go-firewall/csf_linux.go

package firewall

import (
	"bufio"
	"context"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
	"time"
)

const (
	CSFConf  = "/etc/csf/csf.conf"
	CSFAllow = "/etc/csf/csf.allow"
	CSFDeny  = "/etc/csf/csf.deny"
	// CSFRedirect holds csf's port-forwarding rules, one per line in the
	// pipe-delimited form "IPx|portA|IPy|portB|proto". A destination IP (IPy) of
	// "*" is a local port redirect; a concrete IPy is a forward to another host.
	CSFRedirect = "/etc/csf/csf.redirect"
	// CSFHook is the csf pre-hook, run after csf flushes and before it loads its
	// own rules, so injected rules land at the top of the chains and are re-added
	// on every reload. This library writes the iptables rules for features csf's
	// native config cannot express directly into this hook. (csf sources both
	// /usr/local/csf/bin/csfpre.sh and /etc/csf/csfpre.sh when present, so this
	// /etc/csf hook always runs.)
	CSFHook = "/etc/csf/csfpre.sh"
)

// CSF manages a ConfigServer Security & Firewall (csf) installation, mapping
// rules onto its config files (csf.conf, csf.allow, csf.deny, csf.redirect) and
// a managed pre-hook for features csf's native config cannot express.
type CSF struct {
	// rulePrefix tags rules this library creates so they can be told apart
	// from foreign rules. In csf.allow/csf.deny it is prepended to the
	// comment written on the line above each rule; csf.conf port-list rules
	// carry no per-rule comment and so cannot carry the tag.
	rulePrefix string
	// ipv6Enabled mirrors csf.conf's IPV6. With it off (the shipped default) csf
	// enforces no IPv6 at all: csf.pl's linefilter silently drops a csf.allow/
	// csf.deny line resolving to an IPv6 address, the TCP6_IN/UDP6_IN port lists
	// go unread, and ip6tables is never flushed on (re)load. AddRule therefore
	// rejects every concrete-IPv6 rule (see ipv6Unavailable) rather than write one
	// csf will never enforce.
	ipv6Enabled bool
}

// NewCSF constructs a CSF manager, verifying the csf service is enabled and its
// config files are present, and reading whether csf's own IPv6 handling is on.
func NewCSF(ctx context.Context, rulePrefix string) (*CSF, error) {
	csf := new(CSF)
	csf.rulePrefix = rulePrefix

	// Confirm csf is enabled under whatever init system the host uses
	// (systemd, chkconfig, update-rc.d, OpenRC, Slackware rc.d, or rc.local).
	if !serviceEnabled(ctx, "csf") {
		return nil, fmt.Errorf("the csf service is not enabled on this server")
	}

	// Confirm config files exist.
	files := []string{CSFConf, CSFAllow, CSFDeny}
	for _, f := range files {
		if _, err := os.Stat(f); err != nil {
			return nil, fmt.Errorf("the config file %s is missing", f)
		}
	}

	// Confirm its not disabled.
	if _, err := os.Stat("/etc/csf/csf.disable"); err == nil {
		return nil, fmt.Errorf("csf is currently disabled")
	}

	// Read whether csf's own IPv6 handling is turned on.
	useIPv6, err := readConfValue(CSFConf, "IPV6")
	if err != nil {
		return nil, fmt.Errorf("error reading %s: %s", CSFConf, err)
	}
	csf.ipv6Enabled = useIPv6 == "1"

	// Return the new csf object.
	return csf, nil
}

// Type reports the backend identifier, "csf".
func (f *CSF) Type() string {
	return CSFType
}

// Capabilities reports the firewall features csf supports.
func (f *CSF) Capabilities() Capabilities {
	return Capabilities{
		Output:  true,
		Forward: true,
		// ICMPv6 mirrors ipv6Enabled: with csf.conf's IPV6 off, csf never touches
		// ip6tables, so neither its native config nor the raw-iptables hook yields a
		// rule csf will keep in sync across a reload (see ipv6Unavailable).
		ICMPv6: f.ipv6Enabled,
		// A csf.conf port list (TCP_IN="80,443,...") stores each port independently
		// and reads back as one rule per port, so a discrete multi-port rule does
		// not round-trip as a single rule (a range, kept as one token, does). Report
		// PortList as unsupported to reflect that; callers open several ports by
		// adding a rule per port, which is how csf stores them anyway.
		PortList:       false,
		ConnState:      true,
		InterfaceMatch: true,
		Logging:        true,
		RateLimit:      true,
		ConnLimit:      true,
		NAT:            true,
		RuleOrdering:   false,
		DefaultPolicy:  false,
		RuleCounters:   false,
		AddressSets:    true,
		Comments:       true,
	}
}

// GetZone reports no zone; csf has no concept of zones.
func (f *CSF) GetZone(ctx context.Context, iface string) (zoneName string, err error) {
	return "", nil
}

// ParseConnLimit decodes a csf.conf CONNLIMIT value ("port;limit,...") into
// connection-limit rules: csf caps concurrent new TCP connections per source and
// rejects the excess with a TCP reset, so each entry becomes an inbound tcp
// reject rule carrying a per-source ConnLimit.
func (f *CSF) ParseConnLimit(val string) (rules []*Rule) {
	for _, entry := range strings.Split(val, ",") {
		entry = strings.TrimSpace(entry)
		if entry == "" {
			continue
		}
		portTok, limitTok, ok := strings.Cut(entry, ";")
		if !ok {
			continue
		}
		port, err := strconv.ParseUint(strings.TrimSpace(portTok), 10, 16)
		if err != nil {
			continue
		}
		limit, err := strconv.ParseUint(strings.TrimSpace(limitTok), 10, 32)
		if err != nil {
			continue
		}
		// CONNLIMIT is a single config key, but csf.pl only installs its IPv6
		// CONNLIMIT rule (ip6tables) when csf.conf's IPV6 is enabled (ConfigServer/
		// Config.pm, csf.pl); on the shipped default (IPV6="0") CONNLIMIT is IPv4
		// only. Report FamilyAny when IPv6 handling is on — so a FamilyAny desired
		// connlimit rule reconciles with its dual-stack read-back rather than
		// churning every Sync — and IPv4 otherwise, matching what csf actually
		// enforces.
		fam := IPv4
		if f.ipv6Enabled {
			fam = FamilyAny
		}
		rules = append(rules, &Rule{
			Family:    fam,
			Proto:     TCP,
			Port:      uint16(port),
			Action:    Reject,
			ConnLimit: &ConnLimit{Count: uint(limit), PerSource: true},
		})
	}
	return
}

// csfAddrFamily parses an address (IP or CIDR) and reports its family, or false
// when the value is not a valid address.
func csfAddrFamily(v string) (Family, bool) {
	cidrIP, _, err := net.ParseCIDR(v)
	ip := net.ParseIP(v)
	if err != nil && ip == nil {
		return FamilyAny, false
	}
	if (cidrIP != nil && cidrIP.To4() == nil) || (ip != nil && ip.To4() == nil) {
		return IPv6, true
	}
	return IPv4, true
}

// parseAddr classifies a csf advanced-rule address field. It returns the
// address, its family, and whether the value is an address at all (a non-address
// value is a port list or ICMP type). A zero "any" network (0.0.0.0/0 or ::/0)
// is normalized to an empty address so a port-only rule written with the "any"
// placeholder round-trips against a rule that carries no address.
func (f *CSF) parseAddr(v string) (addr string, fam Family, ok bool) {
	family, ok := csfAddrFamily(v)
	if !ok {
		return "", FamilyAny, false
	}
	if _, network, err := net.ParseCIDR(v); err == nil {
		if ones, _ := network.Mask.Size(); ones == 0 && network.IP.IsUnspecified() {
			return "", family, true
		}
	}
	return v, family, true
}

// parseAdvPorts parses a csf advanced-rule port value: a comma list whose
// entries are single ports or underscore ranges (e.g. "22,80,2000_3000").
func (f *CSF) parseAdvPorts(val string) ([]PortRange, error) {
	var specs []PortRange
	for _, tok := range strings.Split(val, ",") {
		tok = strings.TrimSpace(tok)
		if tok == "" {
			continue
		}
		lo, hi, isRange := strings.Cut(tok, "_")
		start, err := strconv.ParseUint(strings.TrimSpace(lo), 10, 16)
		if err != nil {
			return nil, fmt.Errorf("invalid port %q", lo)
		}
		pr := PortRange{Start: uint16(start), End: uint16(start)}
		if isRange {
			end, err := strconv.ParseUint(strings.TrimSpace(hi), 10, 16)
			if err != nil {
				return nil, fmt.Errorf("invalid port %q", hi)
			}
			pr.End = uint16(end)
			if pr.End < pr.Start {
				return nil, fmt.Errorf("port range end below start")
			}
		}
		specs = append(specs, pr)
	}
	if len(specs) == 0 {
		return nil, fmt.Errorf("no ports")
	}
	return specs, nil
}

