Thursday, December 29, 2011

How To Compile Linux Kernel

Compiling custom kernel has its own advantages and disadvantages.Compiling kernel needs to understand few things and then just type couple of commands. This step by step howto covers compiling Linux kernel version 2.6.xx under Debian GNU Linux.

Few step to compile kernel.

# cd /usr/src
# wget

Notes : Make sure GCC and GNU make utility installed on your system before compile kernel.

# tar cvjf linux-x.y.z.tar.bz2
# cd linux-x.y.z

Compile kernel

# make menuconfig

Start compiling to create a compressed kernel image, enter:
# make

compiling to kernel modules
# make modules

Install kernel modules
# make modules_install

To install kernel
# make install

It will install three files into /boot directory as well as modification to your kernel grub configuration file

Create an initrd image for new installed kernel
# cd /boot
# mkinitrd -o initrd.img-2.6.25 2.6.25

Modify Grub configuration file - /boot/grub/menu.lst
# vi /boot/grub/menu.lst

title Debian GNU/Linux, kernel 2.6.25 Default
root (hd0,0)
kernel /boot/vmlinuz root=/dev/hdb1 ro
initrd /boot/initrd.img-2.6.25

Note: its hard to edit this file without knowledge of options so better way is use update-grub command it will do it automaticaly for you.

# update-grub

Now time comes up for testing reboot your system and boot with new kernel and enjoy your system. Good Bye

Netcat - Swiss army knife

One of the Linux command line tools I had initially under-estimated is netcat or just nc. By default, netcat creates a TCP socket either in listening mode (server socket) or a socket that is used in order to connect to a server (client mode). Actually, netcat does not care whether the socket is meant to be a server or a client. All it does is to take the data from stdin and transfer it to the other end across the network.

The simplest example of its usage is to create a server-client chat system. Although this is a very primitive way to chat, it shows how netcat works. In the following examples it is assumed that the machine that creates the listening socket (server) has the IP address. So, create the chat server on this machine and set it to listen to 3333 TCP port:

$ nc -l 3333

On the other end, connect to the server with the following:

$ nc 3333

In this case, the keyboard acts as the stdin. Anything you type in the server machine’s terminal is transfered to the client machine and vice-versa.

Transfering Files

In the very same way it can be used to transfer files between two computers. You can create a server that serves the file with the following:

$ cat backup.iso | nc -l 3333

Receive backup.iso on the client machine with the following:

$ nc 3333 > backup.iso

As you may have noticed, netcat does not show any info about the progress of the data transfer. This is inconvenient when dealing with large files. In such cases, a pipe-monitoring utility like pv can be used to show a progress indicator. For example, the following shows the total amount of data that has been transfered in real-time on the server side:

$ cat backup.iso | pv -b | nc -l 3333

Of course, the same can be implemented on the client side by piping netcat’s output through pv:

$ nc 3333 | pv -b > backup.iso

Other Examples

Netcat is extremely useful for creating a partition image and sending it to a remote machine on-the-fly:

$ dd if=/dev/hdb5 | gzip -9 | nc -l 3333

On the remote machine, connect to the server and receive the partition image with the following command:

$ nc 3333 | pv -b > myhdb5partition.img.gz

This might not be as classy as the partition backups using partimage, but it is efficient.

Another useful thing is to compress the critical files on the server machine with tar and have them pulled by a remote machine:

$ tar -czf - /etc/ | nc -l 3333

As you can see, there is a dash in the tar options instead of a filename. This is because tar’s output needs to be passed to netcat.

On the remote machine, the backup is pulled in the same way as before:

$ nc 3333 | pv -b > mybackup.tar.gz


It is obvious that using netcat in the way described above, the data travels in the clear across the network. This is acceptable in case of a local network, but, in case of transfers across the internet, then it would be a wise choice to do it through an SSH tunnel.

Using an SSH tunnel has two advantages:

1. The data is transfered inside an encrypted tunnel, so it is well-protected.
2. You do not need to keep any open ports in the firewall configuration of the machine that will act as the server, as the connections will take place through SSH.

You pipe the file to a listening socket on the server machine in the same way as before. It is assumed that an SSH server runs on this machine too.

