Reload guard.yaml without losing the bounties

systemctl reload guard, guard reload or SIGHUP reads the configuration
again: a watch keeps its bounties and counts under the same name and
takes its new settings at once; its source starts again only if it
changed; watches gone stop, new ones start; a file with a mistake is
refused. The Housecarl connection, state, socket and log wait for a
restart, and guard reload says so.

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

CHANGELOG.md

@@ -22,8 +22,14 @@ All notable changes to Guard are documented here. The format follows
   `postfix-sasl`, `dovecot`, `nginx-auth`, `housecarl-drops`
   (Housecarl's log of dropped packets, TCP only by default).
 - Commands: `guard run`, `check`, `config [--example]`, `presets`,
-  `test` (a watch's patterns over a log), `status` (through a local
-  socket), `bans`, `ban`, `unban`, `version`.
+  `test` (a watch's patterns over a log), `status` and `reload` (through
+  a local socket), `bans`, `ban`, `unban`, `version`.
+- Reloading: `systemctl reload guard`, `guard reload` or SIGHUP reads
+  guard.yaml again without losing the bounties. A watch keeps its
+  bounties under the same name and takes its new settings at once; its
+  source starts again only if it changed; watches gone stop, new ones
+  start. A file with a mistake is refused. The Housecarl connection,
+  state, socket and log wait for a restart (`guard reload` says so).
 - A Debian package with a sandboxed systemd unit, as the `guard` user in
   the `adm` and `systemd-journal` groups, published with each release to
   the Gitea Debian registry (`stable`, or `testing` for a release

README.md

@@ -49,6 +49,8 @@ watch:
 * **IPv6 by /64**, as the ban list says: one customer usually has a
   whole /64 and moves around in it.
 * **Housecarl away?** Bans wait and are sent once it answers.
+* **`systemctl reload guard`** puts a changed guard.yaml to work
+  without losing what Guard has counted.
 * **`guard test`** shows what a watch's patterns find in a log, and whom
   they would ban, before you trust it.
 

cmd/guard/main.go

@@ -53,6 +53,7 @@ keeps it from this machine (or from what the list says).
   guard check                  check guard.yaml and reach Housecarl
   guard test ssh /var/log/auth.log   what a watch's patterns find in a log
   guard status                 the watches, their highest bounties, recent bans
+  guard reload                 read guard.yaml again, keeping the bounties
 
 Exit codes: 0 ok, 1 error.`,
 		SilenceUsage:  true,
@@ -61,7 +62,7 @@ Exit codes: 0 ok, 1 error.`,
 	}
 	root.PersistentFlags().StringVarP(&flagConfig, "config", "c", config.DefaultPath, "configuration file")
 	root.PersistentFlags().BoolVar(&flagJSON, "json", false, "print JSON instead of tables")
-	root.AddCommand(runCmd(), checkCmd(), configCmd(), presetsCmd(), testCmd(), statusCmd(), bansCmd(), banCmd(), unbanCmd(), versionCmd())
+	root.AddCommand(runCmd(), checkCmd(), configCmd(), presetsCmd(), testCmd(), statusCmd(), reloadCmd(), bansCmd(), banCmd(), unbanCmd(), versionCmd())
 	if err := root.Execute(); err != nil {
 		fmt.Fprintln(os.Stderr, "guard:", err)
 		os.Exit(1)
@@ -132,8 +133,19 @@ func runCmd() *cobra.Command {
 			if err != nil {
 				return err
 			}
+			d.Load = load
 			c, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 			defer stop()
+			// SIGHUP reloads, as guard reload does.
+			hup := make(chan os.Signal, 1)
+			signal.Notify(hup, syscall.SIGHUP)
+			go func() {
+				for range hup {
+					if _, err := d.ReloadFile(); err != nil {
+						log.Error("reload refused; the configuration stays as it was", "err", err)
+					}
+				}
+			}()
 			log.Info("starting", "version", version.String(), "config", flagConfig)
 			return d.Run(c)
 		},
@@ -507,6 +519,32 @@ func clip(s string, n int) string {
 
