Modern Proxmox LXC containers (cgroup v2 + systemd) don't expose the CTID
inside the guest namespace. The auto-detection in DetectLXCCTID() works
for older LXC setups and when hostname is numeric, but fails for most
production containers where users set custom hostnames.
Changes:
- Added PULSE_LXC_CTID environment variable override in router.go:490-495
- Graceful fallback: auto-detect first, then check env var, then show placeholder
- UI already handles missing CTID by showing "pct exec <ctid>" placeholder
This provides a robust solution for thousands of users:
- Stock Proxmox LXC: Shows `pct exec <ctid>` placeholder (user substitutes manually)
- Custom hostname containers: Can set PULSE_LXC_CTID=171 in compose/systemd
- Numeric hostname containers: Auto-detected (backwards compatible)
Related: FirstRunSetup.tsx already has graceful fallback (line 336-339)
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