Linux Book Center – Page 5 – Right choice to learn technology

No supported authentication methods available

Error:

Disconnected: No supported authentication methods available (server sent: publickey,gssapi-keyex,gssapi-with-mic)

No supported authentication methods available

Receiving error When trying to access Linux server through putty or other tool using ssh service.

Solution:

Edit /etc/ssh/sshd_config configuration file using vi editor.
```
    [root@server ~]# vi /etc/ssh/sshd_config
```

Check and replace with yes for below entry

 PasswordAuthentication no

changed

PasswordAuthentication yes

save and exit from the file using :wq
Than restart the ssh service using below command.
```
systemctl restart sshd
```
Restart the network service using below command.
```
systemctl restart network
```
Now try to access the server and sure it will work. Still if you are facing error in accessing the server, that there might be issue with firewall configuration.

Reference: Super user

ACL in RHEL7/Cent OS 7

In this post we are going to securing files and directories using ACL in RHEL7/Cent OS 7.

In this post we are going to see how to secure files and directories using ACL.

As first step need to check kernel compatibility for ACL using below command.

[root@server ~]# grep -i acl /boot/config*
CONFIG_EXT4_FS_POSIX_ACL=y
CONFIG_XFS_POSIX_ACL=y
CONFIG_BTRFS_FS_POSIX_ACL=y
CONFIG_FS_POSIX_ACL=y
CONFIG_GENERIC_ACL=y
CONFIG_TMPFS_POSIX_ACL=y
CONFIG_NFS_V3_ACL=y
CONFIG_NFSD_V2_ACL=y
CONFIG_NFSD_V3_ACL=y
CONFIG_NFS_ACL_SUPPORT=m
CONFIG_CEPH_FS_POSIX_ACL=y
CONFIG_CIFS_ACL=y

Above output will says that this kernel is compatible with ACL access since we could see all are marked as yes POSIX_ACL=y.

If it is set as N. Than we need to rebuild the kernel.

Next need to install the packages.

Required packages for ACL:

acl

nfs4-acl-tools

libacl

Now install all the above three packages using yum:

Link to see how to configure yum locally click here

[root@server ~]# yum -y install nfs4-acl* acl libacl

Will assign read, write and execute permission to files and directories using ACL and will mention characters ugo/rwx in commands for permissions respectively.

Now will see a example which will help us to understand clearly.

Create three users and one group respectively like below.

[root@server ~]# useradd lbcuser1
[root@server ~]# useradd lbcuser2
[root@server ~]# useradd lbcuser3
[root@server ~]# groupadd lbcgroup
[root@server ~]# passwd lbcuser1
Changing password for user lbcuser1.
New password:
BAD PASSWORD: The password is a palindrome
Retype new password:
passwd: all authentication tokens updated successfully.

Above screen password has been generated for only lbcuser1.Same like that need to set password for other 2 users.

Now add the lbcgroup group as secondry group for lbcuser1 and lbcuser2 users.

[root@server ~]# usermod -aG lbcgroup lbcuser1
[root@server ~]# usermod -aG lbcgroup lbcuser2

Create a directory and a file inside of that directory to assign and check permissions using acl.

[root@server ~]# mkdir /tmp/data
[root@server ~]# touch /tmp/data/testfile.txt

Now change the group as lbcgroup to the file like below.

[root@server ~]# chown :lbcgroup /tmp/data/testfile.txt
[root@server ~]# ll /tmp/data/testfile.txt
-rw-r--r--. 1 root lbcgroup 0 Dec 15 21:14 /tmp/data/testfile.txt

set the permission 770 using chmod command to the testfile.txt.

Now we can login as lbcuser1 and lbcuser2 and than will try to insert content in testfile.txt.

Sure both users can able to insert content in the file. Because, both users and files group is same(lbcgroup).

[root@server ~]# su lbcuser1
[lbcuser1@server root]$ echo "My name is lbcuser1..." > /tmp/data/testfile.txt
[lbcuser1@server root]$ exit
exit
[root@server ~]# su lbcuser2
[lbcuser2@server root]$ echo "My name is lbcuser2..." > /tmp/data/testfile.txt
[lbcuser2@server root]$ exit
exit
[root@server ~]#

and now will try to insert content as lbcuser3. It will give error. Since, its not the owner and member of lbcgroup for that file.

