Enable Wifi N access point with hostapd

I use an odroid (a rasberry-pi like mini-pc but more powerfull) as a Wifi access point for my smartphone and my camera since quite a long time. I forgot that my USB Wifi dongle was compatible with Wifi N (only on 2.4Ghz), so my hostapd config file was :


Here is the speed result with iperf :

[ 4] local port 5001 connected with port 48727
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.2 sec 18.4 MBytes 15.1 Mbits/sec

Normaly, this should be 56Mbits/s, but we know wifi is crap…

And to enable Wifi N :

hw_mode=g   #Yes, this is not an error. Wifi N builds on top of G ;)


And the speed result is now :

[ 4] local port 5001 connected with port 48754
[ 4] 0.0-10.1 sec 30.6 MBytes 25.4 Mbits/sec

Better, but still not the 150Mbits/s of wifi N… But it’s better !

Limit rate of mail sent by motion detection daemon (motiond) for webcam surveillance

Maybe you use the “on_picture_save” to send yourself an e-mail with motiond when your webcam detect an  intruder. But when someone enters, you’ll receive a lot of pictures. And if it was just you forgetting to stop the detection before entering, your smartphone will become a christmas tree for nothing…

I just published my last program “limitrate” on Github :


This programs allows you to limit the rate of launching a certain command. Instead of launching

/usr/bin/program argument1 argument2

call :

limitrate 30 “/usr/bin/program % argument2” argument1

To launch only this command at maximum once every 30 seconds. If you call multiple times the last line in 30 seconds, limitrate will run the command only after 30 seconds replacing the “%” by the concatenation of all “argument 1”. For example to send an alert e-mail containing files (for example webcam picture) at max every 30 seconds :

10:24:30 : limitrate 30 "echo \"Alert!\" | mailx % destination@mail.com" "-A /path/to/file1"
--> Send a mail with file1
10:24:32 : limitrate 30 "echo \"Alert!\" | mailx % destination@mail.com" "-A /path/to/file2"
10:24:38 : limitrate 30 "echo \"Alert!\" | mailx % destination@mail.com" "-A /path/to/file3"
10:24:41 : limitrate 30 "echo \"Alert!\" | mailx % destination@mail.com" "-A /path/to/file4"
10:25:05 : limitrate 30 "echo \"Alert!\" | mailx % destination@mail.com" "-A /path/to/file5"
--> Send a mail with file 2,3,4 and 5

Example configuration for motion detection daemon (Camera surveillance) :

on_picture_save /usr/local/bin/limitrate 15 “echo \”See attachment below.\” | mailx -s ‘Motion in the saloon !’ %destination@tombarbette.be” “-A %f”

When an intrusion is detected, you’ll receive a first e-mail with one image. 15 seconds later you’ll receive all the new images in only one mail, and so on.

ZSH : Open terminal where you left, for each session

There is some snippets for ZSH configuration which allow you to re-open the session in the folder where it was last closed available on the web. The problem is that you often launch 3 sessions at the same time, work on them and then quit/reboot/loose SSH connections/… So you will re-log 3 sessions which will start in the same last opened folder.

I propose a version allowing to keep the last folder per-session. Each ZSH session receive a number and write the current folder in a per-session file. When you open a new session it opens the file number associated to the session number.


Add somewhere in .zshrc :

mkdir -p ~/.cwd/
session_num=`pgrep zsh | wc -l`
function cd() {
builtin cd "$@";
echo "$PWD" > ~/.cwd/$session_num
echo "$PWD" > ~/.cwd/last
export cd
function cwd() {
if [ -e ~/.cwd/$session_num ] ; then
cd "$(cat ~/.cwd/$session_num)"
cd "$(cat ~/.cwd/last)"
echo "This is session #$session_num"

And at the bottom of the file :



Installing Spotify 9.10 (may 20), 9.11 (july 2) on fedora 20 64 bits

UPDATE : Tested for 9.11 on July 2


Remove any old version of spotify :

yum remove “*spotify*”

rm -rf /usr/local/share/spotify*


Remove any configuration file of the last spotify version :

rm -rf /home/$USER/.config/spotify

rm -rf /home/$USER/.cache/spotify


Download the debian package at http://repository.spotify.com/pool/non-free/s/spotify/spotify-client_0.9.10.17.g4129e1c.78-1_amd64.deb  http://repository.spotify.com/pool/non-free/s/spotify/spotify-client_0.9.11.26.g995ec04.78-1_amd64.deb:

wget http://repository.spotify.com/pool/non-free/s/spotify/spotify-client_0.9.11.26.g995ec04.78-1_amd64.deb


