ORG.FREEDESKTOP.LOGCONTROL1(5)

NAME
org.freedesktop.LogControl1 - D-Bus interface to query and set logging configuration

INTRODUCTION

org.freedesktop.LogControl1 is a generic interface that is intended to be used by any daemon which allows the log level and target to be set over D-Bus. It is implemented by various daemons that are part of the systemd(1) suite.

It is assumed that those settings are global for the whole program, so a fixed object path is used. The interface should always be available under the path /org/freedesktop/LogControl1.

DESCRIPTION

The following interface is exposed:

    node /org/freedesktop/LogControl1 {
      interface org.freedesktop.LogControl1 {
        properties:
          @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
          @org.freedesktop.systemd1.Privileged("true")
          readwrite s LogLevel = '...';
          @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
          @org.freedesktop.systemd1.Privileged("true")
          readwrite s LogTarget = '...';
          @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
          readonly s SyslogIdentifier = '...';
      };
      interface org.freedesktop.DBus.Peer { ... };
      interface org.freedesktop.DBus.Introspectable { ... };
      interface org.freedesktop.DBus.Properties { ... };
    };

  Properties

LogLevel describes the syslog(3)-style log-level, and should be one of "emerg", "alert", "crit", "err", "warning", "notice", "info", "debug", in order of increasing verbosity.

LogTarget describes the log target (mechanism). It should be one of "console" (log to the console or standard output), "kmsg" (log to the kernel ring buffer), "journal" (log to the journal natively, see systemd-journald.service(8)), "syslog" (log using the syslog(3) call).

    Write Access

    The LogLevel and LogTarget properties are supposed to be writable. Care should be taken to ensure that only appropriately privileged clients can modify them.

SyslogIdentifier is a read-only property that shows the "syslog identifier". It is a short string that identifies the program that is the source of log messages that is passed to the syslog(3) call.

TOOLS

journalctl option -p/--priority= may be used to filter log messages by log level, option -t/--identifier= may be used to by the syslog identifier, and filters like "_TRANSPORT=syslog", "_TRANSPORT=journal", and "_TRANSPORT=kernel" may be used to filter messages by the mechanism through which they reached systemd-journald.

systemctl log-level and systemctl log-target verbs may be used to query and set the LogLevel and LogTarget properties of the service manager. systemctl service-log-level and systemctl service-log-target may similarly be used for individual services. (Services must have the BusName= property set and must implement the interface described here. See systemd.service(5) for details about BusName=.)

