feat(migration): export UniFi config and generate VyOS DHCP+DNS from it

Groundwork for replacing the USG with the VyOS pair without anything on the
network noticing. Three pieces:

migration/unifi-export.py pulls 13 endpoints off the classic controller into
timestamped JSON plus a normalised inventory: 11 networks, 31 DHCP
reservations, 4 port forwards, 2 firewall rules, 0 static routes. Two things
this turned up that a naive export would have lost:

  - 23 of the 31 reservations carry no network_id at all -- UniFi simply does
    not store the binding -- so they are resolved by subnet containment
    instead. Without that, three quarters of the reservations have no subnet
    to be placed in.
  - 30 of the 31 sit INSIDE the DHCP pool, which UniFi's dhcpd tolerates and
    which is flagged as a warning rather than discovered at cutover.

migration/unifi-to-vyos.py turns that inventory into VyOS `set` commands for
DHCP and DNS only -- the services the USG owns that VyOS must reproduce. Not a
general converter. --prod and --sim come from one code path so the config
proven in the sim and the config applied to the firewalls cannot drift. Prod
mode hard-fails if any reservation is missing, since a silent drop is the
failure mode that matters.

DNS is included because the USG resolves for 5 of 6 VLANs today: UniFi hands
out the gateway's own address whenever dhcpd_dns is empty, verified by
labmaster resolving against 192.168.8.1. Replacing the USG without a forwarder
would take DNS away from those VLANs entirely.

labsim/labsim-dhcp-test.sh proves it by booting throwaway VMs with real
production MACs -- the one piece of production config that transplants
verbatim. Safe because ovs-labsim has no physical NIC, so those MACs cannot
reach the real LAN.

Result on VyOS 2026.08 (kea), 4/4: printer1 got 172.31.10.46 from inside the
pool, sonoff-matter got 172.31.11.67 across the /23 boundary, Hubitat got its
out-of-pool .2, and an unreserved MAC got an unreserved address. kea honours
in-pool host reservations -- the open question blocking the cutover.

Supporting changes to labsim:

  - VLAN 10 widened to /23. Every reservation is in LoT and LoT spans 10.0.0.x
    and 10.0.1.x, which a /24 cannot represent.
  - LoT's host leg moved to .3, because 10.0.0.2 is a real reservation
    (Hubitat) that maps onto the host's own address.
  - vlans.conf gained optional masklen and host_octet fields, defaulting to
    24 and 2 so the other five VLANs are untouched.
  - Fixed /etc/network/interfaces hardcoding 255.255.255.0. That file is what
    actually takes effect on these Alpine guests -- cloud-init's
    network-config is ignored -- so any non-/24 VLAN was silently wrong.

Two generator bugs found by VyOS rejecting the output: static-mapping names
are validated as hostnames, so underscores fail; and two devices named
"espressif" plus two named "thebeast" collided into single names, which would
have overwritten one reservation with another's address.

The raw export holds WiFi passphrases and the WAN PPPoE credentials and is
gitignored.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DMVzWZgiKW2wquf5z8S1yH

migration/.gitignore

@@ -0,0 +1,4 @@
+# Raw UniFi export: contains WiFi passphrases (wlanconf) and controller auth
+# material (setting). The inventory is regenerable — never commit it.
+export/
+__pycache__/

