Category: RHEL7

Will see the Booting process in RHEL7/ CentOS 7 in this post. It’s very important to know the booting process for all operating system. It will help us to troubleshoot the booting issues.

We are going to see about RHEL / Cent OS 7 booting process.

Below are booting stages:

BIOS:

BIOS stands for basic input output system.

It will do a POST (Power on self-test) to check system hardware.

And it will search, load and execute MBR  in memory(Master Boot Recorder).

MBR:

MBR Stands for Master boot recorder.

MBR will be available in first 512 bytes of the boot drive. In this 512 bytes 446 bytes contains boot loader information, in 64 bytes contains partition table information available and remaining 2 bytes for MBR validation check. Boot drive will be anything like Hard disk, Pendrive, Floppy.

MBR will search and load the GRUB2 boot loader in memory and control moved to the bootloader.

GRUB2 Bootloader:

GRUB stands for Grand Unified Bootloader.

GRUB will be the default boot loader in RHEL7.

GRUB configuration file is available in this path: /boot/grub2/grub.cfg. Editing this file directly is not advisable.

GRUB file configuration:

# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root rhgb quiet net.ifnames=0"
GRUB_DISABLE_RECOVERY="true"

To change these configuration settings will edit this /etc/default/grub file.

Once modified have to execute the below command to make this change affect in the main configuration file (/boot/grub2/grub.cfg).

# grub2-mkconfig –o /boot/grub2/grub.cfg

grub has kernel image(vmlinuz) and initramfs image details.

grub will search and load the kernel image into memory and it will extract the initramfs image contents into memory based filesystem which is called tmpfs.

initramfs stands for initial ramdisk

initial ramdisk will load the block devices (HDD, CD, Floppy, etc). So that it will find the exact root filesystem and will mount it. Till mounting this exact root filesystem, initramfs will act as a temporary root filesystem.

kernel mounts the initramfs image as two-stage boot process.

Will use the lsinitrd command to view initramfs content.

Kernel:

Kernel will start systemd process and PID is 1 for this, as this will be the first process.

root 1 0 0 02:10 ? 00:00:02 /usr/lib/systemd/systemd --switched-root --system --deserialize 23

Systemd:

Systemd will be the first process.

This process will read the /etc/systemd/system/default.target file to determine the default system target. The system target file defines the services which is started by systemd.

Systemd will bring the system based on the system target runlevel and perform system initialization process such as hostname, network settings, initializing SELinux, Printing welcome banner and mounting all filesystems.

WHat is LDAP?

LDAP: Light weight Directory Access Protocol

This authentication will allow client machine to access any centralized   data/service from different places.

Keeping services/data centralized is very simple to maintain and Grant/Deny permission centrally.

Installing Openldap:

[root@server ~]# yum -y install openldap*

Make sure that SELinux should not deny LDAP. So for that we have to enable the below Boolean values in both server and client.

[root@server ~]# setsebool -P allow_ypbind=0 authlogin_nsswitch_use_ldap=0

Now enable ldap permanently in this session and start the ldap service

[root@server ~]# systemctl enable slapd
[root@server ~]# systemctl start slapd

Since ldap service is running ldap user should be the owner for /var/lib/ldap directory. By default this will be modified by root user as its administrator.
So, we have to change owner and group as ldap user. Before that make a copy of configuration file for slapd into /var/lib/ldap with the DB_CONFIG name.

Note: slapd configuration file will be available under /usr/share/openldap-servers/DB_CONFIG.example

[root@server ~]# cp /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG
cp: overwrite ‘/var/lib/ldap/DB_CONFIG’? y

Now change the owenr as ldap user recursively to that directory.

[root@server ~]# chown -R ldap:ldap /var/lib/ldap

Now set password for ldap admin use and take a copy of create password for future use.

[root@server ~]# slappasswd
New password:
Re-enter new password:
{SSHA}Kp/mCASZRf/dBKNHKOxg4O5lEt7vyiRB

LDAP configuration files are available under /etc/openldap/slapd.d/.
We need to update the “olcSuffix” and “olcRootDN” varialbles.

olcSuffix: Its domain for the LDAP Server to provided the information.
olcRootDN: This is administrator user entry who has all the permission to do operation like root (Root Distinguished Name)

olcRootPW: Password of above user.

