labsim: default-deny firewall policy, proven in the sim

Policy: internal VLANs reach each other and the internet; the internet
initiates nothing inward. That was already the effect of the IPv4 ruleset, but
built as a blacklist -- default-action accept plus explicit drops per WAN
interface. Identical behaviour right up until a WAN is added, at which point it
is open and nothing looks wrong. This expresses it as a whitelist.

Two findings from the sim, both of which would have been outages in production:

`set` on a rule number is ADDITIVE. The sim already had a rule 10 carrying
inbound/outbound interface constraints; `set ... rule 10 state established`
ANDed onto it, producing a stateful-accept that applied to one interface pair
only. Return traffic from the internet then matched no rule and hit the default
drop, so LAN hosts could reach nothing outbound. The generator now deletes each
filter before rebuilding it, so the code owns the subtree. It is one commit, so
nftables is rebuilt atomically -- there is no window without a firewall.

DHCP lease renewal is unicast UDP to port 68 and conntrack does not reliably
cover it. Without an explicit rule the WAN keeps working until the lease
expires and then dies -- a delayed failure that looks nothing like a firewall
change. Also added a loopback accept for both families, absent from the v6
policy since it went default-deny.

Verified in labsim: inter-VLAN ok, LAN-to-internet ok, internet-to-router
dropped, and internet-to-LAN dropped with the drop counter incrementing by
exactly the packets sent, after routing the test through the router rather than
around it via the hypervisor.

Also extends the drift check to the firewall subtree, which it did not cover --
so it had been reporting "in sync" while that subtree was uncaptured.

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

labsim/README.md

@@ -143,6 +143,49 @@ unreserved MAC gets an unreserved address.
 - **http://localhost:9101/metrics** — `labsim_reachable{src,dst,proto}` and
   `labsim_rtt_ms{src,dst}`.
 
+## Routing: BGP, dual WAN, and the ISP VMs
+
+`sim-ha-config.py` covers the LAN side of the routers. `sim-net-config.py`
+covers everything that makes this a rehearsal for production *routing*:
+
+| role | VM | what it generates |
+|---|---|---|
+| `primary` | `labsim-vyos` | BGP + dual WAN + health-checked failover |
+| `secondary` | `labsim-vyos2` | BGP only |
+| `isp-dhcp` | `labsim-isp-dhcp` | 10gig-equivalent ISP on VLAN 53 |
+| `isp-pppoe` | `labsim-isp-pppoe` | Vodafone-equivalent PPPoE ISP on VLAN 51 |
+
+Both ISP VMs are VyOS with two NICs: one on the OVS trunk facing the sim
+router, one on libvirt's `default` network, NATing customers to the real
+internet. They use RFC 5737 documentation ranges (`203.0.113.0/24`,
+`198.51.100.0/24`) so a leaked sim route cannot blackhole anything real.
+
+```sh
+./sim-net-apply.sh check     # VM state vs what the code says — run this first
+./sim-net-apply.sh apply     # push generated config over the serial console
+```
+
+`check` is the important one. All of this previously existed only as running
+state, applied by hand over SSH; rebuilding a VM lost it, and nothing recorded
+why any of it was shaped the way it was.
+
+### Known gaps vs production
+
+- **WAN is on the primary router only.** Production has WAN on both. Two PPPoE
+  clients sharing one credential against a single access concentrator is a
+  failure mode production does not have, so the sim does not model it. VRRP and
+  conntrack failover are still exercised.
+- **ISP VM interface names are not stable across a rebuild** — `isp-dhcp` came
+  up as `eth0`/`eth1` and `isp-pppoe` as `eth2`/`eth3` from identical XML.
+  Check `show interfaces` and pass `--wan-if` / `--uplink-if` rather than
+  trusting the defaults.
+- **`eth2` on the primary router** is a libvirt-NAT uplink predating the ISP
+  VMs: a third default route with no production equivalent that masks real WAN
+  failures during a failover test. `--drop-scaffold` removes it.
+- **Committing on `isp-pppoe` drops the router's PPPoE session**, and the
+  client does not redial promptly. After any change there, check `pppoe0` on
+  the router and `sudo systemctl restart ppp@pppoe0` if it is missing.