migration/_unifi.py

@@ -0,0 +1,51 @@
+"""Shared UniFi API client for the migration tooling.
+
+The controller is a CLASSIC self-hosted UniFi Network app (server_version
+10.4.x), not UniFi OS: login is /api/login and data lives under
+/api/s/<site>/... . UniFi OS would use /api/auth/login + /proxy/network/api.
+Credentials come from the mcpctl server definition so they are not duplicated
+here.
+"""
+from __future__ import annotations
+
+import json, re, ssl, subprocess, urllib.request, http.cookiejar
+
+
+def client():
+    raw = subprocess.run(["mcpctl", "describe", "server", "unifi-network"],
+                         capture_output=True, text=True).stdout
+    m = re.search(r"UNIFI_TARGETS\s+(\[.*)", raw)
+    if not m:
+        raise SystemExit("could not read UNIFI_TARGETS from mcpctl")
+    blob = m.group(1).strip()
+    try:
+        targets = json.loads(blob)
+    except json.JSONDecodeError:
+        targets = json.loads(blob + "}" * (blob.count("{") - blob.count("}")))
+    t = targets[0]
+    base = t["base_url"].rstrip("/")
+    auth = t.get("auth", {})
+    site = t.get("default_site", "default")
+
+    ctx = ssl.create_default_context()
+    ctx.check_hostname = False
+    ctx.verify_mode = ssl.CERT_NONE
+    opener = urllib.request.build_opener(
+        urllib.request.HTTPCookieProcessor(http.cookiejar.CookieJar()),
+        urllib.request.HTTPSHandler(context=ctx))
+    req = urllib.request.Request(
+        f"{base}/api/login",
+        data=json.dumps({"username": auth.get("username"),
+                         "password": auth.get("password")}).encode(),
+        headers={"Content-Type": "application/json"})
+    opener.open(req, timeout=20).read()
+    return opener, base, site
+
+
+def get(opener, base, site, path):
+    """GET /api/s/<site>/<path>, returning the `data` list (never raising)."""
+    try:
+        body = opener.open(f"{base}/api/s/{site}/{path}", timeout=30).read()
+        return json.loads(body).get("data", [])
+    except Exception as exc:
+        return {"__error__": f"{type(exc).__name__}: {exc}"}

