How to Install and Configure NTP Server and Client in RHEL/CentOS6?

NTP Server

1) Install NTP package using yum

yum -y install ntp

2) Setup Restrict Values

vim /etc/ntp.conf               # Add below given lines

# Permit time synchronization with our time source, but do not # permit the source to query or modify the service on this system.

restrict default kod nomodify notrap nopeer noquery

restrict -6 default kod nomodify notrap nopeer noquery

• noquery prevents dumping status data from ntpd.
• notrap prevents control message trap service.
• nomodify prevents all ntpq queries that attempts to modify the server.
• nopeer prevents all packets that attempts to establish a peer association.
• Kod – Kiss-o-death packet is to be sent to reduce unwanted queries

The value -6 in the second line allows forces the DNS resolution to the IPV6 address resolution. For more information on the access parameters list, Please refer to documentation on “man ntp_acc”The value -6 in the second line allows forces the DNS resolution to the IPV6 address resolution.

3) Allow network / IP range for synchronizing time from this server

restrict 10.0.0.0 mask 255.255.255.0 nomodify notrap
restrict 127.0.0.1

4) Add local clock as backup

In case NTP server disconnected from internet, NTP server will provide time from its local system clock

server 127.127.1.0 # local clock

fudge 127.127.1.0 stratum 10

5) Specify log files

driftfile /var/lib/ntp/ntp.drift

logfile /var/log/ntp.log

6) Finally start NTP server

service ntpd start

NTP Client

1) Modify /etc/ntp.conf configuration file, 

Sample entries, for internet servers :

server 0.centos.pool.ntp.org iburst

server 1.centos.pool.ntp.org iburst

server 2.centos.pool.ntp.org iburst

server 3.centos.pool.ntp.org iburst

iburst: After every poll, a burst of eight packets is sent instead of one. When the server is not responding, packets are sent 16s interval. When the server responds, packets are sent every 2s.

Edit your NTP.conf to reflect appropriate entries for your own NTP server.

server 10.0.0.1 prefer

prefer: If this option is specified that server is preferred over other servers.

2) Start NTP service

service ntpd start

3) Finally check NTP status

ntpq -p

4) Set date and time synchronized with desired server

ntpdate -u SERVER-NAME

5) Get the current status of  ntpd:

ntpdc -c sysinfo

system peer:          web10.hnshosting.com

system peer mode:     client

leap indicator:       00

stratum:              3

precision:            -24

root distance:        0.20857 s

root dispersion:      0.27481 s

reference ID:         [120.88.46.10]

reference time:       d7d65c1f.14c6b054  Wed, Oct  1 2014 16:25:27.081

system flags:         auth monitor ntp kernel stats

jitter:               0.014099 s

stability:            0.000 ppm

broadcastdelay:       0.000000 s

authdelay:            0.000000 s

RHEL / CentOS 6 - How to setup iscsi target and initiator ?

How to setup ISCSI Target?

Server Side

1) Install scsi-target-utils using yum

yum -y install scsi-target-utils

2) Edit iSCSI target configuration

vim /etc/tgt/targets.conf

initiator-address 192.168.56.102              ##Initiator IP/Name
incominguser myuser redhat                    ##Username = myuser and password = redhat

"iqn.2014-09.net.node1:server.target01" ##Can be anything

There are 2 types of storage devices:

backing-store - defines a virtual device on the target.

direct-store  - defines a direct mapped device with the same properties as the physical device (such as VENDOR_ID, SERIAL_NUM, etc.)

3) Start iSCSI target and on boot

service tgtd start

Start on boot

chkconfig tgtd on

4) Enable 3260 port at firewall

5) Check iSCSI target configuration

tgtadm --mode target --op show

6) Check if everything works as expected

tgt-admin -s

How to setup iSCSI initiator?

Client Side

1) Install iscsi-initiator-utils using yum

yum -y install iscsi-initiator-utils

2) Edit initiator configuration file

vim /etc/iscsi/initiatorname.iscsi

InitiatorName=iqn.2014-09.net.node1:san.initiator01

InitiatorAlias=node2

3) Edit iscsi client configuration /etc/iscsi/iscsid.conf

node.session.auth.authmethod = CHAP

node.session.auth.username = manoj

node.session.auth.password = redhat

4) Start iscsi client service

service iscsid start

5) Start iscsi client on boot

chkconfig iscsid on

6) Discovering targets in our iSCSI server

iscsiadm -m discovery -t st --portal Target-server-IP

7) Login with iSCSI LUN

iscsiadm -m node --targetname iqn.2014-09.net.node1:server.target01 --portal Target-Server-IP --login

8) Checking session status with the target

iscsiadm --mode session --op show

Thats it!!

Please do not forget to update with your feedback :-)

Encrypting Disks With LUKS (RHEL 6 / CentOS 6)

LUKs a concept used to encrypt disks for securing data. Partitions will not be accessible without decrypting that device with an defined PASSPHRASE

How to setup encrypted disks with LUKs?

