linux guide

If we have logical volume we can dynamicly add or remove diskspace to or from it. Any disk partitions to used as physical volumes need to have their partition types set to 8e, Linux LVM. After creating Primary Volu me, we must create a Volume Group with any name.After that we will create Logical Volume. Not only to the Logical Volume, we can extend or reduce the size of a Volume Group.
Lets start -
Create a Partition of LVM type using fdisk and reboot the system. –

fdisk /dev/hda
n
(define/accept the first cylinder)
(define the last cylinder) +100M
n
(define/accept the first cylinder)
(define the last cylinder) +150M
p (check the newly created partitions at the bottom, note down the partition no.
We are assuming /dev/hda10 and /dev/hda11 are created)
t
10 (partition no)
l ( to list codes - please note the code for Linux LVM, it is 8e)
8e
t
11 (partition no)
8e
w ( to write to partition table and exit from fdisk)

reboot ( reboot your system)

Initialize the LVM Configuration Files
vgscan

Initialize the LVM partitions as physical volumes.
pvcreate /dev/hda10 /dev/hda11

Create Vouume Group.
vgcreate vg1 /dev/ hda10
Here, vg1 is volume group name

Create Logical Volume.

lvcreate -L 50M -n lvm1 vg1
Here, lvm1 is logical volume name

Make filesystem.

mkfs /dev/vg1/lvm1
Mount the newly created logical volume to a directory, say /lvmone
mount /dev/vg1/lvm1 /lvmone

Check the volume size.

lvdisplay /dev/vg1/lvm1


If you want to resize it, first unmount the partion and run
e2fsadm -L+25M /dev/vg1/lvm1

Check the volume size once again. If you want to reduce the partition size run -
e2fsadm -L-20M /dev/vg1/lvm1

Run the following command to see the volume group size and space left on it.
vgdisplay

To extend the volume group size

vgextend vg1 /dev/hda11

Run ‘vgdisplay’ once again to check the volume group size, allocated space and free space.

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To Change owner of a file
chown user_name file_name
Example: You want to make gautam as owner of beragautam.tx
chown gautam beragautam.tx

To change owner and group of a file
chown user_name.group_name file_name

You can also use this command as follows –
chown user_nam:group_name file_name
(Please note, you can use either “:” or “.”)

If you want to assign a file would be used by a group use the following command.
chgrp special atanu.tx
Here group name is special and file name is beragautam.tx

What is a group?
A group is used to represent users, who have similar characteristics,exercise similar tasks and
enjoy similar permission and environment in a network. Now think about the situation here.
You have a group of users who need similar type of access to a file, beragautam.tx. What you will do? Very simple

Create a group.

Issue the command that enables the group to access the file

Give file access permission to this group. (Example, all permissions to owner and read and
execute permissions to group)

Make the users members of a group.

Do these tasks as defined below.
groupadd newgroup
chgrp newgroup bera.tx
chmod 750 beragautam.tx

And then add the user to this group.

vi /etc/group
At the bottom of the file verify the entry related to newgroup and add the user name. It
will look like –
newgroup:x:600:beragautam,gautam

Here, beragautam and gautam are members of the newly created group, ‘newgroup’. The thing
must be noted is, usernames are specified by comma (,) separated form. However,
instead of editing th e /etc/group file directly, we can use commands to add or remove
users from a group.

If we want to add members (in our example, the name of the group is acct and the
members are red and green), we can use the following command –
gpasswd –M red,green acct

If we want to add user ‘blue’ in the ‘acct’ group we can use the following command –
gpasswd –a blue acct

To remove user red from the acct group, we can use the following command –
gpasswd –d red acct

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Remote Administration Telnet, SSH and telnet:
Telnet
Telnet is basically a virtual terminal program that helps you to configure a system remotely. In production environment, telnet is rarely used as it transfer keystroke in clear text.To enable telnet all you have to do is, run the following two commands
chkconfig telnet on
service xinetd restart
By default, telnet to remote system using root user is disabled. You have to log into the telnet server as a non-privileged user then you can change your identity as root using the following command –
su – root

SSH

Unlike telnet, ssh is considered a Secure Shell Service. Ssh runs on public/private key infrastructure using rsa or dsa technology.
It is very easy to start ssh. Execute service sshd restart command.
By default any user can establish ssh session. However, this can be controlled by modifying the /etc/ssh/sshd_config file. See the following screen shot. Here I have denied atanu and allowed all other users to establish an ssh session.
We can also allow or deny one or multiple group using AllowGroups or DenyGroups directive, like AllowGroups acct mrkt.
The modification can only be effected when you restart the ssh server.
Check the last two lines in the screen shot in the page –


In my network, I always use ssh to configure remote servers. Generally I configure my linux laptop as an ssh client. I have a RHEL 4 system and I use this particular computer to administer my proxy server that is running on 172.16.1.12. See, what I usually do –In my laptop I execute the following command to create the key –
ssh-keygen –t dsa
You can also use rsa here. See the following screen shot –


Check the output, in the screen shot, carefully. In the 3rd Line, it is saying the key will be saved in /root/.ssh directory. If the .ssh directory is not exist in /root, the sshkeygen command will create it, check the 4th line. After creating /root/.ssh directory, the system will ask you to supply the passphrase. I enter empty passphrase, means just press enter twice. My identification is now saved in /root/.ssh/id_dsa file and the public key is id_dsa.pub.
It is time to copy the id_dsa.pub in the /root/.ssh/ directory in my proxy server and the name of the file will be authorized_keys. I will be using the following command assuming in my proxy server, running on 172.16.1.12, the /root/.ssh directory exists.
scp –rp /root/.ssh/id_dsa.pub root@172.16.1.12:/root/.ssh/authorized_keys
See the output in the following screenshot. After executing the above command the system will ask your confirmation, type yes and press enter, supply root’s password for 172.16.1.12 and the file will be copied securely.

The scp will be discussed shortly.
Now, from your client computer if you execute ssh 172.16.1.12 the system will never ask you to supply the password!!


SCP

Using scp, you can copy files from or to a remote linux system. In ssh section above, see how we use scp to copy id_dsa.pub. To refer a remote system we use user@machin_name_or_ip_address:/any/directory and a source can also be a remote system.
While working with Windows system, I use winscp to communicate with linux system. The winscp382setup.exe is freely available for download in sourceforge.net.
Please be noted, when you establish an ssh or scp session with a computer for the first time, the remote system related information is added in /root/.ssh/known_hosts text file. And if the remote system is re-installed you will be failed to establish ssh or scp session until you remove the system’s information from the /root/.ssh/known_hosts file.

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