Install alien to convert the deb to RPM :

sudo yum install alien


Convert the deb to RPM with alien :

fakeroot alien -r spotify-client_0.9.10.17.g4129e1c.78-1_amd64.deb


Install the rpm :

sudo rpm -ivh –nodeps –force spotify-client-


You will very probably have problems of dependencies to old libraries when launching spotify, like for libssl.so.1.0.0 , libudev.so.0 and libcrypto.so.1.0.0, I made a tarball with them.


Download oldlibs.tar :

wget https://www.tombarbette.be/wp-content/uploads/2014/05/oldlibs.tar.gz


Untar them :

tar -zxvf oldlibs.tar.gz


Copy them to /usr/lib64 :

cp -rf oldlibs/* /usr/lib64/


Remove the tarball :

rm -rf oldlibs*


If you don’t have any shorcut to launch spotify, copy the desktop entry :

cp -rf /opt/spotify/spotify-client/spotify.desktop /usr/share/applications/


If when you type “spotify” it doesn’t launch anything (and say this command is not found) :

sudo ln -s /opt/spotify/spotify-client/spotify /usr/bin/spotify


Maybe you already know the program “top”, “htop” is its enhanced version. And is very usefull to see how your systems handle its load and where is the load.




You’ve got your CPUs load per core on top. Here I’ve got two processors with 8 cores each, and having hyperthreading activated, so 32 logical cores. The part in green is the percentage of time spent in your programs, and the read is the percentage of time spent in kernel. You also have the memory usage and programs.



Top – Remember…

Post it – What to save when reinstalling a server

More a post-it for myself, what to save when formatting or doing a major upgrade of my linux servers.


– database SQL – Saving /var/lib/mysql is possible but you’ll have to change some maintenance passwords. The easiest is to export your databases with the export function of phpmyadmin or with mysqldump.
– /var/svn – If you have an SVN server
– /var/www – Websites
– /etc – Configuration. Do not re-apply everything ! Choose only some config files for example :

  • /etc/dhcp/dhcpd.conf – Dhcp server config file
  • /etc/php/php.ini (can change according to linux distributions) – Php configuration
  • /etc/apache/sites-enbabled – Apache websites
  • /etc/apache/apache2.conf – Apache config
  • /etc/passwd /etc/shadow /etc/group – Users, groups, and passwords. But I may be a good idea to force everyone to change passwords at the same time, and to clean…
  • And many many mores…

– /home – If you want to keep user data. You should not have to save that when re-formatting your system because /home should always be another partition than the system “/” partition.
– ~/public_html – If not saving the homes, saves your local websites…


Any other ideas?

Understanding sleeping mechanism in Linux Kernel

The students of INFO-0940 were asked to remove a task from it’s current scheduler class when calling a new syscall and put it back when we call again that syscall. The question is : what to do if the task is currently sleeping? Cause the timer will expire at one moment and maybe put back the state to TASK_RUNNING and run the task which should haven’t been able to run again. So either it expires after the processus is back in its previous scheduler, and that’s okay as it will reset it in a runnable state. Or it expires while it’s still removed, and that’s a problem as it will put it back in its previous state (or not, depending on how the syscall is implemented).
Warning : this post is just the result of a quick look. More to give you some ideas and basis for understanding the sleeping mechanism.


So we should try to understand how sleep works.


Let’s look at the nanosleep syscall (which is called when you user sleep() or usleep() functions in C).

SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp,
		struct timespec __user *, rmtp)
	struct timespec tu;

	if (copy_from_user(&tu, rqtp, sizeof(tu)))
		return -EFAULT;

	if (!timespec_valid(&tu))
		return -EINVAL;

	return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);

It copies the userspace data, check that the time is valid and call hrtimer_nanosleep.

long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,
		       const enum hrtimer_mode mode, const clockid_t clockid)
	struct restart_block *restart;
	struct hrtimer_sleeper t;
	int ret = 0;
	unsigned long slack;

	slack = current->timer_slack_ns;
	if (rt_task(current))
		slack = 0;

	hrtimer_init_on_stack(&t.timer, clockid, mode);
	hrtimer_set_expires_range_ns(&t.timer, timespec_to_ktime(*rqtp), slack);
	if (do_nanosleep(&t, mode))
		goto out;

	/* Absolute timers do not update the rmtp value and restart: */
	if (mode == HRTIMER_MODE_ABS) {
		goto out;

	if (rmtp) {
		ret = update_rmtp(&t.timer, rmtp);
		if (ret <= 0)
			goto out;

	restart = &current_thread_info()->restart_block;
	restart->fn = hrtimer_nanosleep_restart;
	restart->nanosleep.clockid = t.timer.base->clockid;
	restart->nanosleep.rmtp = rmtp;
	restart->nanosleep.expires = hrtimer_get_expires_tv64(&t.timer);

	return ret;

It initialize a timer and call do_nanosleep giving it the timer. After do_nanosleep it will destroy/free the timer structure.

static int __sched do_nanosleep(struct hrtimer_sleeper *t, enum hrtimer_mode mode)
	hrtimer_init_sleeper(t, current);

	do {
		hrtimer_start_expires(&t->timer, mode);
		if (!hrtimer_active(&t->timer))
			t->task = NULL;

		if (likely(t->task))


	} while (t->task && !signal_pending(current));


	return t->task == NULL;

This is the interesting part.


The first thing done is calling hrtimer_init_sleeper(); which will set parameters of the timer to call the function hrtimer_wakeup when the timer expires, and specify the task to wakeup at that time. hrtimer_wakeup will simply set the task to NULL and call wake_up_process() on that task. We’ll come back to wake_up_process(task) later.


We see that after that the state is changed to TASK_INTERRUPTIBLE. Then it starts the timer strictly speaking, and go in schedule().


Remember that schedule(), before scheduling a new task, will test the state of the current (which is now “previous”) task, and if the state is not TASK_RUNNING, it will remove that task from its runqueue (and that is the case as it is in the TASK_INTERRUPTIBLE state).


Schedule() goes on and start scheduling another task, if there is not, it will run the “idle” task.


At some point, the timer will expire. The timer rely on an hardware timer which will cause an interrupt, leaving any current task to process the interrupt handler (this is not the schedule() handler and so on !). The interrupt handler for the hardware timer is  hrtimer_interrupt() which will run hrtimer_wakeup() for all expired timer. As said before, outr timer for the sleeping mechanism will set the timer task as NULL, and call wake_up_process(). But that functions just put back the process in the runqueue and set its state to TASK_RUNNING, not actually scheduling it. The interrupt handler will finish and the CPU will go back to it’s currently running process (maybe the idle process, running the cpu_idle() function). 


That currently running process will eventually finish its processing or be preempted (the normal scheduling mechanism), and the process which was sleeping will re-run again when it will be picked again by pick_next_task().


But where will this process restart? After the sleep() or usleep() call? No ! Where it was… in the nanosleep syscall. What does the syscall do after its call to schedule()? Put the state back to TASK_RUNNING and effectively running. That seems obvious as we said before that the syscall is taking care of destroying the timer structure, so it should not restart after the syscall.


Note that I skipped the part where do_nanosleep() is looping and so on… I tried usleep(1) and sleep(1), and the loop never happend. I suspect it’s for very long timers, the kernel would’nt launch a timer of 1 hour long… But it’s just a guess.


Home server : auto-shutdown if no other computers are running

I wanted to shutdown my linux home media server if there is no running computer on my network. So I wrote this little programs which reads all known ips from DHCP configuration and lease files and send a ping to them. If the ping respond, one PC of my LAN is up…

To re-start the computer in the morning, I use the BIOS RTC alarm (the thing you have by pressing F1 or ESC on reboot). You could also add a script/a program on each of your computers to send the magic packet to your home server to wake it by lan (see “wake on lan” on google).

This script can take any command. But if you want to do shutdown like proposed in the title, you can use :

sudo ./autoshut "poweroff"

Where is your local IP adress. To compile the program, simply use (after saving the code as “autoshut.c”) :

gcc -o autoshut autoshut.c

In my case, I wanted to launch the command only after midnight, so I used cron. Cron will launch that command every 5 minutes from midnight to eight o’clock. So if I stay up late, my server won’t shutdown if my own computer is not down too. That’s the whole purpose.