[root@server ~]# su lbcuser3
[lbcuser3@server root]$ echo "My name is lbcuser3..." > /tmp/data/testfile.txt
bash: /tmp/data/testfile.txt: Permission denied

So, now will provide read and write permission using ACL without adding the lbcuser3 in lbcgroup and will check it again to insert content into the file.

[root@server ~]# setfacl -R -m u:lbcuser3:rw /tmp/data/testfile.txt
[root@server ~]# su lbcuser3
[lbcuser3@server root]$ echo "My name is lbcuser3..." > /tmp/data/testfile.txt
[lbcuser3@server root]$ cat /tmp/data/testfile.txt
My name is lbcuser3...

Since we user single > symbol to redirect the echo command out into the file, its showing our last content which is “My name is lbcuser3…”

To set permission for group will use g in above command where we used u and groupname where we given username like below.

[root@server ~]# setfacl -R -m g:lbcgroup:rw /tmp/data/testfile.txt

To check the existing ACL permission of a file use getfacl command.

[root@server ~]# getfacl /tmp/data/testfile.txt
getfacl: Removing leading '/' from absolute path names
# file: tmp/data/testfile.txt
# owner: root
# group: lbcgroup
user::rwx
user:lbcuser3:rw-
group::rwx
mask::rwx
other::---

Same like file will set permission to directory as well.

Command to set permission for directory:

below command will help us to set read permission alone for other users which is not owner/group of the directory.

[root@server ~]# setfacl -m d:o:r /tmp/data
[root@server ~]# getfacl /tmp/data
getfacl: Removing leading '/' from absolute path names
# file: tmp/data
# owner: root
# group: root
user::rwx
group::r-x
other::r-x
default:user::rwx
default:group::r-x
default:other::r--

User/Group disk quota enabling in Linux/Unix

Now we are going to see how to enable User/Group disk quota enabling in Linux/Unix in this post. Sometimes we might have low space in on local disk. To avoid this will allocate disk size to Users/Groups by enabling and configuring quota in /home directory.

As a first step we should enable quota in Filesystem.

by editing and adding usrquota and grpquota in home directory entry at /etc/fstab file will enable quota.

[root@server ~]# vi /etc/fstab

# /etc/fstab
# Created by anaconda on Fri Nov 24 17:31:25 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/rhel-root / xfs defaults 0 0
UUID=2f2c635e-e5fb-4c81-823a-855a334ca04c /boot xfs defaults 0 0
/dev/mapper/rhel-swap swap swap defaults 0 0
/dev/rootvg/lv_tmp1 /home xfs defaults,usrquota,grpquota 0 0

save and exit from the file.

Now remount the filesystem using below command.

[root@server ~]# mount -o remount /home

Then check whether the quota has been enabled or not in /home mountpoint.

[root@server ~]# mount | grep /home
 /dev/mapper/rootvg-lv_tmp1 on /home type ext4 (rw,relatime,seclabel,quota,usrquota,grpquota,data=ordered)

Creating database using below command

[root@server ~]# quotacheck -cugv /home

C: Key to create new quota file

U: User quota

G: Group quota

V: Verbose mode

And now turn on the quota in /home directory using below command

[root@server ~]# quotaon /home

Now will assign quota using edquota command to User/Group

Syntax to create quota on user:

#edquota -u <username>

Syntax to create quota on group:

#edquota -g <groupname>

Will see a example of creating quota on user called abu

[root@server ~]# edquota -u abu

Now the above command will open quota file like below

Disk quotas for user abu (uid 1001):
 Filesystem                 blocks soft hard inodes soft hard
 /dev/mapper/rootvg-lv_tmp1    0   5000 6000   0      0    0

Above data has two quota limits. One is based on blocks and another one based on inode.

