steward/internal/config/config.go

// Package config loads stewardd's configuration file. It only says how
// the daemon runs (where it listens, its files, logging); the DHCP
// configuration itself is in the database.
package config

import (
	"bytes"
	"fmt"
	"net"
	"net/netip"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"time"

	"go.yaml.in/yaml/v3"
)

// DefaultPath is where stewardd looks for its configuration.
const DefaultPath = "/etc/steward/steward.yaml"

// Config is the daemon configuration.
type Config struct {
	// Listen are the web UI's and API's addresses.
	Listen         []string `yaml:"listen"`
	TLS            TLS      `yaml:"tls"`
	TrustedProxies []string `yaml:"trusted_proxies"`
	// Socket is a local unix socket for the API, where root and
	// stewardd's own user need no token ("" for none).
	Socket   string  `yaml:"socket"`
	Database string  `yaml:"database"`
	Log      Log     `yaml:"log"`
	Session  Session `yaml:"session"`
	Backup   Backup  `yaml:"backup"`
	// Jarl is the Jarl server Steward uses for DNS (docs/jarl.md).
	Jarl Jarl `yaml:"jarl"`
}

// TLS is the web UI's certificate: from files, or obtained through ACME
// with Jarl publishing the DNS-01 challenge (docs/certificates.md).
type TLS struct {
	CertFile string `yaml:"cert_file"`
	KeyFile  string `yaml:"key_file"`
	ACME     ACME   `yaml:"acme"`
}

// ACME obtains the web UI's certificate (docs/certificates.md).
type ACME struct {
	// Domain is the certificate's name ("" turns ACME off).
	Domain string `yaml:"domain"`
	Email  string `yaml:"email"`
	// Directory is letsencrypt, letsencrypt-staging (the default, for a
	// first certificate browsers don't trust) or an ACME directory URL.
	Directory string `yaml:"directory"`
	// AgreeTOS says you accept the CA's terms of service; no account is
	// registered without it.
	AgreeTOS bool `yaml:"agree_tos"`
	// Resolvers are public recursive resolvers, to see the challenge as
	// the CA would.
	Resolvers []string `yaml:"resolvers"`
	// CheckPropagation also asks each of the zone's public name servers
	// directly before the CA is told to look.
	CheckPropagation bool `yaml:"check_propagation"`
	// CABundle is a PEM file of extra CAs to trust for the ACME server
	// (a private CA such as step-ca, or Pebble in tests).
	CABundle string `yaml:"ca_bundle"`
}

// Directory URLs for the names tls.acme.directory accepts.
const (
	LetsEncrypt        = "https://acme-v02.api.letsencrypt.org/directory"
	LetsEncryptStaging = "https://acme-staging-v02.api.letsencrypt.org/directory"
)

// DirectoryURL is the ACME directory's URL.
func (a ACME) DirectoryURL() string {
	switch a.Directory {
	case "", "letsencrypt-staging":
		return LetsEncryptStaging
	case "letsencrypt":
		return LetsEncrypt
	}
	return a.Directory
}

// Staging reports whether the directory is Let's Encrypt's staging one.
func (a ACME) Staging() bool { return a.DirectoryURL() == LetsEncryptStaging }

// Jarl is a Jarl server and an API token for it.
type Jarl struct {
	// Server is Jarl's URL ("" when Steward doesn't use Jarl).
	Server string `yaml:"server"`
	// Token, or TokenFile holding it (better: the file can be readable by
	// the steward user alone).
	Token     string `yaml:"token"`
	TokenFile string `yaml:"token_file"`
	// CACert is a PEM file with a CA to trust for Jarl's certificate.
	CACert   string `yaml:"ca_cert"`
	Insecure bool   `yaml:"insecure"`
}

// Enabled reports whether a Jarl server is configured.
func (j Jarl) Enabled() bool { return j.Server != "" }

// ReadToken returns the token, from the setting or the file.
func (j Jarl) ReadToken() (string, error) {
	if j.TokenFile == "" {
		return j.Token, nil
	}
	b, err := os.ReadFile(j.TokenFile)
	if err != nil {
		return "", fmt.Errorf("jarl.token_file: %w", err)
	}
	return strings.TrimSpace(string(b)), nil
}

// Log configures logging to stderr (the journal under systemd).
type Log struct {
	Level  string `yaml:"level"`  // debug, info, warn, error
	Format string `yaml:"format"` // text or json
}

// Session configures web UI logins.
type Session struct {
	Lifetime    Duration `yaml:"lifetime"`
	IdleTimeout Duration `yaml:"idle_timeout"`
	// CookieSecure is auto (secure over HTTPS), always or never.
	CookieSecure string `yaml:"cookie_secure"`
}