 // ---- status
 
+// askDaemon makes a request of the running Guard, on its socket.
+func askDaemon(cfg *config.Config, method, path string) (*http.Response, error) {
+	hc := &http.Client{Timeout: 30 * time.Second, Transport: &http.Transport{DialContext: func(c context.Context, _, _ string) (net.Conn, error) {
+		var d net.Dialer
+		return d.DialContext(c, "unix", cfg.Socket)
+	}}}
+	req, err := http.NewRequest(method, "http://guard"+path, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := hc.Do(req)
+	switch {
+	case errors.Is(err, os.ErrNotExist), errors.Is(err, syscall.ECONNREFUSED):
+		return nil, fmt.Errorf("Guard isn't running (nothing answers on %s)", cfg.Socket)
+	case errors.Is(err, os.ErrPermission):
+		return nil, fmt.Errorf("no permission for %s: run this as root or the guard user", cfg.Socket)
+	case err != nil:
+		var ue *url.Error
+		if errors.As(err, &ue) {
+			err = ue.Err
+		}
+		return nil, fmt.Errorf("Guard's socket %s: %w", cfg.Socket, err)
+	}
+	return resp, nil
+}
+
 func statusCmd() *cobra.Command {
 	return &cobra.Command{
 		Use: "status", Short: "The running Guard: its watches, highest bounties, recent bans", Args: cobra.NoArgs,
@@ -515,22 +553,9 @@ func statusCmd() *cobra.Command {
 			if err != nil {
 				return err
 			}
-			hc := &http.Client{Timeout: 10 * time.Second, Transport: &http.Transport{DialContext: func(c context.Context, _, _ string) (net.Conn, error) {
-				var d net.Dialer
-				return d.DialContext(c, "unix", cfg.Socket)
-			}}}
-			resp, err := hc.Get("http://guard/status")
-			switch {
-			case errors.Is(err, os.ErrNotExist), errors.Is(err, syscall.ECONNREFUSED):
-				return fmt.Errorf("Guard isn't running (nothing answers on %s)", cfg.Socket)
-			case errors.Is(err, os.ErrPermission):
-				return fmt.Errorf("no permission for %s: run this as root or the guard user", cfg.Socket)
-			case err != nil:
-				var ue *url.Error
-				if errors.As(err, &ue) {
-					err = ue.Err
-				}
-				return fmt.Errorf("Guard's socket %s: %w", cfg.Socket, err)
+			resp, err := askDaemon(cfg, http.MethodGet, "/status")
+			if err != nil {
+				return err
 			}
 			defer resp.Body.Close()
 			var s daemon.Status
@@ -575,6 +600,50 @@ func statusCmd() *cobra.Command {
 	}
 }
 
+// ---- reload
+
+func reloadCmd() *cobra.Command {
+	return &cobra.Command{
+		Use: "reload", Short: "Make the running Guard read guard.yaml again, keeping its bounties", Args: cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			// Checked here first, so a mistake is shown before Guard sees it.
+			cfg, err := load()
+			if err != nil {
+				return err
+			}
+			resp, err := askDaemon(cfg, http.MethodPost, "/reload")
+			if err != nil {
+				return err
+			}
+			defer resp.Body.Close()
+			if resp.StatusCode != http.StatusOK {
+				var e struct{ Error string }
+				json.NewDecoder(resp.Body).Decode(&e)
+				return fmt.Errorf("reload refused: %s", e.Error)
+			}
+			var r daemon.Reloaded
+			if err := json.NewDecoder(resp.Body).Decode(&r); err != nil {
+				return err
+			}
+			if flagJSON {
+				return printJSON(r)
+			}
+			if len(r.Changes) == 0 {
+				fmt.Println("Reloaded: nothing changed")
+			} else {
+				fmt.Println("Reloaded:")
+				for _, c := range r.Changes {
+					fmt.Println("  " + c)
+				}
+			}
+			if len(r.Restart) > 0 {
+				fmt.Printf("Changed, but only on a restart (systemctl restart guard): %s\n", strings.Join(r.Restart, ", "))
+			}
+			return nil
+		},
+	}
+}
+
 // plural is "1 ban", "2 bans", "2 matches".
 func plural(n int, what string) string {
 	if n == 1 {

deploy/systemd/guard.service

@@ -9,6 +9,8 @@ Wants=network-online.target
 
 [Service]
 ExecStart=/usr/local/bin/guard --config /etc/guard/guard.yaml run
+# systemctl reload guard: guard.yaml again, keeping the bounties.
+ExecReload=/usr/local/bin/guard --config /etc/guard/guard.yaml reload
 Restart=on-failure
 RestartSec=5s
 

docs/cli.md

@@ -12,6 +12,7 @@ One binary is the daemon and its tools. Every command reads
 | `guard presets [PRESET]` | the presets, or one's patterns |
 | `guard test WATCH [FILE\|-] [--lines N] [--all]` | what a watch's patterns find in a log, and whom they would ban ([presets.md](presets.md)) |
 | `guard status` | the running Guard: each watch's lines, matches, bans and highest bounties, the recent bans and why any failed, and bans waiting for Housecarl. Asks the socket: as root or the guard user |
+| `guard reload` | make the running Guard read guard.yaml again, keeping its bounties, and show what changed (as `systemctl reload guard` does; [configuration.md](configuration.md#reloading)). Asks the socket |
 | `guard bans [--list L]` | the ban list's active bans, from Housecarl |
 | `guard ban ADDRESS [--list L] [--protocol tcp --port 22]... [--time 1d] [--reason R]` | ban by hand |
 | `guard unban ADDRESS [--list L]` | lift an address's bans |

docs/configuration.md

@@ -3,8 +3,9 @@
 `/etc/guard/guard.yaml` is Guard's whole configuration. `guard config
 --example` prints a commented one. Unknown settings are an error;
 `guard check` checks the file, compiles every pattern and asks Housecarl
-for the ban lists. Guard reads it when it starts:
-`systemctl restart guard` after a change.
+for the ban lists. After a change, `systemctl reload guard` (or `guard
+reload`) puts it to work without losing what Guard has counted (see
+[Reloading](#reloading)).
 
