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>

internal/api/types.go

@@ -40,8 +40,18 @@ type Whoami struct {
 	TokenID   int64         `json:"token_id,omitempty"`
 	CSRFToken string        `json:"csrf_token,omitempty"`
 	Build     version.Build `json:"build"`
+	// Mode is enforcing, or shadow (docs/shadow.md).
+	Mode string `json:"mode"`
 }
 
+// Modes: housecarld's ruleset decides, or (shadow mode, the
+// configuration file's shadow.enabled) it only shows what it would do,
+// beside another firewall.
+const (
+	ModeEnforcing = "enforcing"
+	ModeShadow    = "shadow"
+)
+
 // Login is the login request.
 type Login struct {
 	Username string `json:"username"`
@@ -155,15 +165,23 @@ type LogEntry struct {
 	TCPFlags string    `json:"tcp_flags,omitempty"`
 	RuleMark string    `json:"mark,omitempty"`
 	Verdict  string    `json:"verdict,omitempty"`
+	// Shadow: logged in shadow mode, where a drop or reject is what would
+	// have been done; the packet went on.
+	Shadow bool `json:"shadow,omitempty"`
 }
 
 // Status is the dashboard's overview.
 type Status struct {
-	Build      version.Build `json:"build"`
-	Started    time.Time     `json:"started"`
-	Database   string        `json:"database,omitempty"`
-	Applied    bool          `json:"applied"`
-	ApplyError string        `json:"apply_error,omitempty"`
+	Build    version.Build `json:"build"`
+	Started  time.Time     `json:"started"`
+	Database string        `json:"database,omitempty"`
+	// Mode is enforcing or shadow; ShadowPosition (shadow mode) after or
+	// before: where Housecarl's filter chains run beside the other
+	// firewall's.
+	Mode           string `json:"mode"`
+	ShadowPosition string `json:"shadow_position,omitempty"`
+	Applied        bool   `json:"applied"`
+	ApplyError     string `json:"apply_error,omitempty"`
 	// Unconfigured: the configuration is empty, and no ruleset was
 	// applied yet.
 	Unconfigured bool               `json:"unconfigured,omitempty"`
@@ -414,6 +432,9 @@ type TracePacket struct {
 	// Verdict is accepted or dropped; DecidedBy the step that decided.
 	Verdict   string    `json:"verdict"`
 	DecidedBy TraceStep `json:"decided_by"`
+	// WouldDrop, in shadow mode, is the step of Housecarl's where it
+	// would have dropped (or rejected) the packet, had it been enforcing.
+	WouldDrop *TraceStep `json:"would_drop,omitempty"`
 }
 
 // TraceStatus is the trace running (or the last one).