selfpost/internal/domain/opendkim.go

package domain

import (
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"os"
	"path"
	"path/filepath"
	"sort"
	"strings"

	"github.com/mixeme/selfpost/internal/configsafe"
	"github.com/mixeme/selfpost/internal/supervisor"
)

// OpenDKIM manages the on-disk OpenDKIM state the panel is responsible for
// (architecture.md § OpenDKIM): per-domain signing keys under keysDir and the
// KeyTable/SigningTable that map domains to those keys. After rewriting the
// tables it asks OpenDKIM to reload them.
type OpenDKIM struct {
	keysDir          string
	keyTablePath     string
	signingTablePath string

	// reload sends the running OpenDKIM a reload signal. It is a field so tests
	// can substitute a no-op; the default drives supervisord (see reloadViaSupervisor).
	reload func() error
}

// NewOpenDKIM builds a manager rooted at dir (typically /data/opendkim), the
// same layout entrypoint.sh prepares. The default reload path signals OpenDKIM
// through supervisord.
func NewOpenDKIM(dir string) *OpenDKIM {
	return &OpenDKIM{
		keysDir:          filepath.Join(dir, "keys"),
		keyTablePath:     filepath.Join(dir, "KeyTable"),
		signingTablePath: filepath.Join(dir, "SigningTable"),
		reload:           reloadViaSupervisor,
	}
}

// SigningDomain is one row's worth of signing configuration.
type SigningDomain struct {
	Name     string
	Selector string
}

// keyPath is the private-key path for a domain/selector, matching the KeyTable.
func (o *OpenDKIM) keyPath(domainName, selector string) string {
	return filepath.Join(o.keysDir, domainName, selector+".private")
}

// EnsureKey makes sure a signing key exists for the domain. An existing key is
// reused untouched — critical because overwriting it would silently invalidate
// the DKIM record already published in DNS (architecture.md § OpenDKIM).
// Returns whether a new key was generated.
func (o *OpenDKIM) EnsureKey(domainName, selector string) (bool, error) {
	if err := assertConfigSafe(domainName, selector); err != nil {
		return false, err
	}
	path := o.keyPath(domainName, selector)
	if _, err := os.Stat(path); err == nil {
		return false, nil // reuse existing key
	} else if !os.IsNotExist(err) {
		return false, fmt.Errorf("stat dkim key: %w", err)
	}
	// setgid on keysDir (entrypoint.sh) makes the per-domain dir inherit the
	// shared `selfpost` group so OpenDKIM can traverse into it.
	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
		return false, fmt.Errorf("create key dir: %w", err)
	}
	key, err := generateDKIMKey()
	if err != nil {
		return false, err
	}
	if err := writePrivateKeyPEM(path, key); err != nil {
		return false, err
	}
	return true, nil
}

// RemoveKey deletes a domain's key directory (architecture.md § OpenDKIM). A
// missing directory is not an error.
func (o *OpenDKIM) RemoveKey(domainName string) error {
	if err := assertConfigSafe(domainName, "x"); err != nil {
		return err
	}
	if err := os.RemoveAll(filepath.Join(o.keysDir, domainName)); err != nil {
		return fmt.Errorf("remove key dir for %s: %w", domainName, err)
	}
	return nil
}

// KeyDomains lists the names that have a key directory.
func (o *OpenDKIM) KeyDomains() ([]string, error) {
	entries, err := os.ReadDir(o.keysDir)
	if os.IsNotExist(err) {
		return nil, nil
	}
	if err != nil {
		return nil, fmt.Errorf("list key dirs: %w", err)
	}
	var out []string
	for _, e := range entries {
		if e.IsDir() {
			out = append(out, e.Name())
		}
	}
	return out, nil
}

// ExportKey returns a domain's DKIM private key as PKCS#1 PEM, for carrying in
// a domain export so the receiving instance signs with the same key and the
// DNS TXT record never has to change (architecture.md § Persistence). It
// re-marshals the parsed key rather than returning the raw file, so a
// malformed on-disk key is caught here.
func (o *OpenDKIM) ExportKey(domainName, selector string) ([]byte, error) {
	if err := assertConfigSafe(domainName, selector); err != nil {
		return nil, err
	}
	key, err := loadPrivateKeyPEM(o.keyPath(domainName, selector))
	if err != nil {
		return nil, err
	}
	block := &pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(key)}
	return pem.EncodeToMemory(block), nil
}

