vyos: move PPPoE off the config plane onto a gated systemd unit

PPPoE HA could not work as written, and the reason is structural rather than a
bug: `set interfaces pppoe pppoe0 disable` and `delete` are handled identically
by interfaces_pppoe.py -- both UNLINK /etc/ppp/peers/pppoe0. That path is pppd's
own options file, so the resting state destroyed exactly what the promotion path
needed, and `ppp@pppoe0` restart-looped against it (observed: 47 restarts, zero
sessions at the access concentrator). It also made op-mode `connect interface
pppoe0` unusable, and put every failover behind a priority-322 commit where one
unrelated invalid node fails the whole thing -- which has already taken the
10 gig down once.

pppoe0 is now configured identically and ENABLED on both routers, so the peers
file always exists, and dialling is gated by a drop-in on the unit:

    ConditionPathExists=/run/vrrp-wan/may-dial
    ConditionPathExists=/etc/ppp/peers/pppoe0

/run is tmpfs, so the gate is shut at boot and neither box can dial before VRRP
has decided. That matters more than it first appears: with the node enabled,
interfaces_pppoe.py restarts ppp on EVERY commit touching the pppoe subtree when
the daemon is not running -- so the backup actively tries to dial whenever
anything commits. The gate is the only thing making that a no-op, which is why
vrrp-wan-reconcile now refuses to bless a box whose drop-in is missing: /etc is
per-image, and a VyOS upgrade would otherwise silently remove the protection.

may-dial is a LEASE, not a flag. ConditionPathExists is evaluated at start only
-- it can prevent a dial, never revoke one -- so a reconciler that stops running
while its box is demoted would keep the one ISP session for ever. The reconciler
renews the lease; a new 5s vrrp-wan-guard revokes it, and only ever revokes. It
fired correctly first time: "GUARD: lease stale (81s > 75s)".

Also: remove-then-stop on release (the file's absence blocks a NEW start that a
concurrent commit would trigger); a flap damper, because two routers that both
believe they hold the VIP will both dial and each dial kills the other's session
-- against a real ISP that is how an account gets rate-limited; and a guard on
`cfg` returning empty under commit-lock contention, which had already produced
one spurious "releasing" on a box that needed nothing.

GRACE 90 -> 180. accel-ppp's dead-peer budget is lcp-echo-interval(30) x
failure(3) = 90s, so the old value sat exactly on the boundary: a hard failover
into an AC that does not replace the stale session would fail its own check,
shed the VIPs, and leave both routers in FAULT.

The sim could not have tested any of this. Both routers now get the identical
WAN -- the secondary had none "because two PPPoE clients sharing one credential
is a different failure mode than anything production has", which is backwards:
that IS production. It also left the pair incomparable, ten NAT rules against
none. Safety now comes from resting state, not asymmetry.

Three more things the sim was hiding:
  - the drift check's secondary regex omitted interfaces pppoe/bonding, nat
    source and protocols failover, so it reported "in sync" for a box with no
    WAN at all;
  - the VRRP health-check and transition-script hooks existed on both live VMs
    and in NEITHER generator -- the mechanism under test was pure undetected
    drift;
  - labsim-vyos's only default route was the libvirt-NAT scaffold, so every
    "the LAN still has internet" verdict on it was answered by eth2 rather than
    the WAN. --drop-scaffold applied; the earlier DHCP-failover proof is being
    re-run because of it.

vrrp-wan-install ends the other half of that: the sim's previous proof came from
scripts hand-`sed`-ed in place, so the tested behaviour was not the committed
behaviour. `--check` now makes that a hard failure.

First green run: master holds both WANs, backup released, and the AC reports
exactly ONE session. sim-net-apply.sh check: all four in sync.

