nbdmux 0.6.0

HTTP-controlled NBD-export multiplexer for a small lab (boot Linux images over the network with overlayfs+tmpfs writes)

nbdmux

HTTP-controlled NBD-export multiplexer for a small lab. Register local disk-image files as named NBD exports over an HTTP control plane; nbdmux keeps an nbd-server subprocess alive that serves all registered exports on a single TCP port. Targets nbd-client against that port from an initramfs and boot the image with overlayfs over tmpfs for writes (see bty's ramboot boot mode for the canonical consumer).

Designed as a peer to withcache: small lab, single sidecar container, no third-party Python deps. Operationally:

[ bty-web ] --HTTP--> [ nbdmux ]  --supervises-->  [ nbd-server ]
                          |                              |
                          |                            TCP 10809
                          |                              |
                          v                              v
                     SQLite state                  [ target's
                     (exports table)                nbd-client ]

Components

Path	What it is
src/nbdmux/server.py	The daemon. HTTP control plane + nbd-server subprocess management + operator UI (Bootstrap 5 + Bootstrap Icons + HTMX, matches bty's chrome)
src/nbdmux/client.py	Stdlib-only Python client library for other tools
deploy/Containerfile	Single-image deploy (Python + nbd-server)
deploy/compose.yml	Reference compose stack

System dependency

nbdmux runs nbd-server (from the classical nbd project) as a subprocess. Install at the OS level:

# Debian / Ubuntu
sudo apt install nbd-server

# Fedora
sudo dnf install nbd

The container deploy bundles it. Also make sure the nbd kernel module + nbd-client are available on the consuming Linux box (the target you're booting); they're in the same nbd package.

Install

pipx install nbdmux            # or: uv tool install nbdmux

Run the daemon (development; the container deploy is the recommended production path):

nbdmux-server --data-dir ./data --port 8082 --nbd-port 10809

Register an image. Two body shapes are accepted:

Pre-warmed -- point at a file the operator has already placed on disk:

curl -X POST http://localhost:8082/exports \
     -H 'Content-Type: application/json' \
     -d '{"name": "debian-sysdev", "file": "/path/to/debian-sysdev.img", "readonly": true}'

Warm via withcache -- nbdmux fetches src_url through the configured withcache, decompresses on the fly (gzip / zstd / xz), and lands the raw .img under <images-dir>/<name>.img. Requires NBDMUX_WITHCACHE_URL set on the daemon:

curl -X POST http://localhost:8082/exports \
     -H 'Content-Type: application/json' \
     -d '{"name": "debian-sysdev", "src_url": "https://catalog/debian-sysdev.img.zst", "readonly": true}'

Then on a target Linux box:

modprobe nbd
nbd-client <nbdmux-host> 10809 -name debian-sysdev /dev/nbd0
fdisk -l /dev/nbd0   # the .img's partition table

HTTP control plane

Method	Path	Body	Returns
GET	/exports	-	array of exports
POST	/exports	{name, file, readonly?: bool} (pre-warmed) OR {name, src_url} (warm via withcache)	the new export
DELETE	/exports/{name}	-	204 (warm-created also unlinks the .img)
POST	/admin/create_export	form-encoded src_url=...	303 to /ui/exports?error=<msg> on failure, else /ui/exports
GET	/healthz	-	ok (200) when nbd-server is up, nbd-server not running (503) when down
GET	/ui/exports	-	operator dashboard

POST /admin/create_export is what the operator UI's create-export subnav form submits to. The form only carries src_url; the export name is derived from the URL basename (sanitised to the export-name allowlist) so operators don't pick names by hand. Validation failures 303 back to /ui/exports?error=<msg> and the page renders the error inline.

Operator UI

The dashboard at http://<host>:8082/ui/exports shows all registered exports plus (top-right of the sub-navigation strip) a create-export <select> populated from <NBDMUX_WITHCACHE_URL>/catalog. It uses Bootstrap 5 + Bootstrap Icons + HTMX bundled offline; the same chrome as bty and withcache, only the primary hue differs (magenta -- the terminus of the trio's navy -> dark-magenta -> magenta gradient) so operators tell the three consoles apart at a glance.

Withcache floor

Requires withcache >= 0.11.0. That release changed GET /catalog to return only downloaded entries; the nbdmux picker lists exactly what's exportable without a defensive downloaded_at filter. Older withcache releases will still fetch a catalog but staged-not-yet-downloaded entries will surface in the picker and the resulting export will fail at fetch time.

Auth

Single-tenant, server-signed cookie -- same pattern as withcache. Set NBDMUX_ADMIN_PASSWORD to gate the operator UI + the HTTP control plane; unset = open with a startup warning.

NBDMUX_SESSION_SECRET pins the HMAC key that signs session cookies. Unset (or blank) = the daemon generates a fresh 64-hex key at first start and persists it under <data-dir>/session-secret. Set it explicitly to keep cookies valid across a container rebuild that wipes the data volume, or to rotate the secret on demand.

The NBD port itself is unauthenticated (nbd-server's classical model); LAN-only assumption, firewall is the operator's responsibility.

License

BSD-3-Clause.