+
 ## Notes for whoever extends this
 
 Things that cost time the first time round, all verified on this image:

labsim/sim-net-apply.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Apply -- or drift-check -- the labsim routing config on all four VMs.
+#
+#   ./sim-net-apply.sh check    what the VMs run vs what sim-net-config.py says
+#   ./sim-net-apply.sh apply    push the generated config over the serial console
+#
+# `check` is the one you want most of the time. The whole failure mode this
+# guards against is somebody (including me) fixing something on a VM over SSH
+# and never writing it down, so the next rebuild silently loses it.
+#
+# Applied over the serial console rather than SSH because a freshly installed
+# sim router holds the same addresses as its peer -- there is a window where it
+# is not safely reachable over the network at all. See console-apply.py.
+set -uo pipefail
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ACTION="${1:-check}"
+WORK="$(mktemp -d)"; trap 'rm -rf "$WORK"' EXIT
+
+# role : vm : address : regex selecting the subtrees this generator owns
+TARGETS=(
+  "primary:labsim-vyos:172.31.1.252:^set (protocols (bgp|failover|static)|policy (prefix-list|route-map)|nat source rule 1[12]0|interfaces (pppoe|bonding bond0 vif 5[13])|firewall (group interface-group LAN|ipv4|ipv6))"
+  "secondary:labsim-vyos2:172.31.1.253:^set (protocols bgp|policy (prefix-list|route-map)|firewall (group interface-group LAN|ipv4|ipv6))"
+  "isp-dhcp:labsim-isp-dhcp:192.168.122.136:^set (interfaces ethernet|nat source|service dhcp-server|firewall ipv4 forward|system host-name)"
+  "isp-pppoe:labsim-isp-pppoe:192.168.122.63:^set (interfaces ethernet|nat source|service pppoe-server|firewall ipv4 forward|system host-name)"
+)
+# Sim-only credential; these VMs hold nothing real and are not reachable from
+# outside the hypervisor.
+SSH_OPTS=(-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null
+          -o LogLevel=ERROR -o PreferredAuthentications=password -o ConnectTimeout=5)
+live() { timeout 30 sshpass -p vyos ssh "${SSH_OPTS[@]}" "vyos@$1" \
+           "/opt/vyatta/bin/vyatta-op-cmd-wrapper show configuration commands" 2>/dev/null; }
+norm() { sed "s/'//g" | grep -v 'hw-id\|offload' | sort -u; }
+
+rc=0
+for t in "${TARGETS[@]}"; do
+  IFS=: read -r role vm addr rx <<<"$t"
+  "$HERE/sim-net-config.py" --role "$role" >"$WORK/$role.conf" 2>/dev/null || {
+    printf '  %-11s GENERATE FAILED\n' "$role"; rc=1; continue; }
+
+  if [ "$ACTION" = apply ]; then
+    printf '  %-11s applying to %s over console...\n' "$role" "$vm"
+    "$HERE/console-apply.py" --vm "$vm" --config "$WORK/$role.conf" || rc=1
+    continue
+  fi
+
+  if ! live "$addr" >"$WORK/$role.live" || [ ! -s "$WORK/$role.live" ]; then
+    printf '  %-11s UNREACHABLE (%s)\n' "$role" "$addr"; rc=1; continue
+  fi
+  grep -E '^set ' "$WORK/$role.conf" | norm >"$WORK/$role.g"
+  grep -E "$rx" "$WORK/$role.live" | norm >"$WORK/$role.l"
+  if d="$(diff "$WORK/$role.g" "$WORK/$role.l")" && [ -z "$d" ]; then
+    printf '  %-11s in sync (%s commands)\n' "$role" "$(wc -l <"$WORK/$role.g")"
+  else
+    printf '  %-11s DRIFT — "<" only in code, ">" only on the VM:\n' "$role"
+    printf '%s\n' "$d" | sed 's/^/      /'
+    rc=1
+  fi
+done
+exit $rc

labsim/sim-net-config.py

@@ -0,0 +1,409 @@
+#!/usr/bin/env python3
+"""Generate the labsim routing + WAN config: BGP, dual WAN, and the two ISP VMs.
