Month: October 2012

Installing a DHCP server in Centos 6.x


1. Install CentOS 6.x or find an adequate server to run the dhcpd daemon

2. Run the following steps:

sudo yum install dhcp

3. Configuration steps

Edit the following file as shown below:

vim /etc/dhcp/dhcp.conf

Edit to fit your environment

# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.sample
# see 'man 5 dhcpd.conf
subnet netmask {

option routers; #Default Gateway
option subnet-mask;
option domain-name "";
option domain-name-servers;
range dynamic-bootp; #DHCP Range to assign
default-lease-time 43200;
max-lease-time 86400;

3.1 Configure DHCP to listen on a specific interface

If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interface, configure the DHCP server to start only on that device. In /etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:

# Command line options here

3.2  Starting the DHCP service

To start the DHCP service, use the command

/sbin/service dhcpd start

To stop the DHCP server, use the command 

/sbin/service dhcpd stop

3.3    DHCP server logs

On the DHCP server, the file /var/lib/dhcp/dhcpd.leases stores the DHCP client lease database. This file should not be modified by hand. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.

All times in the lease database are in Greenwich Mean Time (GMT), not local time.
The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The dhcpd.leases file is renamed dhcpd.leases~, and the temporary lease database is written to dhcpd.leases.

The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all the old leases will be lost and cause many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.

3.4   DHCP relay agent

The DHCP Relay Agent (dhcrelay) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets. When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.

The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in /etc/sysconfig/dhcrelay with the INTERFACES directive.

To start the DHCP Relay Agent, use the command 

service dhcrelay start

vSphere 5.1 review


I recently took time out to review the new features of vsphere 5.1 and this is what I found:

1. Larger and more powerful Virtual Machines

Virtual machines can grow two times larger than in any previous release to support even the most advanced applications. Virtual machines can now have up to 64 virtual CPUs (vCPUs) and 1TB of virtual RAM (vRAM). Why would we want that size of VMs. Well I have never seen a virtual machine that will utilise 64 vcpus, but large applications that run in high performance compute grid networks may just have this requirement and it’s another reason why you would use VMware over the other competitors, XenServer, Hyper-V for example, to run your mission critical intensive application on a the vmware hypervisor.

usefulness : 5/10

2. A new virtual machine format

New features in the virtualmachine format (version 9) in vSphere 5.1 include support for larger virtual machines, CPU performance counters and virtual shared graphics acceleration designed for enhanced performance.

This goes hand in hand with the first feature but notice we have a nice shared graphics acceleration feature. NVIDIA added vSGA (Virtual Shared Graphics Acceleration) that allows the presentation of a physical graphics processing unit (GPU) from the underlying host to virtual desktops guests. By virtualizing the physical GPU, its resources can be allocated and shared across several virtual desktop instances. This provides several different benefits. Using the physical GPU and vRAM frees the underlying CPU and memory from the host to be used for other tasks. Using a GPU for hardware-accelerated graphics also allows customers to provide a more rich and interactive graphical experience across an even broader set of use cases, especially implementation of vmware view.

Usefulness 6/10

3. Storage enhancements

Flexible, space-efficient storage for virtual desktop infrastructure (VDI). A new disk format enables the correct balance between space efficiency and I/O throughput for the virtual desktop.

Usefulness 6/10

4. vSphere Distributed Switch enhancements

Enhancements such as NetworkHealth Check, Configuration Backup and Restore, Roll Back andRecovery, and Link Aggregation Control Protocol support anddeliver more enterprise-class networking functionality and a more robust foundation for cloud computing.

Anything distributed switch is good and useful – it is a great part of the product and really helps define network policies more efficiently, especially when you have large numbers of hosts.

usefulness 7/10

5. Single-root I/O virtualization (SR-IOV) support

Support forSR-IOV optimizes performance for sophisticated applications. SR-IOV is a specification that allows a PCIe device to appear to be multiple separate physical PCIe devices. Here is a great video explain SR-IOV and how resources are assigned to each particular function. This all helps with over performance and with point 1 and larger machines, all feeds back in to the fact that VMware ESX can handle bigger workloads

usefulness 7/10

6. AvailabilityvSphere vMotion enhancements

Leverage the advantages of vMotion(zero-downtime migration) without the need for shared storage configurations. This new vMotion capability appliesto the entire network. This is great – This means you can migrate virtual machines live without needing “shared storage”. In other words you can vMotion virtual machines between ESXi hosts with only local storage

usefulness 10/10

7. vSphere Data Protection changes

Simple and cost effective backupand recovery for virtual machines. vSphere Data Protection isa newly architected solution based on EMC Avamar technology that allows admins to back up virtual machine data to diskwithout the need of agents and with built-in deduplication.  This feature replaces the vSphere Data Recovery product available with previous releases of vSphere.