 ## housecarl
 
@@ -116,6 +117,29 @@ every 30 seconds, for the time it has left. A ban Housecarl refuses (an
 address it never bans, a list the token doesn't cover) isn't sent again:
 `guard status` and the log say why.
 
+## Reloading
+
+`systemctl reload guard`, `guard reload` or a SIGHUP makes the running
+Guard read guard.yaml again. `guard reload` checks the file first and
+shows what changed; a file with a mistake is refused, and Guard goes on
+as it was.
+
+* A watch is known by its **name**: one that keeps its name keeps its
+  bounties and counts, and takes its new patterns, hits, window and ban
+  at once. If its source changed (another file, journal unit, …), the
+  source starts again; a file is then followed from its end, as when
+  Guard starts.
+* Watches gone (or disabled) stop; new ones start.
+* `never_ban` and `defaults.repeat` apply to the next matches and bans.
+  Bans already made stay as they are, in Housecarl.
+* `housecarl`'s connection (`url`, `socket`, the token, `ca_cert`,
+  `insecure`), `state`, `socket` and `log` only change on a restart:
+  `guard reload` says so. `housecarl.list` reloads (it only fills in the
+  watches' lists).
+
+Changing `hits` doesn't ban at once: an address over the new hits is
+banned on its next match.
+
 ## state, socket, log
 
 | Setting | Default | |

internal/bounty/bounty.go

@@ -46,6 +46,13 @@ func (t *Tracker) Add(p netip.Prefix, at time.Time) (int, bool) {
 	return len(ts), false
 }
 
+// Set changes the hits and window (a reload); the bounties stay.
+func (t *Tracker) Set(hits int, within time.Duration) {
+	t.mu.Lock()
+	t.Hits, t.Within = hits, within
+	t.mu.Unlock()
+}
+
 // keep drops the times before from (times are in order).
 func keep(ts []time.Time, from time.Time) []time.Time {
 	i := 0

internal/config/example.yaml

@@ -1,6 +1,6 @@
-# Guard's configuration (docs/configuration.md). Guard reads it when it
-# starts: systemctl restart guard after a change, and 'guard check' to
-# check it first.
+# Guard's configuration (docs/configuration.md). After a change,
+# 'systemctl reload guard' (or 'guard reload') puts it to work and keeps
+# the bounties; 'guard check' checks it first.
 
 # Housecarl, which does the banning. Give Guard a token for its ban list
 # alone; on the firewall:

internal/daemon/daemon.go

@@ -37,15 +37,19 @@ type Banner interface {
 
 // Daemon is a running Guard.
 type Daemon struct {
-	cfg     *config.Config
 	log     *slog.Logger
 	hc      Banner
+	logs    source.LogReader
 	state   *state.State
-	never   *NeverBan
-	watches []*Watch
 	started time.Time
+	ctx     context.Context // Run's, for what a reload starts
 
 	mu sync.Mutex
+	// cfg, never and watches change on a reload.
+	cfg       *config.Config
+	never     *NeverBan
+	neverStop context.CancelFunc
+	watches   []*Watch
 	groups    map[string]int // ban list → IPv6 grouping
 	// pending are the bans Housecarl hasn't taken yet, by list and group.
 	pending map[string]state.Ban
@@ -54,6 +58,11 @@ type Daemon struct {
 	// once an hour.
 	refused map[string]time.Time
 
+	// Load reads the configuration again for a reload (guard run sets it).
+	Load func() (*config.Config, error)
+	// Sources makes a watch's source (tests); nil: from its configuration.
+	Sources func(*config.Watch) source.Source
+
 	// Now and RetryEvery can be changed by tests.
 	Now        func() time.Time
 	RetryEvery time.Duration
@@ -61,18 +70,30 @@ type Daemon struct {
 
 // Watch is one watch at work.
 type Watch struct {
-	Cfg     *config.Watch
-	Matcher *match.Matcher
 	Source  source.Source
 	Tracker *bounty.Tracker
 
+	// mu guards Cfg and Matcher, which a reload replaces.
+	mu      sync.Mutex
+	Cfg     *config.Watch
+	Matcher *match.Matcher
+
+	stop context.CancelFunc
+	done chan struct{}
+
 	lines, matches, bans atomic.Uint64
 }
 