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

migration/vrrp-wan-install

@@ -0,0 +1,103 @@
+#!/bin/bash
+# Install (or verify) the WAN-follows-VRRP mechanism on a VyOS router.
+#
+# This exists because on 2026-09-05 the sim's failover proof was obtained from
+# scripts that had been hand-`sed`-ed in place: /config/vrrp-wan-health and
+# -reconcile differed from git by an edited VIP, so the tested behaviour was not
+# the committed behaviour and any reinstall would have silently reverted it.
+# `--check` makes that class of drift a hard failure instead of a discovery.
+#
+# It installs ONLY the mechanism -- scripts, units, drop-in, settings. It never
+# touches VyOS configuration: the `interfaces pppoe` node, `vif 53 disable` and
+# the VRRP sync-group hooks are config and belong in the config model
+# (labsim/sim-*.py for the sim, infra/vyos/subtrees/overrides.json for
+# production), not in an installer.
+#
+#   vrrp-wan-install --vip 192.168.1.1 [--host vyos@10.0.1.253]
+#   vrrp-wan-install --check [--host ...]     # exits non-zero on any drift
+#
+# With no --host it operates on the local machine, so it can be scp'd to a
+# router and run there.
+set -uo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+VIP=""; HOST=""; MODE=install; PW="${VYOS_PW:-vyos}"
+
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --vip)   VIP="$2"; shift 2 ;;
+    --host)  HOST="$2"; shift 2 ;;
+    --check) MODE=check; shift ;;
+    *) echo "usage: $0 [--vip A.B.C.D] [--host user@ip] [--check]" >&2; exit 2 ;;
+  esac
+done
+
+SSH_OPTS=(-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null
+          -o LogLevel=ERROR -o ConnectTimeout=8 -o PreferredAuthentications=password)
+run() {  # run a command on the target
+  if [ -n "$HOST" ]; then timeout 60 sshpass -p "$PW" ssh "${SSH_OPTS[@]}" "$HOST" "$@"
+  else bash -c "$*"; fi
+}
+put() {  # copy a file to the target
+  if [ -n "$HOST" ]; then timeout 60 sshpass -p "$PW" scp "${SSH_OPTS[@]}" "$1" "$HOST:$2" >/dev/null
+  else cp "$1" "$2"; fi
+}
+
+# script -> destination. take/release are hooks keepalived calls; both exec the
+# reconciler, so there is one code path.
+SCRIPTS="vrrp-wan-reconcile vrrp-wan-apply vrrp-wan-health vrrp-wan-guard vrrp-wan-take vrrp-wan-release"
+UNITS="vrrp-wan-reconcile.service vrrp-wan-reconcile.timer vrrp-wan-guard.service vrrp-wan-guard.timer"
+GATE_DIR=/etc/systemd/system/ppp@pppoe0.service.d
+GATE=$GATE_DIR/10-vrrp-wan-gate.conf
+
+if [ "$MODE" = check ]; then
+  rc=0
+  for f in $SCRIPTS; do
+    local_sum=$(md5sum "$HERE/$f" | cut -d' ' -f1)
+    remote_sum=$(run "md5sum /config/$f 2>/dev/null | cut -d' ' -f1")
+    [ "$local_sum" = "$remote_sum" ] || { echo "  DRIFT /config/$f"; rc=1; }
+  done
+  for f in $UNITS; do
+    local_sum=$(md5sum "$HERE/$f" | cut -d' ' -f1)
+    remote_sum=$(run "md5sum /etc/systemd/system/$f 2>/dev/null | cut -d' ' -f1")
+    [ "$local_sum" = "$remote_sum" ] || { echo "  DRIFT /etc/systemd/system/$f"; rc=1; }
+  done
+  gate_sum=$(md5sum "$HERE/ppp-vrrp-gate.conf" | cut -d' ' -f1)
+  remote_gate=$(run "md5sum $GATE 2>/dev/null | cut -d' ' -f1")
+  [ "$gate_sum" = "$remote_gate" ] || { echo "  DRIFT $GATE (a VyOS upgrade wipes /etc -- both routers would dial)"; rc=1; }
+  run "[ -r /config/vrrp-wan.conf ]" || { echo "  MISSING /config/vrrp-wan.conf"; rc=1; }
+  for t in vrrp-wan-reconcile.timer vrrp-wan-guard.timer; do
+    [ "$(run "systemctl is-enabled $t 2>/dev/null")" = enabled ] || { echo "  NOT ENABLED $t"; rc=1; }
+  done
+  [ "$rc" -eq 0 ] && echo "  vrrp-wan in sync"
+  exit "$rc"
+fi
+
+[ -n "$VIP" ] || { echo "--vip is required to install" >&2; exit 2; }
+
+for f in $SCRIPTS; do
+  put "$HERE/$f" "/tmp/$f"
+  # root:vyattacfg 0775 -- vrrp-wan-apply enters config mode, which requires
+  # membership of vyattacfg.
+  run "sudo install -o root -g vyattacfg -m 0775 /tmp/$f /config/$f"
+done
+
+# Settings, with the VIP substituted. One file, read by BOTH the reconciler and
+# the health check -- keepalived invokes the latter with no environment at all,
+# so an Environment= line in the unit would be read by one and not the other.
+sed "s|^VRRP_WAN_VIP=.*|VRRP_WAN_VIP=${VIP}|" "$HERE/vrrp-wan.conf" > /tmp/vrrp-wan.conf.gen
+put /tmp/vrrp-wan.conf.gen /tmp/vrrp-wan.conf.gen
+run "sudo install -o root -g vyattacfg -m 0664 /tmp/vrrp-wan.conf.gen /config/vrrp-wan.conf"
+
+for f in $UNITS; do
+  put "$HERE/$f" "/tmp/$f"
+  run "sudo install -m 0644 /tmp/$f /etc/systemd/system/$f"
+done
+
+put "$HERE/ppp-vrrp-gate.conf" /tmp/ppp-vrrp-gate.conf
+run "sudo mkdir -p $GATE_DIR && sudo install -m 0644 /tmp/ppp-vrrp-gate.conf $GATE"
+
+run "sudo systemctl daemon-reload && sudo systemctl enable --now vrrp-wan-reconcile.timer vrrp-wan-guard.timer" >/dev/null 2>&1
+
+echo "  installed (vip=$VIP)"
+run "sudo /config/vrrp-wan-reconcile --status"