migration/unifi-export.py

@@ -0,0 +1,335 @@
+#!/usr/bin/env python3
+"""Export everything from the UniFi controller that the VyOS cutover must preserve.
+
+The point is that nothing quietly stops working after the switch. That means
+capturing not just the networks but every DHCP reservation, every port forward
+and every firewall rule — the things nobody remembers configuring until they
+break.
+
+Writes one JSON file per endpoint into ./export/ (raw, unmodified — the source
+of truth) plus inventory.json, a normalised view used by the VyOS generator.
+
+  ./unifi-export.py                 # export to ./export/
+  ./unifi-export.py --out /tmp/x    # elsewhere
+  ./unifi-export.py --summary       # print a human summary of what was found
+
+WARNING: the raw export contains secrets (wlanconf holds WiFi passphrases,
+setting holds RADIUS/auth material). ./export/ is gitignored — keep it that way.
+"""
+from __future__ import annotations
+
+import argparse
+import ipaddress
+import json
+import os
+import sys
+
+import _unifi
+
+# endpoint -> why it matters for the cutover
+ENDPOINTS = {
+    "rest/networkconf": "networks: VLANs, subnets, DHCP ranges, DNS, lease time",
+    "rest/user":        "known clients — this is where fixed DHCP reservations live",
+    "stat/sta":         "currently active clients and their live IPs",
+    "rest/firewallrule": "firewall rules",
+    "rest/firewallgroup": "address/port groups referenced by rules",
+    "rest/portforward": "port forwards (inbound NAT)",
+    "rest/routing":     "static routes",
+    "rest/dhcpoption":  "custom DHCP options",
+    "rest/wlanconf":    "wireless networks (VLAN bindings)",
+    "stat/device":      "switches/APs incl. per-port VLAN config",
+    "rest/setting":     "controller settings (incl. USG/gateway config)",
+    "rest/usergroup":   "bandwidth groups referenced by clients",
+    "rest/dynamicdns":  "dynamic DNS",
+}
+
+
+def build_inventory(raw: dict) -> dict:
+    """Normalise the parts a migration actually has to reproduce."""
+    nets_by_id = {n["_id"]: n for n in raw.get("rest/networkconf", []) if isinstance(n, dict)}
+
+    networks = []
+    for n in raw.get("rest/networkconf", []):
+        if not isinstance(n, dict):
+            continue
+        networks.append({
+            "id": n.get("_id"),
+            "name": n.get("name"),
+            "purpose": n.get("purpose"),
+            "vlan": n.get("vlan"),
+            "vlan_enabled": n.get("vlan_enabled"),
+            "subnet": n.get("ip_subnet"),
+            "domain_name": n.get("domain_name"),
+            "dhcp_enabled": n.get("dhcpd_enabled"),
+            "dhcp_start": n.get("dhcpd_start"),
+            "dhcp_stop": n.get("dhcpd_stop"),
+            "dhcp_lease": n.get("dhcpd_leasetime"),
+            "dhcp_dns": [n.get(f"dhcpd_dns_{i}") for i in (1, 2, 3, 4) if n.get(f"dhcpd_dns_{i}")],
+            "dhcp_gateway": n.get("dhcpd_gateway") or n.get("dhcpd_gateway_enabled"),
+            "dhcp_ntp": [n.get(f"dhcpd_ntp_{i}") for i in (1, 2) if n.get(f"dhcpd_ntp_{i}")],
+            "igmp_snooping": n.get("igmp_snooping"),
+            "enabled": n.get("enabled", True),
+        })
+
+    # ip_subnet is the GATEWAY address with a prefix ("10.0.0.1/23"), not the
+    # network address — so derive the real subnet before matching against it.
+    subnets = []
+    for n in networks:
+        if not n["subnet"]:
+            continue
+        try:
+            iface = ipaddress.ip_interface(n["subnet"])
+        except ValueError:
+            continue
+        subnets.append((iface.network, n))
+
+    def resolve_net(ip: str | None, network_id: str | None) -> dict:
+        """Which network does this reservation belong to?