+func (w *Watch) setup() (*config.Watch, *match.Matcher) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	return w.Cfg, w.Matcher
+}
+
 // New makes a Daemon. hc may be a fake; sources come from the watches'
 // configuration unless a test sets them (Watch.Source).
 func New(cfg *config.Config, hc Banner, logs source.LogReader, st *state.State, log *slog.Logger) (*Daemon, error) {
-	d := &Daemon{cfg: cfg, log: log, hc: hc, state: st, never: NewNeverBan(cfg.NeverBan, log), groups: map[string]int{},
+	d := &Daemon{cfg: cfg, log: log, hc: hc, logs: logs, state: st, never: NewNeverBan(cfg.NeverBan, log), groups: map[string]int{},
 		pending: map[string]state.Ban{}, wake: make(chan struct{}, 1), refused: map[string]time.Time{},
 		Now: time.Now, RetryEvery: 30 * time.Second}
 	for _, b := range st.Pending {
@@ -86,16 +107,7 @@ func New(cfg *config.Config, hc Banner, logs source.LogReader, st *state.State,
 		if err != nil {
 			return nil, fmt.Errorf("watch %s: %v", w.Name, err)
 		}
-		wt := &Watch{Cfg: w, Matcher: m, Tracker: bounty.New(w.Hits, w.Within.D())}
-		switch w.Kind() {
-		case "file":
-			wt.Source = &source.File{Pattern: w.File, Log: log.With("watch", w.Name)}
-		case "journal":
-			wt.Source = &source.Journal{Units: w.Journal.Unit, Identifiers: w.Journal.Identifier, Log: log.With("watch", w.Name)}
-		case config.SourceHousecarlDrops:
-			wt.Source = &source.HousecarlDrops{Client: logs, Protocol: w.Protocol, Shadow: w.Shadow, Log: log.With("watch", w.Name)}
-		}
-		d.watches = append(d.watches, wt)
+		d.watches = append(d.watches, &Watch{Cfg: w, Matcher: m, Tracker: bounty.New(w.Hits, w.Within.D())})
 	}
 	if len(d.watches) == 0 {
 		return nil, errors.New("every watch is disabled: nothing to do")
@@ -103,23 +115,93 @@ func New(cfg *config.Config, hc Banner, logs source.LogReader, st *state.State,
 	return d, nil
 }
 
-// Watches are the watches at work (tests set their sources).
-func (d *Daemon) Watches() []*Watch { return d.watches }
+// newSource makes a watch's source from its configuration.
+func (d *Daemon) newSource(w *config.Watch) source.Source {
+	if d.Sources != nil {
+		return d.Sources(w)
+	}
+	log := d.log.With("watch", w.Name)
+	switch w.Kind() {
+	case "journal":
+		return &source.Journal{Units: w.Journal.Unit, Identifiers: w.Journal.Identifier, Log: log}
+	case config.SourceHousecarlDrops:
+		return &source.HousecarlDrops{Client: d.logs, Protocol: w.Protocol, Shadow: w.Shadow, Log: log}
+	}
+	return &source.File{Pattern: w.File, Log: log}
+}
+
+// Watches are the watches at work (tests set their sources before Run).
+func (d *Daemon) Watches() []*Watch {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return append([]*Watch(nil), d.watches...)
+}
+
+func (d *Daemon) config() *config.Config {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.cfg
+}
+
+func (d *Daemon) neverBan() *NeverBan {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.never
+}
 
 // Run watches until ctx ends.
 func (d *Daemon) Run(ctx context.Context) error {
 	d.started = d.Now()
+	d.ctx = ctx
 	d.loadGroups(ctx)
-	go d.never.Run(ctx)
-	var wg sync.WaitGroup
+	d.mu.Lock()
+	d.startNever()
 	for _, w := range d.watches {
-		lines := make(chan source.Line, 256)
-		wg.Add(2)
+		d.start(w)
+	}
+	d.mu.Unlock()
+	var wg sync.WaitGroup
+	wg.Add(1)
 	go func() {
 		defer wg.Done()
+		d.sender(ctx)
+	}()
+	if sock := d.config().Socket; sock != "" {
+		if err := d.serveStatus(ctx); err != nil {
+			d.log.Warn("no status socket (guard status won't work)", "socket", sock, "err", err)
+		}
+	}
+	<-ctx.Done()
+	wg.Wait()
+	for _, w := range d.Watches() {
+		<-w.done
+	}
+	d.savePending()
+	return nil
+}
+
+// startNever looks never_ban's names up until a reload replaces it
+// (d.mu held).
+func (d *Daemon) startNever() {
+	c, stop := context.WithCancel(d.ctx)
+	d.neverStop = stop
+	go d.never.Run(c)
+}
+
+// start runs a watch: its source, and its lines through Line, until
+// stopped or Run's context ends (d.mu held).
+func (d *Daemon) start(w *Watch) {
+	if w.Source == nil {
+		w.Source = d.newSource(w.Cfg)
+	}
+	ctx, stop := context.WithCancel(d.ctx)
+	w.stop, w.done = stop, make(chan struct{})
+	lines := make(chan source.Line, 256)
+	src := w.Source
+	go func() {
 		defer close(lines)
 		for {
-				err := w.Source.Run(ctx, lines)
+			err := src.Run(ctx, lines)
 			if ctx.Err() != nil {
 				return
 			}
@@ -132,28 +214,13 @@ func (d *Daemon) Run(ctx context.Context) error {
 		}
 	}()
 	go func() {
-			defer wg.Done()
+		defer close(w.done)
 		for l := range lines {
 			d.Line(ctx, w, l)
 		}
 	}()
 	d.log.Info("watching", "watch", w.Cfg.Name, "source", Describe(w.Cfg), "patterns", len(w.Cfg.Match), "hits", w.Cfg.Hits, "within", w.Cfg.Within.String(), "list", w.Cfg.Ban.List)
 }
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		d.sender(ctx)
-	}()
-	if d.cfg.Socket != "" {
-		if err := d.serveStatus(ctx); err != nil {
-			d.log.Warn("no status socket (guard status won't work)", "socket", d.cfg.Socket, "err", err)
-		}
-	}
-	<-ctx.Done()
-	wg.Wait()
-	d.savePending()
-	return nil
-}
 
