Sunday, March 22, 2015

Block Access during certain period using EBTABLES

Say, we want to block any packets coming from a device with mac address 00:01:02:03:04:05 (in other words, our router/switch should just silently drop any packets coming from this MAC address) during period of time 00:00 (00:00 AM) to 6:00 AM, do:

#ebtables -A INPUT -s 00:01:02:03:04:05  --timestart 0:0 --timestop 06:00 -j DROP

If we just want to drop IPv4 packets for the above:

#ebtables -A INPUT -p IPv4 -s 00:01:02:03:04:05  --timestart 0:0 --timestop 06:00 -j DROP

So, parameters for ebtables are actually similar (yet subset) of iptables (netfilter).

Wednesday, March 11, 2015

To fix Mute button keyboard shortcut issue on LXDE

Create a script call "amixertoggle" and save it in /usr/local/bin:


amixer $1 sset Headphone toggle
amixer $1 sset Speaker toggle
amixer $1 sset PCM toggle
amixer $1 sset Master toggle

Edit file $HOME/.config/openbox/lxde-rc.xml and replace block that has "XF86AudioMute" to call our script.  For example:


    <keybind key="XF86AudioMute">
      <action name="Execute">
              <command>amixertoggle -c 1 toggle</command>


(" -c 1" above is for machine, where the mixer control is actually on card 1.  If doesn't work, we can try different number)

Reload the modified file lxde-rc.xml by doing:

openbox --reconfigure

That's it.  Everytime we press "Mute" button on our PC keyboard, the mute/unmute will toggle.

Thursday, February 26, 2015

Raspberry Pi 2

Got this New Raspberry Pi 2 I ordered a few weeks ago.  It's significantly faster than the first version.  I only wish it had USB 3.0 ports so I can have much bigger and faster storage.

Monday, October 13, 2014

Cheap ATMEL AVR ISP ICE from QinHeng

I bought this small USB stick from eBay.  It supposedly can do JTAG and many other cool stuff for ATMEL microcontrollers.

Anyway, here is the detail how to use it:

The other end of the stick has dual-line 10-pin connector.  The pins are:

Pin  Purpose
1    TCK
2    GND
3    TDO
4    VTref
5    TMS
6    nSRT
7    VSupply
8    (nSRT)
9    TDI
10   GND

These pins are actually JTAG pins.  The connections to the Microcontroller is as follow (the pullup resistors are all 4.7k):

The USB id:

$ lsusb
Bus 003 Device 006: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter

Upon inserting the stick to my PC's USB (running Ubuntu Linux):

[118845.955500] ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
[118845.955541] ch341 3-2:1.0: device disconnected
[118851.692044] usb 3-2: new full-speed USB device number 6 using ohci-pci
[118851.901261] usb 3-2: New USB device found, idVendor=1a86, idProduct=7523
[118851.901271] usb 3-2: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[118851.901276] usb 3-2: Product: USB2.0-Serial
[118851.904683] ch341 3-2:1.0: ch341-uart converter detected
[118851.940428] usb 3-2: ch341-uart converter now attached to ttyUSB0

To use it as programmer, we can use avrdude:

$ sudo avrdude -c avrisp -P /dev/ttyUSB0 -pm328p <other options>

To debug, we can use another opensource, AvaRICE (from ubuntu, just do "apt-get install avarice").

For example (this is just to show how to use it, as my ICE stick is not connected to any target device yet so it reports it "No configuration available for device ID: ffff"):

$ sudo avarice --jtag /dev/ttyUSB0 -1
AVaRICE version 2.11, Jan 17 2014 02:53:19

Defaulting JTAG bitrate to 250 kHz.

JTAG config starting.
Hardware Version: 0xc3
Software Version: 0x80
Reported JTAG device ID: 0xFFFF
No configuration available for device ID: ffff

The avarice can be set as a gdb server mode, so we can debug the target removely. For example, to make it as gdb-server listening on port 4242.

$ sudo avarice --jtag /dev/ttyUSB0 :4242

From another PC (or the same PC, if host and server is on the same machine), we open gdb, and connect:

$ gdb
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
Find the GDB manual and other documentation resources online at:
For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) target remote

Using PicKit2 Programmer to program Atmel's microcontrollers

Many hobbists are familiar with Arduino kit.  This beast uses various Microcontrollers from Atmel.  For example, Arduino UNO uses AtMega 328p.