+
+`sim-ha-config.py` covers the LAN side of the sim routers (addresses, VRRP,
+conntrack-sync, DHCP). This covers everything that makes the sim a rehearsal for
+production routing rather than just a LAN:
+
+  * eBGP between the sim routers and the k3s nodes, carrying the service range
+  * dual WAN -- DHCP on VLAN 53, PPPoE on VLAN 51 -- with health-checked failover
+  * the two ISP VMs that terminate those WANs and NAT to the real internet
+
+All of it previously existed only as running state on the VMs, applied by hand
+over SSH. Rebuilding a VM lost the rehearsal, and nothing recorded *why* any of
+it was shaped the way it is. That is the entire reason this file exists.
+
+  ./sim-net-config.py --role primary   > r1-net.conf
+  ./console-apply.py --vm labsim-vyos  --config r1-net.conf
+
+or apply all four at once with ./sim-net-apply.sh.
+"""
+from __future__ import annotations
+
+import argparse
+import sys
+
+# ---------------------------------------------------------------------------
+# BGP. Numbers match production so what is proven here ports over unchanged.
+# ---------------------------------------------------------------------------
+ROUTER_AS = 65000
+CLUSTER_AS = 65001
+# The service range Cilium advertises. Chosen against a survey of third-party
+# RFC1918 defaults (docker-desktop, tailscale, k3s, EKS...) so it cannot collide
+# with something we adopt later. Production uses the same /22 -- keep them equal.
+SERVICE_CIDR = "10.61.0.0/22"
+K8S_VLAN = 2
+K8S_NODES = ["172.31.2.11", "172.31.2.12", "172.31.2.13"]
+PEER_GROUP = "K8S"
+PFX_LIST = "K8S-SERVICE-IPS"
+RM_IN, RM_OUT = "K8S-IN", "K8S-OUT"
+# One route per node; ECMP across all three. 4 leaves headroom for a fourth node
+# without a config change.
+MAX_PATHS = 4
+# A safety valve, not a capacity plan: a misconfigured Cilium that starts
+# advertising pod CIDRs should tear the session down, not quietly fill the FIB.
+MAX_PREFIX = 100
+
+# ---------------------------------------------------------------------------
+# Dual WAN. The sim ISPs deliberately use TEST-NET-3 (203.0.113.0/24) and
+# TEST-NET-2 (198.51.100.0/24) from RFC 5737: documentation ranges that are
+# guaranteed never to be real destinations, so a leaked sim route cannot
+# blackhole something that matters.
+# ---------------------------------------------------------------------------
+WAN_DHCP_VLAN = 53          # "10gig-equivalent" -- the primary in production
+WAN_PPPOE_VLAN = 51         # "Vodafone-equivalent" -- the backup
+ISP_DHCP_NET = "203.0.113.0/24"
+ISP_DHCP_GW = "203.0.113.1"
+ISP_DHCP_POOL = ("203.0.113.100", "203.0.113.150")
+ISP_PPPOE_NET = "198.51.100.0/24"
+ISP_PPPOE_GW = "198.51.100.1"
+ISP_PPPOE_POOL = ("198.51.100.100", "198.51.100.150")
+# Sim-only fake credentials. Both ends are in this file on purpose: they
+# authenticate nothing real, and splitting them across a secret store would make
+# the sim unreproducible for no security gain. The PRODUCTION PPPoE password
+# lives in /config/wan-secrets on the router and is never in git.
+PPPOE_USER, PPPOE_PASS = "simdsl", "simpass"
+PPPOE_MTU = 1492            # 1500 - 8 bytes of PPPoE header
+PPPOE_AC = "sim-isp"
+
+# Failover probe targets. NOT 8.8.8.8/8.8.4.4: those are `system name-server`,
+# so a probe failure and a DNS failure would be the same event and the router
+# would flap the WAN every time DNS hiccuped.
+PROBE_TARGETS = ["9.9.9.9", "208.67.222.222"]
+# The bug this shape fixes (WI-8, found here, fixed in production): `ping -I
+# bond0.53` binds the SOURCE address but does not make the kernel use that
+# interface's gateway. On a cold boot where PPPoE won the default route, probes
+# for the 10 gig egressed via PPPoE, succeeded, and the 10 gig was still never
+# selected -- the house ran on the backup line silently. Pinning each target as
+# a /32 via `dhcp-interface` forces the probe onto the line being tested.