// ImportKey writes an imported DKIM private key to disk for a domain
// (architecture.md § Persistence). The PEM is parsed and re-marshalled through
// the same writer used for generated keys, so only a well-formed PKCS#1 RSA
// key is ever stored. Unlike EnsureKey it overwrites: an import (re-)creates
// the domain with exactly this key, which is the whole point of keeping the
// published DNS record valid.
func (o *OpenDKIM) ImportKey(domainName, selector string, pemKey []byte) error {
	if err := assertConfigSafe(domainName, selector); err != nil {
		return err
	}
	block, _ := pem.Decode(pemKey)
	if block == nil || block.Type != "RSA PRIVATE KEY" {
		return fmt.Errorf("import dkim key for %s: not a PKCS#1 RSA private key", domainName)
	}
	key, err := x509.ParsePKCS1PrivateKey(block.Bytes)
	if err != nil {
		return fmt.Errorf("import dkim key for %s: %w", domainName, err)
	}
	path := o.keyPath(domainName, selector)
	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
		return fmt.Errorf("create key dir: %w", err)
	}
	return writePrivateKeyPEM(path, key)
}

// Record returns the published DKIM DNS record for a domain, recomputed from the
// private key on disk (product.md).
func (o *OpenDKIM) Record(domainName, selector string) (DKIMRecord, error) {
	key, err := loadPrivateKeyPEM(o.keyPath(domainName, selector))
	if err != nil {
		return DKIMRecord{}, err
	}
	return dkimRecord(selector, domainName, &key.PublicKey)
}

// SigningTablePath is the SigningTable's location: the names this server
// signs for, which the sender check reads (internal/milter/sender.go).
func (o *OpenDKIM) SigningTablePath() string {
	return o.signingTablePath
}

// Rebuild regenerates KeyTable and SigningTable from the full domain set and
// reloads OpenDKIM (architecture.md § OpenDKIM). Full regeneration (rather
// than incremental edits) keeps the files a pure function of the registry, so
// add and delete share one idempotent path. Both files are written atomically
// before the reload signal is sent.
func (o *OpenDKIM) Rebuild(domains []SigningDomain) error {
	keyTable, signingTable, err := renderTables(o.keysDir, domains)
	if err != nil {
		return err
	}
	if err := writeFileAtomic(o.keyTablePath, keyTable, 0o640); err != nil {
		return err
	}
	if err := writeFileAtomic(o.signingTablePath, signingTable, 0o640); err != nil {
		return err
	}
	return o.reload()
}

// signs reports whether the tables on disk sign name with the key of
// selector: a line for the name in the SigningTable, and in the KeyTable the
// line that gives it that selector. Tables that do not exist yet sign nothing.
// The selector is part of the question because the panel shows and checks the
// record of one key; tables that name another would have OpenDKIM sign with a
// key whose record nobody is told to publish.
//
// OpenDKIM 2.11.0~beta2, opendkim/util.c, dkimf_mkregexp(): "*@name" is
// matched as ^.*@name$ (case aside), so it signs name and none of its
// subdomains (see Signers.Signs in internal/milter/sender.go).
func (o *OpenDKIM) signs(name, selector string) (bool, error) {
	has := func(path, prefix string) (bool, error) {
		data, err := os.ReadFile(path)
		if os.IsNotExist(err) {
			return false, nil
		}
		if err != nil {
			return false, fmt.Errorf("read %s: %w", filepath.Base(path), err)
		}
		for _, line := range strings.Split(string(data), "\n") {
			if strings.HasPrefix(line, prefix) {
				return true, nil
			}
		}
		return false, nil
	}
	signed, err := has(o.signingTablePath, "*@"+name+" ")
	if err != nil || !signed {
		return false, err
	}
	return has(o.keyTablePath, name+" "+name+":"+selector+":")
}

// Reload asks OpenDKIM to re-read its tables without regenerating them. It
// backs the panel's manual reload button (architecture.md § Panel HTTP
// surface).
func (o *OpenDKIM) Reload() error {
	return o.reload()
}