// ParseAdvRule decodes a csf advanced allow/deny rule of the form
// tcp/udp/icmp|in/out|s/d=port(s)|s/d=ip. The port field accepts a comma
// multiport list and underscore ranges; for icmp it holds the ICMP type.
func (f *CSF) ParseAdvRule(val string, action Action) (r *Rule) {
	r = &Rule{
		Action: action,
	}

	fields := strings.Split(val, "|")
	for _, fld := range fields {
		switch {
		case strings.EqualFold(fld, "tcp"):
			r.Proto = TCP
		case strings.EqualFold(fld, "udp"):
			r.Proto = UDP
		case strings.EqualFold(fld, "icmp"):
			r.Proto = ICMP
		case strings.EqualFold(fld, "in"):
			r.Direction = DirInput
		case strings.EqualFold(fld, "out"):
			r.Direction = DirOutput
		case strings.HasPrefix(fld, "s="):
			// The source field is either an address or, when it is not, an ICMP type
			// for icmp rules or a source port list/range otherwise. csf reuses the
			// port position for the ICMP type in both s= and d= (csf.pl maps
			// `s=<n>` to `--icmp-type <n>` for an icmp rule), so mirror the d= branch.
			v := strings.TrimPrefix(fld, "s=")
			if addr, fam, ok := f.parseAddr(v); ok {
				r.Family = fam
				r.Source = addr
				continue
			}
			if r.Proto == ICMP {
				n, ok := parseICMPType(v)
				if !ok {
					return nil
				}
				r.ICMPType = Ptr(n)
				continue
			}
			specs, err := f.parseAdvPorts(v)
			if err != nil {
				return nil
			}
			sourcePortSpecsToRule(r, specs)
		case strings.HasPrefix(fld, "d="):
			v := strings.TrimPrefix(fld, "d=")
			// A destination value is either an address, or (when it is not) an
			// ICMP type for icmp rules or a port list/range otherwise.
			if addr, fam, ok := f.parseAddr(v); ok {
				r.Family = fam
				r.Destination = addr
				continue
			}
			if r.Proto == ICMP {
				n, ok := parseICMPType(v)
				if !ok {
					return nil
				}
				r.ICMPType = Ptr(n)
				continue
			}
			specs, err := f.parseAdvPorts(v)
			if err != nil {
				return nil
			}
			portSpecsToRule(r, specs)
		case strings.HasPrefix(fld, "u=") || strings.HasPrefix(fld, "g="):
			return nil
		}
	}

	return
}

// ParseIPList reads a csf.allow/csf.deny file into rules, stamping each with the
// given action and any full-line comment that precedes it.
func (f *CSF) ParseIPList(filePath string, action Action) (rules []*Rule, err error) {
	// Read the allow/deny IP rule list.
	fd, err := os.Open(filePath)
	if err != nil {
		return nil, err
	}

	scanner := bufio.NewScanner(fd)
	// A full-line comment immediately above a rule is that rule's comment.
	// Consecutive comment lines accumulate (joined by a space); a blank line
	// detaches a comment from any rule that follows.
	var pendingComment string
	flushComment := func() (string, bool) {
		text, hasPrefix := prefixedComment(f.rulePrefix, pendingComment)
		pendingComment = ""
		return text, hasPrefix
	}
	for scanner.Scan() {
		raw := scanner.Text()
		trimmed := strings.TrimSpace(raw)

		// A full-line comment is held as a candidate rule comment.
		if trimmed != "" && strings.HasPrefix(trimmed, "#") {
			text := strings.TrimSpace(strings.TrimPrefix(trimmed, "#"))
			// A prefix tag starts a fresh comment block for the rule that
			// follows, so header/section comments above it are not absorbed into
			// the rule's comment and prefix detection stays reliable.
			if f.rulePrefix != "" && (text == f.rulePrefix || strings.HasPrefix(text, f.rulePrefix+" ")) {
				pendingComment = text
			} else if pendingComment != "" {
				pendingComment += " " + text
			} else {
				pendingComment = text
			}
			continue
		}

		// Strip an inline trailing comment (not a rule comment).
		if ci := strings.IndexByte(trimmed, '#'); ci >= 0 {
			trimmed = trimmed[:ci]
		}
		trimmed = strings.TrimSpace(trimmed)

		// A blank line detaches a pending comment from any later rule.
		if len(trimmed) == 0 {
			pendingComment = ""
			continue
		}

		comment, hasPrefix := flushComment()

		if strings.Contains(trimmed, "|") {
			rule := f.ParseAdvRule(trimmed, action)
			if rule == nil {
				continue
			}
			rule.Comment = comment
			rule.HasPrefix = hasPrefix
			rules = append(rules, rule)
		} else {
			// Try to parse IP.
			family, ok := csfAddrFamily(trimmed)
			if !ok {
				continue
			}

			// A plain IP line matches the host in both directions, so it is one
			// bidirectional DirAny rule, authored in the inbound frame (Source=X).
			rules = append(rules, &Rule{
				Direction: DirAny,
				Family:    family,
				Source:    trimmed,
				Action:    action,
				Comment:   comment,
				HasPrefix: hasPrefix,
			})
		}
	}

	_ = fd.Close()
	if serr := scanner.Err(); serr != nil {
		return nil, serr
	}
	return
}

// ParsePorts decodes a csf.conf port-list value into one accept rule per port
// token for the given family, protocol, and direction.
func (f *CSF) ParsePorts(val string, family Family, proto Protocol, out bool) (rules []*Rule) {
	ports := strings.Split(val, ",")
	for _, port := range ports {
		port = strings.TrimSpace(port)
		if port == "" {
			continue
		}

		// A csf.conf port token is a single port or a colon range.
		pr, err := ParsePortRange(port)
		if err != nil {
			continue
		}
		rule := &Rule{
			Family:    family,
			Proto:     proto,
			Direction: directionFromOutput(out),
			Action:    Accept,
		}
		portSpecsToRule(rule, []PortRange{pr})
		rules = append(rules, rule)
	}
	return
}

// dropActions reads csf.conf's DROP (inbound) and DROP_OUT (outbound)
// settings, which decide whether a csf.deny entry is dropped or rejected: csf
// builds its DENYIN chain with `-j $DROP` and its DENYOUT chain with
// `-j $DROP_OUT`, so a deny rule's effective action follows its direction.
// Only "DROP" and "REJECT" are valid values; anything else (or an unreadable
// file) falls back to stock csf defaults — DROP drops inbound, DROP_OUT rejects
// outbound.
func (f *CSF) dropActions() (dropIn, dropOut Action) {
	dropIn, dropOut = Drop, Reject
	fd, err := os.Open(CSFConf)
	if err != nil {
		return
	}
	defer func() { _ = fd.Close() }()

	scanner := bufio.NewScanner(fd)
	for scanner.Scan() {
		line := scanner.Text()
		if ci := strings.IndexByte(line, '#'); ci >= 0 {
			line = line[:ci]
		}
		key, val, found := strings.Cut(strings.TrimSpace(line), "=")
		if !found {
			continue
		}
		key = strings.TrimSpace(key)
		val = strings.ToUpper(trimQuotes(strings.TrimSpace(val)))
		switch key {
		case "DROP":
			if val == "REJECT" {
				dropIn = Reject
			} else {
				dropIn = Drop
			}
		case "DROP_OUT":
			if val == "DROP" {
				dropOut = Drop
			} else {
				dropOut = Reject
			}
		}
	}
	return
}

// hook returns the managed pre-hook script used to inject iptables rules for
// features csf's native config cannot express.
func (f *CSF) hook() *hookScript {
	return &hookScript{
		rulePrefix: f.rulePrefix,
		hookPath:   CSFHook,
		hookPerm:   0700,
	}
}

// GetRules reads all filter rules from csf's config files and the managed
// pre-hook, merging family and protocol fan-outs back to their written form.
func (f *CSF) GetRules(ctx context.Context, zoneName string) (rules []*Rule, err error) {
	// Read the standard configuration.
	fd, err := os.Open(CSFConf)
	if err != nil {
		return nil, err
	}

	// Scan each line.
	scanner := bufio.NewScanner(fd)
	for scanner.Scan() {
		// Get the line.
		line := scanner.Text()