+
+        Most reservations here (23 of 31 as of the first export) carry no
+        network_id at all — UniFi simply does not bind them. VyOS needs the
+        subnet to place a static-mapping, so fall back to containment.
+        """
+        if network_id and network_id in nets_by_id:
+            nid = nets_by_id[network_id]
+            return {"name": nid.get("name"), "vlan": nid.get("vlan"), "by": "network_id"}
+        if ip:
+            try:
+                addr = ipaddress.ip_address(ip)
+            except ValueError:
+                return {"name": None, "vlan": None, "by": "unresolved"}
+            for net, meta in subnets:
+                if addr in net:
+                    return {"name": meta["name"], "vlan": meta["vlan"], "by": "subnet"}
+        return {"name": None, "vlan": None, "by": "unresolved"}
+
+    # Fixed reservations: the single most important thing to carry over, and
+    # the easiest to lose — nobody has these written down anywhere else.
+    reservations = []
+    for u in raw.get("rest/user", []):
+        if not isinstance(u, dict) or not u.get("use_fixedip"):
+            continue
+        net = resolve_net(u.get("fixed_ip"), u.get("network_id"))
+        reservations.append({
+            "mac": (u.get("mac") or "").lower(),
+            "ip": u.get("fixed_ip"),
+            "name": u.get("name") or u.get("hostname") or "",
+            "hostname": u.get("hostname") or "",
+            "network_id": u.get("network_id"),
+            "network_name": net["name"],
+            "network_vlan": net["vlan"],
+            "resolved_by": net["by"],
+            "note": (u.get("note") or "").strip(),
+        })
+    reservations.sort(key=lambda r: tuple(int(p) for p in r["ip"].split(".")) if r["ip"] else (0,))
+
+    # Active leases without a reservation: these devices work today by luck of
+    # the lease database. After a DHCP server swap they get a NEW address.
+    reserved_macs = {r["mac"] for r in reservations}
+    dynamic = []
+    for c in raw.get("stat/sta", []):
+        if not isinstance(c, dict):
+            continue
+        mac = (c.get("mac") or "").lower()
+        if mac in reserved_macs or not c.get("ip"):
+            continue
+        dynamic.append({
+            "mac": mac,
+            "ip": c.get("ip"),
+            "name": c.get("name") or c.get("hostname") or "",
+            "network": c.get("network"),
+        })
+    dynamic.sort(key=lambda r: tuple(int(p) for p in r["ip"].split(".")) if r["ip"] else (0,))
+
+    port_forwards = [{
+        "name": p.get("name"), "enabled": p.get("enabled"),
+        "proto": p.get("proto"), "src": p.get("src"),
+        "dst_port": p.get("dst_port"), "fwd": p.get("fwd"),
+        "fwd_port": p.get("fwd_port"), "log": p.get("log"),
+    } for p in raw.get("rest/portforward", []) if isinstance(p, dict)]
+
+    firewall_rules = [{
+        "name": r.get("name"), "enabled": r.get("enabled"), "action": r.get("action"),
+        "ruleset": r.get("ruleset"), "rule_index": r.get("rule_index"),
+        "protocol": r.get("protocol"),
+        "src_address": r.get("src_address"), "dst_address": r.get("dst_address"),
+        "src_firewallgroup_ids": r.get("src_firewallgroup_ids"),
+        "dst_firewallgroup_ids": r.get("dst_firewallgroup_ids"),
+        "src_networkconf_id": r.get("src_networkconf_id"),
+        "dst_networkconf_id": r.get("dst_networkconf_id"),
+    } for r in raw.get("rest/firewallrule", []) if isinstance(r, dict)]
+
+    firewall_groups = [{
+        "id": g.get("_id"), "name": g.get("name"),
+        "type": g.get("group_type"), "members": g.get("group_members"),
+    } for g in raw.get("rest/firewallgroup", []) if isinstance(g, dict)]
