mesh-catalog/modules/fail2ban/cmd/fail2ban-tools/own.go

package main

// The operator's own addresses (novox/hq ADR 0339, issue 238): the public addresses and ranges the operator
// names as theirs, in the mesh-wide setting `own-addresses`. Every jail ignores them beside the private ranges
// ADR 0186 spares, the ban verb refuses them, and a failure a jail counted from one is said as a warning.
//
// **The setting reaches the machine as a mergeable file, so it may be unset.** A `${setting:own-addresses}`
// would leave the module out of every machine until the operator gave a value (ADR 0262: an operator's value
// has no default), and an empty list is what the record says the mesh starts from. The mesh writes the
// setting into SettingFile, whose own content is the empty list; the watch, as root, writes the entries
// into ListFile, one per line, and every jail reads that through `ignoreip = … file:<ListFile>`, which fail2ban
// (1.1 and later) reloads within a second of the file changing — no restart, no ban lost. A failure ignored
// because of it is logged as `Ignore <address> by file:<ListFile>`, which is how the watch tells the
// operator's own addresses from the private ranges, which are logged `by ip`.

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"net/netip"
	"os"
	"path/filepath"
	"sort"
	"strings"
)

// The files, as the machine has them.
const (
	// SettingFile is the setting as the mesh writes it: {"own-addresses": […]} or a string of entries.
	SettingFile = "/etc/fail2ban/mesh-own-addresses.json"
	// StateDir is the module's own directory, where the watch writes.
	StateDir = "/var/lib/mesh-fail2ban"
	// ListFile is the entries one per line, as fail2ban reads them.
	ListFile = StateDir + "/own-addresses"
	// StateFile is what the watch saw, for the health tool to read.
	StateFile = StateDir + "/watch.json"
	// LogFile is the daemon's log, where it says what it ignored.
	LogFile = "/var/log/fail2ban.log"
	// SettingKey is the setting's name (ADR 0339 §1).
	SettingKey = "own-addresses"
	// IgnoredBy is how every jail's ignore list names the operator's own addresses, and how the daemon's log
	// names them as the reason it ignored a failure.
	IgnoredBy = "file:" + ListFile
)

// Own is the setting read: its entries, and each one that is not an address or a range, with why.
type Own struct {
	Entries  []netip.Prefix `json:"-"`
	Listed   []string       `json:"entries"`
	Problems []string       `json:"problems,omitempty"`
}

// ReadOwn reads the setting from SettingFile under root. A file not written yet is no entry: the mesh has
// not written the setting there, and the record starts from none. Any other failure to read it is an error,
// so a caller that refuses on it fails closed.
func ReadOwn(root string) (Own, error) {
	raw, err := os.ReadFile(filepath.Join(root, SettingFile))
	if errors.Is(err, fs.ErrNotExist) {
		return Own{Listed: []string{}}, nil
	}
	if err != nil {
		return Own{}, fmt.Errorf("the operator's own addresses cannot be read from %s: %w", SettingFile, err)
	}
	var doc map[string]any
	if err := json.Unmarshal(raw, &doc); err != nil {
		return Own{}, fmt.Errorf("the operator's own addresses in %s are not JSON: %w", SettingFile, err)
	}
	return ParseOwn(doc[SettingKey])
}

// ParseOwn reads the setting's value: a list of entries, or one string of entries separated by spaces or
// commas, as `settings set` may give either. Each entry is an address or a range in either family; anything
// else is a problem, named, and left out — a typo never widens what is spared.
func ParseOwn(value any) (Own, error) {
	var words []string
	switch v := value.(type) {
	case nil:
	case string:
		words = strings.FieldsFunc(v, func(r rune) bool { return r == ',' || r == ' ' || r == '\t' || r == '\n' })
	case []any:
		for _, item := range v {
			s, ok := item.(string)
			if !ok {
				return Own{}, fmt.Errorf("%s holds %v, which is not text: each entry is an address or a range", SettingKey, item)
			}
			words = append(words, strings.TrimSpace(s))
		}
	default:
		return Own{}, fmt.Errorf("%s is %T: it is a list of addresses and ranges, or one line of them", SettingKey, value)
	}
	own := Own{Listed: []string{}}
	seen := map[netip.Prefix]bool{}
	for _, w := range words {
		if w == "" {
			continue
		}
		p, err := entry(w)
		if err != nil {
			own.Problems = append(own.Problems, err.Error())
			continue
		}
		if seen[p] {
			continue
		}
		seen[p] = true
		own.Entries = append(own.Entries, p)
	}
	sort.Slice(own.Entries, func(i, j int) bool { return own.Entries[i].String() < own.Entries[j].String() })
	for _, p := range own.Entries {
		own.Listed = append(own.Listed, show(p))
	}
	return own, nil
}

// entry is one address or range; an address is the range of itself.
func entry(word string) (netip.Prefix, error) {
	if strings.Contains(word, "/") {
		p, err := netip.ParsePrefix(word)
		if err != nil {
			return netip.Prefix{}, fmt.Errorf("%q is not an address or a range", word)
		}
		return p.Masked(), nil
	}
	a, err := netip.ParseAddr(word)
	if err != nil {
		return netip.Prefix{}, fmt.Errorf("%q is not an address or a range", word)
	}
	a = a.Unmap()
	return netip.PrefixFrom(a, a.BitLen()), nil
}

// show is an entry as a person reads it: a single address without its length.
func show(p netip.Prefix) string {
	if p.IsSingleIP() {
		return p.Addr().String()
	}
	return p.String()
}

// Lies is the entry ip lies on, or "".
func (o Own) Lies(ip netip.Addr) string {
	return liesOn(ip, o.Entries)
}

func liesOn(ip netip.Addr, entries []netip.Prefix) string {
	ip = ip.Unmap()
	for _, p := range entries {
		if p.Contains(ip) {
			return show(p)
		}
	}
	return ""
}

// ListText is the entries as fail2ban reads ListFile: one per line, under a line saying whose it is.
func (o Own) ListText() string {
	var b strings.Builder
	b.WriteString("# The operator's own addresses (novox/hq ADR 0339), written by the fail2ban module's watch from the\n")
	b.WriteString("# mesh-wide setting own-addresses. Every jail ignores them. Replaced on every change; edit nothing here.\n")
	for _, l := range o.Listed {
		b.WriteString(l + "\n")
	}
	return b.String()
}

// WriteList writes ListFile under root when its content differs, by a rename so the daemon never reads half
// of it. Says whether it wrote.
func WriteList(root string, o Own) (bool, error) {
	path := filepath.Join(root, ListFile)
	want := o.ListText()
	if have, err := os.ReadFile(path); err == nil && string(have) == want {
		return false, nil
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return false, err
	}
	tmp := path + ".new"
	if err := os.WriteFile(tmp, []byte(want), 0o644); err != nil {
		return false, err
	}
	if err := os.Rename(tmp, path); err != nil {
		return false, err
	}
	return true, nil
}