$ cat backup.iso | nc -l 3333

On the client machine connect to the listening socket through an SSH tunnel:

$ ssh -f -L 23333: me@ sleep 10; \
nc 23333 | pv -b > backup.iso

This way of creating and using the SSH tunnel has the advantage that the tunnel is automagically closed after file transfer finishes. For more information and explanation about it please read my article about auto-closing SSH tunnels.

Telnet-like Usage

Netcat can be used in order to talk to servers like telnet does. For example, in order to get the definition of the word “server” from the “WordNet” database at the dictionary server, I’d do:

$ nc 2628
220 ..............some WELCOME.....
DEFINE wn server
150 1 definitions retrieved
151 "server" wn "WordNet (r) 2.0"
n 1: a person whose occupation is to serve at table (as in a
restaurant) [syn: {waiter}]
2: (court games) the player who serves to start a point
3: (computer science) a computer that provides client stations
with access to files and printers as shared resources to a
computer network [syn: {host}]
4: utensil used in serving food or drink
250 ok [d/m/c = 1/0/18; 0.000r 0.000u 0.000s]
221 bye [d/m/c = 0/0/0; 16.000r 0.000u 0.000s]

Works as a Port Scanner too

A useful command line flag is -z. When it is used, netcat does not initiate a connection to the server, but just informs about the open port it has found. Also, instead of a single port, it can accept a port-range to scan. For example:

$ nc -z 80-90
Connection to 80 port [tcp/http] succeeded!

In this example, netcat scanned the 80-90 range of ports and reported that port 80 is open on the remote machine.

The man page contains some more interesting examples, so take the time to read it.

All the above examples have been performed on Fedora 5/6. Netcat syntax may vary slightly among Linux distributions, so read the man page carefully.

Netcat provides a primitive way to transfer data between two networked computers. I wouldn’t say it’s an absolutely necessary tool in the everyday use, but there are times that this primitive functionality is very useful.

Friday, December 2, 2011

SFTP/SCP autologin Perl script

I have wrote script to autologin scp and transfer specified file. I am using perl Net::SCP::Expect module for it.

Requirement:   Net::SCP::Expect  perl module is required for script.
Install required module:

perl -MCPAN -e 'install Net::SCP::Expect' 

Script name is


#!/usr/bin/perl -w

use strict;
use Net::SCP::Expect;

if ($#ARGV != 0 ) {
print "usage: \n";

my $server=$ARGV[0];

my $user = "orion";
my $password = 'mypassword';
my $remotedir = "/home/local/mydata/";
my $filelocation = "/home/remote/backup/";

print "Login...Starting scp...";
my $scpe = Net::SCP::Expect->new(host=>$server, user=>$user, password=>$password, recursive=>'1', auto_yes => '1', auto_quote => '0');

print "\nFILELOCATION:" . $filelocation . "*\n";
print "REMOTEDIR: " . $remotedir . "\n";

$scpe->scp($filelocation, $remotedir);

print "SCP complete\n";


To run script type.


This script will copy /home/mydata local directory to remove server "" at /home/remote/backup directory.

Friday, September 9, 2011

Migrate / Move MySQL Database And Users To New Server

Get Current MySQL, Usernames, Hostname, And Database Names

Type the following command at shell prompt to list username and hostname list, enter:

mysql -u root -B -N -p -e "SELECT user, host FROM user" mysql

Sample outputs:

root localhost

The first column is mysql username and second one is network host names. Now, type the following command to get exact details about grants and password for each user from above list:

mysql -u root -p -B -N -e"SHOW GRANTS FOR 'userName'@hostName"
mysql -u root -p -B -N -e"SHOW GRANTS FOR 'satish'@"

Sample outputs:

GRANT USAGE ON *.* TO 'satish'@'' IDENTIFIED BY PASSWORD 'somePasswordMd5'
GRANT ALL PRIVILEGES ON `blogdb`.* TO 'satish'@''


satiah - MySQL login username - Another server or workstation to access this mysql server
somePasswordMd5 - Password stored in mysql database which is not in a clear text format
blogdb - Your database name