// Backup configures 'stewardd backup'.
type Backup struct {
	Dir       string   `yaml:"dir"`
	Retention Duration `yaml:"retention"`
	// KeepAtLeast backups are kept however old they are.
	KeepAtLeast int `yaml:"keep_at_least"`
}

// Duration is a time.Duration written as "15s" or "12h"; days ("30d")
// work too.
type Duration time.Duration

// UnmarshalYAML implements yaml.Unmarshaler.
func (d *Duration) UnmarshalYAML(n *yaml.Node) error {
	var s string
	if err := n.Decode(&s); err != nil {
		return err
	}
	v, err := ParseDuration(s)
	if err != nil {
		return err
	}
	*d = Duration(v)
	return nil
}

// ParseDuration parses a Go duration, also accepting days ("30d").
func ParseDuration(s string) (time.Duration, error) {
	s = strings.TrimSpace(s)
	if n, ok := strings.CutSuffix(s, "d"); ok {
		var days int
		if _, err := fmt.Sscanf(n, "%d", &days); err != nil || fmt.Sprint(days) != n {
			return 0, fmt.Errorf("invalid duration %q", s)
		}
		return time.Duration(days) * 24 * time.Hour, nil
	}
	return time.ParseDuration(s)
}

// Default returns the defaults for Debian/Ubuntu.
func Default() *Config {
	return &Config{
		Listen:         []string{"127.0.0.1:8067"},
		TrustedProxies: []string{},
		Socket:         "/run/steward/steward.sock",
		Database:       "/var/lib/steward/steward.db",
		Log:            Log{Level: "info", Format: "text"},
		Session:        Session{Lifetime: Duration(12 * time.Hour), IdleTimeout: Duration(2 * time.Hour), CookieSecure: "auto"},
		Backup:         Backup{Dir: "/var/backups/steward", Retention: Duration(30 * 24 * time.Hour), KeepAtLeast: 7},
		TLS:            TLS{ACME: ACME{Directory: "letsencrypt-staging", Resolvers: []string{"1.1.1.1:53", "9.9.9.9:53", "8.8.8.8:53"}, CheckPropagation: true}},
	}
}

// Load reads the file at path over the defaults. A missing file is fine
// when allowMissing is set.
func Load(path string, allowMissing bool) (*Config, error) {
	c := Default()
	b, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) && allowMissing {
			return c, c.check()
		}
		return nil, err
	}
	dec := yaml.NewDecoder(bytes.NewReader(b))
	dec.KnownFields(true)
	if err := dec.Decode(c); err != nil && err.Error() != "EOF" {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	if err := c.check(); err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	return c, nil
}

func (c *Config) check() error {
	if len(c.Listen) == 0 {
		return fmt.Errorf("listen: at least one address is needed")
	}
	if c.Socket != "" && !filepath.IsAbs(c.Socket) {
		return fmt.Errorf("socket must be an absolute path (or \"\" for none)")
	}
	if len(c.Socket) > 107 {
		return fmt.Errorf("socket: the path is %d bytes long; unix sockets allow at most 107", len(c.Socket))
	}
	if (c.TLS.CertFile == "") != (c.TLS.KeyFile == "") {
		return fmt.Errorf("tls: set both cert_file and key_file")
	}
	if err := c.checkACME(); err != nil {
		return err
	}
	for _, p := range c.TrustedProxies {
		if _, err := ParsePrefix(p); err != nil {
			return fmt.Errorf("trusted_proxies: %v", err)
		}
	}
	if c.Database == "" || !filepath.IsAbs(c.Database) {
		return fmt.Errorf("database must be an absolute path")
	}
	if c.Backup.Dir != "" && !filepath.IsAbs(c.Backup.Dir) {
		return fmt.Errorf("backup.dir must be an absolute path")
	}
	switch c.Session.CookieSecure {
	case "auto", "always", "never":
	default:
		return fmt.Errorf("session.cookie_secure must be auto, always or never")
	}
	switch c.Log.Format {
	case "text", "json":
	default:
		return fmt.Errorf("log.format must be text or json")
	}
	switch c.Log.Level {
	case "debug", "info", "warn", "error":
	default:
		return fmt.Errorf("log.level must be debug, info, warn or error")
	}
	return nil
}