 // Describe says where a watch's lines come from.
 func Describe(w *config.Watch) string {
@@ -199,8 +266,9 @@ func Short(w *config.Watch) string {
 // bounties are counted as the list bans. A list that can't be read now
 // counts by /64, its default.
 func (d *Daemon) loadGroups(ctx context.Context) {
-	for _, w := range d.watches {
-		list := w.Cfg.Ban.List
+	for _, w := range d.Watches() {
+		cfg, _ := w.setup()
+		list := cfg.Ban.List
 		d.mu.Lock()
 		_, done := d.groups[list]
 		d.mu.Unlock()
@@ -238,33 +306,34 @@ func (d *Daemon) group(list string) int {
 // Line handles one line of a watch.
 func (d *Daemon) Line(ctx context.Context, w *Watch, l source.Line) {
 	w.lines.Add(1)
+	cfg, m := w.setup()
 	addr := l.Addr
 	if !addr.IsValid() {
 		var ok bool
-		if addr, ok = w.Matcher.Match(l.Text); !ok {
+		if addr, ok = m.Match(l.Text); !ok {
 			return
 		}
 	}
 	w.matches.Add(1)
-	if why := d.never.Why(addr); why != "" {
-		d.log.Debug("match, but never banned", "watch", w.Cfg.Name, "address", addr, "why", why)
+	if why := d.neverBan().Why(addr); why != "" {
+		d.log.Debug("match, but never banned", "watch", cfg.Name, "address", addr, "why", why)
 		return
 	}
-	g := match.Group(addr, d.group(w.Cfg.Ban.List))
+	g := match.Group(addr, d.group(cfg.Ban.List))
 	n, reached := w.Tracker.Add(g, l.Time)
-	d.log.Debug("match", "watch", w.Cfg.Name, "address", addr, "bounty", n, "hits", w.Cfg.Hits, "line", clip(l.Text, 300))
+	d.log.Debug("match", "watch", cfg.Name, "address", addr, "bounty", n, "hits", cfg.Hits, "line", clip(l.Text, 300))
 	if !reached {
 		return
 	}
 	w.bans.Add(1)
-	d.ban(w, addr, g, l.Text)
+	d.ban(cfg, addr, g, l.Text)
 }
 
 // BanTime is how long a group's next ban lasts: the watch's time,
 // times the repeat factor for each earlier ban within forget, up to the
 // most.
 func (d *Daemon) BanTime(w *config.Watch, group string) time.Duration {
-	r := d.cfg.Defaults.Repeat
+	r := d.config().Defaults.Repeat
 	base := w.Ban.Time.D()
 	n := d.state.Repeats(group, d.Now().Add(-r.Forget.D()))
 	t := time.Duration(float64(base) * math.Pow(r.Factor, float64(n)))
@@ -277,12 +346,12 @@ func (d *Daemon) BanTime(w *config.Watch, group string) time.Duration {
 	return t
 }
 
-func (d *Daemon) ban(w *Watch, addr netip.Addr, g netip.Prefix, line string) {
+func (d *Daemon) ban(w *config.Watch, addr netip.Addr, g netip.Prefix, line string) {
 	group := g.String()
-	t := d.BanTime(w.Cfg, group)
-	b := state.Ban{Time: d.Now(), Watch: w.Cfg.Name, List: w.Cfg.Ban.List, Address: addr.String(), Group: group,
-		Protocol: strings.ToLower(w.Cfg.Ban.Protocol), Ports: strings.Join(w.Cfg.Ban.Port, ","), Seconds: int(t / time.Second), Reason: clip(line, 300)}
-	d.log.Info("bounty reached; banning", "watch", w.Cfg.Name, "address", addr, "list", b.List, "ports", portsText(b), "for", config.FormatDuration(t))
+	t := d.BanTime(w, group)
+	b := state.Ban{Time: d.Now(), Watch: w.Name, List: w.Ban.List, Address: addr.String(), Group: group,
+		Protocol: strings.ToLower(w.Ban.Protocol), Ports: strings.Join(w.Ban.Port, ","), Seconds: int(t / time.Second), Reason: clip(line, 300)}
+	d.log.Info("bounty reached; banning", "watch", w.Name, "address", addr, "list", b.List, "ports", portsText(b), "for", config.FormatDuration(t))
 	d.mu.Lock()
 	d.pending[b.List+" "+b.Group+" "+b.Protocol+" "+b.Ports] = b
 	d.mu.Unlock()
@@ -390,10 +459,11 @@ func (d *Daemon) savePending() {
 		p = append(p, b)
 	}
 	d.mu.Unlock()
+	cfg := d.config()
 	d.state.SetPending(p)
-	d.state.Forget(d.Now().Add(-d.cfg.Defaults.Repeat.Forget.D()))
+	d.state.Forget(d.Now().Add(-cfg.Defaults.Repeat.Forget.D()))
 	if err := d.state.Save(); err != nil {
-		d.log.Warn("saving the state failed", "file", d.cfg.State, "err", err)
+		d.log.Warn("saving the state failed", "file", cfg.State, "err", err)
 	}
 }
 
