mtu-dbus.c

#include <dbus/dbus.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


#ifdef LOG_ERRORS
#include <syslog.h>
#endif

/* gcc -Os `pkg-config --cflags --libs dbus-1` mtu-dbus.c -o mtu-dbus */

static void terminateOnError(const char* msg,
                             const DBusError* error) {
  if (dbus_error_is_set(error)) {
#ifdef LOG_ERRORS
    syslog (LOG_ERR, error->name);
    syslog(LOG_ERR, error->message);
#endif
    exit(EXIT_FAILURE);
  }
}

int wifi_connection = 0;

void change_mtu() {
  system("ifconfig wlan0 mtu 1492 > /dev/null");
}

DBusHandlerResult signal_filter(DBusConnection *connection, DBusMessage * message ,
		void *user_data)
{
  DBusError error;
  dbus_error_init(&error);
  DBusMessageIter iter;

  char *key, * value;
  dbus_message_iter_init (message, &iter);
  int current_type;

  while ((current_type = dbus_message_iter_get_arg_type (&iter)) != DBUS_TYPE_INVALID) {
    if (current_type == DBUS_TYPE_STRING) {
      dbus_message_iter_get_basic (&iter, &key);
    }
    else if (current_type == DBUS_TYPE_VARIANT) {
      DBusMessageIter sub_iter;
      dbus_message_iter_recurse(&iter, &sub_iter);

      while ((current_type = dbus_message_iter_get_arg_type (&sub_iter)) != DBUS_TYPE_INVALID) {
        if (current_type == DBUS_TYPE_STRING) {
          dbus_message_iter_get_basic (&sub_iter, &value);
        }
        dbus_message_iter_next (&sub_iter);
      }
    }
    dbus_message_iter_next (&iter);
  }

  if (!strcmp(key, "DefaultTechnology")) {
    wifi_connection = !strcmp(value, "wifi");
  } else if (wifi_connection &&
             !strcmp(key, "ConnectionState") && !strcmp(value, "online")) {
    change_mtu();
  }

  dbus_error_free(&error);
  return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

static void daemonize()
{
    pid_t pid;


    if ((pid = fork()) < 0)
        exit(EXIT_FAILURE);

    if (pid > 0)
        exit(EXIT_SUCCESS);

    if (setsid() < 0)
        exit(EXIT_FAILURE);

    umask(0);

    chdir("/");

    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);
}

int main ()
{
  daemonize();
  DBusConnection* bus = NULL;
  DBusMessage* msg = NULL;
  DBusError error;
  dbus_error_init(&error);

#ifdef LOG_ERRORS
  openlog ("mtu-dbus", LOG_PID, LOG_DAEMON);
#endif

  change_mtu();

  bus = dbus_bus_get(DBUS_BUS_SYSTEM, &error);
  terminateOnError("Failed to open Session bus\n", &error);
  char *rule = "type='signal',interface='org.chromium.flimflam.Manager',member='PropertyChanged'";
  dbus_bus_add_match(bus, rule, &error);
  terminateOnError("adding rule", &error);

  dbus_connection_add_filter(bus, signal_filter, NULL, NULL);
  while (dbus_connection_read_write_dispatch (bus, -1))
    ;
#ifdef LOG_ERRORS
  closelog();
#endif

  return 0;
}