Above entries should be updated in /etc/openldap/slapd.d/cn=config/olcDatabase={2}hdb.ldif file.
Note: Updating the entries manually is nont adviseable. So, we are going to create a ldif file and update it using ldapmodify command.

Now creare ldaprootpasswd.ldif file with following contents:

[root@server cn=config]# vi db.ldif

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcSuffix
olcSuffix: dc=lbcdomain,dc=com

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcRootDN
olcRootDN: cn=ldapadm,dc=lbcdomain,dc=com

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcRootPW
olcRootPW: {SSHA}Kp/mCASZRf/dBKNHKOxg4O5lEt7vyiRB

save and exit.

once file created with above content execute the below command

[root@server cn=config]# ldapmodify -Y EXTERNAL -H ldapi:/// -f db.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
modifying entry "olcDatabase={2}hdb,cn=config"

modifying entry "olcDatabase={2}hdb,cn=config"

modifying entry "olcDatabase={2}hdb,cn=config"

[root@server cn=config]#

Do changes in /etc/openldap/slapd.d/cn=config/olcDatabase={1}monitor.ldif file to restrict monitor access to only ldap root user.

[root@server ~]# vi monitor.ldif

dn: olcDatabase={1}monitor,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to * by dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external, cn=auth" read by dn.base="cn=ldapadm,dc=lbcdomain,dc=com" read by * none

save and exit from the file.

once created file with above content send configuration to ldap server by usinf ldapmodify command.

[root@server cn=config]# ldapmodify -Y EXTERNAL -H ldapi:/// -f monitor.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
modifying entry "olcDatabase={1}monitor,cn=config"

[root@server cn=config]#

Creating LDAP Certificate:

now will create certifiate and private key with ldap server self signed by using below command in /etc/openldap/certs/ directory

[root@server cn=config]# openssl req -new -x509 -nodes -out /etc/openldap/certs/lbcdomainldapcert.pem -keyout /etc/openldap/certs/lbcdomainldapkey.pem -days 365
Generating a 2048 bit RSA private key
..........................................................................................................................................................................................................................+++
................+++
writing new private key to '/etc/openldap/certs/lbcdomainldapkey.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:IN
State or Province Name (full name) []:Tamilnadu
Locality Name (eg, city) [Default City]:Chennai
Organization Name (eg, company) [Default Company Ltd]:Linux Book Center
Organizational Unit Name (eg, section) []:Linux Book Center
Common Name (eg, your name or your server's hostname) []:Server
Email Address []:[email protected]

Now change the ownership and group to key files.

[root@server cn=config]# chown -R ldap:ldap /etc/openldap/certs/*.pem

[root@server certs]# ll | grep pem
-rw-r--r--. 1 ldap ldap 1480 Aug 15 11:32 lbcdomainldapcert.pem
-rw-r--r--. 1 ldap ldap 1708 Aug 15 11:32 lbcdomainldapkey.pem

Create certs.ldif file to configure LDAP to use secure communication using self signed certificates.

[root@server certs]# vi certs.ldif

dn: cn=config
changetype: modify
replace: olcTLSCertificateFile
olcTLSCertificateFile: /etc/openldap/certs/lbcdomainldapcert.pem

dn: cn=config
changetype: modify
replace: olcTLSCertificateKeyFile
olcTLSCertificateKeyFile: /etc/openldap/certs/lbcdomainldapkey.pem

oncefile created import the certificate configuration by usging below ldapmodify command

[root@server certs]# ldapmodify -Y EXTERNAL -H ldapi:/// -f certs.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
modifying entry "cn=config"

modifying entry "cn=config"

[root@server certs]#

Use the below command to verify the configuration:

[root@server certs]# slaptest -u
5992a281 UNKNOWN attributeDescription "CHANGETYPE" inserted.
5992a281 UNKNOWN attributeDescription "REPLACE" inserted.
5992a281 is_entry_objectclass("olcDatabase={2}hdb,cn=config,cn=config", "2.16.840.1.113730.3.2.6") no objectClass attribute
5992a281 is_entry_objectclass("olcDatabase={1}monitor,cn=config,cn=config", "2.16.840.1.113730.3.2.6") no objectClass attribute
config file testing succeeded

LDAP Database creation:

Sample configuration file will be available in /var/lib/ldap  file. Copy the sample file and change the file permission.

