i think docker allows for this configuration. i use a gluetun container for the network definition of the torrent container to prevent leaking. the torrent container knows of no other network than the vpn container.

There's readily available docker containers for it but I wanted to build it by hand. Well, more or less, Extremely hacky but it works, so fine for me.

I started out with cheating and used this wrapper around wg-quick that gives us a persistent network namespace with the tunnel interface in it:

https://github.com/dadevel/wg-netns

cat /etc/systemd/system/wg-qbittorrent.service
[Unit]
Description=WireGuard Network Namespace for qBittorrent
Wants=network-online.target nss-lookup.target
After=network-online.target nss-lookup.target

[Service]
Type=oneshot
Environment=WG_ENDPOINT_RESOLUTION_RETRIES=infinity
Environment=WG_VERBOSE=1
ExecStart=/opt/wg-netns/bin/wg-netns up /etc/wireguard/wgconfig.yaml
ExecStop=/opt/wg-netns/bin/wg-netns down /etc/wireguard/wgconfig.yaml
RemainAfterExit=yes

WorkingDirectory=%E/wireguard
ConfigurationDirectory=wireguard
ConfigurationDirectoryMode=0700

CapabilityBoundingSet=CAP_NET_ADMIN CAP_SYS_ADMIN
LimitNOFILE=4096
LimitNPROC=512
LockPersonality=true
MemoryDenyWriteExecute=true
NoNewPrivileges=true
ProtectClock=true
ProtectHostname=true
RemoveIPC=true
RestrictAddressFamilies=AF_INET AF_INET6 AF_NETLINK
RestrictNamespaces=mnt net
RestrictRealtime=true
RestrictSUIDSGID=true
SystemCallArchitectures=native

[Install]
WantedBy=multi-user.target

Then I built a static binary of qbittorrent using this really neat docker image: https://github.com/userdocs/qbittorrent-nox-static

...and stuffed the result into a systemd service that runs it in the namespace wg-netns provides:

cat /etc/systemd/system/qbittorrent-nox.service 

[Unit]
Description=qBittorrent-nox service
Wants=network-online.target wg-qbittorrent.service 
After=local-fs.target network-online.target nss-lookup.target wg-qbittorrent.service 

[Service]
Type=simple
PrivateTmp=false
#User=qbittorrent
ExecStart=/usr/sbin/ip netns exec ns-qbittorrent sudo -u qbittorrent /opt/qbittorrent/qbittorrent-nox
TimeoutStopSec=1800
RestartSec=15
RestartMaxDelaySec=600
RestartSteps=10
Restart=always

[Install]
WantedBy=multi-user.target

To get the webui out of that I stuck two instances of socat together at the stdout and from there it depends on whatever you want to use as a reverse proxy on the host - or you bind to a network interface if you trust the network:

cat /etc/systemd/system/qbittorrent-webui.service 
[Unit]
Description=qBittorrent-nox webui forwarding into its namespace
Wants=network-online.target wg-qbittorrent.service 
After=local-fs.target network-online.target nss-lookup.target wg-qbittorrent.service 

[Service]
Type=simple
PrivateTmp=false
ExecStart=/opt/qbittorrent/forward-webinterface.sh
TimeoutStopSec=1800
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

cat /opt/qbittorrent/forward-webinterface.sh
#!/bin/sh
set -eu

exec socat tcp6-listen:"8080",reuseaddr,fork,range=[::1]/128 "exec:ip netns exec ns-qbittorrent socat stdio 'tcp-connect:127.0.0.1:8080',nofork"

Works, is reboot safe, stopped caring about beauty at that point.

https://github.com/jamesmcm/vopono