For block usage:

Soft: Soft limit will warn the user if the user exceeds the limit. But, user allowed to write data in home directory till reaching the hard limit. In above example, we have provided 5000KB(nearby 5MB)

hard: Hard limit will not allow user to write data in home directory once reached hard limit. In above example, we have provided 6000KB(6MB) as hard limit.

Will login and try to create 8MB of file using dd command to check the quota on user.

[root@server ~]# su abu
[abu@server ~]$ dd if=/dev/zero of=bgfile bs=1M count=8
dm-3: warning, user block quota exceeded.
dm-3: write failed, user block limit reached.
dd: error writing ‘bgfile’: Disk quota exceeded
6+0 records in
5+0 records out
6127616 bytes (6.1 MB) copied, 0.00498719 s, 1.2 GB/s

Command to display report on user quota:

[root@server ~]# repquota -as
*** Report for user quotas on device /dev/mapper/rootvg-lv_tmp1
Block grace time: 7days; Inode grace time: 7days
 Space limits File limits
User used soft hard grace used soft hard grace
----------------------------------------------------------------------
root -- 20K 0K 0K 2 0 0
abu +- 6000K 5000K 6000K 6days 6 0 0

Will configure grace period for the user quota. Once grace period has been reached than the soft limit will be come hard limit.

Command to create grace period:

[root@server ~]# edquota -t

Grace period before enforcing soft limits for users:
Time units may be: days, hours, minutes, or seconds
 Filesystem Block grace period Inode grace period
 /dev/mapper/rootvg-lv_tmp1 7days 7days

Grace period also has two types which based on blocks and inodes.

Thanks for reading this post.

Directory structure in Linux

We are going to see Directory structure in Linux/ Unix and what was the use of those directories.

Directory structure in Linux

/: Root

Root is a parent directory for all the directories and files.

Each and every directories and files will comes under root only.

Only root user only will do any changes in this directory.

For root user /root is the home directory and for others home directory will comes under /home

/home

All the users home directory will be created under /home to store their files. Ex: /home/user

/boot

This directory contains boot loader information.

Boot loader file contains kernel and initramfs image details.

/bin

Contains all the executable binary files which are

commands which we are using in linux/unix.

/sbin

/sbin also contains binary files like same as /bin.

But, this commands are typically used by system administrator.

/etc

Contains configuration files of all the application/programs used in Linux/Unix.

and startup scripts also stored in this location.

/dev

This directory contains all the device files and drivers as well. Like CD Drive, HDD, USB, tty

/tmp

This directory is for temporary use only. All the temps files and directories stored here which is created by user or system.

Files will be deleted after reboot of the system.

/opt

Stands for optional.

This directory contains applications installed which all are separate vendor.

/var

Contains all the variable files and logs and this can be grow in future based on the usage.

Ex: /var/log/dmesg, /var/log/secure,etc…

/mnt

This will be used to mount devices temporary purpose.

/usr

This directory contains libraries, variables, binaries. /usr/bin directory contains binary files for user level programs and /usr/sbin contains binary files foe system administrator levels.

Help command and Data Types in Python

In this post we are going to see Help command and Data Types in Python.

Use help along with command which you want need to know more that command.

Here is the example:

Below command will shows the help about print command.

[root@server ~]# python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> help ('print')

Data types:

Numbers
Strings
Lists
Tuple
Dictionary

Numbers:

Three types of numbers data type available in Python.

Integers: 10

Floating point: 2.1, 3.45

Complex numbers: (4+1J), (3.6 – 5.4a).

If..else.. statement in Bash shell scripting

We are going to see if statement in bash shell scripting in Linux/UNIX environment.

If statement is playing the most important role in bash shell scripting and other programming languages. Its working based on condition and we can write another if statement inside of an if statement.

Syntax of If statement:

if [ condition..]
then
  < commands...>
else
  < commands... >
fi

Basic if statement says that, if a condition if valid then, it will execute the single/ set of commands which is comes under then will execute and if a condition not valid then, it will execute the single/ set of commands which comes under else. Finally, it will be ended.

We have some operator which is useful in if statement conditions for validation. Below they are.