+
+    static_routes = [{
+        "name": r.get("name"), "enabled": r.get("enabled"),
+        "network": r.get("static-route_network"),
+        "nexthop": r.get("static-route_nexthop"),
+        "distance": r.get("static-route_distance"),
+        "type": r.get("static-route_type"),
+    } for r in raw.get("rest/routing", []) if isinstance(r, dict)]
+
+    return {
+        "networks": networks,
+        "reservations": reservations,
+        "dynamic_clients": dynamic,
+        "port_forwards": port_forwards,
+        "firewall_rules": firewall_rules,
+        "firewall_groups": firewall_groups,
+        "static_routes": static_routes,
+        "warnings": find_warnings(networks, reservations),
+    }
+
+
+def find_warnings(networks: list, reservations: list) -> list:
+    """Things that are fine under UniFi but bite when rebuilt on VyOS."""
+    warns = []
+
+    for n in networks:
+        if not n["subnet"]:
+            continue
+        try:
+            iface = ipaddress.ip_interface(n["subnet"])
+        except ValueError:
+            warns.append({"kind": "bad_subnet", "network": n["name"], "detail": n["subnet"]})
+            continue
+        # UniFi stores the gateway in ip_subnet. A gateway equal to the network
+        # address is legal in a /23 but plenty of tooling rejects it, so it must
+        # not be discovered during the cutover window.
+        if iface.ip == iface.network.network_address:
+            warns.append({
+                "kind": "gateway_is_network_address", "network": n["name"],
+                "detail": f"gateway {iface.ip} is the network address of {iface.network}",
+            })
+
+    # Reservations that sit inside the dynamic pool. UniFi's dhcpd tolerates
+    # this; whether VyOS does depends on its DHCP backend, so every one of these
+    # is a config that must be proven on the sim before cutover.
+    ranges = []
+    for n in networks:
+        if n["dhcp_enabled"] and n["dhcp_start"] and n["dhcp_stop"]:
+            try:
+                ranges.append((n["name"], ipaddress.ip_address(n["dhcp_start"]),
+                               ipaddress.ip_address(n["dhcp_stop"])))
+            except ValueError:
+                pass
+    inside = []
+    for r in reservations:
+        if not r["ip"]:
+            continue
+        try:
+            addr = ipaddress.ip_address(r["ip"])
+        except ValueError:
+            continue
+        for name, lo, hi in ranges:
+            if lo <= addr <= hi:
+                inside.append(f"{r['ip']} ({r['name'] or r['mac']}) in {name} pool")
+                break
+    if inside:
+        warns.append({"kind": "reservation_inside_dhcp_pool",
+                      "count": len(inside), "detail": inside})
+
+    unresolved = [f"{r['ip']} {r['mac']} {r['name']}"
+                  for r in reservations if r["resolved_by"] == "unresolved"]
+    if unresolved:
+        warns.append({"kind": "reservation_matches_no_subnet",
+                      "count": len(unresolved), "detail": unresolved})
+    return warns
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--out", default=os.path.join(os.path.dirname(os.path.abspath(__file__)), "export"))
+    ap.add_argument("--summary", action="store_true")
+    args = ap.parse_args()
+
+    os.makedirs(args.out, exist_ok=True)
+    opener, base, site = _unifi.client()
+    print(f"controller {base}  site {site}")
+
+    raw: dict = {}
+    errors = []
+    for path, why in ENDPOINTS.items():
+        data = _unifi.get(opener, base, site, path)
+        if isinstance(data, dict) and "__error__" in data:
+            errors.append((path, data["__error__"]))