[root@server certs]# cp /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG
[root@server certs]# chown ldap:ldap /var/lib/ldap/*

Add cosine ans nis LDAP schemas

ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/cosine.ldif
ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/nis.ldif
ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/inetorgperson.ldif

[root@server certs]# ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/cosine.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
adding new entry "cn=cosine,cn=schema,cn=config"
ldap_add: Other (e.g., implementation specific) error (80)
 additional info: olcAttributeTypes: Duplicate attributeType: "0.9.2342.19200300.100.1.2"

[root@server certs]# ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/nis.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
adding new entry "cn=nis,cn=schema,cn=config"
ldap_add: Other (e.g., implementation specific) error (80)
 additional info: olcAttributeTypes: Duplicate attributeType: "1.3.6.1.1.1.1.2"

[root@server certs]# ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/inetorgperson.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
adding new entry "cn=inetorgperson,cn=schema,cn=config"
ldap_add: Other (e.g., implementation specific) error (80)
 additional info: olcAttributeTypes: Duplicate attributeType: "2.16.840.1.113730.3.1.1"

Now create ldif file for our domain

[root@server certs]# vi base.ldif

dn: dc=lbcdomain,dc=com
dc: lbcdomain
objectClass: top
objectClass: domain

dn: cn=ldapadm ,dc=lbcdomain,dc=com
objectClass: organizationalRole
cn: ldapadm
description: LDAP Manager

dn: ou=People,dc=lbcdomain,dc=com
objectClass: organizationalUnit
ou: People

dn: ou=Group,dc=lbcdomain,dc=com
objectClass: organizationalUnit
ou: Group

Now build the directory structure

[root@server certs]# ldapadd -x -W -D "cn=ldapadm,dc=lbcdomain,dc=com" -f base.ldif
Enter LDAP Password:
adding new entry "dc=lbcdomain,dc=com"
ldap_add: Already exists (68)

We configured LDAP server and now will use ldap credentials to access centralized data/ service like NFS share.

What is Kickstart installation?

Kick start installation is unattended installation, where we can install the operating system
without fully seated in front of system till completing the installation.

Here we need to create a kickstart configuration file which contains all the details like,
Language, Timezone, Network details, Partition details, Package details, etc…

1. Creating a Kickstart configuration File

by default kickstart configuration file will be available in /root directory with the name of anaconda-ks.cfg in linux machine which is already installed. by editing this file will create new kickstart file for our requirement. But comparing to this method will create kickstart file in Graphical mode which will be very easier way.

GUI mode should be working in our machine to create kickstart file in Graphical utility.

Login the server in Graphical mode as root user and issue the below command to open the kickstart file configuration window.

[root@server ~]# system-config-kickstart

will get a window now to configure new kickstart file.

Using above window we need provide the required details in each section by selecting the option which is available in left pane.

Basic Configuration:

Default and keyboard Layout Language, Time Zone, Root password, Architecture and Reboot option after installation the Operating system.

Installation Method:

Whether its going to be a new installation or upgrade and which source we are going to use for OS media.

Boot Loader Option:

Its advisable to use “Install new boot loader” option.

Partition Information: 

Depends on your requirement you can create partitions or you may leave it to create automatically by the OS.

Network Configuration:

Based on your requirement you can provide static or DHCP IP’s and you should select the adapter first for that.

Authentication, Firewall Configuration & Display Configuration:

You may leave it by default. If you have any specific reason you can select the option.

Package Selection:

Here we need to provide what type of server you are going to install. Like, Web server,  Minimal package, DNS Server, Application Server, etc…

By default it will be minimal only.

Pre and Post Installation Script:

You may leave it.