1) Install LUKs utilities

yum -y install crypt*

2) Load dm_crypt module

modprobe dm_crypt

3) Add dm_crypt in system, so that after reboot this should be loaded

cat > /etc/sysconfig/modules/dm_crypt.modules
#!/bin/bash
modprobe dm_crypt

save and exit with  CTRL+D

chmod 755 /etc/sysconfig/modules/dm_crypt.modules

NOTE: dm_crypt.modules script created to loaod module


4) Lets say /dev/sdb is the disk to encrypt, First Create setup and format /dev/sdb with following command:

cryptsetup luksFormat /dev/sdb

This will ask for passphrase which will be used to decrypt/open this disk before using this disk

To mount or use /dev/sdb disk follow below given steps as at this point /dev/sdb will not be available to mount

5) Open LUKs disk/device by mapping/assigning an name, this will create and file/mapping in /dev/mapper

cryptsetup luksOpen /dev/sdb myluksdev_map

This will ask for passphrase to decrypt disk. Enter passphrase set in step number 4

NOTE : myluksdev_map given name can be anything as per your desire

6) As above command has opened/decrypted disk, now we can format with ext4

mkfs.ext4 /dev/mapper/myluksdev_map

7) Create an folder and mount /dev/sdb

mkdir /mnt/my_secret_dir

mount /dev/mapper/myluksdev_map /mnt/my_secret_dir

[Above /dev/mapper/myluksdev_map is name which is given at the time of opening/decrypting /dev/sdb device]

8) If we want to close encryption, have to follow given steps

umount /mnt/my_secret_dir
cryptsetup luksClose myluksdev_map

8) To make partition persistent or permanent after reboot we should add MOUNT entry in /etc/fstab but for mounting encrypted file system, we should decrypt disk/device. So, we should follow given steps to mount disks at the time of boot

    a) Create a file /etc/crypttab
    b) Update with given content

        MAPname         Device/path     ------------------- example

        In our case MAPname = myluksdev_map , Device Path = /dev/sdb

    c) Content should be

         cat /etc/crypttab
         myluksdev_map      /dev/sdb

   d) Now update /etc/fstab file with below content
 
       /dev/mapper/myluksdev_map                /mnt/my_secret_dir          ext4      defaults        0 0


Now reboot your server, this will ask for PASSPHRASE while mounting device /dev/mapper/myluksdev_map

To automatically unlock passphrase or avoiding asking password use below given steps:

    a) Create an file, lets say /root/cpasswd which will store password
       
         echo -n "password" > /root/cpasswd
         chmod 600 /root/cpasswd

   b) Now update /etc/crypttab like this

       MAPNAME         /DEVICE/PATH      /PASSWORD/FILE

       So, in our case this should be

       myluksdev_map     /dev/sdb           /root/cpasswd

Now try rebooting your server, this should not ask for password.

Thats it!!!

Please comment with your feedback :-)

Troubleshooting Kdump error "Memory for crashkernel is not reserved"

In case getting following error while starting kdump service

Memory for crashkernel is not reserved
Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel

If your server is having more than 4GB of RAM then crashkernel=auto (in /etc/grub.conf line starting with kernel)will automatically Reserve memory else need to specify in following format:


crashkernel=0M-2G:128M,2G-6G:256M

If physical memory is 0MB to 2GB then 128MB of memory will be reserved
If physical memory is 2MB to 6GB then 256MB of memory will be reserved

Now REBOOT server for kdump changes to take effect (1st time after kdump installed, a new INITRD will be generated)

Thats it!!!

How To setup Kdump on Linux (Redhat/CentOS 6)?

1) Install kdump package using YUM

yum -y install kexec-tools

2) Once installed, check /etc/init.d/kdump file/service should be available

ls -l /etc/init.d/kdump

3) Enable kdump at boot

chkconfig kdump on

4) Reboot server for kdump to take effect (new initrd will be generated)

5) Once rebooted kdump status should be Operational

service kdump status

if it's not Operational check error with

service kdump restart/start

If error is "Memory for crashkernel is not reserved" follow given link

http://linuxtroubleshoot.blogspot.in/2014/09/troubleshooting-kdump-error-memory-for.html

How to set QLogic HBA Adapter Speed?

Here we are going to discuss steps which needs to follow for QLogic HBA Adapter

Reboot the server and go to Qlogic HBA BIOS by entering ctrl + Q when it appears on the screen.

1) Ctrl + Q



2) HBA BIOS, select the HBA port and press enter



3) Select "Configuration Settings"



4) Select "Data Rate"

5) Finally Select Speed as per requirement



Thats it!!

Linux login issue - Can not login: Resource temporarily not available

Sometime we get following error while user login:

$su - username
Coud not login: Resource temporarily not available

Most of the times this is because of processes or number of files available to user is already used. So, could not arrange more files/processes to login, in this case we required to increase the limits for user in /etc/security/limits.conf (PAM module)

username        soft    nproc           3000
username        hard    nproc          4096
username        soft     nofile           6000
username        hard    nofile          8192


Set these values as per requirement else setting up these values higher may result in server inaccessible in case that particular uses all file descriptors, process and higher memory of server.