		// Remove comments.
		ci := strings.IndexByte(line, '#')
		if ci >= 0 {
			line = line[:ci]
		}

		// Trim spaces.
		line = strings.TrimSpace(line)

		// Ignore zero lines.
		if len(line) == 0 {
			continue
		}

		// Parse key/value.
		key, val, found := strings.Cut(line, "=")
		if !found {
			continue
		}
		key = strings.TrimSpace(key)
		val = trimQuotes(strings.TrimSpace(val))

		// Parse rules.
		switch key {
		case "TCP_IN":
			rules = append(rules, f.ParsePorts(val, IPv4, TCP, false)...)
		case "TCP_OUT":
			rules = append(rules, f.ParsePorts(val, IPv4, TCP, true)...)
		case "UDP_IN":
			rules = append(rules, f.ParsePorts(val, IPv4, UDP, false)...)
		case "UDP_OUT":
			rules = append(rules, f.ParsePorts(val, IPv4, UDP, true)...)
		case "TCP6_IN":
			rules = append(rules, f.ParsePorts(val, IPv6, TCP, false)...)
		case "TCP6_OUT":
			rules = append(rules, f.ParsePorts(val, IPv6, TCP, true)...)
		case "UDP6_IN":
			rules = append(rules, f.ParsePorts(val, IPv6, UDP, false)...)
		case "UDP6_OUT":
			rules = append(rules, f.ParsePorts(val, IPv6, UDP, true)...)
		case "CONNLIMIT":
			rules = append(rules, f.ParseConnLimit(val)...)
		}
	}

	_ = fd.Close()
	if err := scanner.Err(); err != nil {
		return nil, err
	}

	// Read the allowed IP rule list.
	ipRules, err := f.ParseIPList(CSFAllow, Accept)
	if err != nil {
		return nil, err
	}
	rules = append(rules, ipRules...)

	// Read the denied IP rule list. A csf.deny entry takes effect as the DROP
	// (inbound) or DROP_OUT (outbound) action from csf.conf, so stamp each rule
	// with the action its direction actually gets rather than a fixed Reject —
	// otherwise a Drop rule the caller manages reads back as Reject, never
	// compares equal to the desired rule, and churns on every Sync.
	dropIn, dropOut := f.dropActions()
	ipRules, err = f.ParseIPList(CSFDeny, dropIn)
	if err != nil {
		return nil, err
	}
	for _, r := range ipRules {
		if r.IsOutput() {
			r.Action = dropOut
		}
	}
	rules = append(rules, ipRules...)

	// Read the iptables rules injected through the csf pre-hook (state,
	// interface, logging, rate-limit, icmpv6).
	hookRules, err := f.hook().getRules()
	if err != nil {
		return nil, err
	}
	rules = append(rules, hookRules...)
	return
}

// confPortToken renders a port spec for a csf.conf port list, where a range
// is written with a colon (e.g. "30000:35000").
func (f *CSF) confPortToken(pr PortRange) string {
	pr = pr.normalized()
	if pr.Start == pr.End {
		return strconv.FormatUint(uint64(pr.Start), 10)
	}
	return fmt.Sprintf("%d:%d", pr.Start, pr.End)
}

// editConnLimit renders the csf.conf CONNLIMIT line with a port's per-source
// limit added or removed, preserving the other entries.
func (f *CSF) editConnLimit(val string, port uint16, limit uint, remove bool) string {
	portStr := strconv.Itoa(int(port))
	var kept []string
	present := false
	for _, tok := range strings.Split(val, ",") {
		tok = strings.TrimSpace(tok)
		if tok == "" {
			continue
		}
		p, _, ok := strings.Cut(tok, ";")
		if ok && strings.TrimSpace(p) == portStr {
			present = true
			if remove {
				continue
			}
			kept = append(kept, fmt.Sprintf("%d;%d", port, limit))
			continue
		}
		kept = append(kept, tok)
	}
	if !remove && !present {
		kept = append(kept, fmt.Sprintf("%d;%d", port, limit))
	}
	return fmt.Sprintf(`CONNLIMIT = "%s"`, strings.Join(kept, ","))
}

// isConnLimitRule reports whether a rule maps onto csf.conf's CONNLIMIT: a
// per-source cap on concurrent new connections to a single inbound TCP port with
// no address. csf's CONNLIMIT chain rejects the excess with a TCP reset
// (`-j REJECT --reject-with tcp-reset`), so the excess action is Reject, not Drop.
func (f *CSF) isConnLimitRule(r *Rule) bool {
	return r.ConnLimit != nil && r.ConnLimit.PerSource &&
		!r.IsOutput() && r.Proto == TCP && r.Source == "" && r.Destination == "" &&
		r.HasPorts() && !r.HasPortSet() && r.Action == Reject
}

// EditRulePort returns the config line for key with the rule's port added or
// removed, leaving lines the rule does not apply to unchanged.
func (f *CSF) EditRulePort(orig, key, val string, r *Rule, remove bool) string {
	// A connection-limit rule is expressed solely through the CONNLIMIT config;
	// it must never also add or remove its port from an accept port list, or
	// RemoveRule would close a port the caller never opened and a round-trip
	// would report a spurious accept rule alongside the connlimit.
	if r.ConnLimit != nil && key != "CONNLIMIT" {
		return orig
	}

	// Determine if this key needs edits.
	switch key {
	case "TCP_IN":
		if r.IsOutput() || r.Family == IPv6 || r.Proto == UDP {
			return orig
		}
	case "TCP_OUT":
		if !r.IsOutput() || r.Family == IPv6 || r.Proto == UDP {
			return orig
		}
	case "UDP_IN":
		if r.IsOutput() || r.Family == IPv6 || r.Proto == TCP {
			return orig
		}
	case "UDP_OUT":
		if !r.IsOutput() || r.Family == IPv6 || r.Proto == TCP {
			return orig
		}
	case "TCP6_IN":
		if r.IsOutput() || r.Family == IPv4 || r.Proto == UDP {
			return orig
		}
	case "TCP6_OUT":
		if !r.IsOutput() || r.Family == IPv4 || r.Proto == UDP {
			return orig
		}
	case "UDP6_IN":
		if r.IsOutput() || r.Family == IPv4 || r.Proto == TCP {
			return orig
		}
	case "UDP6_OUT":
		if !r.IsOutput() || r.Family == IPv4 || r.Proto == TCP {
			return orig
		}
	case "CONNLIMIT":
		// CONNLIMIT tokens are "port;limit", edited independently of the port
		// lists above.
		if !f.isConnLimitRule(r) {
			return orig
		}
		// isConnLimitRule guarantees a single discrete port, which may be
		// carried in either Port or a one-element Ports; read it via PortSpecs so
		// a rule expressing its port through Ports is not written as port 0.
		return f.editConnLimit(val, r.PortSpecs()[0].Start, r.ConnLimit.Count, remove)
	default:
		return orig
	}

	// The rule may carry one or more ports (a single port, a range, or a list).
	// Add or remove each of the rule's port tokens from the config list,
	// preserving any existing tokens the rule does not touch.
	specs := r.PortSpecs()
	present := make(map[string]bool)
	var kept []string
	for _, tok := range strings.Split(val, ",") {
		tok = strings.TrimSpace(tok)
		if tok == "" {
			continue
		}
		// Preserve tokens we cannot parse untouched.
		pr, err := ParsePortRange(tok)
		if err != nil {
			kept = append(kept, tok)
			continue
		}
		if remove && portRangeInSpecs(pr, specs) {
			continue
		}
		kept = append(kept, tok)
		present[f.confPortToken(pr)] = true
	}
	if !remove {
		for _, sp := range specs {
			tok := f.confPortToken(sp)
			if !present[tok] {
				kept = append(kept, tok)
				present[tok] = true
			}
		}
	}

	// Re-create the configuration with new port list.
	return fmt.Sprintf(`%s = "%s"`, key, strings.Join(kept, ","))
}

// EditConf rewrites csf.conf to add or remove a port-list or CONNLIMIT rule.
func (f *CSF) EditConf(ctx context.Context, r *Rule, remove bool) error {
	// For port only rules, open the standard config file.
	fd, err := os.Open(CSFConf)
	if err != nil {
		return err
	}

	// Stage the rewrite, preserving csf.conf's mode and ownership.
	af, err := newAtomicFile(CSFConf, 0644)
	if err != nil {
		_ = fd.Close()
		return err
	}
	defer af.Abort()

	// Parse config one line at a time, adding the port rule.
	scanner := bufio.NewScanner(fd)
	for scanner.Scan() {
		// Get the line.
		orig := scanner.Text()
		line := orig[:]