The line in my crontab :

0,10,20,30,40,50 1,2,3,4,5,6,7 * * * root /home/tom/autoshut/autoshut "poweroff"

Why do all that ? Energy consumption…

#include <stdio.h>
#include <stdlib.h>
#include <regex.h>
#include <string.h>

#define LEASE_FILE "/var/lib/dhcp/dhcpd.leases"
#define DHCP_CONFIG_FILE "/etc/dhcp/dhcpd.conf"

int in_array(char** ar, char* str) {
int i = 0;
while (ar[i] != NULL) {
if (strcmp(ar[i],str) == 0) return 1;
return 0;

char* extractIP (char* filename, char** list, int* listNum) {
FILE *pfile;

pfile = fopen(filename, "rb");

if(pfile == NULL){
printf("Sorry, can't open %s\n", filename);
return '\0';
regex_t reg;
int err = regcomp (&reg, "(10\\.[0-1]\\.0\\.([1-9]|[0-9]{2,3}))", REG_EXTENDED);
if (err != 0)
return '\0';
char ligne[255];

while(!feof(pfile)) {
fgets(ligne, 254 ,pfile);
int match;
size_t nmatch = 0;
regmatch_t *pmatch = NULL;

nmatch = reg.re_nsub;
pmatch = malloc (sizeof (*pmatch) * nmatch);
match = regexec (&reg, ligne, nmatch, pmatch, 0);
if (match == 0)
char *ip = NULL;
int start = pmatch->rm_so;
int end = pmatch->rm_eo;
size_t size = end - start;

char* str = malloc(sizeof(char) * 15);
strncpy (str, &ligne[start], size);
str[size] = '\0';

if (!in_array(list, str)) {
list[*listNum] = str;
printf("%s\n", str);




int ping(char* address) {

char cmd[30] = "ping -W 1 -q -c 1 ";
int ret=system(cmd);
printf("\nResultat : %d\n\n",ret);
return !ret;
TODO : maybe an option?
int checkCableStatus(const char* interface) {

int main(int argc, char** argv) {

if (argc <= 2) {
printf("Usage : %s Command Local-IP [Local-IP-2]\n\tCommand : a command to execute if ping does not work\n\tLocal-IP : Ip to ignore\n\tLocal-IP-2 : Optional second ip to ignore",argv[0]);
return -1;
int listNum = 0;
char** list = malloc(sizeof(char*) * 255);
extractIP(DHCP_CONFIG_FILE, list, &listNum);
extractIP(LEASE_FILE, list, &listNum);
int i = 0;
while (list[i] != NULL) {
if (strcmp(list[i],argv[2])!=0 && (argc==3 ||strcmp(list[i],argv[3])!=0)) {
if (ping (list[i])) {
printf("%s responded. Command aborted !\n",list[i]);

Can a task have multiple states?

A little correction about what I said in class… I said that technically, a task could have multiple states because it’s a bitmask but not in practice. It seems that was before that, and searching for TASK_KILLABLE in the linux kernel code (and not sched.c only, that was my mistake), you’ll find for example in timer.c :




And TASK_KILLABLE is a combinaison of two states… So it’s not just used for convenient “multiple test at once”.

Automatically find all returned e-mails from “Undelivered messages”

If you’ve got your mails under a text format in a folder (like the unix Maildir) you can use this command to extract the e-mails with a 550 return error.


cat * | grep --text -Pzoi '([a-z0-9._-]+@[a-z0-9.]+)(?=.*host.*said.*55[0-9])'


The first command in the pipe, cat, send all files content to the next command in the pipe : grep. Grep is removing everything except what is an adress, and only if it is followed by “host * said * 55[0-9] ” where * can be everything and [0-9] is a number between 0 and 9. We also use the –text parameter because some mails could contain binary data.


As grep give you the mail separated by new line, and an sql command takes a list of strings separated by comma, you can copy the list in gedit or notepad++ and use search->replace to change them in the format ‘mail1’, ‘mail2’, … You have to put “(.*)” in the search field, “‘\1’,” in the replace by field, and select “regular expression”. You then place the result in the parenthesis after IN, in SQL command below :


UPDATE contact SET mail='' WHERE mail IN ('bad@hotmail.com', 'error@mail.com')