@@ -430,9 +500,10 @@ type WatchStatus struct {
 // Status reports the watches, their highest bounties and the recent bans.
 func (d *Daemon) Status() Status {
 	s := Status{Started: d.started}
-	for _, w := range d.watches {
-		s.Watches = append(s.Watches, WatchStatus{Name: w.Cfg.Name, Source: Short(w.Cfg), List: w.Cfg.Ban.List, Hits: w.Cfg.Hits,
-			Within: w.Cfg.Within.String(), Lines: w.lines.Load(), Matches: w.matches.Load(), Bans: w.bans.Load(), Top: w.Tracker.Top(10, d.Now())})
+	for _, w := range d.Watches() {
+		cfg, _ := w.setup()
+		s.Watches = append(s.Watches, WatchStatus{Name: cfg.Name, Source: Short(cfg), List: cfg.Ban.List, Hits: cfg.Hits,
+			Within: cfg.Within.String(), Lines: w.lines.Load(), Matches: w.matches.Load(), Bans: w.bans.Load(), Top: w.Tracker.Top(10, d.Now())})
 	}
 	s.Recent, s.Pending = d.state.Snapshot()
 	if len(s.Recent) > 50 {
@@ -444,7 +515,7 @@ func (d *Daemon) Status() Status {
 // serveStatus answers GET /status on the local socket (root and the
 // guard user only: the socket is 0660 in /run/guard).
 func (d *Daemon) serveStatus(ctx context.Context) error {
-	path := d.cfg.Socket
+	path := d.config().Socket
 	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
 		return err
 	}
@@ -459,6 +530,16 @@ func (d *Daemon) serveStatus(ctx context.Context) error {
 		w.Header().Set("Content-Type", "application/json")
 		json.NewEncoder(w).Encode(d.Status())
 	})
+	mux.HandleFunc("POST /reload", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		res, err := d.ReloadFile()
+		if err != nil {
+			w.WriteHeader(http.StatusUnprocessableEntity)
+			json.NewEncoder(w).Encode(map[string]string{"error": err.Error()})
+			return
+		}
+		json.NewEncoder(w).Encode(res)
+	})
 	srv := &http.Server{Handler: mux, ReadHeaderTimeout: 5 * time.Second}
 	go srv.Serve(ln)
 	go func() {

