org.freedesktop.LogControl1 — D-Bus interface to query and set logging configuration
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.
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 { ... };
};
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).
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.
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 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.