		// Remove comments.
		ci := strings.IndexByte(line, '#')
		if ci >= 0 {
			line = line[:ci]
		}

		// Trim spaces.
		line = strings.TrimSpace(line)

		// Ignore zero lines.
		if len(line) == 0 {
			_, _ = fmt.Fprintln(af, orig)
			continue
		}

		// Parse key/value.
		key, val, found := strings.Cut(line, "=")
		if !found {
			_, _ = fmt.Fprintln(af, orig)
			continue
		}
		key = strings.TrimSpace(key)
		val = trimQuotes(strings.TrimSpace(val))

		// Parse rules.
		orig = f.EditRulePort(orig, key, val, r, remove)
		_, _ = fmt.Fprintln(af, orig)
	}

	_ = fd.Close()

	// A read error means the rewritten file is truncated; discard it.
	if serr := scanner.Err(); serr != nil {
		return serr
	}

	// Move new file into place, preserving mode and ownership.
	return af.Commit()
}

// advPortValue renders port specs for a csf advanced rule, which uses a comma
// list and an underscore range (e.g. "22,80,2000_3000").
func (f *CSF) advPortValue(specs []PortRange) string {
	parts := make([]string, len(specs))
	for i, pr := range specs {
		pr = pr.normalized()
		if pr.Start == pr.End {
			parts[i] = strconv.FormatUint(uint64(pr.Start), 10)
		} else {
			parts[i] = fmt.Sprintf("%d_%d", pr.Start, pr.End)
		}
	}
	return strings.Join(parts, ",")
}

// MarshalAdvRule encodes a rule as a csf advanced allow/deny line. A csf
// advanced rule must carry a source or destination address.
func (f *CSF) MarshalAdvRule(r *Rule) (string, error) {
	if r.Proto == ICMPv6 {
		return "", fmt.Errorf("csf advanced rules do not support icmpv6")
	}
	if r.Source == "" && r.Destination == "" {
		return "", fmt.Errorf("a csf advanced rule requires a source or destination address")
	}
	// A csf advanced rule holds a single address field, so a rule matching both a
	// source and a destination cannot be expressed; reject it rather than silently
	// dropping the destination (installing a broader rule and leaving it
	// unremovable). Mirrors the dual-port guard below.
	if r.Source != "" && r.Destination != "" {
		return "", fmt.Errorf("csf advanced rules cannot match both a source and destination address")
	}
	// A csf advanced rule carries a single port-flow field, so a rule cannot match
	// both a source and a destination port at once.
	if r.HasPorts() && r.HasSourcePorts() {
		return "", fmt.Errorf("csf advanced rules cannot match both a source and destination port")
	}
	// An ICMP advanced rule needs a concrete type for the port-flow field: without
	// one the address would land there and csf would parse it as `--icmp-type <ip>`
	// and drop the rule (see checkICMP). Refuse to emit such a line.
	if r.Proto == ICMP && r.ICMPType == nil {
		return "", fmt.Errorf("a csf icmp advanced rule requires a concrete icmp type")
	}
	// A single advanced line carries a single protocol token, so a TCPUDP port rule
	// with an address cannot be expressed here: a protocol-less line is defaulted to
	// `-p tcp` by csf.pl's linefilter, applying the rule to TCP only while the
	// library reads it back as both transports — leaving UDP open on a deny (or
	// unallowed on an accept). The address-less form is fanned into a tcp and a udp
	// line by the caller (portOnlyDenyLines); one line cannot fan, so reject it here
	// rather than under-apply it.
	if r.Proto == TCPUDP {
		return "", fmt.Errorf("a csf advanced rule cannot carry both transports; expand it to tcp and udp lines: %w", ErrUnsupported)
	}
	// csf.pl's linefilter reads an advanced line by fixed field position, not by
	// tag: it always looks for the port-flow field (d=/s=) before the address
	// field. A tcp/udp rule with an address but no port shifts the address into
	// the port-flow slot instead, where it is parsed as a garbage --sport/--dport
	// value and the address field is left empty — linefilter then requires both
	// an address and a port to install anything, so the rule is silently dropped.
	if r.Proto != ICMP && !r.HasPorts() && !r.HasSourcePorts() {
		return "", fmt.Errorf("a csf tcp/udp advanced rule with an address requires a port: %w", ErrUnsupported)
	}

	var parts []string
	switch r.Proto {
	case TCP:
		parts = append(parts, "tcp")
	case UDP:
		parts = append(parts, "udp")
	case ICMP:
		parts = append(parts, "icmp")
	}
	if r.IsOutput() {
		parts = append(parts, "out")
	} else {
		parts = append(parts, "in")
	}

	// The port-flow field: an ICMP type, a source port, or a destination port.
	switch {
	case r.Proto == ICMP:
		if r.ICMPType != nil {
			parts = append(parts, fmt.Sprintf("d=%d", *r.ICMPType))
		}
	case r.HasSourcePorts():
		parts = append(parts, "s="+f.advPortValue(r.SourcePortSpecs()))
	case r.HasPorts():
		parts = append(parts, "d="+f.advPortValue(r.PortSpecs()))
	}

	// Address.
	if r.Source != "" {
		parts = append(parts, "s="+r.Source)
	} else if r.Destination != "" {
		parts = append(parts, "d="+r.Destination)
	}

	return strings.Join(parts, "|"), nil
}

// advMatch reports whether a parsed advanced-rule line (the existing row) matches
// target. A TCPUDP port rule is written as a separate tcp line and udp line, so a
// TCPUDP target matches either concrete-transport line; that coverage, like the
// family and direction coverage the add and remove paths need, is folded into
// EqualForDedup / EqualForRemoval.
func (f *CSF) advMatch(parsed, target *Rule, remove bool) bool {
	if remove {
		return parsed.EqualForRemoval(target, true)
	}
	return parsed.EqualForDedup(target, true)
}

// portOnlyDenyLines returns the advanced csf.deny lines a port-only deny (no
// address) fans out to: one per transport (tcp and udp for a TCPUDP rule) and per
// family placeholder (0.0.0.0/0 for IPv4, ::/0 for IPv6, both for a family-neutral
// rule). It mirrors the emit path so the add logic can compare against the lines
// already in the file and write only the missing ones — a single "exists" flag
// would skip the whole fan-out when just one family/protocol line was already
// present, leaving the other family/protocol open.
func (f *CSF) portOnlyDenyLines(r *Rule) []string {
	dir := "in"
	if r.IsOutput() {
		dir = "out"
	}
	port := f.advPortValue(r.PortSpecs())
	var protos []string
	for _, sub := range expandProtocols(r) {
		protos = append(protos, sub.Proto.String())
	}
	var placeholders []string
	switch r.impliedFamily() {
	case IPv6:
		placeholders = []string{"::/0"}
	case IPv4:
		placeholders = []string{"0.0.0.0/0"}
	default:
		placeholders = []string{"0.0.0.0/0", "::/0"}
	}
	var lines []string
	for _, ph := range placeholders {
		for _, p := range protos {
			tokens := []string{p, dir, "d=" + port}
			if r.IsOutput() {
				tokens = append(tokens, "d="+ph)
			} else {
				tokens = append(tokens, "s="+ph)
			}
			lines = append(lines, strings.Join(tokens, "|"))
		}
	}
	return lines
}

// EditIPList adds or removes a rule in a csf.allow/csf.deny list, rewriting it in
// place. A port-only deny fans out per transport and family placeholder, so a
// TCPUDP rule reads back as the rule that was written rather than being re-added on
// every reconcile and never matched for removal.
func (f *CSF) EditIPList(ctx context.Context, filePath string, action Action, r *Rule, remove bool) error {
	// Read the allow/deny IP rule list.
	fd, err := os.Open(filePath)
	if err != nil {
		return err
	}