+WAN_DHCP_DISTANCE = 210     # NOT `no-default-route`, which blanks new_routers
+PPPOE_DISTANCE = 10         #   in the lease file, leaving failover no gateway
+                            #   to install and silently handing the default
+                            #   route to the backup line.
+SIM_LAN = "172.31.0.0/16"
+
+# The sim routers' own libvirt-NAT uplink, from before the ISP VMs existed. It
+# is a third default route that does not exist in production and quietly masks
+# WAN failures during a failover test. `--drop-scaffold` removes it.
+SCAFFOLD_IF = "eth2"
+SCAFFOLD_NAT_RULE = 100
+
+
+def bgp(role: str) -> list[str]:
+    """eBGP toward the k3s nodes. Identical on both routers except router-id."""
+    octet = 252 if role == "primary" else 253
+    out = [
+        f"# --- BGP: AS{ROUTER_AS} <-> AS{CLUSTER_AS} (k3s/Cilium) ---",
+        # FRR enforces RFC 8212: an eBGP session with no policy establishes but
+        # exchanges ZERO prefixes, silently. Both directions need a policy or
+        # the session looks perfectly healthy and carries nothing.
+        f"set policy prefix-list {PFX_LIST} rule 10 action permit",
+        f"set policy prefix-list {PFX_LIST} rule 10 prefix {SERVICE_CIDR}",
+        # `le 32` because Cilium advertises individual /32 service addresses out
+        # of the pool, not the aggregate.
+        f"set policy prefix-list {PFX_LIST} rule 10 le 32",
+        f"set policy route-map {RM_IN} rule 10 action permit",
+        f"set policy route-map {RM_IN} rule 10 match ip address prefix-list {PFX_LIST}",
+        # Deny everything outbound. The cluster must never learn a default route
+        # from us -- Cilium would install it and blackhole pod egress.
+        f"set policy route-map {RM_OUT} rule 10 action deny",
+        f"set protocols bgp system-as {ROUTER_AS}",
+        f"set protocols bgp parameters router-id 172.31.{K8S_VLAN}.{octet}",
+        f"set protocols bgp address-family ipv4-unicast maximum-paths ebgp {MAX_PATHS}",
+        f"set protocols bgp peer-group {PEER_GROUP} remote-as {CLUSTER_AS}",
+        f"set protocols bgp peer-group {PEER_GROUP} address-family ipv4-unicast route-map import {RM_IN}",
+        f"set protocols bgp peer-group {PEER_GROUP} address-family ipv4-unicast route-map export {RM_OUT}",
+        f"set protocols bgp peer-group {PEER_GROUP} address-family ipv4-unicast maximum-prefix {MAX_PREFIX}",
+    ]
+    out += [f"set protocols bgp neighbor {n} peer-group {PEER_GROUP}" for n in K8S_NODES]
+    out.append("")
+    return out
+
+
+def wan(drop_scaffold: bool) -> list[str]:
+    """Dual WAN + health-checked failover. Primary router only -- see README."""
+    out = [
+        "# --- WAN: DHCP (primary) + PPPoE (backup), health-checked ---",
+        f"set interfaces bonding bond0 vif {WAN_PPPOE_VLAN} description "
+        f"'WAN1 Vodafone-equivalent (sim ISP PPPoE)'",
+        f"set interfaces bonding bond0 vif {WAN_DHCP_VLAN} address dhcp",
+        f"set interfaces bonding bond0 vif {WAN_DHCP_VLAN} description "
+        f"'WAN3 10gig-equivalent (sim ISP DHCP)'",
+        f"set interfaces bonding bond0 vif {WAN_DHCP_VLAN} dhcp-options "
+        f"default-route-distance {WAN_DHCP_DISTANCE}",
+        f"set interfaces pppoe pppoe0 source-interface bond0.{WAN_PPPOE_VLAN}",
+        f"set interfaces pppoe pppoe0 authentication username {PPPOE_USER}",
+        f"set interfaces pppoe pppoe0 authentication password {PPPOE_PASS}",
+        f"set interfaces pppoe pppoe0 default-route-distance {PPPOE_DISTANCE}",
+        f"set interfaces pppoe pppoe0 mtu {PPPOE_MTU}",
+        # The ISP's resolvers would otherwise overwrite ours in resolv.conf every
+        # time the session comes up.
