Kernel Panic - Fallback to Old Kernel

Few days back, i was testing a kernel module, unfortunately it didn't work and instantly after reboot, kernel Panic occurs. Well i was clueless what goes wrong, but now I cannot select the older kernel version to boot. But, there is a way out, if we choose these first before reboot (off course after installing new kernel), and this is inbuilt in GRUB. This mechanism is called Grub Fall-back. In this case you can set your older/tested kernel as the Fallback kernel in case new kernel panics. And here is how you do it:All you have to do is to open your /boot/grub/grub.conf and add a few lines in it like this

—————————————————————————————————————
default=saved

timeout=15
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
fallback 0 1

title CentOS LIDS (2.6.18-128)
root (hd0,0)
kernel /vmlinuz ro root=/dev/VolGroup00/LogVol00
initrd /initrd-lids-2.6.18-128.img
savedefault fallback

title CentOS (2.6.18-128.el5xen)
root (hd0,0)
kernel /vmlinuz-2.6.18-128.el5 ro root=/dev/VolGroup00/LogVol00
initrd /initrd-2.6.18-128.el5.img
savedefault

—————————————————————————————————————

If the above grub.conf is booted, then first Grub.conf will try to boot the system with “CentOS Testing” and if and only if the kenrel panics, it will call “CentOS Working” and boot the system

Hope this quick howto helps.

Kernel Panic - Fallback to Old Kernel

Few days back, i was testing a kernel module, unfortunately it didn't work and instantly after reboot, kernel Panic occurs. Well i was clueless what goes wrong, but now i can not select the older kernel version to boot. But, there is a way out, if we choose these first before reboot (off course after installing new kernel), and this is inbuilt in GRUB. This mechanism is called Grub Fall-back. In this case you can set your older/tested kernel as the Fallback kernel in case new kernel panics. And here is how you do it:All you have to do is to open your /boot/grub/grub.conf and add a few lines in it like this

Citrix XenServer – iSCSI LUN

Hi all, here is my new Mini-Howto for Citrix XenServer – Storage Repository Over Software iSCSI LUN

Topic contains:
How to Create NFS and iSCSI LUN on Linux (I will CentOS)
How to mount them on Citrix Xen Server using Command line and GUI (that includes Screen Shots)

STORE SSH KEYS IN LDAP

We need password less ssh login, so here i have tried to make the keys centralized by storing them in LDAP

LDAP Server Setup

[root@ldap]#  yum install openldap{,-clients,-devel,} nss_ldap

make entry in /etc/hosts

192.168.1.3    directory.domain.com

DNS QUERY TIMEOUT ON LINUX CLIENT

Hi all, here i have come with a situation, when we have multiple DNS, basically Nameserver entry in resolve.conf at client, and sometimes we also face dam slow resolves in queries, this is may also due to DNS server issue, may be DNS has gone away or some other issues that is making it slow to resolve queries.By default, linux client attempts 3 times to Nameserver for queries. Here it make the process slow.
So what we can do, is to set timeout in resolve.conf, this will help client to choose another Nameserver for AnswersHere we go:

Multiple Purpose Expect Script

DISK ALERT - Expect ScriptHi All, here's I have bring a expect script, that this we can Run command on Remote Linux Servers, copy file using SSHSo here it is:
[root@virtual-vm ~]#  yum install expect  (if expect package is not installed)

Load Balance Mysql Slaves Using MySQLProxy

I am sure you all have read about MySQL replication, like Master-Master, Single Master - Multiple Slaves and their topologies.
Here I am going to describe how you can load balance several MySQL Slaves and query logging.

Linux Commands - Not Usual

Linux Command Tips

Check Validity of a Certificate File (PEM File) # openssl x509 -in Certificates.pem -inform PEM -text -noout -enddate

Monitor File Activities - Using iWatch

Monitor File Activities - iWatch

We frequently meet situation, like some files or directories that supposed to secure are having some operations on it, like read, open, close, write etc...

And many times people would like to have a monitoring tool to monitor files activities.

Here’s a solution for this, I got a tool “iWatch”, written in Perl and can also work in Daemon mode.

This tool can be used as to monitor/detect unwanted manipulation on file system and report it to system admins.

RSYNC Server - A Remote File Transfer Protocol - Server Migration Tool

RSYNC SERVER

Remote file transfer protocol

Rsync is a linux in-built file transfer program that is similar to RCP or SCP. It includes the added feature of allowing just the differences between two sets of files on two machines to be transferred across the network.  

Rsync sends un-encrypted traffic over the network, but can this can also be tunneled through SSH.

The Rsync listens on port 873.

Create RAID on CentOS

RAID On CentOS

First let's understand the term RAID
RAID basically stands for Redundant Array of Inexpensive Disk
RAID is used for Implementing Fastness in reading, writing, and Redundancy on DISks

RAID types:
1: Software RAID
2: Hardware RAID
3: Fake RAID

LDAP - Authentication server for SSH and SVN

LDAP as Auth server for Users  ---- With SSH and SVNThis HOW TO belongs to those, who have some funda about LDAP, SVN, Apache

Prepare LDAP Server:

yum install openldap{,-clients,-devel,} nss_ldap