Migrating from RHEL6 to CentOS6...

Why would anyone ever want to do this???
mkdir TMP
yum remove rhnlib abrt-plugin-bugzilla redhat-release-notes*
rpm -e --nodeps redhat-release redhat-indexhtml
cd TMP
wget http://mirror.centos.org/centos/6/os/x86_64/Pack\
wget http://mirror.centos.org/centos/6/os/x86_64/Pack\
wget http://mirror.centos.org/centos/6/os/x86_64/Pack\
wget http://mirror.centos.org/centos/6/os/x86_64/Pack\
rpm -Uvh *.rpm
cd ..
rm -rf TMP
yum clean all
yum upgrade

How to Migrate from RHEL5 to CentOS5...

# cp /etc/redhat-release /etc/redhat-release-saved
# rpm -e --nodeps redhat-release-notes redhat-release \
yum-rhn-plugin redhat-logos
# rpm -ivh \
CentOS/centos-release-5-5.el5.centos.1.i386.rpm  \
CentOS/centos-release-notes-5.5-0.i386.rpm \
# yum update

How to make a RHEL Auto-Install DVD

This is how I make a bootable Red Hat Enterprise Linux installation DVD, that includes a customized kickstart file. Installation is automatic and no questions are asked. 
Make a directory called ./dvd/

Loop mount the ISO image of the DVD and copy all of its contents (intact) to ./dvd/

Unmount the ISO image.

The kickstart file is named ks.cfg and is placed in the root directory of the DVD.

In the ks.cfg file:


This specifies cdrom as the installation source and causes the machine to power off when done installing, so we can remove the DVD from the drive.

In the isolinux/isolinux.cfg file:

default linux    
prompt 1
timeout 6        
display boot.msg
label linux
  kernel vmlinuz
  append initrd=initrd.img method=cdrom ks=cdrom:/ks.cfg   

Change the timeout to something quicker. The default statement specifies which menu option is selected automatically. In the menu option, we specify cdrom as the installation method, and the location and name of the kickstart file (which has to be named ks.cfg).

Create the new ISO file from the contents of the ./dvd/ directory:

/usr/bin/mkisofs -o dvd.iso -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -R -J -v -T ./dvd/

The -o option lets you name the ISO image file anything you want. 
Copy the resulting dvd.iso file to your PC and burn it to a DVD.

How to create an SSH tunnel for vncviewer

What I'm doing is using a vncviewer to connect to a vncserver, via an intermediary host (such as a firewall), using a SSH Tunnel between HostA and HostC, to encrypt communications.

I have the same unprivileged userid on all three hosts. Also, I used ssh keys for authentication, just to speed things up. Otherwise, you can specify username@host and enter the password when asked.

$ ssh -N -T -X -f HostB.example.com -L 5905:HostC.example.com:5905 --sleep +30m
$ vncviewer localhost:5905 &

Change the “+30m” to however long you want the ssh tunnel to stay open. You can also use “+8h” or “+1d”.

+----------+        +----------------+        +-------------+
| Host A   |        |                |        |   Host C    |
|          |------->|  Host B        |------->| destination |
| vncviewer|        | "intermediary" |        | vncserver   |
+----------+        +----------------+        +-------------+