internal/daemon/daemon_test.go

@@ -177,3 +177,140 @@ socket: ` + filepath.Join(dir, "guard.sock") + `
 		t.Errorf("saved state: %v %+v", err, st2.Offences)
 	}
 }
+
+// stoppable is a chanSource that says when it stopped.
+type stoppable struct {
+	chanSource
+	stopped chan struct{}
+}
+
+func (s *stoppable) Run(ctx context.Context, out chan<- source.Line) error {
+	err := s.chanSource.Run(ctx, out)
+	close(s.stopped)
+	return err
+}
+
+// TestReload: a reload keeps the bounties of a watch under the same name
+// and applies its new settings; a watch whose source changed gets a new
+// one; watches gone stop, new ones start; settings for a restart are
+// reported; a configuration with nothing to watch is refused.
+func TestReload(t *testing.T) {
+	dir := t.TempDir()
+	base := `housecarl: {url: https://fw:8070, list: web}
+defaults: {hits: 3, within: 10m, ban: 1h}
+state: ` + filepath.Join(dir, "state.json") + `
+socket: ` + filepath.Join(dir, "guard.sock") + `
+`
+	parse := func(extra string) *config.Config {
+		t.Helper()
+		cfg, err := config.Parse([]byte(base + extra))
+		if err != nil {
+			t.Fatal(err)
+		}
+		return cfg
+	}
+	cfg := parse(`watch:
+  - {name: ssh, preset: sshd, file: /var/log/auth.log}
+  - {name: web, file: /var/log/web.log, match: ['bad <IP>$']}
+`)
+	st, _ := state.Load(cfg.State)
+	hc := &fakeHC{}
+	d, err := New(cfg, hc, nil, st, slog.New(slog.NewTextHandler(io.Discard, nil)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var mu sync.Mutex
+	srcs := map[string]*stoppable{}
+	d.Sources = func(w *config.Watch) source.Source {
+		mu.Lock()
+		defer mu.Unlock()
+		s := &stoppable{chanSource{make(chan source.Line)}, make(chan struct{})}
+		srcs[w.Name+" "+w.File] = s
+		return s
+	}
+	src := func(key string) *stoppable {
+		mu.Lock()
+		defer mu.Unlock()
+		return srcs[key]
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { d.Run(ctx); close(done) }()
+	waitFor(t, "the sources", func() bool { return src("ssh /var/log/auth.log") != nil && src("web /var/log/web.log") != nil })
+	fail := func(s *stoppable, addr string, n int) {
+		for i := 0; i < n; i++ {
+			s.ch <- source.Line{Text: "Failed password for root from " + addr + " port 4000 ssh2", Time: time.Now()}
+		}
+	}
+	ssh := src("ssh /var/log/auth.log")
+	fail(ssh, "203.0.113.9", 2)
+
+	res, err := d.Reload(parse(`never_ban: [198.51.100.0/24]
+watch:
+  - {name: ssh, preset: sshd, file: /var/log/auth.log, ban: {time: 2h}}
+  - {name: mail, preset: dovecot, file: /var/log/mail.log}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := strings.Join(res.Changes, "; "); got != "never_ban; watch ssh: ban; watch mail: added; watch web: removed" || len(res.Restart) != 0 {
+		t.Errorf("reload: %q, restart %v", got, res.Restart)
+	}
+	select {
+	case <-src("web /var/log/web.log").stopped:
+	case <-time.After(2 * time.Second):
+		t.Error("the removed watch's source still runs")
+	}
+	if src("mail /var/log/mail.log") == nil {
+		t.Error("the added watch has no source")
+	}
+	// The bounty of 2 was kept: one more bans, for the new time.
+	fail(ssh, "203.0.113.9", 1)
+	waitFor(t, "the ban after the reload", func() bool { return hc.count() == 1 })
+	if b := hc.last(); b.Time < 7190 || b.Time > 7200 {
+		t.Errorf("ban after reload: %ds, want 2h", b.Time)
+	}
+	fail(ssh, "198.51.100.7", 3)
+
+	// A new file: a new source; the bounty stays; the hits change.
+	fail(ssh, "203.0.113.50", 2)
+	res, err = d.Reload(parse(`log: {level: debug}
+watch:
+  - {name: ssh, preset: sshd, file: /var/log/secure, hits: 4, ban: {time: 2h}}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := strings.Join(res.Changes, "; "); got != "never_ban; watch ssh: bounty 4 in 10m, source (started again); watch mail: removed" ||
+		strings.Join(res.Restart, ",") != "log" {
+		t.Errorf("reload: %q, restart %v", got, res.Restart)
+	}
+	select {
+	case <-ssh.stopped:
+	case <-time.After(2 * time.Second):
+		t.Error("the old source still runs")
+	}
+	secure := src("ssh /var/log/secure")
+	fail(secure, "203.0.113.50", 1)
+	time.Sleep(50 * time.Millisecond)
+	if hc.count() != 1 {
+		t.Fatal("banned below the new hits")
+	}
+	fail(secure, "203.0.113.50", 1)
+	waitFor(t, "the ban at 4", func() bool { return hc.count() == 2 && hc.last().Address == "203.0.113.50" })
+	if d.config().Log.Level != "info" {
+		t.Error("the log level changed without a restart")
+	}
+
+	// Nothing to watch: refused, the old configuration stays.
+	if _, err := d.Reload(parse(`watch:
+  - {name: ssh, preset: sshd, disabled: true}
+`)); err == nil {
+		t.Error("a configuration with every watch disabled was taken")
+	}
+	if s := d.Status(); len(s.Watches) != 1 || s.Watches[0].Hits != 4 || s.Watches[0].Bans != 2 || s.Watches[0].Lines != 10 {
+		t.Errorf("status after reloads %+v", s.Watches)
+	}
+	cancel()
+	<-done
+}

