit `settings.json` only while the daemon is **stopped** —
transmission rewrites the whole file on shutdown and will silently
discard changes made underneath it.