+        "set interfaces pppoe pppoe0 no-peer-dns",
+        "",
+        "# Failover: prefer the DHCP WAN, fall back to PPPoE when probes fail.",
+        f"set protocols failover route 0.0.0.0/0 dhcp-interface bond0.{WAN_DHCP_VLAN} metric 1",
+        f"set protocols failover route 0.0.0.0/0 dhcp-interface bond0.{WAN_DHCP_VLAN} check type icmp",
+        f"set protocols failover route 0.0.0.0/0 dhcp-interface bond0.{WAN_DHCP_VLAN} check timeout 5",
+        # any-available, not all: one unreachable public resolver is a normal
+        # internet event, not a reason to abandon a working 10 gig line.
+        f"set protocols failover route 0.0.0.0/0 dhcp-interface bond0.{WAN_DHCP_VLAN} check policy any-available",
+    ]
+    for t in PROBE_TARGETS:
+        out.append(f"set protocols failover route 0.0.0.0/0 dhcp-interface "
+                   f"bond0.{WAN_DHCP_VLAN} check target {t}")
+    out.append("")
+    out.append("# Pin the probe targets to the line under test (WI-8 -- see above).")
+    for t in PROBE_TARGETS:
+        out.append(f"set protocols static route {t}/32 dhcp-interface bond0.{WAN_DHCP_VLAN}")
+    out += [
+        "",
+        "# Masquerade out of whichever WAN currently holds the default route.",
+        f"set nat source rule 110 outbound-interface name bond0.{WAN_DHCP_VLAN}",
+        f"set nat source rule 110 source address {SIM_LAN}",
+        "set nat source rule 110 translation address masquerade",
+        "set nat source rule 120 outbound-interface name pppoe0",
+        f"set nat source rule 120 source address {SIM_LAN}",
+        "set nat source rule 120 translation address masquerade",
+        "",
+    ]
+    if drop_scaffold:
+        out += [
+            "# Remove the pre-ISP-VM libvirt-NAT uplink: a third default route",
+            "# that has no production equivalent and hides real WAN failures.",
+            f"delete interfaces ethernet {SCAFFOLD_IF} address",
+            f"delete nat source rule {SCAFFOLD_NAT_RULE}",
+            "",
+        ]
+    return out
+
+
+# ---------------------------------------------------------------------------
+# Firewall. The policy is: internal VLANs talk to each other and to the
+# internet; the internet initiates nothing inward.
+#
+# That was already the *effect* of the previous IPv4 ruleset, but it was built
+# as a blacklist -- `default-action accept` plus explicit drops on each WAN
+# interface. The result is identical right up until someone adds a WAN, at
+# which point it is wide open and nothing looks wrong. This is the same policy
+# expressed as a whitelist, so a new interface is closed until it is named.
+# ---------------------------------------------------------------------------
+LAN_IFACES = ["bond0", "bond0.2", "bond0.3", "bond0.9", "bond0.10", "bond0.200"]
+LAN_GROUP = "LAN"
+
+
+def firewall(wan_dhcp_if: str | None = f"bond0.{WAN_DHCP_VLAN}") -> list[str]:
+    """wan_dhcp_if=None on a router with no DHCP WAN -- a firewall rule naming
+    an interface that does not exist is rejected at commit."""
+    out = [f"# --- firewall: LAN-to-anywhere, internet-to-nothing ---"]
+    # Delete each filter before rebuilding it. `set` on a rule number is
+    # ADDITIVE: if a rule 10 already exists carrying an inbound-interface
+    # constraint, `set ... rule 10 state established` silently ANDs onto it,
+    # and you get a stateful-accept rule that only applies to one interface
+    # pair. Observed in labsim: return traffic from the internet matched
+    # neither that rule nor the LAN rule and hit the default drop, so LAN
+    # hosts could reach nothing outbound. Everything here is one commit, so
+    # nftables is rebuilt atomically -- there is no window with no firewall.