+            print(f"  {path:22} FAILED  {data['__error__'][:50]}")
+            continue
+        raw[path] = data
+        fname = path.replace("/", "_") + ".json"
+        with open(os.path.join(args.out, fname), "w") as fh:
+            json.dump(data, fh, indent=2, sort_keys=True)
+        print(f"  {path:22} {len(data):4d}  -> {fname}")
+
+    inventory = build_inventory(raw)
+    with open(os.path.join(args.out, "inventory.json"), "w") as fh:
+        json.dump(inventory, fh, indent=2, sort_keys=True)
+
+    print(f"\nwrote {args.out}/inventory.json")
+    print(f"  networks           {len(inventory['networks'])}")
+    print(f"  DHCP reservations  {len(inventory['reservations'])}")
+    print(f"  dynamic clients    {len(inventory['dynamic_clients'])}  (no reservation — see summary)")
+    print(f"  port forwards      {len(inventory['port_forwards'])}")
+    print(f"  firewall rules     {len(inventory['firewall_rules'])}")
+    print(f"  static routes      {len(inventory['static_routes'])}")
+    if errors:
+        print(f"  endpoints failed   {len(errors)}: {[e[0] for e in errors]}")
+
+    if args.summary:
+        print_summary(inventory)
+    return 0
+
+
+def print_summary(inv: dict) -> None:
+    print("\n=== networks ===")
+    print(f"{'name':22} {'vlan':>5}  {'subnet':20} {'dhcp range':32} lease")
+    for n in sorted(inv["networks"], key=lambda x: (x["vlan"] or 0)):
+        rng = f"{n['dhcp_start']} - {n['dhcp_stop']}" if n["dhcp_enabled"] else "(dhcp off)"
+        print(f"{(n['name'] or '')[:22]:22} {str(n['vlan'] or '-'):>5}  "
+              f"{(n['subnet'] or '-'):20} {rng:32} {n['dhcp_lease'] or '-'}")
+
+    print(f"\n=== DHCP reservations ({len(inv['reservations'])}) ===")
+    for r in inv["reservations"]:
+        print(f"  {r['ip']:16} {r['mac']:18} {(r['network_name'] or '?')[:14]:14} {r['name'][:30]}")
+
+    if inv["port_forwards"]:
+        print(f"\n=== port forwards ({len(inv['port_forwards'])}) ===")
+        for p in inv["port_forwards"]:
+            state = "" if p["enabled"] else "  [DISABLED]"
+            print(f"  {p['proto']:6} {str(p['src']):16}:{str(p['dst_port']):11} -> "
+                  f"{p['fwd']}:{p['fwd_port']}   {p['name']}{state}")
+
+    if inv["firewall_rules"]:
+        print(f"\n=== firewall rules ({len(inv['firewall_rules'])}) ===")
+        for r in inv["firewall_rules"]:
+            state = "" if r["enabled"] else "  [DISABLED]"
+            print(f"  {str(r['ruleset']):22} {str(r['action']):8} {r['name']}{state}")
+
+    if inv["warnings"]:
+        print(f"\n=== {len(inv['warnings'])} things to settle before cutover ===")
+        for w in inv["warnings"]:
+            n = w.get("count")
+            print(f"  [{w['kind']}]" + (f"  x{n}" if n else ""))
+            det = w["detail"]
+            for line in (det if isinstance(det, list) else [det])[:6]:
+                print(f"      {line}")
+            if isinstance(det, list) and len(det) > 6:
+                print(f"      ... and {len(det) - 6} more (see inventory.json)")
+
+    n_dyn = len(inv["dynamic_clients"])
+    if n_dyn:
+        print(f"\n=== {n_dyn} active clients WITHOUT a reservation ===")
+        print("  These hold their address only via the current lease database. A DHCP")
+        print("  server swap hands them a different one — fine for phones, not fine for")
+        print("  anything another host reaches by IP. Review before cutover:")
+        for c in inv["dynamic_clients"][:40]:
+            print(f"  {c['ip']:16} {c['mac']:18} {(c['network'] or '')[:14]:14} {c['name'][:30]}")
+        if n_dyn > 40:
+            print(f"  ... and {n_dyn - 40} more (see inventory.json)")
+
+
+if __name__ == "__main__":
+    sys.exit(main())