Normally, we use the pcb kit to program the chip, as the Arduino (or the clones) use USB to serial chip to translate  USB to serial needed to program Arduino.

For people who has done hacking on PIC microcontrollers from MicroChip and wants to use bare-metal chip, they can program the Atmel chips using the existing cheap PicKit2 programmer.  What they need just the Pickit2 programmer and a software tool called avrdude.

The pin connections:

AVR   - PICKit2 (pin):
RST   - VPP/MCLR (1)
VDD   - VDD Target (2) -- optional if AVR self powered
GND   - GND (3)
MISO  - PGD (4)
SCLK  - PDC (5)
MOSI  - AUX (6) 
Some of command examples are shown below:

To write our firmware to the flash in AtMega 328p:

$ avrdude -c pickit2 -p m328p -v -V -U flash:w:"myfirmware.hex":a 

To upload bootloader (e.g. to upload ARDUINO bootloader):
$ avrdude -c pickit2 -p m328p -v -V -U boot:w:"boot.hex":a 


Left shift by negative number used to test unique characters in a string

What is the outcome of this?

mask = 1<<val;

I tried to execute it on Python, it failed with error message.

But if I do it in C or C++, I get something else.  Turn out, this depends o the compiler and CPU we use.  As I use Pentium4 and GCC to compile this, the following snippet of the generated assembly language is interesting:

        movsbl  (%edx), %ecx
        movl    %esi, %edi
        addl    $1, %edx
        subl    $97, %ecx
        sall    %cl, %edi
        testl   %eax, %edi
        jg      .L5
        orl     %edi, %eax


What is "sall" above? It is Intel CPU's mnemonic for "Shift Arithmetic Left for Long Integer".

In the machine code, -32 is transformed to Second-complement of 32:

32        = 0b00100000
~32      = 0b11011111
~32+1 = 0b11100000 = -32

According Wikipedia,  arithmetic shift left maintains the MSBit, so in this case MSB is always 1. Also, Intel CPUs only shifts the first 5-bits of the shifting bits (in this case, 5-bit LSB of 0b11100000 which is 0), so 1<<-32 is actuall 1<<0 = 1.

What about 1<<-33?

33        = 0b00100001
~33      = 0b11011110
~32+1 = 0b11100001 = -33

The first five bits is 1, so 1<<-33 = 1<<1 = 2

What about 1<<-31?

31        = 0b00011111
~31      = 0b11100000
~31+1 = 0b11100001 = -31

The first five bits is 1, so 1<<-31 = 1<<1 = 0x00000010

What about 1<<-7?

7        = 0b00000111
~7      = 0b11111000
~7+1 = 0b11111001 = -7

The first five bits is 1, so 1<<-7 = 1<<25 = 0x02000000

Now, how do we use this to check a uniqueness of characters in a string? 
Alphabet 'A' to 'Z' has 26 characters. Alphabet 'a' to 'z' has also 26 characters.

The difference between the character i in a string is: str[i] - 'a'.  Now this is the interesting part: if we declare a 32-bit variable (which is enough to cover 'A' to 'Z') where each bit corresponds to the occurence of each character in alphabet, we can determine if the character has been used before and tell the string has unique characters or not (e.g, bit 1 to tell if character 'A' has been used or not, etc.)

'A' - 'a' = -32
'Z' - 'a' = -7
'a' - 'a' = 0
'Z' - 'a' = 25

if we shift 1 to -32, it is the same as 1<<0.  If we shift 1 to -33 it is the same as 1<<1, so on.

1<<-32 = 0x01
1<<-31 = 0x02
1<<-7 = 1<<25 = 0x2000000
1<<0 = 0x01
1<<1 = 0x02
1<<25 = 0x2000000

 1<<('A'-'a') == 1<<('a'-'a') = 0x01
 1<<('B'-'a') == 1<<('b'-'a') = 0x02
1<<('Z'-'a') == 1<<('z'-'a') = 0x2000000

So the code is:

for (i=0; i<strlen(s); i++)
    val = s[i] - 'a';
    if (flags & (1 << val))
        return 0; /* the string is not unique */
    flags |= 1 << val;
return 1;

Notice this algorithm might not work on different machine with different compiler (as shifting by negative number is undefined in standard C).