# labsim — libvirt replica of the lab network

A throwaway copy of the production VLAN topology for testing routing, firewall
rules and failover **without touching the real network**. Same VLAN IDs and
roles as UniFi, deliberately different IP ranges so nothing can be confused for
production.

## Topology

Each VLAN is its own isolated libvirt network with one tiny Alpine VM on it.

| VLAN | Name | Sim subnet | VM address | Mirrors production |
|-----:|------|------------|-----------|--------------------|
| 1 | management | 172.31.1.0/24 | 172.31.1.10 | 192.168.1.0/24 |
| 2 | k8s | 172.31.2.0/24 | 172.31.2.10 | 192.168.8.0/23 |
| 3 | kvm | 172.31.3.0/24 | 172.31.3.10 | 192.168.3.0/24 |
| 9 | private | 172.31.9.0/24 | 172.31.9.10 | 10.0.9.0/23 |
| 10 | lot | 172.31.10.0/24 | 172.31.10.10 | 10.0.0.0/23 |
| 200 | roomates | 172.31.200.0/24 | 172.31.200.10 | 192.168.2.0/24 |

The sim subnet always encodes the VLAN id: `172.31.<vlan>.0/24`.

Address plan, identical on every VLAN:

| Address | Role |
|---------|------|
| `.1` | gateway under test — a router VM you add (not created by default) |
| `.2` | host bridge — how you reach the VMs from this workstation |
| `.10` | the VLAN's micro VM |
| `.254` | reserved for a VRRP VIP, mirroring production |

The host sits at `.2` purely so you can SSH in. It is deliberately **not** the
VMs' default route — that is `.1` — so inter-VLAN tests fail loudly when no
router is present instead of being silently served by the host's own routing
table. libvirt also installs reject rules that stop these networks forwarding
to each other, so traffic between VLANs only works once a router VM bridges
them.

## Usage

```bash
./labsim-up.sh              # bring up every VLAN (idempotent)
./labsim-up.sh 2 3          # only VLANs 2 and 3
./labsim-down.sh            # destroy VMs + networks, keep the base image
./labsim-down.sh --purge    # also delete the downloaded Alpine image
```

Each VM: 256 MB, 1 vCPU, a copy-on-write overlay on one shared 176 MB Alpine
image (so six VMs cost a few MB of disk, not 1 GB).

## Access

```bash
ssh alpine@172.31.2.10       # normal user (password: labsim)
ssh root@172.31.2.10         # privileged — this image has no sudo
curl http://172.31.2.10/     # hello-world page naming the VLAN
```

Console, when the network is the thing that is broken:

```bash
sudo virsh console labsim-2-k8s     # root / labsim
```

## Watching it

```bash
./labsim-matrix.py --watch 2     # terminal grid, changed cells highlighted
./monitoring-up.sh               # topology page + Prometheus + Grafana
```

- **http://localhost:9101/** — live mesh: a node per VLAN, the router in the
  middle, one line per pair coloured green/red with the ICMP RTT on it. Hover a
  line for per-direction detail. Refreshes every 5s. This is the one to watch
  while changing firewall rules.
- **http://localhost:3000/d/labsim-matrix** — Grafana (anonymous, no login) for
  *history*: when did a path flip, and how has latency moved.
- **http://localhost:9101/metrics** — `labsim_reachable{src,dst,proto}` and
  `labsim_rtt_ms{src,dst}`.

## Notes for whoever extends this

Things that cost time the first time round, all verified on this image:

- **No `sudo`.** Alpine ships `doas`; cloud-init's `sudo:` directive is inert
  here. Use `root@` for privileged work.
- **cloud-init leaves users locked** (`!*` in `/etc/shadow`) unless
  `lock_passwd: false`, and sshd then refuses key auth for that user.
- **One failing `runcmd` aborts every command after it.** Each entry is
  `|| true` for that reason.
- **busybox here has no `httpd` applet**, and the VMs have no internet to
  `apk add` one — so the hello-world server is `python3 -m http.server`
  (python3 is already present because cloud-init depends on it).
- **`start-stop-daemon --exec /usr/bin/python3` matches cloud-init's own
  python3** at boot and refuses to start anything.
- **busybox `pgrep -f PATTERN` matches its own argv**, so a "skip if already
  running" guard always fires. Verified: `guard_exit=0` with nothing listening.

## Not modelled (yet)

VLANs are separate L2 segments rather than one 802.1Q trunk, so this exercises
inter-VLAN routing but not a `bond0.<vif>` trunk config specifically. A router
VM would attach one NIC per VLAN. Adding a tagged-trunk variant is the obvious
next step if the bond/vif config itself needs testing.