	// Stage the rewrite, preserving the list file's mode and ownership.
	af, err := newAtomicFile(filePath, 0644)
	if err != nil {
		_ = fd.Close()
		return err
	}
	defer af.Abort()

	scanner := bufio.NewScanner(fd)
	exists := false
	// A port-only deny (no address) fans out into several csf.deny lines across
	// family and protocol. Track which of those lines are already in the file so the
	// add path can write only the missing ones: the single "exists" flag below marks
	// the whole rule present as soon as any one fan-out line matches, which would
	// otherwise leave the other family/protocol open. wantDeny is empty for every
	// other rule shape, so this tracking is inert unless the fan-out applies.
	wantDeny := map[string]bool{}
	if !remove && action != Accept && r.HasPorts() && r.Source == "" && r.Destination == "" {
		for _, l := range f.portOnlyDenyLines(r) {
			wantDeny[l] = true
		}
	}
	presentDeny := map[string]bool{}
	// csf.allow/csf.deny encode no action of their own — the file decides it
	// (csf.allow is accept, csf.deny is a deny). A rule read from a file is stamped
	// with that file's action, so match an incoming rule with its action coerced
	// the same way: otherwise a rule added as Drop (written to csf.deny, read back
	// as the deny action) could never be found and removed.
	match := *r
	match.Action = action
	// Full-line comments immediately above a rule are held back so they can be
	// dropped together with a removed rule (they are its comment) and written
	// ahead of a kept one. A blank line detaches them.
	var pending []string
	flush := func() {
		for _, c := range pending {
			_, _ = fmt.Fprintln(af, c)
		}
		pending = nil
	}
	drop := func() { pending = nil }
	for scanner.Scan() {
		orig := scanner.Text()
		trimmed := strings.TrimSpace(orig)

		// A full-line comment is held as a candidate rule comment.
		if trimmed != "" && strings.HasPrefix(trimmed, "#") {
			// Mirror ParseIPList: a prefix tag starts a fresh comment block, so
			// any header/section comments above it are not part of the rule's comment
			// and must survive its removal. Flush them now and begin the rule's block
			// at the tag, so drop() only discards the tag and the rule's own comment.
			if f.rulePrefix != "" {
				if text := strings.TrimSpace(strings.TrimPrefix(trimmed, "#")); text == f.rulePrefix || strings.HasPrefix(text, f.rulePrefix+" ") {
					flush()
				}
			}
			pending = append(pending, orig)
			continue
		}

		// Strip an inline trailing comment for matching, but preserve the
		// original line (with its inline note) when copying it through.
		line := trimmed
		if ci := strings.IndexByte(line, '#'); ci >= 0 {
			line = line[:ci]
		}
		line = strings.TrimSpace(line)

		// A blank line detaches a pending comment; write it and the blank.
		if len(line) == 0 {
			flush()
			_, _ = fmt.Fprintln(af, orig)
			continue
		}

		// Note a fan-out line that is already present so the add path below skips it
		// and writes only the missing family/protocol lines. Every such line is
		// preserved (written back) by the pass-through branches below.
		if wantDeny[line] {
			presentDeny[line] = true
		}

		if strings.Contains(line, "|") {
			rule := f.ParseAdvRule(line, action)
			if rule == nil {
				flush()
				_, _ = fmt.Fprintln(af, orig)
				continue
			}
			if f.advMatch(rule, &match, remove) {
				exists = true
				if !remove {
					flush()
					_, _ = fmt.Fprintln(af, orig)
				} else {
					drop()
				}
			} else {
				flush()
				_, _ = fmt.Fprintln(af, orig)
			}
		} else {
			// Try to parse IP.
			family, ok := csfAddrFamily(line)
			if !ok {
				flush()
				_, _ = fmt.Fprintln(af, orig)
				continue
			}

			// A plain IP line is one bidirectional DirAny rule; match the target
			// against it in the inbound frame (canonicalMatch), so a DirAny or a
			// concrete-direction input/output target that names this host lines up.
			plainRule := &Rule{
				Direction: DirAny,
				Family:    family,
				Source:    line,
				Action:    action,
			}
			if plainRule.EqualBase(match.canonicalMatch(), false) {
				exists = true
				if !remove {
					flush()
					_, _ = fmt.Fprintln(af, orig)
				} else {
					drop()
				}
			} else {
				flush()
				_, _ = fmt.Fprintln(af, orig)
			}
		}
	}
	// Write any trailing comments that followed the last rule.
	flush()

	// If not exists and not remove, try adding the rule.
	if !exists && !remove {
		writeComment := func() {
			if c := combineComment(f.rulePrefix, r.Comment); c != "" {
				_, _ = fmt.Fprintln(af, "# "+c)
			}
		}
		hasIP := r.Source != "" || r.Destination != ""
		switch {
		case hasIP && (r.HasPorts() || r.HasSourcePorts() || r.Proto.IsICMP()):
			// A port/ICMP rule with an address is an advanced rule.
			line, err := f.MarshalAdvRule(r)
			if err != nil {
				_ = fd.Close()
				return err
			}
			writeComment()
			_, _ = fmt.Fprintln(af, line)
		case hasIP:
			// A bare all-protocol host allow/deny: a single address matching every
			// protocol. csf.allow/csf.deny hold no other portless address shape — a
			// concrete-protocol host or a source+destination pair — so AddRule diverts
			// those to the raw-iptables hook (hostNeedsHook) and never reaches here
			// with one. A direct caller of this exported writer that supplies such a
			// shape gets a best-effort single-address write, not a guard.
			writeComment()
			if r.Source != "" {
				_, _ = fmt.Fprintln(af, r.Source)
			} else {
				_, _ = fmt.Fprintln(af, r.Destination)
			}
		}
	}

	// A port-only deny (no address) fans out into a csf.deny line per family and
	// protocol. Unlike the single-line cases above it must NOT be gated on the
	// whole-rule "exists" flag: when only a subset of the fan-out lines is already
	// present (e.g. the IPv4 line but not the IPv6 one, from a prior single-family
	// add or a manual edit), the missing lines must still be written or that
	// family/protocol stays open while the library reports the port blocked. Emit
	// only the lines not already present (presentDeny, noted during the scan).
	//
	// csf's advanced-rule handler only emits an iptables rule when the line carries
	// an address, so each fan-out line uses the "any" network placeholder (0.0.0.0/0
	// or ::/0) as the address; parseAddr normalizes it back to an empty address on
	// read, so each family's line reads back as its own rule. The transport is named
	// explicitly (a
	// protocol-less line defaults to tcp in csf's linefilter), and a TCPUDP deny fans
	// to both tcp and udp.
	if len(wantDeny) > 0 {
		writeComment := func() {
			if c := combineComment(f.rulePrefix, r.Comment); c != "" {
				_, _ = fmt.Fprintln(af, "# "+c)
			}
		}
		for _, line := range f.portOnlyDenyLines(r) {
			if presentDeny[line] {
				continue
			}
			writeComment()
			_, _ = fmt.Fprintln(af, line)
		}
	}

	_ = fd.Close()

	// A read error means the rewritten file is truncated; discard it.
	if serr := scanner.Err(); serr != nil {
		return serr
	}

	// Move new file into place, preserving mode and ownership.
	return af.Commit()
}

// checkConnLimit rejects a connection-limit request csf's CONNLIMIT cannot
// express, so an inexpressible one is reported rather than silently dropped.
func (f *CSF) checkConnLimit(r *Rule) error {
	if r.ConnLimit == nil || f.isConnLimitRule(r) {
		return nil
	}
	return fmt.Errorf("csf connection limiting (CONNLIMIT) applies only to a single inbound tcp port with no address, per source, rejecting the excess with a tcp reset: %w", ErrUnsupportedConnLimit)
}

// checkICMP rejects ICMP rules csf cannot express: csf advanced rules are
// built on iptables ICMP (IPv4) and require an address, and must also carry a
// concrete type: csf's linefilter treats the single port-flow field as the
// icmp-type, so an address with no type would put the address in that field
// (`--icmp-type <ip>`), which csf then fails to parse and drops silently (csf.pl
// linefilter). There is no csf advanced-rule encoding for "any icmp type from an
// address" — a bare host rule already covers all protocols — so reject it rather
// than emit a dropped line. ICMPv6 never reaches this check: ruleNeedsHook routes it
// to the pre-hook (or the IPv6 gate rejects it) before addRule/RemoveRule call checkICMP.
func (f *CSF) checkICMP(r *Rule) error {
	if r.Proto == ICMP {
		if r.Source == "" && r.Destination == "" {
			return fmt.Errorf("a csf icmp rule requires a source or destination address: %w", ErrUnsupported)
		}
		if r.ICMPType == nil {
			return fmt.Errorf("a csf icmp rule with an address requires a concrete icmp type: %w", ErrUnsupported)
		}
	}
	return nil
}

// checkPortProto rejects a port match on a protocol csf cannot express as a port.
// csf's port lists are TCP_IN/UDP_IN only, so a port on a concrete non-tcp/udp
// protocol (e.g. sctp) would otherwise be wrongly written into BOTH lists, and a
// port on ProtocolAny — which matches every IP protocol, not just the two csf can
// carry — has no faithful form at all. TCPUDP is allowed: it fans out to both lists
// on an address-less accept, and to a tcp and a udp csf.deny advanced line
// otherwise.
func (f *CSF) checkPortProto(r *Rule) error {
	switch r.Proto {
	case TCP, UDP, TCPUDP:
		return nil
	}
	if r.HasPorts() || r.HasSourcePorts() {
		return fmt.Errorf("csf requires a tcp or udp protocol for a port match: %w", ErrUnsupported)
	}
	return nil
}

