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/server/server.go

@@ -18,6 +18,7 @@ import (
 	"syscall"
 	"time"
 
+	"housecarl/internal/api"
 	"housecarl/internal/auth"
 	"housecarl/internal/certs"
 	"housecarl/internal/engine"
@@ -45,6 +46,11 @@ type Config struct {
 	// ACME keeps the certificate housecarld obtains itself (tls.acme; nil
 	// when off).
 	ACME *certs.Manager
+	// Shadow is shadow mode (docs/shadow.md): nothing goes on probation,
+	// takeovers and closing connections are refused. ShadowPosition is
+	// after or before, for the status.
+	Shadow         bool
+	ShadowPosition string
 }
 
 // Interfaces lists the host's interfaces with their addresses, and
@@ -58,6 +64,8 @@ type Interfaces interface {
 // engine (internal/nft.Firewall).
 type Firewall interface {
 	Render(st *model.State) *render.Ruleset
+	// RenderEnforcing is the ruleset outside shadow mode.
+	RenderEnforcing(st *model.State) *render.Ruleset
 	Table(ctx context.Context) (*nft.Table, error)
 	AddElement(ctx context.Context, set, addr string, timeout int) error
 	DeleteElement(ctx context.Context, set, addr string) error
@@ -131,6 +139,22 @@ func New(cfg Config, eng *engine.Engine, fw Firewall, ifaces Interfaces, log *sl
 	return s
 }
 
+// mode is housecarld's mode: enforcing, or shadow.
+func (s *Server) mode() string {
+	if s.cfg.Shadow {
+		return api.ModeShadow
+	}
+	return api.ModeEnforcing
+}
+
+// refuseInShadow refuses what would change traffic in shadow mode.
+func (s *Server) refuseInShadow(what string) error {
+	if !s.cfg.Shadow {
+		return nil
+	}
+	return conflict("housecarld is in shadow mode, where it changes nothing about traffic, so it doesn't %s; leave shadow mode first (shadow.enabled in housecarl.yaml, docs/shadow.md)", what)
+}
+
 // Handler returns the HTTP handler with all middleware.
 func (s *Server) Handler() http.Handler {
 	return s.recoverer(s.securityHeaders(s.logRequests(s.mux)))