Rule files, and bans for a clear sign of attack (closes #24)

Every *.rules file in SWWAF_RULES_DIR is read at start and on each
change, and each request is checked against the rules after the rate
limits: log notes a match, block refuses with 403, ban refuses and bans
the netblock for SWWAF_ATTACK_BAN_DURATION, made permanent by its next
request or clear sign of attack. path, query and uri are matched as the
request line sent them. bans.json gains each ban's cause, and ban notes
count earlier bans by cause. The image ships 00-default.rules.

Judgement call: a header sent twice is matched with its values joined
by ", ".
Judgement call: SWWAF_MAX_BAN_DURATION does not cap a ban for an attack.
Not in this unit: offences for rule matches, with the error burst.

Model: opus-5-5

internal/state/state.go

@@ -48,6 +48,7 @@ var (
 	errVersion = errors.New("unknown version")
 	// errMissing is for an entry without a field it needs.
 	errMissing = errors.New("has no")
+	errCause   = errors.New("is not limit or attack")
 )
 
 // Params are what Load needs.
@@ -95,11 +96,12 @@ type bansFile struct {
 }
 
 // banEntry is a ban as bans.json holds it: a permanent ban's expires is
-// null.
+// null, and a ban an admin added may have no cause.
 type banEntry struct {
 	Netblock netip.Prefix `json:"netblock"`
 	Start    time.Time    `json:"start"`
 	Expires  *time.Time   `json:"expires"`
+	Cause    string       `json:"cause,omitempty"`
 	Notes    bans.Notes   `json:"notes"`
 }
 
@@ -444,7 +446,9 @@ func (f *Files) encode(name string) ([]byte, error) {
 
 // newBanEntry returns ban as bans.json holds it.
 func newBanEntry(ban bans.Ban) banEntry {
-	entry := banEntry{Netblock: ban.Netblock, Start: ban.Start, Notes: ban.Notes}
+	entry := banEntry{
+		Netblock: ban.Netblock, Start: ban.Start, Cause: ban.Cause, Notes: ban.Notes,
+	}
 	if !ban.Permanent() {
 		entry.Expires = &ban.Expires
 	}
@@ -454,7 +458,7 @@ func newBanEntry(ban bans.Ban) banEntry {
 
 // ban returns the ban an entry of bans.json holds.
 func (e banEntry) ban() bans.Ban {
-	ban := bans.Ban{Netblock: e.Netblock, Start: e.Start, Notes: e.Notes}
+	ban := bans.Ban{Netblock: e.Netblock, Start: e.Start, Cause: e.Cause, Notes: e.Notes}
 	if e.Expires != nil {
 		ban.Expires = *e.Expires
 	}
@@ -466,7 +470,8 @@ func (e banEntry) ban() bans.Ban {
 // client, a start, from which the length of the netblock's next ban is
 // worked out, or an expires, which would make it permanent. A permanent
 // ban's expires is null, which Bans cannot tell from a missing one, so
-// each expires is read again as written.
+// each expires is read again as written. A cause other than limit or
+// attack, most likely misspelt, is refused too.
 func (f *bansFile) check(data []byte) error {
 	var written struct {
 		Bans []struct {
@@ -487,6 +492,9 @@ func (f *bansFile) check(data []byte) error {
 			return missing(i, "start")
 		case written.Bans[i].Expires == nil:
 			return missing(i, "expires")
+		case entry.Cause != "" && entry.Cause != bans.CauseLimit &&
+			entry.Cause != bans.CauseAttack:
+			return fmt.Errorf("entry %d's cause %q %w", i+1, entry.Cause, errCause)
 		}
 	}