func (c *Config) checkACME() error {
	j := c.Jarl
	if j.Server != "" {
		if u, err := url.Parse(j.Server); err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
			return fmt.Errorf("jarl.server %q: give Jarl's URL, e.g. https://dns.example.com:8053", j.Server)
		}
		if j.Token == "" && j.TokenFile == "" {
			return fmt.Errorf("jarl: set token or token_file (a Jarl API token, docs/jarl.md)")
		}
		if j.Token != "" && j.TokenFile != "" {
			return fmt.Errorf("jarl: set token or token_file, not both")
		}
	}
	a := c.TLS.ACME
	if a.Domain == "" {
		return nil
	}
	if c.TLS.CertFile != "" {
		return fmt.Errorf("tls: cert_file and acme can't both be set; choose one")
	}
	if !j.Enabled() {
		return fmt.Errorf("tls.acme needs Jarl to publish the challenge: set jarl.server and its token (docs/certificates.md)")
	}
	d := strings.TrimSuffix(strings.ToLower(a.Domain), ".")
	if strings.HasPrefix(d, "*.") || strings.Count(d, ".") < 1 || strings.ContainsAny(d, " /:") {
		return fmt.Errorf("tls.acme.domain %q: give the full name the web UI is reached by, e.g. dhcp.example.com", a.Domain)
	}
	if u := a.DirectoryURL(); !strings.HasPrefix(u, "https://") {
		return fmt.Errorf("tls.acme.directory %q: use letsencrypt, letsencrypt-staging or an https:// ACME directory URL", a.Directory)
	}
	for _, r := range a.Resolvers {
		if _, _, err := net.SplitHostPort(r); err != nil {
			return fmt.Errorf("tls.acme.resolvers: %q must be HOST:PORT, e.g. 1.1.1.1:53", r)
		}
	}
	return nil
}

// ParsePrefix parses an address or a prefix.
func ParsePrefix(s string) (netip.Prefix, error) {
	if strings.Contains(s, "/") {
		return netip.ParsePrefix(s)
	}
	a, err := netip.ParseAddr(s)
	if err != nil {
		return netip.Prefix{}, err
	}
	return netip.PrefixFrom(a, a.BitLen()), nil
}

// Proxies returns the trusted proxies.
func (c *Config) Proxies() []netip.Prefix {
	var out []netip.Prefix
	for _, p := range c.TrustedProxies {
		if pf, err := ParsePrefix(p); err == nil {
			out = append(out, pf.Masked())
		}
	}
	return out
}

// Example is a commented example configuration with the defaults.
const Example = `# Steward daemon configuration (/etc/steward/steward.yaml).
# Every setting is optional; the values shown are the defaults. The DHCP
# configuration (subnets, pools, reservations, options) is not here: it is
# in the database, changed with the steward CLI or the web UI.

# Addresses the web UI and API listen on. Keep this off the internet:
# bind to localhost or a management network, and use TLS (below) or a
# TLS-terminating reverse proxy.
listen:
  - "127.0.0.1:8067"

# HTTPS for the web UI and API: a certificate from files you renew yourself
# (reread when they change)...
# tls:
#   cert_file: /etc/steward/tls/fullchain.pem
#   key_file: /etc/steward/tls/privkey.pem
#
# ...or one Steward obtains and renews itself from Let's Encrypt, with Jarl
# publishing the DNS-01 challenge (needs jarl: below; docs/certificates.md).
# Start with the staging directory: its certificate isn't trusted by
# browsers, but it proves everything works without Let's Encrypt's rate
# limits. Then set directory: letsencrypt.
tls:
  acme:
    domain: ""                       # e.g. dhcp.example.com; "" is off
    email: ""
    directory: letsencrypt-staging   # letsencrypt, or an ACME directory URL
    agree_tos: false                 # you accept the CA's terms of service
    # Public resolvers, to see the challenge as the CA does.
    resolvers: ["1.1.1.1:53", "9.9.9.9:53", "8.8.8.8:53"]
    # Also ask the zone's public name servers directly.
    check_propagation: true
    ca_bundle: ""                    # extra CA for a private ACME server

# Jarl, the BIND manager, for DNS records (docs/jarl.md). Give Steward a
# token that may only write its challenge record; keep it in a file only
# the steward user can read.
# jarl:
#   server: https://dns.example.com:8053
#   token_file: /etc/steward/jarl.token
#   ca_cert: ""                      # a CA to trust for Jarl's certificate
#   insecure: false

# Reverse proxies whose X-Forwarded-For and X-Forwarded-Proto are
# believed.
trusted_proxies: []

# A local socket for the API: root and stewardd's own user use it without
# a token ('sudo steward status' just works). "" turns it off.
socket: /run/steward/steward.sock

database: /var/lib/steward/steward.db

log:
  level: info     # debug, info, warn or error
  format: text    # text or json

session:
  lifetime: 12h
  idle_timeout: 2h
  cookie_secure: auto   # auto (secure over HTTPS), always or never

# 'stewardd backup' (the daily steward-backup.timer) writes copies of the
# database here and deletes those older than retention, keeping at least
# keep_at_least.
backup:
  dir: /var/backups/steward
  retention: 30d
  keep_at_least: 7
`