// checkSourcePort rejects a source-port match csf cannot express. csf source
// ports live in an advanced rule, which requires an address, so a source-port
// rule without one has nowhere to go.
func (f *CSF) checkSourcePort(r *Rule) error {
	if r.HasSourcePorts() && r.Source == "" && r.Destination == "" {
		return fmt.Errorf("a csf source-port rule requires a source or destination address: %w", ErrUnsupportedSourcePort)
	}
	return nil
}

// denyAction returns the action a csf.deny entry takes in the given direction,
// following csf.conf's DROP (inbound) / DROP_OUT (outbound) settings. A deny
// rule this library writes must carry exactly this action: csf.deny encodes no
// action of its own, so a rule read back is stamped with what csf would apply,
// and a caller asking for the opposite action could never reconcile against it.
func (f *CSF) denyAction(output bool) Action {
	dropIn, dropOut := f.dropActions()
	if output {
		return dropOut
	}
	return dropIn
}

// addRule is AddRule's implementation, with the IPv6 gate optional so Restore
// can reproduce a prior snapshot's inert entries rather than be rejected by a
// gate meant to catch fresh no-op writes.
func (f *CSF) addRule(ctx context.Context, zoneName string, r *Rule, enforceIPv6Gate bool) error {
	// Reject a concrete-IPv6 rule when csf's own IPv6 handling is off, ahead of every
	// routing decision below: neither csf's config nor the pre-hook can carry one that
	// csf will keep in sync (see ipv6Unavailable). Checking here rather than past the
	// hook branches also keeps a DirAny rule from writing its input half before its
	// output half is rejected.
	if enforceIPv6Gate && ipv6Unavailable(f.ipv6Enabled, r) {
		return fmt.Errorf("csf's IPv6 handling is disabled (csf.conf IPV6 is not \"1\"): %w", ErrUnsupported)
	}

	// A DirAny rule maps to a single native construct only as a bare-host plain line;
	// every other DirAny shape fans out into a concrete input rule plus its swapped
	// output rule, each routed independently (a half may itself need the hook).
	if r.Direction == DirAny && !dirAnyPlainLine(r) {
		for _, sub := range expandDirections(r) {
			if err := f.addRule(ctx, zoneName, sub, enforceIPv6Gate); err != nil {
				return err
			}
		}
		return nil
	}

	// csf has no both-transports construct: its port lists are a TCP list and a UDP
	// list, and csf.pl's linefilter silently reads a protocol-less advanced line as
	// `-p tcp`. So a TCPUDP rule fans out into a tcp rule and a udp rule, each routed
	// independently, and each reads back as its own rule. Unlike apf — whose trust
	// parser derives both transports from one protocol-less line — csf cannot express
	// the pair in a single line anywhere.
	if r.Proto == TCPUDP {
		for _, sub := range expandProtocols(r) {
			if err := f.addRule(ctx, zoneName, sub, enforceIPv6Gate); err != nil {
				return err
			}
		}
		return nil
	}

	// Features csf's native config cannot express (connection-state, per-rule
	// interface, logging, rate limiting, icmpv6) are injected as iptables rules
	// through the csf pre-hook.
	if ruleNeedsHook(r) {
		_, err := f.hook().edit(r, false)
		return err
	}
	// A one-way bare host allow/deny cannot be a plain csf line (bidirectional) nor
	// an advanced rule (needs a port), so it is written to the hook. A DirAny bare
	// host takes the plain-line path below.
	if bareHostOneWay(r) {
		_, err := f.hook().edit(r, false)
		return err
	}
	// A concrete-protocol host or a source+destination pair likewise has no csf.allow/
	// csf.deny form (see hostNeedsHook), so it too is injected as a raw iptables rule.
	if hostNeedsHook(r) {
		_, err := f.hook().edit(r, false)
		return err
	}
	// A bare protocol match with no address and no port has no native csf construct —
	// csf.conf keys on a port, an advanced rule on address+port, and csf.allow/csf.deny
	// on an address — but iptables expresses it directly, so it is injected through the
	// pre-hook (see bareProtoNeedsHook). ICMP keeps its own handling (checkICMP) and is
	// excluded there.
	if bareProtoNeedsHook(r) {
		_, err := f.hook().edit(r, false)
		return err
	}
	if err := f.checkSourcePort(r); err != nil {
		return err
	}
	if err := f.checkConnLimit(r); err != nil {
		return err
	}
	if err := f.checkICMP(r); err != nil {
		return err
	}
	if err := f.checkPortProto(r); err != nil {
		return err
	}

	// A connection-limit rule maps onto the csf.conf CONNLIMIT list.
	if r.ConnLimit != nil {
		return f.EditConf(ctx, r, false)
	}

	// A port-only accept maps to a csf.conf port list rather than csf.allow.
	if r.Source == "" && r.Destination == "" && r.HasPorts() && r.Action == Accept {
		err := f.EditConf(ctx, r, false)
		if err != nil {
			return err
		}
	}

	// Edit csf.allow if accept is the action, otherwise edit csf.deny. A csf.deny
	// entry carries no action of its own — csf applies csf.conf's action by direction
	// (DROP inbound, DROP_OUT outbound) — so a deny whose action matches is written
	// natively, while one that differs has no native form and is injected through the
	// pre-hook instead, whose iptables rule carries the exact action. A DirAny bare-host
	// deny is expanded to its two concrete directions first, since each hook line is
	// one-way.
	if r.Action == Accept {
		err := f.EditIPList(ctx, CSFAllow, Accept, r, false)
		if err != nil {
			return err
		}
	} else {
		denyAction := f.denyAction(r.IsOutput())
		if r.Action != denyAction {
			for _, sub := range expandDirections(r) {
				if _, err := f.hook().edit(sub, false); err != nil {
					return err
				}
			}
			return nil
		}
		err := f.EditIPList(ctx, CSFDeny, denyAction, r, false)
		if err != nil {
			return err
		}
	}
	return nil
}

// AddRule adds a filter rule to the appropriate csf construct: a csf.conf port
// list, an advanced rule, a bare address list, CONNLIMIT, or the pre-hook.
func (f *CSF) AddRule(ctx context.Context, zoneName string, r *Rule) error {
	return f.addRule(ctx, zoneName, r, true)
}

// InsertRule is unsupported: CSF organizes rules in config files, not an ordered list.
func (f *CSF) InsertRule(ctx context.Context, zoneName string, position int, r *Rule) error {
	return unsupportedOrdering(f.Type())
}

// MoveRule is unsupported for the same reason as InsertRule.
func (f *CSF) MoveRule(ctx context.Context, zoneName string, r *Rule, position int) error {
	return unsupportedOrdering(f.Type())
}

// removePlainHost drops the bidirectional plain csf.allow/csf.deny line backing the
// DirAny rule e, choosing the list by the rule's action.
func (f *CSF) removePlainHost(ctx context.Context, e *Rule) error {
	if e.Action == Accept {
		return f.EditIPList(ctx, CSFAllow, Accept, e, true)
	}
	return f.EditIPList(ctx, CSFDeny, f.denyAction(false), e, true)
}

// removeBareHostOneWay removes a one-way bare-address host rule. Such a rule is
// stored either as its own hook rule or as one direction of a bidirectional plain
// csf.allow/csf.deny line (a DirAny rule). When a matching plain line exists, split
// it: drop the line and re-add the surviving opposite direction as a hook rule so
// the untargeted direction keeps its coverage.
func (f *CSF) removeBareHostOneWay(ctx context.Context, zoneName string, r *Rule) error {
	existing, err := f.GetRules(ctx, zoneName)
	if err != nil {
		return err
	}
	for _, e := range existing {
		if !e.IsAny() || !e.EqualForRemoval(r, true) {
			continue
		}
		// The host is stored as a bidirectional plain line; drop it, then re-add the
		// surviving direction as a hook rule.
		if err := f.removePlainHost(ctx, e); err != nil {
			return err
		}
		if s := splitDualRowDirection(e, r); s != nil {
			_, err := f.hook().edit(s, false)
			return err
		}
		return nil
	}
	// Not stored as a plain line; remove the one-way hook rule.
	_, err = f.hook().edit(r, true)
	return err
}