A great white paper from VMware regarding this:

usefulness: 9/10

8. vSphere Replication

vSphere Replication enables efficientarray-agnostic replication of virtual machine data over the LAN or WAN. vSphere Replication simplifies management enablingreplication at the virtual machine level and enables RPOs as low as 15 minutes.

I like this one. Again another feature rich offering from Vmware.

usefulness: 9/10

9. Reduced downtime upgrade for VMware Tools

After you upgrade to the VMware Tools available with version 5.1, reboots have been reduced or eliminated for subsequent VMware Tools upgrades on Windows.

Been a while coming due to the challenges of upgrading locked files within an operating system most probably not the best and easiest to work within when you are VMware. We here and expect some more enhancements coming in the later versions.

usefulness 8/10

10. Additional security enhancements

VMware vShield Endpoint delivers a proven endpointsecurity solution to any workload with an approach that is simplified, efficient, and cloud-aware. vShield Endpoint enables3rd party endpoint security solutions to eliminate the agentfootprint from the virtual machines, offload intelligence to a security virtual appliance, and run scans with minimal impact.

This was once bluelane and its now bundled in the product. It makes a lot of sense when you are running multiple instances of security as you can now limit the overhead.

usefulness: 8/10

11. vSphere Storage DRS and Profile-Driven Storage

New integration with VMware vCloud® Director™ enables further storage efficiencies and automation in a private cloud environment. This is cool – a feature that allows us to DRS storage IO. Not tested in anger but a great performance tool in an already feature rich product.

usefulness: 9/10

12. vSphere Auto Deploy

Two new methods for deployingnew vSphere hosts to an environment make the Auto Deploy process more highly available then ever before. This is my favourite. Stateless and stateful ESXi deployments. Got to be worth a bucket of comfort knowing that whatever happens, the ESXi host will also boot up.

usefulness 10/10

13. VMware vCenter™ Operations Manager Foundation

This enables you to leverage comprehensive views into health, risk and efficiency scores of your vSphere environment infrastructure. Quickly drill down to see what’s causing current workloadconditions, pinpoint potential problems in the future andidentify areas with inefficient use of resources.

usefulness 9/10

14. vCenter Orchestrator

Orchestrator simplifies installation and configuration of the powerful workflow engine in vCenter Server. Newly designed workflows enhance ease of use, and canalso be launched directly from the new vSphere Web Client. vCO is a powerful tool and one that you can do truck loads with – alot of large service providers would use this to prep a machine when it is being provisioned, for example, creating a naming convention, putting this VM in the right container within a management domain, installing management tools . Always a good thing

usefulness 8/10

15. Management using vSphere Web Client

The vSphere Web Client is now the core administrative interface for vSphere. This new flexible, robust interface simplifies vSphere control through shortcut navigation, custom tagging, enhanced scalability, and the ability to manage from anywhere with Internet Explorer orFirefox-enabled devices.

This is a big winner for me. Still we need a windows installer and an operating system to run on but its a big move in the right direction.

usefulness 10/10

16. vCenter Single Sign-On

vCenter SSO is sometimes misunderstood and this is a great step in the right direction. SSO will act as a proxy rather than a authenticator and what I mean by this is that you can now have mulitple directory services configured to authenticate your user on to a vCenter environment. There is a great blog from Arnim Van Lieshout which describes this in more detail –

usefulness 8/10


VMware are still ahead of the game. XenServer is catching up and have a release later on in the year called the Augusta release which has features like dom0 disaggregation and Hyper-V 2012 has all the features now, but as VMware still have a huge foot in the industry, they need to now move up the stack and take more advantage of workflows and the automation features that are possible,


well its a 9/10 for me. Keep up the good work VMware, you really are some software provider who have changed the face of infrastructure.