+    out += [f"delete firewall {fam} {hook} filter"
+            for fam in ("ipv4", "ipv6") for hook in ("forward", "input")]
+    out += [f"set firewall group interface-group {LAN_GROUP} interface {i}"
+            for i in LAN_IFACES]
+    out += [
+        "",
+        # INPUT -- traffic terminating ON the router.
+        # Loopback first. Under a default-drop input policy, services talking to
+        # 127.0.0.1 are filtered like anything else, and the failures are
+        # bizarre and hard to attribute. Nothing off-box can forge iif lo.
+        "set firewall ipv4 input filter rule 5 action accept",
+        "set firewall ipv4 input filter rule 5 description 'loopback'",
+        "set firewall ipv4 input filter rule 5 inbound-interface name lo",
+        "set firewall ipv4 input filter rule 10 action accept",
+        "set firewall ipv4 input filter rule 10 description 'established/related'",
+        "set firewall ipv4 input filter rule 10 state established",
+        "set firewall ipv4 input filter rule 10 state related",
+        # One rule covers VRRP, conntrack-sync, kea HA, SSH, DNS and BGP,
+        # because every one of them arrives on a LAN interface. Enumerating the
+        # protocols instead would mean a new firewall rule every time the pair
+        # gains a feature -- and a lockout the day someone forgets.
+        f"set firewall ipv4 input filter rule 20 action accept",
+        f"set firewall ipv4 input filter rule 20 description 'trusted LAN to the router'",
+        f"set firewall ipv4 input filter rule 20 inbound-interface group {LAN_GROUP}",
+        # DHCP client. Lease RENEWAL is unicast UDP to port 68 and conntrack
+        # does not reliably cover it, so without this the WAN keeps working
+        # until the lease expires and then dies -- a delayed failure that looks
+        # nothing like a firewall change.
+        "set firewall ipv4 input filter default-action drop",
+        "",
+        # FORWARD -- traffic passing THROUGH the router.
+        "set firewall ipv4 forward filter rule 10 action accept",
+        "set firewall ipv4 forward filter rule 10 description 'established/related'",
+        "set firewall ipv4 forward filter rule 10 state established",
+        "set firewall ipv4 forward filter rule 10 state related",
+        # Inter-VLAN *and* LAN-to-internet in one rule: both are "came in on a
+        # LAN interface". Deliberately no restriction between internal VLANs --
+        # segmenting them is a separate decision, not a side effect of this one.
+        f"set firewall ipv4 forward filter rule 20 action accept",
+        f"set firewall ipv4 forward filter rule 20 description 'LAN to anywhere (inter-VLAN + internet)'",
+        f"set firewall ipv4 forward filter rule 20 inbound-interface group {LAN_GROUP}",
+        "set firewall ipv4 forward filter default-action drop",
+        "",
+        # IPv6 already runs default-deny. It only lacks the loopback rule.
+        "set firewall ipv6 input filter rule 5 action accept",
+        "set firewall ipv6 input filter rule 5 description 'loopback'",
+        "set firewall ipv6 input filter rule 5 inbound-interface name lo",
+        "set firewall ipv6 input filter rule 10 action accept",
+        "set firewall ipv6 input filter rule 10 description 'replies to our own traffic'",
+        "set firewall ipv6 input filter rule 10 state established",
+        "set firewall ipv6 input filter rule 10 state related",
+        # RFC 4890: filtering ICMPv6 wholesale breaks ND and PMTUD, which
+        # presents as "IPv6 works until something large", not as a block.