After creating file use file-> save option to save the configuration. Here i’m saving this file under /root  with  ks.cfg name.

If you are going to create same configuration Operating system you can open the existing file which is available in you machine under /root directory.

2. Verifying the Kickstart File

After created the kickstart configuration file will verify using below command. We should install the package called ypkickstart to run the command.

ksvalidator command will help to verify the created kickstart file

[root@server ~]# yum install ypkickstart

[root@server ~]# ksvalidator /root/ks.cfg

3. Making the Kickstart File Available

Now i’m going to use http service to access created kickstart file through network for unattended  installation.

Simply install httpd package and start the service in remote machine where we are going to kept kickstart file.

Then copying created kickstart file from /root to /var/www/html/kickstart/  which is default location where the pages available to access using http service on browser.

[root@server ~]# yum install httpd
[root@server ~]# systemctl start httpd
[root@server ~]# cp /root/ks.cfg /var/www/html/kickstart/

4. Making the Installation Source Available

This section is pointing that which source of OS media we are going to use for this installation. We can use Local media, NFS, HTTP, HTTPS, FTP.

For Local media installation simply will use OS media or OS image file, if we are going to install in Virtual Machine and for other options OS image will be placed in remote machine and will be accessed using anyone of mentioned protocol.

In our case going to install in VMWare using Local media(OS Image file) source with kickstart file, which will be placed in remote machine and going to use http service to access kickstart file.

5. Starting the Kickstart Installation

Now start the virtual machine which is create in VMWare and interrupt by pressing Tab  key in keyborad and will get the screen like below.

Now insert the below entry to mention the kickstart file http location like below.

inst.ks=http://192.168.43.226/kickstart/ks.cfg

Then press enter to begin the installation.

What is ?

Two or more than that network adapters in corporate with single logical network pipe.

Logical network interface work as master and physical network interfaces are work as slave.

IP Address is assigned to  logical interface instead of assigning physical interfaces.

Logical interface only will work on connecting any internal/external network traffic.

It has several modes of operations based on behavior.

Modes:

1. Round robin
2. Active backup
3. XOR
4. Broadcast

Advantages:

1. Better throughput
2. Load balancing
3. Fault tolerance

Issue ip a command to check available interfaces in server.

in my server, i have two insterfaces called enp0s3  and enp0s8

Using mobprobe command load the bonding kernel if its not loaded already.

[root@server ~]# modprobe bonding

To check whether the bonding driver in kernel  use the modinfo command and that will list the details like below once its loaded.

[root@server ~]# modinfo bonding
 filename: /lib/modules/3.10.0-514.el7.x86_64/kernel/drivers/net/bonding/bonding.ko
 author: Thomas Davis, [email protected] and many others
 description: Ethernet Channel Bonding Driver, v3.7.1
 version: 3.7.1
 license: GPL
 alias: rtnl-link-bond
 rhelversion: 7.3
 srcversion: B664145ACFBCC961505C750
 depends:
 intree: Y
 vermagic: 3.10.0-514.el7.x86_64 SMP mod_unload modversions

Create a file called ifcfg-bond0 which will work as logical interface in bonding using vi editor with below mentioned settings under /etc/sysconfig/network-scripts

[root@server ~]# vi /etc/sysconfig/network-scripts/ifcfg-bond0

DEVICE=bond0
 TYPE=Bond
 NAME=bond0
 BONDING_MASTER=yes
 BOOTPROTO=none
 ONBOOT=yes
 IPADDR=192.168.43.250
 NETMASK=255.255.255.0
 GATEWAY=192.168.43.1
 BONDING_OPTS="mode=5 miimon=100"
 ZONE=public

:wq

save and exit from the file.

now the bond0 logical interface has been created and have to set the slave interface by following below steps.

in our case already we have two interfaces called enp0s3 and enp0s8. for the same configuration file also already there under /etc/sysconfig/network-scripts with below name’s

ifcfg-enp0s3

ifcfg-enp0s8

edit the above two files using vi editor and make entry for MASTER and SLAVE. Set the MASTER=bond0 and SLAVE=Yes for the both interface configuration files like below.