Operator	Description
=, ==, -eq	This is to check string/numeric value is equal to another string/ numeric value.
!=,-ne	This is to check string/numeric value is not equal to another string/ numeric value.
<=, -le	This is to check string/numeric value is less than or equal to another string/ numeric value.
>=, -ge	This is to check string/numeric value is greater than or equal to another string/ numeric value.
<, -lt	This is to check string/numeric value is less than to another string/ numeric value.
>, -gt	This is to check string/numeric value is greater than to another string/ numeric value.

Example:

[root@server ~]# vi newscript.sh

#!/bin/bash
if [[ $1 -gt "100" ]]
then
 echo "Given values is greater than 100.."
else
 echo "Given value is equal to or less than 100..."
fi

Then provide the execute permission to the script using chmod command.

[root@server ~]# chmod +x newscript.sh

Now will run the script by passing a parameter

[root@server ~]# ./newscript.sh 101
Given values is greater than 100..

Basic’s of Python scripting

In this post we are going to see Basic’s of Python scripting.

Use Python command to move to python interpreter like below and using quit() to exit from the interpreter.

and here we used simle print command to print any string.

[root@server ~]# python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print ('Hello World')
Hello World
>>> quit()
[root@server ~]#

As you all aware will write bash script in linux/unix. In Python also will create shell script. For that, we need to do simple change in first line like below and even we can create with extension with .sh.

#!/usr/bin/env python

Now we will create our first simple python script to display set of strings. To create this script will use any editor and here i’m going to use vi editor.

[root@server ~]# vi first.py
#!/usr/bin/env python
print "Hello, This is my first script"

use :wq to save and exit from this file and now should provide execute permission using chmod command to run this script.

[root@server ~]# chmod +x first.py

Finally will run the script using ./ like shell scripting.

[root@server ~]# ./first.py
Hello, This is my first script

We successfully created our first python script.

Python installation in Linux/Unix

In RedHat by default python installed. We are going to see the Python installation in Linux/Unix.

First we have to prepare our system to install Python.

Preparing system:

Use subscription-manager to know whether you have access to RedHat software repository or not.

[root@server ~]# subscription-manager repos --list-enabled

if you don’t see any repository enabled, than your machine not registered or not having subscription.

Now update using yum command.

[root@server ~]# yum update

Once you executed the above command with valid subscription, all the packages will get updated in your OS.

Now setup your environment:

As you aware already, we are going to use yum to install and check Python package.

Check whether python installed or not using yum.

[root@server ~]# yum list installed | grep python

If the package not installed, use below command to install python.

[root@server ~]# yum install python

above command will ask your confirmation to proceed install. Simply type Y and press enter.

Once executed above command, again check whether its installed or not.

[root@server ~]# yum list installed | grep python

Use python command to run python in interactive mode.

[root@server ~]# python
Python 2.7.5 (default, Aug 2 2016, 04:20:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> quit()
[root@server ~]#

Finally we installed python in linux.

Creating a new filesystem in Linux/Unix

We are going to see creating a new filesystem in Linux/Unix and especially in RHEL7.

As a first step we should know how many physical disks available in our machine and available free space in those disks.

For that we can use below commands. fdisk command will list all the physical disks with partitions and size of the disk.

[root@server ~]# fdisk -l

Disk /dev/sda: 21.5 GB, 21474836480 bytes, 41943040 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x00060f18

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 1026047 512000 83 Linux
/dev/sda2 1026048 17803263 8388608 83 Linux
/dev/sda3 17803264 21997567 2097152 83 Linux
/dev/sda4 21997568 41943039 9972736 5 Extended
/dev/sda5 21999616 30388223 4194304 82 Linux swap / Solaris
/dev/sda6 30390272 41943039 5776384 8e Linux LVM

Disk /dev/sdb: 8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x1f51ec05

Device Boot Start End Blocks Id System
/dev/sdb1 2048 12584959 6291456 8e Linux LVM
/dev/sdb2 12584960 16777215 2096128 8e Linux LVM

Disk /dev/sdc: 8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

df command will list the free space in the mentioned disk

[root@server ~]# df -h /dev/sdc
Filesystem Size Used Avail Use% Mounted on
devtmpfs 1.4G 0 1.4G 0% /dev

We are going to use /dev/sdc disk, as its having free space and none FS created in this.

