lab/migration/PPPOE-HA.md

# PPPoE high availability

Proven in labsim. **Not applied to production.**

## What it does

One consumer ISP account, two routers. The 10 gig lease is bound to a cloned MAC
(`f0:9f:c2:12:9b:4f`, the retired USG's) and the Vodafone line to a single
credential, so neither may be live on both boxes. The WAN follows VRRP
mastership — but the two halves use different control planes, and that is the
whole design:

| | plane | why |
|---|---|---|
| `bond0.53` (10 gig) | VyOS **config** (`disable`) | only config can move a MAC |
| `pppoe0` (Vodafone) | **systemd** unit gate | see below |

## Why PPPoE cannot live on the config plane

`interfaces_pppoe.py` treats `disable` and `delete` identically: both **unlink
`/etc/ppp/peers/pppoe0`**, call `PPPoEIf.remove()` (withdrawing the FRR default
route) and stop the unit. That path is pppd's own options file
(`ExecStart=/usr/sbin/pppd call %I`), so the resting state destroyed exactly what
the promotion path needed. `ppp@pppoe0` then restart-looped against the missing
file — 47 restarts observed, zero sessions at the access concentrator — and
never tripped systemd's limiter, because `RestartSec=5s` against the default
10s/5-burst window is only two restarts per interval.

It also made op-mode `connect interface pppoe0` unusable (it refuses without the
peers file), and put every failover behind a priority-322 commit where one
unrelated invalid node fails the whole thing.

## The gate

`pppoe0` is configured identically and **enabled on both** routers, so the peers
file always exists. Dialling is gated by
`/etc/systemd/system/ppp@pppoe0.service.d/10-vrrp-wan-gate.conf`:

```ini
ConditionPathExists=/run/vrrp-wan/may-dial
ConditionPathExists=/etc/ppp/peers/pppoe0
StartLimitIntervalSec=600
StartLimitBurst=6
```

`/run` is tmpfs, so the gate is shut at boot and neither box can dial before VRRP
has decided. **This is load-bearing, not a nicety:** 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 (`pulumi up`, a hand commit, the boot-time config load). The
gate is the only thing making that a no-op, which is why `vrrp-wan-reconcile`
**refuses to bless a box whose drop-in is missing**: `/etc` is per-image, so a
VyOS upgrade silently removes the protection, and failing closed turns that into
"PPPoE never dials" rather than "both routers dial".

`may-dial` is a **lease, not a flag**. `ConditionPathExists` is evaluated at
start only — it can prevent a dial, never revoke one. `vrrp-wan-reconcile`
renews it every 30s; `vrrp-wan-guard` runs every 5s and only ever revokes, on
either "I do not hold the VIP" or "the lease is stale".

## Measured in labsim

| | |
|---|---|
| Clean failover (`force-fault`) | `pppoe0` moves in **20-26s**, reproducible |
| 10 gig down → PPPoE | route falls to `pppoe0`; LAN back online in **5s** |
| Stale lease | guard hangs up within ~5s |
| Missing peers file | `NRestarts=0` — no loop |
| Flap holdoff vs live session | session survives a forged 900s holdoff |
| Invariant | AC never showed two `simdsl` sessions |

## Two failure modes found by running it, not by reading it

**The flap damper tore down a healthy WAN.** `ppp_dial()` checked the hold-off
and returned *before* renewing `may-dial`. That file is a lease the guard
expires after `LEASE_TTL`, so tripping the damper stopped the renewal and the
guard hung up `pppoe0` **on the master** ~80s later:

```
DIAL FLAP: >=6 attempts in 600s -- holding off 900s
GUARD: lease stale (81s > 75s) -- hanging up pppoe0
```

A damper meant to suppress repeated *dials* was destroying an established
session instead. An active session now renews the lease and returns before
every other check; everything below only decides whether to start a **new**
session. T12 is the regression test.

**A missing peers file is silent.** `/etc/ppp/peers/pppoe0` is both pppd's
options file and the gate's second condition, and it is only written by a commit
that touches the pppoe subtree. Without it systemd logs
`skipped because of an unmet condition check` exactly once and then nothing —
a router that cannot dial at all looks identical to a healthy backup.
`ppp_dial()` now says so on every tick, and distinguishes the two causes:
configured-but-not-rendered (re-commit the subtree) versus no `pppoe0` in the
config at all.

The second cause is the one to watch in production: **a commit that was never
`save`d reverts on reboot and takes `pppoe0` with it.** That is exactly how the
sim secondary lost its WAN and spent hours looking like an ISP problem. After
any hand commit to the pppoe subtree, `save` — or the next reboot produces a
standby that can never take over.

## Deploying (not yet done)

1. `sudo /config/vyos-known-good save` on both.
2. `migration/vrrp-wan-install --vip 192.168.1.1 --host vyos@10.0.1.253` then the
   same for `.252`. Then `--check` on both. **No config change yet** — verify
   nothing dials.
3. Confirm `/config/wan-secrets` is present and identical on both.
4. **vyos002 first** (the non-master), on its own commit — `interfaces pppoe` is
   priority 322 and one bad node fails everything:
   `delete interfaces pppoe pppoe0 disable`, `commit-confirm 10`.
5. Verify vyos002 did **not** dial — check on the wire, not from state:
   `sudo tcpdump -i bond0.51 -nn pppoed` should show no PADI. Then `confirm`; `save`.
6. vyos001: nothing to change; it already has `pppoe0` enabled.
7. Confirm `vif 53 disable` is in **both** `config.boot`s.
8. Add the drop-in re-install to the VyOS image-upgrade runbook.

**Rollback**, from either box: `set interfaces pppoe pppoe0 disable` on both and
`rm /run/vrrp-wan/may-dial`. That restores today's behaviour exactly.

## What the sim cannot prove

- **Vodafone's `session-control`.** The sim's accel-ppp defaults to `replace`, so
  a new auth kills the old session immediately. A real BRAS may `deny` and hold
  the session for its own dead-peer timer. The matrix runs all three modes to
  bracket the risk; it cannot tell you which one you will meet, and account
  rate-limiting or lockout on repeated dials has no sim analogue at all. The
  flap damper (6 dials / 600s → 15 min hold-off) exists for that.
- **Whether Vodafone honours our LCP Terminate / PADT** on a graceful stop.
- **The cloned-MAC lease** — whether the 10 gig ISP re-issues `87.192.101.48` to
  `f0:9f:c2:12:9b:4f` arriving on a different switch port. That risk belongs to
  `bond0.53`, not PPPoE, and is the largest untested item in the failover.
- **Real dial time and MTU/MSS under load.** PPPoE was proven on the *USG*;
  VyOS dialling Vodafone has never been done.
- **Timing under load.** The sim routers are idle 2-vCPU VMs; commit latency on
  the VP2440s under kea + BGP + conntrack will be worse, and commit latency is
  the dominant term in the `bond0.53` half of a failover.

## A model hazard to fix before applying

The imported baseline records the **running** state, not the safe one: vyos001
has no `vif 53 disable` (it is master), vyos002 does. So an apply performed while
vyos002 held the VIP would enable vyos001's WAN as well, putting the cloned MAC
on both boxes. An override must assert `vif 53 disable` on **both** routers, with
the reconciler re-enabling whichever holds the VIP. Note the consequence: an
apply then briefly disables the current master's 10 gig until the reconciler
restores it (≤30s).