[root@server ~]# vi /etc/sysconfig/network-scripts/ifcfg-enp0s3

TYPE=Ethernet
 NAME=enp0s3
 UUID=f42b80f0-95dd-4126-a5f8-62db4f4a368f
 DEVICE=enp0s3
 ONBOOT=yes
 MASTER=bond0
 SLAVE=Yes

[root@server ~]# vi /etc/sysconfig/network-scripts/ifcfg-enp0s8

TYPE=Ethernet
 NAME=enp0s8
 DEVICE=enp0s8

UUID=f42b80f0-95dd-4126-a5f8-62db4f4a368f
 ONBOOT=yes
 MASTER=bond0
 SLAVE=Yes

Now use ifdown and ifup command to bring down and bring up the bond0 interface.

[root@server ~]# ifdown bond0
../network: line 2: NETWORKING: command not found
Device 'bond0' successfully disconnected.
[root@server ~]# ifup bond0
../network: line 2: NETWORKING: command not found
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/16)

Then check the ip of bond0 to confirm whether our bonding has been working or not.

[root@server ~]# ip a | grep bond0
6: bond0: <NO-CARRIER,BROADCAST,MULTICAST,MASTER,UP> mtu 1500 qdisc noqueue state DOWN qlen 1000
 inet 192.168.43.250/24 brd 192.168.43.255 scope global bond0

We configured bonding successfully. Now will communicate from remote server to this server using bond0 interface ip address (192.168.43.250)

Thanks for your support 🙂

We are going to see User and Group Management in Linux/Unix in this post.

 /etc/passwd   ->  This file contains all the users details.

[root@server ~]# more /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin

/etc/passwd file has all the user details with some parameters.

Here i’m taking root user details to explain with details.

root:x:0:0:root:/root:/bin/bash

root – User name

x – Password

0 – User ID

0 – Group ID

Root – Comments/ User description

/root – Home directory

/bin/bash  – shell

Home directory and Group will be created with the same name of user, once we created a user.

Command to create user:

Useradd <username>

adduser <username>

Example:

[root@server ~]# useradd anand

[root@server ~]# adduser anand

Command to delete user account:

deluser <username>

[root@server ~]# userdel anand

The above command will delete a user but will not delete home directory of the user.

Command to delete a user account with home directory:

Use option -r to delete user with home directory.

[root@server ~]# userdel –r deepak

Use id command to know whether a user available in system and if the user exist, it will display User ID, Group ID and Primary group and Secondary group details.

[root@server ~]# id abu
uid=1000(abu) gid=1011(Technology) groups=1011(Technology),1012(DBcheck)

Command to add secondary group to a user:

[root@server ~]# usermod –G Technology abu

Command to change primary group:

[root@server ~]# usermod –g Technology abu

Customizing home directory:

by default home directory will be created under /home for all the users. We can set some other directory as home directory as well.

we have two options to change the home directory.

1. we can edit /etc/passwd file using vi editor to change the home directory
2. Using usermod command will change home directory

Changing home directory by editing /etc/passwd  file

[root@server ~]# vi /etc/passwd
testuser:x:1004:1004::/testuser:/bin/bash

Command to change home directory:

[root@server ~]# usermod -m -d /testuser testuser

-m:    option to move the home directory

-d:      option to mention the home directory

Assigning expiry date to user:

Command to check expiry details for a user

[root@server ~]# chage -l testuser
Last password change : May 27, 2017
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7

Command to set expiry date for a user:

[root@server ~]# usermod --expiredate=2017-07-20 testuser

Again will check whether the expire date has been set or not.

[root@server ~]# chage -l testuser
Last password change : May 27, 2017
Password expires : never
Password inactive : never
Account expires : Jul 20, 2017
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7

We successfully set expire date for testuser.

Group Management:

Group is used to manage more than one users without touching the each and every user alone.

Group is created based on the Team on an organization.

All the group details available in /etc/group file, once we created a group.

[root@server ~]# more /etc/group

Using Groupadd command will create group

[root@server ~]# groupadd Technology