// RemoveRule removes a filter rule from whichever csf construct holds it.
func (f *CSF) RemoveRule(ctx context.Context, zoneName string, r *Rule) error {
	// A non-plain-line DirAny target fans out into its two concrete-direction rules,
	// mirroring addRule, so each half is removed from wherever it was written.
	if r.Direction == DirAny && !dirAnyPlainLine(r) {
		for _, sub := range expandDirections(r) {
			if err := f.RemoveRule(ctx, zoneName, sub); err != nil {
				return err
			}
		}
		return nil
	}

	// A TCPUDP target fans out into its two concrete-transport rules, mirroring
	// addRule, so each is removed from whichever list or line it was written to. A
	// caller removing one transport targets that transport directly and leaves the
	// other in place.
	if r.Proto == TCPUDP {
		for _, sub := range expandProtocols(r) {
			if err := f.RemoveRule(ctx, zoneName, sub); err != nil {
				return err
			}
		}
		return nil
	}

	// Hook-injected rules (see AddRule) are removed from the managed script.
	if ruleNeedsHook(r) {
		_, err := f.hook().edit(r, true)
		return err
	}
	// A one-way bare host rule is stored either as its own hook rule or as one
	// direction of a bidirectional plain line; removing it may need to split the
	// plain line (see removeBareHostOneWay).
	if bareHostOneWay(r) {
		return f.removeBareHostOneWay(ctx, zoneName, r)
	}
	// A concrete-protocol host or source+destination pair is stored only as its own
	// hook rule (it has no plain-line form to split), so remove it directly.
	if hostNeedsHook(r) {
		_, err := f.hook().edit(r, true)
		return err
	}
	// A bare protocol match is stored only as its own hook rule (see AddRule), so
	// remove it from the managed script directly. ICMP keeps its own handling below.
	if bareProtoNeedsHook(r) {
		_, err := f.hook().edit(r, true)
		return err
	}
	if err := f.checkSourcePort(r); err != nil {
		return err
	}
	if err := f.checkConnLimit(r); err != nil {
		return err
	}
	if err := f.checkICMP(r); err != nil {
		return err
	}
	if err := f.checkPortProto(r); err != nil {
		return err
	}

	// This rule is natively expressible, but a copy of it may nonetheless live in the
	// hook — the library's own (a deny whose action differs from csf.conf's is stored
	// there, see AddRule) or one a customer added by hand for a shape csf can also
	// express natively. Clear any hook copy before removing the native entry, so a rule
	// present in both csf's config and the hook is removed from both and a hook-only
	// deny is fully removed here. DirAny is expanded so both one-way hook lines are
	// matched; a rule with no hook copy makes this a harmless no-op.
	for _, sub := range expandDirections(r) {
		if _, err := f.hook().edit(sub, true); err != nil {
			return err
		}
	}

	// A connection-limit rule maps onto the csf.conf CONNLIMIT list.
	if r.ConnLimit != nil {
		return f.EditConf(ctx, r, true)
	}

	// A port-only accept maps to a csf.conf port list rather than csf.allow.
	if r.Source == "" && r.Destination == "" && r.HasPorts() && r.Action == Accept {
		err := f.EditConf(ctx, r, true)
		if err != nil {
			return err
		}
	}

	// Edit csf.allow if accept is the action, otherwise edit csf.deny. A deny whose
	// action differs from csf.conf's action for its direction was only ever in the hook
	// (cleared above), so return rather than match it against csf.deny — where it could
	// coincide with a genuine matching-action entry. A matching-action deny lives in
	// csf.deny.
	if r.Action == Accept {
		err := f.EditIPList(ctx, CSFAllow, Accept, r, true)
		if err != nil {
			return err
		}
	} else {
		denyAction := f.denyAction(r.IsOutput())
		if r.Action != denyAction {
			return nil
		}
		err := f.EditIPList(ctx, CSFDeny, denyAction, r, true)
		if err != nil {
			return err
		}
	}
	return nil
}

// UnmarshalNATRule decodes a csf.redirect line into a NATRule.
func (f *CSF) UnmarshalNATRule(line string) *NATRule {
	fields := strings.Split(line, "|")
	if len(fields) != 5 {
		return nil
	}
	ipx, porta, ipy, portb, proto := fields[0], fields[1], fields[2], fields[3], fields[4]

	parsePort := func(s string) (uint16, bool) {
		if s == "*" || s == "" {
			return 0, true
		}
		n, err := strconv.ParseUint(strings.TrimSpace(s), 10, 16)
		if err != nil {
			return 0, false
		}
		return uint16(n), true
	}

	r := &NATRule{Proto: GetProtocol(proto)}
	if r.Proto != TCP && r.Proto != UDP {
		return nil
	}
	if ipx != "*" && ipx != "" {
		if _, ok := csfAddrFamily(ipx); !ok {
			return nil
		}
		r.Destination = ipx
	}
	pa, ok := parsePort(porta)
	if !ok {
		return nil
	}
	r.Port = pa
	pb, ok := parsePort(portb)
	if !ok {
		return nil
	}
	r.ToPort = pb

	if ipy == "*" || ipy == "" {
		r.Kind = Redirect
		if r.ToPort == 0 || r.Port == 0 {
			return nil
		}
	} else {
		fam, ok := csfAddrFamily(ipy)
		if !ok {
			return nil
		}
		r.Kind = DNAT
		r.ToAddress = ipy
		r.Family = fam
	}
	if r.Family == FamilyAny {
		r.Family = r.impliedFamily()
	}
	return r
}

// GetNATRules reads the NAT rules from csf.redirect.
func (f *CSF) GetNATRules(ctx context.Context, zoneName string) ([]*NATRule, error) {
	fd, err := os.Open(CSFRedirect)
	if err != nil {
		// csf.redirect is optional; a missing file simply has no rules.
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	defer func() { _ = fd.Close() }()

	var rules []*NATRule
	scanner := bufio.NewScanner(fd)
	for scanner.Scan() {
		line := scanner.Text()
		if ci := strings.IndexByte(line, '#'); ci >= 0 {
			line = line[:ci]
		}
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		if r := f.UnmarshalNATRule(line); r != nil {
			rules = append(rules, r)
		}
	}
	if err := scanner.Err(); err != nil {
		return nil, err
	}
	// csf.redirect is CSF's own NAT config with no per-rule prefix marker, so no
	// rule in it carries the configured prefix; HasPrefix stays false (mirroring
	// firewalld's zones).
	return rules, nil
}

// redirectAddr renders an address for a csf.redirect field, using "*" for an
// empty (any) address.
func (f *CSF) redirectAddr(a string) string {
	if a == "" {
		return "*"
	}
	return a
}

// redirectPort renders a single port for a csf.redirect field, using "*" for
// an unset (0) port, which csf reads as "any/unchanged".
func (f *CSF) redirectPort(p uint16) string {
	if p == 0 {
		return "*"
	}
	return strconv.FormatUint(uint64(p), 10)
}

// MarshalNATRule encodes a NAT rule as a csf.redirect line
// ("IPx|portA|IPy|portB|proto"). csf.redirect expresses only destination NAT: a
// Redirect to a local port (IPy = "*") or a DNAT forward to another host
// (IPy = ToAddress). Source NAT, port ranges/lists, and non-tcp/udp protocols
// are not representable.
func (f *CSF) MarshalNATRule(r *NATRule) (string, error) {
	if err := r.validate(); err != nil {
		return "", err
	}
	if r.Kind.isSource() {
		return "", fmt.Errorf("csf.redirect cannot express source NAT: %w", ErrUnsupportedNAT)
	}
	if r.Proto != TCP && r.Proto != UDP {
		return "", fmt.Errorf("csf.redirect requires a tcp or udp protocol")
	}
	if r.HasPortSet() {
		return "", fmt.Errorf("csf.redirect matches a single port, not a range or list")
	}
	if r.Source != "" {
		return "", fmt.Errorf("csf.redirect cannot match a source address")
	}

	ipx := f.redirectAddr(r.Destination)
	porta := f.redirectPort(r.Port)
	switch r.Kind {
	case Redirect:
		// A local port redirect: IPy is "*", portB is the target local port.
		if r.Port == 0 {
			return "", fmt.Errorf("a csf redirect requires a matched port")
		}
		return strings.Join([]string{ipx, porta, "*", f.redirectPort(r.ToPort), r.Proto.String()}, "|"), nil
	case DNAT:
		// A forward to another host: IPy is the translation address. csf.redirect
		// accepts a DNAT only in two shapes (csf.pl "Invalid csf.redirect format"
		// otherwise): a full-IP forward (concrete IPx, both ports "*") or a port
		// forward (concrete IPx and both ports concrete). Reject the shapes csf
		// would refuse rather than emit a line that aborts the whole redirect load —
		// the line still parses back here, so a round-trip check alone misses it.
		if r.Destination == "" {
			return "", fmt.Errorf("csf.redirect requires a destination address for a forward: %w", ErrUnsupportedNAT)
		}
		if (r.Port == 0) != (r.ToPort == 0) {
			return "", fmt.Errorf("csf.redirect forward requires both a matched and a target port, or neither: %w", ErrUnsupportedNAT)
		}
		return strings.Join([]string{ipx, porta, r.ToAddress, f.redirectPort(r.ToPort), r.Proto.String()}, "|"), nil
	}
	return "", fmt.Errorf("csf.redirect cannot express this nat kind: %w", ErrUnsupportedNAT)
}