+        "set firewall ipv6 input filter rule 20 action accept",
+        "set firewall ipv6 input filter rule 20 description 'ICMPv6 - ND/RA/PMTUD'",
+        "set firewall ipv6 input filter rule 20 protocol icmpv6",
+        f"set firewall ipv6 input filter rule 30 action accept",
+        f"set firewall ipv6 input filter rule 30 description 'trusted LAN to the router'",
+        f"set firewall ipv6 input filter rule 30 inbound-interface group {LAN_GROUP}",
+        "set firewall ipv6 input filter default-action drop",
+        "set firewall ipv6 forward filter rule 10 action accept",
+        "set firewall ipv6 forward filter rule 10 description 'replies to our own traffic'",
+        "set firewall ipv6 forward filter rule 10 state established",
+        "set firewall ipv6 forward filter rule 10 state related",
+        "set firewall ipv6 forward filter rule 20 action accept",
+        "set firewall ipv6 forward filter rule 20 description 'ICMPv6 - ND/RA/PMTUD'",
+        "set firewall ipv6 forward filter rule 20 protocol icmpv6",
+        f"set firewall ipv6 forward filter rule 30 action accept",
+        f"set firewall ipv6 forward filter rule 30 description 'trusted LAN interfaces only'",
+        f"set firewall ipv6 forward filter rule 30 inbound-interface group {LAN_GROUP}",
+        "set firewall ipv6 forward filter default-action drop",
+        "",
+    ]
+    if wan_dhcp_if:
+        dhcp = [
+            "set firewall ipv4 input filter rule 140 action accept",
+            "set firewall ipv4 input filter rule 140 description 'DHCP client lease renewal'",
+            "set firewall ipv4 input filter rule 140 protocol udp",
+            "set firewall ipv4 input filter rule 140 destination port 68",
+            f"set firewall ipv4 input filter rule 140 inbound-interface name {wan_dhcp_if}",
+        ]
+        i = out.index("set firewall ipv4 input filter default-action drop")
+        out[i:i] = dhcp
+    return out
+
+
+def isp_dhcp(wan_if: str, uplink_if: str) -> list[str]:
+    """The 10gig-equivalent ISP: hands out a lease, NATs to the real internet."""
+    return [
+        f"# --- sim ISP: DHCP WAN on VLAN {WAN_DHCP_VLAN} ---",
+        "set system host-name isp-dhcp",
+        f"set interfaces ethernet {wan_if} address {ISP_DHCP_GW}/24",
+        f"set interfaces ethernet {wan_if} description "
+        f"'sim ISP - 10gig-equivalent WAN on VLAN{WAN_DHCP_VLAN}'",
+        f"set interfaces ethernet {uplink_if} address dhcp",
+        f"set interfaces ethernet {uplink_if} description 'uplink to the real internet'",
+        f"set service dhcp-server shared-network-name WAN{WAN_DHCP_VLAN} "
+        f"subnet {ISP_DHCP_NET} subnet-id 1",
+        f"set service dhcp-server shared-network-name WAN{WAN_DHCP_VLAN} "
+        f"subnet {ISP_DHCP_NET} option default-router {ISP_DHCP_GW}",
+        f"set service dhcp-server shared-network-name WAN{WAN_DHCP_VLAN} "
+        f"subnet {ISP_DHCP_NET} option name-server 8.8.8.8",
+        f"set service dhcp-server shared-network-name WAN{WAN_DHCP_VLAN} "
+        f"subnet {ISP_DHCP_NET} range CUST start {ISP_DHCP_POOL[0]}",
+        f"set service dhcp-server shared-network-name WAN{WAN_DHCP_VLAN} "
+        f"subnet {ISP_DHCP_NET} range CUST stop {ISP_DHCP_POOL[1]}",
+        "",
+    ] + _isp_common(uplink_if, ISP_DHCP_NET, "sim ISP: NAT customers to the real internet")
+
+
+def isp_pppoe(wan_if: str, uplink_if: str) -> list[str]:
+    """The Vodafone-equivalent ISP: terminates PPPoE, NATs to the real internet."""
