archivmail/internal/ldapauth/client.go

// Package ldapauth provides LDAP/Active Directory authentication helpers.
// It wraps go-ldap/ldap/v3 and exposes a TestConnection probe and an
// Authenticate function used by the auth.Manager LDAP fallback.
package ldapauth

import (
	"crypto/tls"
	"fmt"
	"strings"
	"time"

	ldapv3 "github.com/go-ldap/ldap/v3"
)

// Config holds the parameters required to connect to an LDAP/AD server.
type Config struct {
	URL           string
	BindDN        string
	BindPassword  string
	BaseDN        string
	UserFilter    string // must contain %s as placeholder for the username
	TLS           bool   // true = STARTTLS upgrade after plain connection
	TLSSkipVerify bool
}

// TestResult is the structured output of TestConnection.
type TestResult struct {
	OK          bool   `json:"ok"`
	Message     string `json:"message"`
	LatencyMS   int64  `json:"latency_ms"`
	ServerInfo  string `json:"server_info"`
	UsersFound  int    `json:"users_found"`
	ErrorDetail string `json:"error_detail"`
}

// TestConnection opens a connection to the LDAP server, binds with the service
// account, queries the RootDSE for server info, and counts user objects.
func TestConnection(cfg Config) TestResult {
	start := time.Now()

	conn, err := dial(cfg)
	if err != nil {
		return TestResult{
			OK:          false,
			Message:     "Verbindung fehlgeschlagen",
			LatencyMS:   time.Since(start).Milliseconds(),
			ErrorDetail: err.Error(),
		}
	}
	defer conn.Close()

	// Service-account bind
	if err := conn.Bind(cfg.BindDN, cfg.BindPassword); err != nil {
		return TestResult{
			OK:          false,
			Message:     "Bind fehlgeschlagen",
			LatencyMS:   time.Since(start).Milliseconds(),
			ErrorDetail: err.Error(),
		}
	}

	// Query RootDSE for server info
	serverInfo := queryRootDSE(conn)

	// Count user objects (capped at 500 to avoid large result sets)
	usersFound := countUsers(conn, cfg.BaseDN)

	return TestResult{
		OK:         true,
		Message:    "Verbindung erfolgreich",
		LatencyMS:  time.Since(start).Milliseconds(),
		ServerInfo: serverInfo,
		UsersFound: usersFound,
	}
}

// Authenticate performs a two-step bind against LDAP:
//  1. Bind with the service account to locate the user DN via search.
//  2. Bind with the user DN and the provided password to verify credentials.
//
// Returns a map of user attributes (mail, displayName, memberOf) on success.
func Authenticate(cfg Config, username, password string) (map[string]string, error) {
	conn, err := dial(cfg)
	if err != nil {
		return nil, fmt.Errorf("ldapauth: dial: %w", err)
	}
	defer conn.Close()

	// Step 1: service-account bind
	if err := conn.Bind(cfg.BindDN, cfg.BindPassword); err != nil {
		return nil, fmt.Errorf("ldapauth: service bind: %w", err)
	}

	// Step 2: search for user DN
	filter := strings.ReplaceAll(cfg.UserFilter, "%s", ldapv3.EscapeFilter(username))
	searchReq := ldapv3.NewSearchRequest(
		cfg.BaseDN,
		ldapv3.ScopeWholeSubtree,
		ldapv3.NeverDerefAliases,
		1, // size limit
		30, // time limit seconds
		false,
		filter,
		[]string{"dn", "mail", "displayName", "memberOf"},
		nil,
	)
	result, err := conn.Search(searchReq)
	if err != nil {
		return nil, fmt.Errorf("ldapauth: search user: %w", err)
	}
	if len(result.Entries) == 0 {
		return nil, fmt.Errorf("ldapauth: user not found: %s", username)
	}

	userDN := result.Entries[0].DN
	attrs := map[string]string{
		"mail":        result.Entries[0].GetAttributeValue("mail"),
		"displayName": result.Entries[0].GetAttributeValue("displayName"),
		"memberOf":    strings.Join(result.Entries[0].GetAttributeValues("memberOf"), ","),
	}

	// Step 3: user bind to verify password
	if err := conn.Bind(userDN, password); err != nil {
		return nil, fmt.Errorf("ldapauth: user bind failed")
	}

	return attrs, nil
}

// dial creates an LDAP connection, optionally upgrading to TLS via STARTTLS.
func dial(cfg Config) (*ldapv3.Conn, error) {
	tlsCfg := &tls.Config{InsecureSkipVerify: cfg.TLSSkipVerify} //nolint:gosec // user-configured option

	// ldaps:// URLs use implicit TLS; ldap:// URLs use plain or STARTTLS.
	if strings.HasPrefix(cfg.URL, "ldaps://") {
		return ldapv3.DialURL(cfg.URL, ldapv3.DialWithTLSConfig(tlsCfg))
	}

	conn, err := ldapv3.DialURL(cfg.URL)
	if err != nil {
		return nil, err
	}

	if cfg.TLS {
		if err := conn.StartTLS(tlsCfg); err != nil {
			conn.Close()
			return nil, fmt.Errorf("ldapauth: STARTTLS: %w", err)
		}
	}
	return conn, nil
}

// queryRootDSE fetches vendorName / vendorVersion from the RootDSE.
func queryRootDSE(conn *ldapv3.Conn) string {
	req := ldapv3.NewSearchRequest(
		"",
		ldapv3.ScopeBaseObject,
		ldapv3.NeverDerefAliases,
		1, 10, false,
		"(objectClass=*)",
		[]string{"vendorName", "vendorVersion", "serverType", "isGlobalCatalogReady"},
		nil,
	)
	res, err := conn.Search(req)
	if err != nil || len(res.Entries) == 0 {
		return ""
	}
	e := res.Entries[0]
	parts := []string{}
	for _, attr := range []string{"vendorName", "vendorVersion", "serverType"} {
		v := e.GetAttributeValue(attr)
		if v != "" {
			parts = append(parts, v)
		}
	}
	return strings.Join(parts, " ")
}

// countUsers searches for person/user objects and returns the count (max 500).
func countUsers(conn *ldapv3.Conn, baseDN string) int {
	req := ldapv3.NewSearchRequest(
		baseDN,
		ldapv3.ScopeWholeSubtree,
		ldapv3.NeverDerefAliases,
		500, 30, false,
		"(|(objectClass=person)(objectClass=user))",
		[]string{"dn"},
		nil,
	)
	res, err := conn.Search(req)
	if err != nil {
		return 0
	}
	return len(res.Entries)
}