EXAMPLE

Example 1. Create a simple listener on the bus that implements LogControl1

    /* SPDX-License-Identifier: MIT-0 */
    /* Implements the LogControl1 interface as per specification:
     * https://www.freedesktop.org/software/systemd/man/org.freedesktop.LogControl1.html
     *
     * Compile with 'cc logcontrol-example.c $(pkg-config --libs --cflags libsystemd)'
     *
     * To get and set properties via busctl:
     *
     * $ busctl --user get-property org.freedesktop.Example \
     *                              /org/freedesktop/LogControl1 \
     *                              org.freedesktop.LogControl1 \
     *                              SyslogIdentifier
     *   s "example"
     * $ busctl --user get-property org.freedesktop.Example \
     *                              /org/freedesktop/LogControl1 \
     *                              org.freedesktop.LogControl1 \
     *                              LogTarget
     *   s "journal"
     * $ busctl --user get-property org.freedesktop.Example \
     *                              /org/freedesktop/LogControl1 \
     *                              org.freedesktop.LogControl1 \
     *                              LogLevel
     *   s "info"
     * $ busctl --user set-property org.freedesktop.Example \
     *                              /org/freedesktop/LogControl1 \
     *                              org.freedesktop.LogControl1 \
     *                              LogLevel \
     *                              "s" debug
     * $ busctl --user get-property org.freedesktop.Example \
     *                              /org/freedesktop/LogControl1 \
     *                              org.freedesktop.LogControl1 \
     *                              LogLevel
     *   s "debug"
     */
    #include <errno.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <syslog.h>
    #include <systemd/sd-bus.h>
    #include <systemd/sd-journal.h>
    #define _cleanup_(f) __attribute__((cleanup(f)))
    static int log_error(int log_level, int error, const char *str) {
      sd_journal_print(log_level, "%s failed: %s", str, strerror(-error));
      return error;
    }
    typedef enum LogTarget {
      LOG_TARGET_JOURNAL,
      LOG_TARGET_KMSG,
      LOG_TARGET_SYSLOG,
      LOG_TARGET_CONSOLE,
      _LOG_TARGET_MAX,
    } LogTarget;
    static const char* const log_target_table[_LOG_TARGET_MAX] = {
      [LOG_TARGET_JOURNAL] = "journal",
      [LOG_TARGET_KMSG]    = "kmsg",
      [LOG_TARGET_SYSLOG]  = "syslog",
      [LOG_TARGET_CONSOLE] = "console",
    };
    static const char* const log_level_table[LOG_DEBUG + 1] = {
      [LOG_EMERG]   = "emerg",
      [LOG_ALERT]   = "alert",
      [LOG_CRIT]    = "crit",
      [LOG_ERR]     = "err",
      [LOG_WARNING] = "warning",
      [LOG_NOTICE]  = "notice",
      [LOG_INFO]    = "info",
      [LOG_DEBUG]   = "debug",
    };
    typedef struct object {
      const char *syslog_identifier;
      LogTarget log_target;
      int log_level;
    } object;
    static int property_get(
                    sd_bus *bus,
                    const char *path,
                    const char *interface,
                    const char *property,
                    sd_bus_message *reply,
                    void *userdata,
                    sd_bus_error *error) {
      object *o = userdata;
      if (strcmp(property, "LogLevel") == 0)
        return sd_bus_message_append(reply, "s", log_level_table[o->log_level]);
      if (strcmp(property, "LogTarget") == 0)
        return sd_bus_message_append(reply, "s", log_target_table[o->log_target]);
      if (strcmp(property, "SyslogIdentifier") == 0)
        return sd_bus_message_append(reply, "s", o->syslog_identifier);
      return sd_bus_error_setf(error,
                               SD_BUS_ERROR_UNKNOWN_PROPERTY,
                               "Unknown property '%s'",
                               property);
    }
    static int property_set(
                    sd_bus *bus,
                    const char *path,
                    const char *interface,
                    const char *property,
                    sd_bus_message *message,
                    void *userdata,
                    sd_bus_error *error) {
      object *o = userdata;
      const char *value;
      int r;
      r = sd_bus_message_read(message, "s", &value);
      if (r < 0)
        return r;
      if (strcmp(property, "LogLevel") == 0) {
        int i;
        for (i = 0; i < LOG_DEBUG + 1; i++)
          if (strcmp(value, log_level_table[i]) == 0) {
            o->log_level = i;
            setlogmask(LOG_UPTO(i));
            return 0;
          }
        return sd_bus_error_setf(error,
                                 SD_BUS_ERROR_INVALID_ARGS,
                                 "Invalid value for LogLevel: '%s'",
                                 value);
      }
      if (strcmp(property, "LogTarget") == 0) {
        LogTarget i;
        for (i = 0; i < _LOG_TARGET_MAX; i++)
          if (strcmp(value, log_target_table[i]) == 0) {
            o->log_target = i;
            return 0;
          }
        return sd_bus_error_setf(error,
                                 SD_BUS_ERROR_INVALID_ARGS,
                                 "Invalid value for LogTarget: '%s'",
                                 value);
      }
      return sd_bus_error_setf(error,
                               SD_BUS_ERROR_UNKNOWN_PROPERTY,
                               "Unknown property '%s'",
                               property);
    }
    /* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
     */
    static const sd_bus_vtable vtable[] = {
      SD_BUS_VTABLE_START(0),
      SD_BUS_WRITABLE_PROPERTY(
        "LogLevel", "s",
        property_get, property_set,
        0,
        0),
      SD_BUS_WRITABLE_PROPERTY(
        "LogTarget", "s",
        property_get, property_set,
        0,
        0),
      SD_BUS_PROPERTY(
        "SyslogIdentifier", "s",
        property_get,
        0,
        SD_BUS_VTABLE_PROPERTY_CONST),
      SD_BUS_VTABLE_END
    };
    int main(int argc, char **argv) {
      /* The bus should be relinquished before the program terminates. The cleanup
       * attribute allows us to do it nicely and cleanly whenever we exit the
       * block.
       */
      _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
      object o = {
        .log_level = LOG_INFO,
        .log_target = LOG_TARGET_JOURNAL,
        .syslog_identifier = "example",
      };
      int r;
      /* https://man7.org/linux/man-pages/man3/setlogmask.3.html
       * Programs using syslog() instead of sd_journal can use this API to cut logs
       * emission at the source.
       */
      setlogmask(LOG_UPTO(o.log_level));
      /* Acquire a connection to the bus, letting the library work out the details.
       * https://www.freedesktop.org/software/systemd/man/sd_bus_default.html
       */
      r = sd_bus_default(&bus);
      if (r < 0)
        return log_error(o.log_level, r, "sd_bus_default()");
      /* Publish an interface on the bus, specifying our well-known object access
       * path and public interface name.
       * https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
       * https://dbus.freedesktop.org/doc/dbus-tutorial.html
       */
      r = sd_bus_add_object_vtable(bus, NULL,
                                   "/org/freedesktop/LogControl1",
                                   "org.freedesktop.LogControl1",
                                   vtable,
                                   &o);
      if (r < 0)
        return log_error(o.log_level, r, "sd_bus_add_object_vtable()");
      /* By default, the service is assigned an ephemeral name. Also add a fixed
       * one, so that clients know whom to call.
       * https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
       */
      r = sd_bus_request_name(bus, "org.freedesktop.Example", 0);
      if (r < 0)
        return log_error(o.log_level, r, "sd_bus_request_name()");
      for (;;) {
        /* https://www.freedesktop.org/software/systemd/man/sd_bus_wait.html
         */
        r = sd_bus_wait(bus, UINT64_MAX);
        if (r < 0)
          return log_error(o.log_level, r, "sd_bus_wait()");
        /* https://www.freedesktop.org/software/systemd/man/sd_bus_process.html
         */
        r = sd_bus_process(bus, NULL);
        if (r < 0)
          return log_error(o.log_level, r, "sd_bus_process()");
      }
      /* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html
       */
      r = sd_bus_release_name(bus, "org.freedesktop.Example");
      if (r < 0)
        return log_error(o.log_level, r, "sd_bus_release_name()");
      return 0;
    }

This creates a simple server on the bus. It implements the LogControl1 interface by providing the required properties and allowing to set the writable ones. It logs at the configured log level using sd_journal_print(3).

Note that when porting this example to other D-Bus libraries it might be necessary to add manual client privilege checks, as they typically do not default to the restrictive defaults of sd-bus, where unprivileged access to properties is controlled via the SD_BUS_VTABLE_UNPRIVILEGED flag that is opt-in rather than opt-out.

SEE ALSO

systemd(1), journalctl(1), systemctl(1), systemd.service(5), syslog(3)