// SetReloadHook replaces how configuration is applied after a rebuild. Tests
// that cannot reach supervisord use this to verify file regeneration alone.
func (o *OpenDKIM) SetReloadHook(fn func() error) {
	if fn != nil {
		o.reload = fn
	}
}

// renderTables builds the KeyTable and SigningTable byte contents for a domain
// set, sorted by name so the output is deterministic. Every domain is
// re-checked for shell/config-injection safety before being written
// (security.md) — validation upstream already guarantees this, but the table
// writer refuses to emit anything unsafe as a hard backstop.
func renderTables(keysDir string, domains []SigningDomain) (keyTable, signingTable []byte, err error) {
	sorted := append([]SigningDomain(nil), domains...)
	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Name < sorted[j].Name })

	// OpenDKIM 2.11.0~beta2: KeyTable is a file: table, the key up to the
	// first blank (opendkim/opendkim-db.c, dkimf_db_open(), DKIMF_DB_TYPE_FILE);
	// opendkim/opendkim.c, dkimf_add_signrequest(), reads domain, selector and
	// key from its value, split at ":" (opendkim-db.c, dkimf_db_datasplit()),
	// a key starting with "/" being a file. SigningTable is refile:, a pattern
	// up to the first blank and the key name after it (dkimf_db_open(), case
	// DKIMF_DB_TYPE_REFILE).
	var kt, st strings.Builder
	for _, d := range sorted {
		if err := assertConfigSafe(d.Name, d.Selector); err != nil {
			return nil, nil, err
		}
		keyName := d.Name // one key per domain; the domain name is a fine handle
		// Absolute key path so OpenDKIM resolves it independently of its CWD.
		keyFile := path.Join(keysDir, d.Name, d.Selector+".private")
		// KeyTable:     <key-name>  <domain>:<selector>:<key-path>
		fmt.Fprintf(&kt, "%s %s:%s:%s\n", keyName, d.Name, d.Selector, keyFile)
		// SigningTable (refile): <address-pattern>  <key-name>
		fmt.Fprintf(&st, "*@%s %s\n", d.Name, keyName)
	}
	return []byte(kt.String()), []byte(st.String()), nil
}

// opendkimForbidden is what may never appear in a KeyTable or SigningTable
// value: whitespace and the newline that would end the line, and the colon and
// path separators that carry meaning inside a table entry. (OpenDKIM
// 2.11.0~beta2, opendkim/opendkim-db.c, dkimf_db_datasplit(), splits a
// KeyTable value at ":"; the domain and selector are also the key's path.)
const opendkimForbidden = " \t\r\n:/\\"

// assertConfigSafe rejects any domain/selector value that could break out of a
// single table line. Domains are already whitelisted to [a-z0-9.-] and selectors
// to a similar set before they reach here (security.md); this is defence in depth
// against a validation gap ever letting whitespace, a newline or a field
// separator through into a config file (security.md).
func assertConfigSafe(domainName, selector string) error {
	for _, f := range []struct{ what, value string }{
		{"domain", domainName},
		{"selector", selector},
	} {
		if err := configsafe.Token(f.what, f.value, opendkimForbidden); err != nil {
			return fmt.Errorf("opendkim: %w", err)
		}
	}
	return nil
}

// reloadViaSupervisor asks supervisord (PID 1, running as root) to send the
// OpenDKIM process SIGUSR1, which makes it re-read KeyTable/SigningTable
// (opendkim's documented reload signal). The panel runs unprivileged and cannot
// signal another user's process directly, so it goes through the supervisor
// control socket, reachable via the shared `selfpost` group (security.md).
//
// Arguments are fixed literals — no user input is interpolated into the command,
// and it never goes through a shell (security.md).
//
// OpenDKIM 2.11.0~beta2, opendkim/opendkim.c, dkimf_reloader(): SIGUSR1 sets
// reload when a configuration file was given (-x in build/supervisord.conf);
// dkimf_config_reload(), from mlfi_negotiate() at the next milter connection,
// loads the configuration and opens the tables again (opendkim.8, OPERATION).
// supervisor 4.2.5, supervisor/rpcinterface.py, signalProcess(), and
// process.py, signal(): kill() of the program's own pid - opendkim itself, run
// with -f (not forked into the background) and exec'd with no shell.
func reloadViaSupervisor() error {
	return supervisor.Signal("USR1", "opendkim")
}