Linux/Unix/BSD

I got back home from airport some time ago. It was a very long day for me. My exam started at 9 AM and continued till 4:30 PM including 30 minutes of lunch break. I did extremely well in the exam (I felt that way) and was pretty sure that I am going to be RHCE. After the exam, I grabbed a taxi and went to the Toronto Pearson Airport. But my flight was at 9:45 PM Toronto time, so I had to wait for more than 3 hours. Then after 4.5 hours of journey I came back home, took a shower, and started to check my email. Surprisingly, I found they already emailed me the result of my exam and I lost all of my tiredness after seeing my RHCE certificate (they sent a PDF file of certificate). My end results:

SECTION I: TROUBLESHOOTING AND SYSTEM MAINTENANCE
RHCE requirements: 	completion of compulsory items (50 points)
			overall section score of 80 or higher
RHCT requirements:  	completion of compulsory items (50 points)

Compulsory Section I score:		50.0
Non-compulsory Section I score:		50.0
Overall Section I score:		100

SECTION II: INSTALLATION AND CONFIGURATION
RHCE requirements:	score of 70 or higher on RHCT components (100 points)
			score of 70 or higher on RHCE components (100 points)
RHCT requirement:	score of 70 or higher on RHCT components (100 points)

RHCT components score:			90.3
RHCE components score:			89.3

RHCE Certification:			PASS

It was really a thrilling and challenging exam.

As I am building up a total IPv6 network of Linux servers on VMWare, today I will write about how we can configure DHCPv6 server & client. I tested only on RedHat/Fedora/CentOS based platforms. But it should work on other platforms with some minor changes. So lets start…

Server Configuration: (dhcp6s)

First we will install DHCPv6 RPM on the server:

# yum -y install dhcpv6

After installing the above mentioned RPM we will turn on global IPv6 initialization and will also enable forwarding of IPv6 packets:

# vi /etc/sysconfig/network

NETWORKING_IPV6=yes
IPV6FORWARDING=yes

Then we will turn on IPv6 on a particular device and we will also a set an IPv6 address for that device by adding the following lines:

# vi /etc/sysconfig/network-scripts/ifcfg-eth0

IPV6INIT=yes
IPV6ADDR="2002:1851:c622:1::1/64"

Now we will specify the interface for DHCPv6 server:

# vi /etc/sysconfig/dhcp6s

DHCP6SIF=eth0
DHCP6SARGS=

Next, we will edit the main DHCPv6 server configuration file as following:

# cp /usr/share/doc/dhcpv6-*/dhcp6s.conf /etc/
# vi /etc/dhcp6s.conf

interface eth0 {
	server-preference 255;
	renew-time 60;
	rebind-time 90;
	prefer-life-time 130;
	valid-life-time 200;
	allow rapid-commit;
	option dns_servers 2002:1851:c622:1::1 example.com;
	link AAA {
		pool{
			range 2002:1851:c622:1::2 to 2002:1851:c622:1::9/64;
			prefix 2002:1851:c622:1::/64;
		};
	};
};

Finally, we will start DHCPv6 server daemon:

# service network restart && service dhcp6s start && chkconfig dhcp6s on

Client Configuration: (dhcp6c)

We will start client configuration by installing DHCPv6 client RPM:

# yum –y install dhcpv6_client

Then we will enable IPv6 networking on the client:

# vi /etc/sysconfig/network

NETWORKING_IPV6=yes

Next we will configure a particular device to participate as DHCPv6 client by adding the following lines to that device configuration file:

# vi /etc/sysconfig/network-scripts/ifcfg-eth0

IPV6INIT=yes
DHCPV6C=yes

After that we will edit the main DHCPv6 client configuration file as following:

# cp /usr/share/doc/dhcpv6_client-*/dhcp6c.conf /etc/
# vi /etc/dhcp6c.conf

interface eth0 { 
	send rapid-commit; 
	request domain-name-servers; 
};

Finally we will restart networking on the client to acquire an IPv6 address from the DHCPv6 server:

# service network restart

That’s all. If everything goes right we will see a global IPv6 address on the client by running:

# ifconfig eth0

I will write more about IPv6 stuff later.

Today I created a script for one of the RHCE troubleshooting problem. Here it goes...

#!/bin/bash

# Created by Zakir (ahmed@zakir.net)
# Problem: Run this script as 'root' and then login as 'root'
# Best of luck.


date -s 2007-01-02 &> /dev/null
chage -E 2007-01-01 root &> /dev/null

usermod -s /sbin/nologin root &> /dev/null

for n in {1..11}; do
sed -i "s/^tty$n/#tty$n/" /etc/securetty
done

sed -i "s/$(grep ^passwd /etc/nsswitch.conf)/passwd: nis/g" /etc/nsswitch.conf

chattr +i /etc/shadow &> /dev/null

history -c
reboot

# End of script

This script will make your life hell to login as 'root'.

Today I tested how to access NFSv4 share using SSH tunnel. Now I will go through the steps to demonstrate how can we accomplish this goal on RedHat/Fedora/CentOS based platform:

Server Configuration:

First, in the /etc/exports file, we will export the NFS share by inserting the following line:

# vi /etc/exports

/share 127.0.0.1(rw,sync,fsid=0,insecure)

Then we will export the share and start the nfs service:

# exportfs -rv
# service nfs restart && chkconfig nfs on

Client Configuration:

In the /etc/fstab file, we will add an entry as follows:

# vi /etc/fstab

127.0.0.1:/ /mnt/nfs4 nfs4 rw,hard,intr,proto=tcp,port=3333 0 0

Now we will setup an SSH tunnel with port forwarding. Instead of using TCP 2049 for the local port, we will use TCP 3333 just to show that NFSv4 clients and SSH tunnels do not care which ports they use. So we will open up an SSH session from the NFS client to the NFS server:

# ssh -Nf -L 3333:127.0.0.1:2049

Note: The IP 127.0.0.1 is resolved after the secure shell connection has been established. So when defining local forwarding (outgoing tunnels), 127.0.0.1 refers to the server (remote host computer) we are connected to.After that we will mount the NFS share:

# mount -v /mnt/nfs4

That’s all. The client can now use the NFSv4 filesystem as if it were local, but it is actually an encrypted, remote filesystem.