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 http://www.kernel.org/pub/linux/kernel/v2.6/linux-x.y.z.tar.bz2
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
System.map-2.6.25
config-2.6.25
vmlinuz-2.6.25
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
savedefault
boot
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
Thursday, December 29, 2011
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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 3333 | pv -b > mybackup.tar.gz
Security
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 23333:127.0.0.1:3333 me@192.168.0.1 sleep 10; \
nc 127.0.0.1 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 dict.org dictionary server, I’d do:
$ nc dict.org 2628
220 ..............some WELCOME.....
DEFINE wn server
150 1 definitions retrieved
151 "server" wn "WordNet (r) 2.0"
server
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]
QUIT
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 192.168.0.1 80-90
Connection to 192.168.0.1 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.
Notes
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.
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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 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 192.168.0.1 3333 | pv -b > mybackup.tar.gz
Security
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 23333:127.0.0.1:3333 me@192.168.0.1 sleep 10; \
nc 127.0.0.1 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 dict.org dictionary server, I’d do:
$ nc dict.org 2628
220 ..............some WELCOME.....
DEFINE wn server
150 1 definitions retrieved
151 "server" wn "WordNet (r) 2.0"
server
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]
QUIT
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 192.168.0.1 80-90
Connection to 192.168.0.1 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.
Notes
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:
Script name is autossh.pl
-----------------------START--------------------------
#!/usr/bin/perl -w
use strict;
use Net::SCP::Expect;
if ($#ARGV != 0 ) {
print "usage: autossh.pl\n";
exit;
}
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, $remotedir);
print "SCP complete\n";
-----------------------END--------------------------
To run script type.
#./autossh.pl servername.com
This script will copy /home/mydata local directory to remove server "remoteserver.com" at /home/remote/backup directory.
Requirement: Net::SCP::Expect perl module is required for script.
Install required module:
perl -MCPAN -e 'installNet::SCP::Expect'
Script name is autossh.pl
-----------------------START--------------------------
#!/usr/bin/perl -w
use strict;
use Net::SCP::Expect;
if ($#ARGV != 0 ) {
print "usage: autossh.pl
exit;
}
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";
-----------------------END--------------------------
To run script type.
#./autossh.pl servername.com
This script will copy /home/mydata local directory to remove server "remoteserver.com" 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:
Sample outputs:
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'@192.168.1.5"
Sample outputs:
Where,
satiah - MySQL login username
192.168.1.5 - 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
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:
satish 192.168.1.5
tom 192.168.1.5
blog 192.168.1.7
root localhost
db1.vm.linuxbug.net.in
root db1.vm.linuxbug.net.in
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'@192.168.1.5"
Sample outputs:
GRANT USAGE ON *.* TO 'satish'@'192.168.1.5' IDENTIFIED BY PASSWORD 'somePasswordMd5'
GRANT ALL PRIVILEGES ON `blogdb`.* TO 'satish'@'192.168.1.5'
Where,
satiah - MySQL login username
192.168.1.5 - 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
Friday, August 27, 2010
GDB Example Debugging Session: Segmentation Fault Example
We are going to use gdb to figure out why the following program causes a segmentation fault. The program is meant to read in a line of text from the user and print it. However, we will see that in it's current state it doesn't work as expected...
1 : #include
2 : #include
3 : int main(int argc, char **argv)
4 : {
5 : char *buf;
6 :
7 : buf = malloc(1<<31);
8 :
9 : fgets(buf, 1024, stdin);
10: printf("%s\n", buf);
11:
12: return 1;
13: }
The first step is to compile the program with debugging flags:
prompt> gcc -g segfault.c
Now we run the program:
prompt > a.out
Hello World!
Segmentation fault
prompt >
This is not what we want. Time to fire up gdb:
prompt > gdb a.out
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
(gdb)
We'll just run it and see what happens:
(gdb) run
Starting program: /home/dgawd/cpsc/363/a.out
test string
Program received signal SIGSEGV, Segmentation fault.
0x4007fc13 in _IO_getline_info () from /lib/libc.so.6
So we received the SIGSEGV signal from the operating system. This means that we tried to access an invalid memory address. Let's take a backtrace:
(gdb) backtrace
#0 0x4007fc13 in _IO_getline_info () from /lib/libc.so.6
#1 0x4007fb6c in _IO_getline () from /lib/libc.so.6
#2 0x4007ef51 in fgets () from /lib/libc.so.6
#3 0x80484b2 in main (argc=1, argv=0xbffffaf4) at segfault.c:10
#4 0x40037f5c in __libc_start_main () from /lib/libc.so.6
We are only interested in our own code here, so we want to switch to stack frame 3 and see where the program crashed:
(gdb) frame 3
#3 0x80484b2 in main (argc=1, argv=0xbffffaf4) at segfault.c:10
10 fgets(buf, 1024, stdin)
We crashed inside the call to fgets. In general, we can assume that library functions such as fgets work properly (if this isn't the case, we are in a lot of trouble). So the problem must be one of our arguments. You may not know that 'stdin' is a global variable that is created by the stdio libraries. So we can assume this one is ok. That leaves us with 'buf':
(gdb) print buf
$1 = 0x0
The value of buf is 0x0, which is the NULL pointer. This is not what we want - buf should point to the memory we allocated on line 8. So we're going to have to find out what happened there. First we want to kill the currently-running invocation of our program:
(gdb) kill
Kill the program being debugged? (y or n) y
Now set a breakpoint on line 8:
(gdb) break segfault.c:8
Breakpoint 1 at 0x8048486: file segfault.c, line 8.
Now run the program again:
(gdb) run
Starting program: /home/dgawd/cpsc/363/a.out
Breakpoint 1, main (argc=1, argv=0xbffffaf4) at segfault.c:8
8 buf = malloc(1<<31);
We're going to check the value of buf before the malloc call. Since buf wasn't initialized, the value should be garbage, and it is:
(gdb) print buf
$2 = 0xbffffaa8 "Èúÿ¿#\177\003@t`\001@\001"
Now step over the malloc call and examine buf again:
(gdb) next
10 fgets(buf, 1024, stdin);
(gdb) print buf
$3 = 0x0
After the call to malloc, buf is NULL. If you were to go check the man page for malloc, you would discover that malloc returns NULL when it cannot allocate the amount of memory requested. So our malloc must have failed. Let's go back and look at it again:
7 : buf = malloc(1<<31);
Well, the value of the expression 1 <<>
prompt >
Hello World!
Hello World!
prompt >
So now you know how to debug segmentation faults with gdb. This is extremely useful (I use it more often then I care to admit). The example also illustrated another very important point: ALWAYS CHECK THE RETURN VALUE OF MALLOC! Have a nice day.
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