migration/unifi-to-vyos.py

@@ -0,0 +1,253 @@
+#!/usr/bin/env python3
+"""Turn the UniFi export into the VyOS config that replaces it.
+
+Scope is deliberately narrow: DHCP and DNS. Those are the services the USG owns
+that VyOS must reproduce byte-for-byte in behaviour, because getting them wrong
+means clients lose their addresses or their name resolution. Everything else
+either stays on UniFi (wireless), has no VyOS equivalent (user groups), or is
+hand-written because it is not in the export (WAN, NAT, VRRP).
+
+This is not a general UniFi-to-VyOS converter and should not grow into one.
+
+  ./unifi-to-vyos.py --mode prod          # the cutover artifact
+  ./unifi-to-vyos.py --mode sim           # same MACs, labsim addresses
+  ./unifi-to-vyos.py --mode prod --check  # counts only, no output
+
+Both modes come from one code path on purpose: the config proven in labsim and
+the config applied to the firewalls must not be able to drift apart.
+
+DNS note: UniFi hands out the gateway's own IP as resolver whenever a network
+has no explicit dhcpd_dns -- true for 5 of the 6 VLANs, verified by labmaster
+resolving against 192.168.8.1. So VyOS must run `service dns forwarding` or
+those VLANs lose DNS entirely at cutover. LoT's explicit 10.0.0.194 is preserved
+as-is.
+"""
+from __future__ import annotations
+
+import argparse
+import ipaddress
+import json
+import os
+import re
+import sys
+
+# Where the production upstream resolver lives. Verified authoritative for
+# ad.itaz.eu (SOA nas001.ad.itaz.eu) and also recursive, so it can serve as the
+# single forwarder target.
+UPSTREAM_DNS = "10.0.0.194"
+
+# labsim equivalents, keyed by VLAN id. Only VLAN 10 needs a /23: every one of
+# the 31 reservations is in LoT, which spans 10.0.0.x and 10.0.1.x, and a /24
+# cannot represent that. k8s and Private are also /23 in production but hold no
+# reservations, so they keep their existing /24 and their DHCP range is clamped
+# (reported at generation time -- never silently).
+SIM_SUBNETS = {
+    1:   "172.31.1.0/24",
+    2:   "172.31.2.0/24",
+    3:   "172.31.3.0/24",
+    9:   "172.31.9.0/24",
+    10:  "172.31.10.0/23",
+    200: "172.31.200.0/24",
+}
+
+
+def vlan_of(net: dict) -> int:
+    """VLAN id, treating the untagged Management network as 1.
+
+    UniFi stores vlan=None for the native network; the VyOS side already uses
+    vrid 1 for it (high-availability group 'native'), so 1 is the consistent id.
+    """
+    return int(net["vlan"]) if net.get("vlan") else 1
+
+
+def sanitize(name: str, fallback: str) -> str:
+    """Reduce a UniFi client name to something VyOS will accept as a node name.
+
+    VyOS validates static-mapping names as *hostnames*, so underscores are
+    rejected outright -- verified: `Dongle-M_C0D4` fails with "Invalid static
+    mapping hostname". Letters, digits and hyphens only, no leading digit or
+    hyphen, no trailing hyphen.
+    """
+    cleaned = re.sub(r"[^A-Za-z0-9-]", "-", (name or "").strip())
+    cleaned = re.sub(r"-{2,}", "-", cleaned).strip("-")
+    if cleaned and cleaned[0].isdigit():
+        cleaned = "h" + cleaned
+    return cleaned or fallback
+
+
+class Mapper:
+    """Translates production addresses into the target mode's address space.
+
+    In prod mode this is the identity. In sim mode an address is mapped by its
+    offset from the network address, so the host part is preserved: 10.0.0.46
+    -> 172.31.10.46 and 10.0.1.67 -> 172.31.11.67. That is what makes the sim
+    test meaningful -- the MAC is identical and the host octet is recognisable.
+    """
+
+    def __init__(self, mode: str, networks: list) -> None:
+        self.mode = mode
+        self.clamped: list[str] = []
+        self.map: dict[int, tuple] = {}
+        for n in networks:
+            prod = ipaddress.ip_network(
+                ipaddress.ip_interface(n["subnet"]).network)
+            if mode == "sim":
+                sim = ipaddress.ip_network(SIM_SUBNETS[vlan_of(n)])
+            else:
+                sim = prod
+            self.map[vlan_of(n)] = (prod, sim)
+
+    def net(self, vlan: int) -> ipaddress.IPv4Network:
+        return self.map[vlan][1]
+
+    def addr(self, vlan: int, ip: str, what: str) -> str | None:
+        """Map one address, or None if it does not fit the target subnet."""
+        prod, sim = self.map[vlan]
+        offset = int(ipaddress.ip_address(ip)) - int(prod.network_address)
+        if offset < 0 or offset >= sim.num_addresses:
+            self.clamped.append(f"{what}: {ip} does not fit {sim}")
+            return None
+        return str(ipaddress.ip_address(int(sim.network_address) + offset))
+
+    def gateway(self, vlan: int, net: dict) -> str:
+        """The address clients are told to use as their default route.
+
+        Production: the USG's current address (VIPs move .254 -> .1 at cutover),
+        so no client has to change anything. Sim: the sim router at .1.
+        """
+        if self.mode == "sim":
+            return str(self.net(vlan).network_address + 1)
+        return str(ipaddress.ip_interface(net["subnet"]).ip)
+
+
+def build(inv: dict, mode: str) -> tuple[list[str], dict]:
+    nets = [n for n in inv["networks"] if n["dhcp_enabled"] and n["subnet"]]
+    nets.sort(key=vlan_of)
+    m = Mapper(mode, nets)
+
+    out: list[str] = []
+    used_tags: set[str] = set()
+    stats = {"subnets": 0, "mappings": 0, "dropped": []}
+    by_vlan: dict[int, list] = {}