internal/daemon/reload.go

@@ -0,0 +1,185 @@
+package daemon
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"slices"
+	"strings"
+
+	"guard/internal/bounty"
+	"guard/internal/config"
+	"guard/internal/match"
+)
+
+// Reloaded is what a reload changed.
+type Reloaded struct {
+	Changes []string `json:"changes"`
+	// Restart are the changed settings a reload can't apply: they wait
+	// for a restart.
+	Restart []string `json:"restart,omitempty"`
+}
+
+// ReloadFile reads the configuration again (Load) and reloads it.
+func (d *Daemon) ReloadFile() (*Reloaded, error) {
+	if d.Load == nil {
+		return nil, errors.New("this Guard can't read its configuration again")
+	}
+	cfg, err := d.Load()
+	if err != nil {
+		return nil, err
+	}
+	return d.Reload(cfg)
+}
+
+// Reload puts a new configuration to work without losing what Guard
+// counted: a watch keeps its bounties and counts under the same name,
+// and its source runs on unless it changed. Watches gone stop, new ones
+// start. The housecarl connection, state, socket and log wait for a
+// restart. A configuration with no watch at work is refused, and the
+// old one stays.
+func (d *Daemon) Reload(cfg *config.Config) (*Reloaded, error) {
+	if d.ctx == nil {
+		return nil, errors.New("Guard isn't running")
+	}
+	type next struct {
+		cfg *config.Watch
+		m   *match.Matcher
+	}
+	var want []next
+	for _, w := range cfg.Watch {
+		if w.Disabled {
+			continue
+		}
+		m, err := match.Compile(w.Match, w.Ignore)
+		if err != nil {
+			return nil, fmt.Errorf("watch %s: %v", w.Name, err)
+		}
+		want = append(want, next{w, m})
+	}
+	if len(want) == 0 {
+		return nil, errors.New("every watch is disabled: nothing to do (the configuration stays as it was)")
+	}
+
+	res := &Reloaded{}
+	d.mu.Lock()
+	old := d.cfg
+	oh, nh := old.Housecarl, cfg.Housecarl
+	oh.List, nh.List = "", "" // the default list only fills in watches
+	if oh != nh {
+		res.Restart = append(res.Restart, "housecarl")
+	}
+	if old.State != cfg.State {
+		res.Restart = append(res.Restart, "state")
+	}
+	if old.Socket != cfg.Socket {
+		res.Restart = append(res.Restart, "socket")
+	}
+	if old.Log != cfg.Log {
+		res.Restart = append(res.Restart, "log")
+	}
+	cfg.Housecarl.URL, cfg.Housecarl.Socket, cfg.Housecarl.TokenFile, cfg.Housecarl.Token = old.Housecarl.URL, old.Housecarl.Socket, old.Housecarl.TokenFile, old.Housecarl.Token
+	cfg.Housecarl.CACert, cfg.Housecarl.Insecure = old.Housecarl.CACert, old.Housecarl.Insecure
+	cfg.State, cfg.Socket, cfg.Log = old.State, old.Socket, old.Log
+
+	if !slices.Equal(old.NeverBan, cfg.NeverBan) {
+		n := NewNeverBan(cfg.NeverBan, d.log)
+		n.Lookup = d.never.Lookup
+		d.never.mu.RLock()
+		for _, h := range n.names {
+			if a, ok := d.never.resolved[h]; ok {
+				n.resolved[h] = a
+			}
+		}
+		d.never.mu.RUnlock()
+		d.neverStop()
+		d.never = n
+		d.startNever()
+		res.Changes = append(res.Changes, "never_ban")
+	}
+	if old.Defaults.Repeat != cfg.Defaults.Repeat {
+		res.Changes = append(res.Changes, "defaults.repeat: for the next bans")
+	}
+
+	byName := map[string]*Watch{}
+	for _, w := range d.watches {
+		byName[strings.ToLower(w.Cfg.Name)] = w
+	}
+	var watches []*Watch
+	var stopped []chan struct{}
+	for _, n := range want {
+		w := byName[strings.ToLower(n.cfg.Name)]
+		delete(byName, strings.ToLower(n.cfg.Name))
+		if w == nil {
+			w = &Watch{Cfg: n.cfg, Matcher: n.m, Tracker: bounty.New(n.cfg.Hits, n.cfg.Within.D())}
+			d.start(w)
+			watches = append(watches, w)
+			res.Changes = append(res.Changes, "watch "+n.cfg.Name+": added")
+			continue
+		}
+		watches = append(watches, w)
+		oc, _ := w.setup()
+		if reflect.DeepEqual(oc, n.cfg) {
+			continue
+		}
+		var what []string
+		if !slices.Equal(oc.Match, n.cfg.Match) || !slices.Equal(oc.Ignore, n.cfg.Ignore) {
+			what = append(what, "patterns")
+		}
+		if oc.Hits != n.cfg.Hits || oc.Within != n.cfg.Within {
+			what = append(what, fmt.Sprintf("bounty %d in %s", n.cfg.Hits, n.cfg.Within))
+		}
+		if !reflect.DeepEqual(oc.Ban, n.cfg.Ban) {
+			what = append(what, "ban")
+		}
+		w.Tracker.Set(n.cfg.Hits, n.cfg.Within.D())
+		w.mu.Lock()
+		w.Cfg, w.Matcher = n.cfg, n.m
+		w.mu.Unlock()
+		if sourceKey(oc) != sourceKey(n.cfg) {
+			w.stop()
+			stopped = append(stopped, w.done)
+			w.Source = d.newSource(n.cfg)
+			d.start(w)
+			what = append(what, "source (started again)")
+		}
+		if len(what) == 0 {
+			what = append(what, "changed")
+		}
+		res.Changes = append(res.Changes, "watch "+n.cfg.Name+": "+strings.Join(what, ", "))
+	}
+	for _, w := range d.watches {
+		if byName[strings.ToLower(w.Cfg.Name)] == w {
+			w.stop()
+			stopped = append(stopped, w.done)
+			res.Changes = append(res.Changes, "watch "+w.Cfg.Name+": removed")
+		}
+	}
+	d.watches = watches
+	d.cfg = cfg
+	d.mu.Unlock()
+
+	// A stopped watch may be handling a line, which needs d.mu.
+	for _, done := range stopped {
+		<-done
+	}
+	d.loadGroups(d.ctx)
+	if len(res.Changes) == 0 {
+		res.Changes = []string{}
+	}
+	d.log.Info("reloaded", "changes", strings.Join(res.Changes, "; "))
+	if len(res.Restart) > 0 {
+		d.log.Warn("changed settings that wait for a restart", "settings", strings.Join(res.Restart, ", "))
+	}
+	return res, nil
+}
+
+// sourceKey is what a watch's source is made from: when it changes, the
+// source starts again.
+func sourceKey(w *config.Watch) string {
+	j := ""
+	if w.Journal != nil {
+		j = fmt.Sprint(w.Journal.Unit, w.Journal.Identifier)
+	}
+	return fmt.Sprint(w.Kind(), "|", w.File, "|", j, "|", w.Source, "|", w.Protocol, "|", w.Shadow)
+}