Use the below command to do changes and create filesystem in this disk

[root@server ~]# fdisk /dev/sdc
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x52240e5b.

Command (m for help):

Once you executed fdisk command with disk name(/dev/sdc), you will get in to the fdisk tool to do changes and it will show like above. type ‘m’ and press enter to get to know the keys which will be helpful in this tool

[root@server ~]# fdisk /dev/sdc
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x52240e5b.

Command (m for help): m
Command action
 a toggle a bootable flag
 b edit bsd disklabel
 c toggle the dos compatibility flag
 d delete a partition
 g create a new empty GPT partition table
 G create an IRIX (SGI) partition table
 l list known partition types
 m print this menu
 n add a new partition
 o create a new empty DOS partition table
 p print the partition table
 q quit without saving changes
 s create a new empty Sun disklabel
 t change a partition's system id
 u change display/entry units
 v verify the partition table
 w write table to disk and exit
 x extra functionality (experts only)

Now we are going to create a filesystem/partition. So for that type n to create new filesystem. Once pressed enter you will get prompt to select the partition type. Select Primary.

Than provide the partition number or else just press enter to take default value.

First Sector: Provide the sector value, from there only filesystem/partition will start.

Last Sector: Finally we have to provide the last sector value where the filesystem/partition will get end or in number value with G(GB),M(MB),K(KB). For best practise we can use the number value with + like this +4G and than press enter to complete it.

Command (m for help): n
Partition type:
 p primary (0 primary, 0 extended, 4 free)
 e extended
Select (default p): p
Partition number (1-4, default 1):
First sector (2048-16777215, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-16777215, default 16777215): +4G
Partition 1 of type Linux and of size 4 GiB is set

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

and now type w and press enter to save the changes and quit from the fdisk.

We created partition in physical disk. Use partprobe command to affect the disk changes without restarting the machine in this session.(Note: for hardware changes system restart is must. So here specially we are using this command to avoid machine reboot)

[root@server ~]# partprobe

[root@server ~]# fdisk -l /dev/sdc

Disk /dev/sdc: 8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x61352143

Device Boot Start End Blocks Id System
/dev/sdc1 2048 8390655 4194304 83 Linux

We have to format the partition using anyone of the filesystem type(XFS, EXT4,ETX3…).

Note: if you are going to use this partition in LVM, no need to follow the further steps.

As we are using RHEL7, going to use default one which XFS to format.

Command to format the partition using XFS filesystem.

[root@server ~]# mkfs.xfs /dev/sdc1
meta-data=/dev/sdc1 isize=512 agcount=4, agsize=262144 blks
 = sectsz=512 attr=2, projid32bit=1
 = crc=1 finobt=0, sparse=0
data = bsize=4096 blocks=1048576, imaxpct=25
 = sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal log bsize=4096 blocks=2560, version=2
 = sectsz=512 sunit=0 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0

After formating need to create a directory to mount this and the directory will act as mount point.

[root@server ~]# mkdir /app

Finally filesystem should be mounted under the created directory to make use it.

We ahev two type in mounting filesystem.

Temporary mount: Will mount the filesystem. But, after restarting mount will be lost. Again we need to mount it.

Permanent mount: Need to make entry in /etc/fstab file and this mount will not lost even after restarting the machine as well.

Temporary mount:

[root@server ~]# mount /dev/sdc1 /app

Permanent mount:

Open the /etc/fstab file using vi editor and provide the below entry. Save and exit from the file using :wq.

[root@server ~]# vi /etc/fstab

/dev/sdc1 /app xfs defaults 0 0

Use mount command with grep to check the filesystem whether its listing or not.

[root@server ~]# mount | grep /app
/dev/sdc1 on /app type xfs (rw,relatime,seclabel,attr2,inode64,noquota)

Successfully we created a filesystem now.