Shadow mode: Housecarl beside the firewall in force, changing nothing

shadow.enabled in housecarl.yaml loads Housecarl's ruleset so that it
blocks, rejects and translates nothing: drops and rejects count and
accept (an accept only ends Housecarl's chain), policies accept and log
what they would drop, NAT only counts, MSS clamping and the flowtable
are left out, and raw rules that do more than accept, drop or reject are
left out with a warning. Comments mark what would happen ([would drop]).
Filter chains run after the other firewall's (+10) or before (-10); NAT
chains always first. Every change is checked by nftables in both forms.
No probation; takeovers and closing connections are refused. The boot
fallback filters nothing. internal/tablewatch puts the table back after
a flush ruleset. Mode in status, healthz, whoami, the web UI, the log
and the tracer; hc ruleset --enforcing. docs/shadow.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/configuration.md

@@ -30,9 +30,14 @@ reads the file when it starts.
 | `stats.database` | `/var/lib/housecarl/stats.db` | the statistics' own database; deleting it is safe |
 | `stats.retention.minute`, `.hour`, `.day` | `7d`, `90d`, `730d` | how long per-minute, hourly and daily figures are kept |
 | `stats.top_n` | `100` | entries kept per top list, per hour and per day |
+| `shadow.enabled` | `false` | shadow mode: Housecarl's ruleset is loaded beside the firewall you have, but blocks, rejects and translates nothing; its counters, log and tracer show what it would do. No change is on probation, and takeovers and closing connections are refused ([shadow.md](shadow.md)) |
+| `shadow.position` | `after` | where Housecarl's filter chains run in shadow mode: `after` the other firewall's (`filter + 10`: it sees only what that one lets through, so what it would drop is what switching would break) or `before` them (`filter - 10`: it sees every packet) |
 
 Durations are written `15s`, `12h` or `30d`.
 
+Shadow mode is here, and not a firewall setting, so that no user or API
+token can switch enforcement off; restart housecarld to switch.
+
 The probation's length is a firewall setting, not here: `hc settings set
 --probation 5m`. So is counting traffic per device for the statistics:
 `hc settings set --stats-per-device on`.