+    for r in inv["reservations"]:
+        if r["network_vlan"] is None and r["network_name"] != "Management":
+            # resolved_by == "unresolved"; cannot place it without a subnet
+            stats["dropped"].append(f"{r['ip']} {r['mac']} (no network)")
+            continue
+        by_vlan.setdefault(r["network_vlan"] or 1, []).append(r)
+
+    out.append("# --- DHCP ---------------------------------------------------")
+    for n in nets:
+        vlan = vlan_of(n)
+        sub = m.net(vlan)
+        base = f"set service dhcp-server shared-network-name {sanitize(n['name'], f'vlan{vlan}')} subnet {sub}"
+        gw = m.gateway(vlan, n)
+
+        out.append("")
+        out.append(f"# {n['name']} (VLAN {vlan}) <- {n['subnet']}")
+        # subnet-id is required by kea and must be stable across regenerations;
+        # the VLAN id is already the unique per-network number in this lab.
+        out.append(f"{base} subnet-id {vlan}")
+        out.append(f"{base} option default-router {gw}")
+
+        # Preserve UniFi's explicit resolver where it set one (LoT -> the NAS);
+        # otherwise hand out the gateway, which is what the USG does today and
+        # what `service dns forwarding` below will answer on.
+        for ns in (n["dhcp_dns"] or [gw]):
+            mapped = ns if ns not in (UPSTREAM_DNS,) or mode == "prod" else gw
+            if mode == "sim" and ns == UPSTREAM_DNS:
+                mapped = gw  # no NAS in the sim; the router resolves
+            out.append(f"{base} option name-server {mapped}")
+
+        if n["domain_name"]:
+            out.append(f"{base} option domain-name '{n['domain_name']}'")
+        if n["dhcp_lease"]:
+            out.append(f"{base} lease {n['dhcp_lease']}")
+
+        start = m.addr(vlan, n["dhcp_start"], f"{n['name']} range start")
+        stop = m.addr(vlan, n["dhcp_stop"], f"{n['name']} range stop")
+        if start is None:
+            start = str(sub.network_address + 11)
+        if stop is None:
+            # Clamp to the last usable address rather than dropping the pool.
+            stop = str(sub.broadcast_address - 1)
+        out.append(f"{base} range LAN start {start}")
+        out.append(f"{base} range LAN stop {stop}")
+        stats["subnets"] += 1
+
+        for r in sorted(by_vlan.get(vlan, []), key=lambda x: ipaddress.ip_address(x["ip"])):
+            ip = m.addr(vlan, r["ip"], f"reservation {r['name']}")
+            if ip is None:
+                stats["dropped"].append(f"{r['ip']} {r['mac']} ({r['name']})")
+                continue
+            tag = sanitize(r["name"] or r["hostname"], "host-" + r["mac"].replace(":", ""))
+            # Distinct clients can sanitize to the same name; a collision would
+            # silently overwrite one reservation with another's address.
+            if tag in used_tags:
+                tag = f"{tag}-{r['mac'].replace(':', '')[-4:]}"
+            used_tags.add(tag)
+            out.append(f"{base} static-mapping {tag} mac {r['mac']}")
+            out.append(f"{base} static-mapping {tag} ip-address {ip}")
+            stats["mappings"] += 1
+
+    out.append("")
+    out.append("# --- DNS ----------------------------------------------------")
+    out.append("# The USG resolves for 5 of 6 VLANs today (it hands out its own")
+    out.append("# address when dhcpd_dns is empty). Without this, they lose DNS.")
+    for n in nets:
+        vlan = vlan_of(n)
+        out.append(f"set service dns forwarding listen-address {m.gateway(vlan, n)}")
+        out.append(f"set service dns forwarding allow-from {m.net(vlan)}")
+    if mode == "prod":
+        out.append(f"set service dns forwarding name-server {UPSTREAM_DNS}")
+    else:
+        # The sim has no NAS; forward to whatever the sim host can reach.
+        out.append("set service dns forwarding name-server 1.1.1.1")
+    out.append("set service dns forwarding cache-size 10000")
+
+    stats["clamped"] = m.clamped
+    return out, stats
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    here = os.path.dirname(os.path.abspath(__file__))
+    ap.add_argument("--mode", choices=("prod", "sim"), required=True)
+    ap.add_argument("--inventory", default=os.path.join(here, "export", "inventory.json"))
+    ap.add_argument("-o", "--out")
+    ap.add_argument("--check", action="store_true", help="counts only, no config")
+    args = ap.parse_args()
+
+    with open(args.inventory) as fh:
+        inv = json.load(fh)
+
+    lines, stats = build(inv, args.mode)
+
+    expected = len(inv["reservations"])
+    print(f"mode={args.mode}  subnets={stats['subnets']}  "
+          f"static-mappings={stats['mappings']}/{expected}", file=sys.stderr)
+    for c in stats["clamped"]:
+        print(f"  clamped: {c}", file=sys.stderr)
+    for d in stats["dropped"]:
+        print(f"  DROPPED: {d}", file=sys.stderr)
+
+    if stats["mappings"] != expected and args.mode == "prod":
+        print(f"ERROR: {expected - stats['mappings']} reservation(s) missing from "
+              f"prod output -- every one must survive the cutover", file=sys.stderr)
+        return 1
+
+    if args.check:
+        return 0
+
+    text = "\n".join(lines) + "\n"
+    if args.out:
+        with open(args.out, "w") as fh:
+            fh.write(text)
+        print(f"wrote {args.out}", file=sys.stderr)
+    else:
+        sys.stdout.write(text)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())