+    return [
+        f"# --- sim ISP: PPPoE WAN on VLAN {WAN_PPPOE_VLAN} ---",
+        "set system host-name isp-pppoe",
+        f"set interfaces ethernet {wan_if} description "
+        f"'sim ISP - Vodafone-equivalent WAN on VLAN{WAN_PPPOE_VLAN} (PPPoE)'",
+        f"set interfaces ethernet {uplink_if} address dhcp",
+        f"set interfaces ethernet {uplink_if} description 'uplink to the real internet'",
+        f"set service pppoe-server access-concentrator {PPPOE_AC}",
+        f"set service pppoe-server interface {wan_if}",
+        f"set service pppoe-server gateway-address {ISP_PPPOE_GW}",
+        "set service pppoe-server authentication mode local",
+        f"set service pppoe-server authentication local-users username {PPPOE_USER} "
+        f"password {PPPOE_PASS}",
+        f"set service pppoe-server client-ip-pool CUST range "
+        f"{ISP_PPPOE_POOL[0]}-{ISP_PPPOE_POOL[1]}",
+        "set service pppoe-server default-pool CUST",
+        "set service pppoe-server name-server 8.8.8.8",
+        "",
+    ] + _isp_common(uplink_if, ISP_PPPOE_NET, "sim ISP: NAT PPPoE customers to the real internet")
+
+
+def _isp_common(uplink_if: str, customer_net: str, desc: str) -> list[str]:
+    return [
+        "set nat source rule 100 description " + f"'{desc}'",
+        f"set nat source rule 100 outbound-interface name {uplink_if}",
+        f"set nat source rule 100 source address {customer_net}",
+        "set nat source rule 100 translation address masquerade",
+        "",
+        # An ISP that drops return traffic is not simulating an ISP. The forward
+        # chain defaults to accept here on purpose -- these VMs model the
+        # internet, and the thing under test is the router's firewall, not this.
+        "set firewall ipv4 forward filter default-action accept",
+        "set firewall ipv4 forward filter rule 10 action accept",
+        "set firewall ipv4 forward filter rule 10 state established",
+        "set firewall ipv4 forward filter rule 10 state related",
+        "set firewall ipv4 forward filter rule 10 description conntrack-engage",
+        "",
+    ]
+
+
+def build(role: str, drop_scaffold: bool, wan_if: str, uplink_if: str) -> list[str]:
+    if role == "primary":
+        # WAN lives on the primary only. Production has WAN on both routers;
+        # the sim does not, because two PPPoE clients sharing one credential
+        # against a single access concentrator is a different failure mode than
+        # anything production has. VRRP/conntrack failover is still exercised --
+        # see README, "known gaps".
+        return ([f"# labsim routing -- {role}", ""]
+                + bgp(role) + wan(drop_scaffold) + firewall())
+    if role == "secondary":
+        # The backup has no WAN in the sim, so it has no DHCP client to
+        # exempt -- but it gets the same policy otherwise, because after a VRRP
+        # failover it IS the router and a divergent ruleset would only be
+        # discovered during the failover.
+        return ([f"# labsim routing -- {role}", ""]
+                + bgp(role) + firewall(wan_dhcp_if=None))
+    if role == "isp-dhcp":
+        return isp_dhcp(wan_if, uplink_if)
+    return isp_pppoe(wan_if, uplink_if)
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--role", required=True,
+                    choices=("primary", "secondary", "isp-dhcp", "isp-pppoe"))
+    ap.add_argument("--drop-scaffold", action="store_true",
+                    help="also remove the pre-ISP-VM libvirt-NAT uplink (primary only)")
+    # The ISP VMs' interface names depend on PCI enumeration order, which is not
+    # stable across a rebuild: isp-dhcp came up as eth0/eth1 and isp-pppoe as
+    # eth2/eth3 from identical XML. Check with `show interfaces` before applying
+    # rather than trusting these defaults.
+    ap.add_argument("--wan-if", default=None, help="ISP VM: customer-facing NIC")
+    ap.add_argument("--uplink-if", default=None, help="ISP VM: internet-facing NIC")
+    args = ap.parse_args()
+
+    defaults = {"isp-dhcp": ("eth0", "eth1"), "isp-pppoe": ("eth2", "eth3")}
+    w, u = defaults.get(args.role, ("", ""))
+    w, u = args.wan_if or w, args.uplink_if or u
+
+    if args.drop_scaffold and args.role != "primary":
+        print("--drop-scaffold only applies to --role primary", file=sys.stderr)
+        return 2
+
+    lines = build(args.role, args.drop_scaffold, w, u)
+    sys.stdout.write("\n".join(lines) + "\n")
+    n = len([l for l in lines if l.startswith(("set ", "delete "))])
+    print(f"{args.role}: {n} commands", file=sys.stderr)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())