// editRedirect adds or removes a csf.redirect line, returning without change when
// an add is a duplicate or a remove finds no match.
func (f *CSF) editRedirect(r *NATRule, remove bool) error {
	line, err := f.MarshalNATRule(r)
	if err != nil {
		return err
	}

	data, err := os.ReadFile(CSFRedirect)
	if err != nil {
		if os.IsNotExist(err) {
			if remove {
				return nil
			}
			data = nil
		} else {
			return err
		}
	}
	lines := strings.Split(string(data), "\n")
	// Drop the trailing empty element left by a final newline so repeated adds do
	// not accumulate blank lines.
	if len(lines) > 0 && lines[len(lines)-1] == "" {
		lines = lines[:len(lines)-1]
	}

	out := make([]string, 0, len(lines)+1)
	found := false
	for _, raw := range lines {
		body := raw
		if ci := strings.IndexByte(body, '#'); ci >= 0 {
			body = body[:ci]
		}
		body = strings.TrimSpace(body)
		if body != "" {
			// Keep the match family-aware (EqualForRemoval): a family-scoped removal
			// must not drop an opposite-family twin sharing this file (mirrors the
			// filter-rule and pf/nft NAT family gates).
			if existing := f.UnmarshalNATRule(body); existing != nil && existing.EqualForRemoval(r) {
				found = true
				if remove {
					continue
				}
			}
		}
		out = append(out, raw)
	}

	if remove {
		if !found {
			return nil
		}
	} else {
		if found {
			return nil
		}
		out = append(out, line)
	}

	// Ensure the file ends with a single trailing newline.
	content := strings.Join(out, "\n")
	if !strings.HasSuffix(content, "\n") {
		content += "\n"
	}
	return writeConfigFile(CSFRedirect, []byte(content), 0600)
}

// AddNATRule adds a NAT rule to csf.redirect.
func (f *CSF) AddNATRule(ctx context.Context, zoneName string, r *NATRule) error {
	return f.editRedirect(r, false)
}

// InsertNATRule is unsupported: CSF stores redirects in a config file it applies
// as a whole, with no explicit ordering.
func (f *CSF) InsertNATRule(ctx context.Context, zoneName string, position int, r *NATRule) error {
	return unsupportedOrdering(f.Type())
}

// RemoveNATRule removes a NAT rule from csf.redirect.
func (f *CSF) RemoveNATRule(ctx context.Context, zoneName string, r *NATRule) error {
	return f.editRedirect(r, true)
}

// GetDefaultPolicy is unsupported: csf exposes no chain default policy.
func (f *CSF) GetDefaultPolicy(ctx context.Context, zoneName string) (*DefaultPolicy, error) {
	return nil, unsupportedPolicy(f.Type())
}

// SetDefaultPolicy is unsupported: csf exposes no chain default policy.
func (f *CSF) SetDefaultPolicy(ctx context.Context, zoneName string, policy *DefaultPolicy) error {
	return unsupportedPolicy(f.Type())
}

// GetAddressSets returns the address sets carried by the csf pre-hook.
func (f *CSF) GetAddressSets(ctx context.Context) ([]*AddressSet, error) {
	return f.hook().getAddressSets()
}

// GetAddressSet returns a single address set by name, or an error if absent.
func (f *CSF) GetAddressSet(ctx context.Context, name string) (*AddressSet, error) {
	sets, err := f.hook().getAddressSets()
	if err != nil {
		return nil, err
	}
	for _, s := range sets {
		if s.Name == name {
			return s, nil
		}
	}
	return nil, fmt.Errorf("address set %q not found", name)
}

// AddAddressSet writes a set as ipset commands in the pre-hook; csf -r (Reload)
// sources the hook to create the set. Re-adding a set is idempotent.
func (f *CSF) AddAddressSet(ctx context.Context, set *AddressSet) error {
	if set == nil || set.Name == "" {
		return fmt.Errorf("an address set requires a name")
	}
	_, err := f.hook().editAddressSet(set, false)
	return err
}

// RemoveAddressSet drops a set's ipset commands from the pre-hook. It fails if a
func (f *CSF) RemoveAddressSet(ctx context.Context, name string) error {
	_, err := f.hook().editAddressSet(&AddressSet{Name: name}, true)
	return err
}

// AddAddressSetEntry adds an entry to an existing set in the pre-hook.
func (f *CSF) AddAddressSetEntry(ctx context.Context, name, entry string) error {
	_, err := f.hook().editAddressSetEntry(name, entry, false)
	return err
}

// RemoveAddressSetEntry removes an entry from an existing set in the pre-hook.
func (f *CSF) RemoveAddressSetEntry(ctx context.Context, name, entry string) error {
	_, err := f.hook().editAddressSetEntry(name, entry, true)
	return err
}

// Backup captures the current filter and NAT rules managed by this backend.
func (f *CSF) Backup(ctx context.Context, zoneName string) (*Backup, error) {
	rules, err := f.GetRules(ctx, zoneName)
	if err != nil {
		return nil, err
	}
	natRules, err := f.GetNATRules(ctx, zoneName)
	if err != nil {
		return nil, err
	}
	// Backup captures the full filter and NAT rule state plus the hook's address
	// sets; Restore removes the current rules and re-adds these, so every rule read
	// is preserved.
	backup := &Backup{Rules: rules, NATRules: natRules}
	if err := captureBackupState(ctx, f, zoneName, backup); err != nil {
		return nil, err
	}
	return backup, nil
}

// Restore replaces the managed rules with the contents of a Backup.
func (f *CSF) Restore(ctx context.Context, zoneName string, backup *Backup) error {
	if backup == nil {
		return fmt.Errorf("backup cannot be nil")
	}

	// Remove existing rules.
	existing, err := f.GetRules(ctx, zoneName)
	if err != nil {
		return err
	}
	for _, r := range existing {
		if err := f.RemoveRule(ctx, zoneName, r); err != nil {
			return err
		}
	}
	existingNAT, err := f.GetNATRules(ctx, zoneName)
	if err != nil {
		return err
	}
	for _, r := range existingNAT {
		if err := f.RemoveNATRule(ctx, zoneName, r); err != nil {
			return err
		}
	}

	// Recreate the address sets before the rules so a set-referencing rule resolves
	// when csf sources the hook. The old rules are already gone, and editAddressSet
	// rewrites each set's block idempotently, so cleanFirst is unnecessary.
	if err := restoreBackupSets(ctx, f, backup, false); err != nil {
		return err
	}

	// Re-add rules from backup.
	for _, r := range backup.Rules {
		if err := f.addRule(ctx, zoneName, r, false); err != nil {
			return err
		}
	}
	for _, r := range backup.NATRules {
		if err := f.AddNATRule(ctx, zoneName, r); err != nil {
			return err
		}
	}
	return nil
}

// Reload restarts csf to apply config changes, retrying past csf's transient
// restart lock.
func (f *CSF) Reload(ctx context.Context) error {
	// csf serializes restarts behind a lock, so a reload issued while a previous
	// restart is still finishing fails transiently with "csf is being restarted, try
	// again in a moment" (Resource temporarily unavailable). Wait and retry rather
	// than surfacing that transient condition — the caller asked for a reload, not to
	// race csf's own in-flight restart.
	var err error
	for attempt := 0; attempt < 20; attempt++ {
		if _, err = runCommand(ctx, "csf", "-r"); err == nil {
			return nil
		}
		if !strings.Contains(err.Error(), "being restarted") && !strings.Contains(err.Error(), "temporarily unavailable") {
			return err
		}
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-time.After(500 * time.Millisecond):
		}
	}
	return err
}

// Close releases any resources held by the manager; csf holds none.
func (f *CSF) Close(ctx context.Context) error {
	return nil
}