mac address – Page 2

Detecting duplicate VM MAC Address using vCenter Server Alarm

02.25.2015 by William Lam // 6 Comments

Having a duplicate VM MAC Address in your environment can lead to an extremely painful day of troubleshooting and it can also be tough to prevent depending on how and where you provision your VMs.

There are two cases that I can think of where a duplicate MAC Address can potentially occur:

You manually assign a static MAC Address versus using dynamic assignment (includes VM import) and it conflicts with an already assigned MAC Address
You migrate a VM from one vCenter Server to another and the destination vCenter Server has already assigned the MAC Address of the migrated VM

In both of these scenarios, when a duplicate MAC Address occurs, time is of the essence to quickly pin-point the source of the duplicated entry and quickly resolving the conflict. What would be nice is to be able to automatically detect that a MAC Address conflict has occurred and provide the necessary information of the offending VMs.

UPDATE (4/22) - Thanks to Petr, it turns out there is another MAC Address conflict event which I did not know about specifically for detecting duplicate entries for manually assigned MAC Addresses called "VM static MAC conflict". I definitely recommend creating an alarm for both Events for the vCenter Alarm.

While performing some research in my lab environment the other day, I accidentally stumbled onto this little tidbit in vCenter Server. It turns out there is an out of the box event called "VM Mac conflict" which can be triggered using a vCenter Server alarm when a duplicated MAC Address is detected for a VM. I was actually surprised that this was not one of the pre-created default alarms in vCenter Server as I can see this being extremely useful to have out of the box. In any case, it is simple enough to create a new vCenter Server Alarm and in the example below I called it "Dupe VM Mac Address".

To test our new alarm, I created a new VM called "VM1" which has been configured with static MAC Address that matches "VM2". Once the VM has been created, we can see that the alarm is immediately triggered and by clicking into the alarm details, it provides the details of the MAC Address and the offending VMs.

In my opinion this is an alarm that everyone should create in their environment to ensure that if this problem ever occurs, you can quickly get notified and resolve the problem. I have also reported this internally and asked if we can have this alarm created by default, so hopefully this will not be necessary in the near future 🙂

Considerations when migrating VMs between vCenter Servers

05.02.2014 by William Lam // 19 Comments

Something that I really enjoy when I get a chance to, is to speak with our field folks and learn a bit more about our customer environments and some of the challenges they are facing. Last week I had quick call with one of our TAMs (Technical Account Managers) regarding the topic of Virtual Machine migration between vCenter Servers. The process of migrating Virtual Machines between two vCenter Servers is not particularly difficult, you simply disconnect the ESXi hosts from one vCenter Server and re-connect to the new vCenter Server. This is something I have performed on several occasions when I was a customer and with some planning it works effortlessly.

However, there are certain scenarios and configurations when migrating VMs between vCenter Servers that could potentially cause Virtual Machine MAC Address collisions. Before we jump into these scenarios, here is some background. By default, a Virtual Machine MAC Address is automatically generated by vCenter Server and the algorithm is based on vCenter Server's unique ID (0-63) among few other parameters which is documented here. If you have more than one vCenter Server, a best practice is to ensure that these VC IDs are different, especially if they are in the same broadcast domain.

As you can imagine, if you have two vCenter Servers that are configured with the same VC ID, there is a possibility that a duplicate MAC Address could be generated. You might think this is probably a rare event given the 65,000 possible MAC Address combinations. However, it actually happens more frequently than you think, especially in very large scale environments and/or Dev/Test for continuous build/integration environments which I have worked in the past and I have personally seen these issues before.

Going back to our vCenter Server migration topic, there are currently two main scenarios that I see occurring in customr environments and we can explore each in a bit more detail and their implications:

Migrate ALL Virtual Machines from old vCenter Server to new vCenter Server
Migrate portion of Virtual Machines from old vCenter Server to new vCenter Server

Migrate all Virtual Machines:

In the diagram above, we have vCenter Server 1 and vCenter Server 2 providing a before/after view. To make things easy, lets say they have VC ID 1 and 2. If we migrate ALL Virtual Machines across, we can see their original MAC Addresses will be preserved as we expect. For any new Virtual Machines being created, the 4th octet of the MAC Address will differ as expected and the vCenter Server will guarantee it is unique. If you want to ensure that new Virtual Machines keep a similar algorithm, you could change the vCenter Server ID to 1. No issues here and the migration is very straight forward.

Migrate A portion of Virtual Machines:

In the second diagram, we still have vCenter Server 1 and vCenter Server with unique VC IDs. However, in this scenario we are only migrating a portion of the Virtual Machines from vCenter Server 1 to vCenter Server 2. By migrating VM2 off of vCenter Server 1, the MAC Address of VM2 is no longer registered with that vCenter Server. What this means is that vCenter Server 1 can potentially re-use that MAC Address when it generates a new request. As you can see from the above diagram, this is a concern because VM2 is still using that MAC Address in vCenter Server 2, but vCenter Server 1 is no longer aware of its existence.

The scenario above is what the TAM was seeing at his customer's site and after understanding the challenge, there are a couple of potential solutions:

Range-Based MAC Address allocation - Allows you to specify a range of MAC Addresses to allocate from which may or may not helpful if the migrated MAC Addresses are truly random
Prefix-Based MAC Address allocation - Allows you to modify the default VMware OUI (00:50:56) which would then ensure no conflicts would be created with previously assigned MAC Addresses. Though this could solve the problem, you potentially could run into collisions with other OUI's within your environment
Leave VMs in a disconnected state - This was actually a solution provided by another TAM on an internal thread which ended up working for his customer. The idea was that instead of disconnecting and removing the ESXi host when migrating a set of Virtual Machines, you would just leave it disconnected in vCenter Server 1. You would still be able to connect the ESXi host and Virtual Machines to vCenter Server 2 but from vCenter Server 1 point of view, the MAC Addresses for those Virtual Machines are seen as in use and it would not be reallocated.

I thought option #3 was a pretty interesting and out of the box solution that the customer came up with. The use case that caused them to see this problem in the first place is due to the way they provision remote environments. The customer has a centralized build environment in which everything is built and then shipped off to the remote sites which is a fairly common practice. Since the centralized vCenter Server is not moving, you can see how previously used MAC Addresses could be re-allocated.

Although option #3 would be the easiest to implement, I am not a fan of seeing so many disconnected systems from an operational perspective as it is hard to tell if there is an issue with the ESXi host and Virtual Machines or because it has been migrated. I guess one way to help with that is to create a Folder called "Migrated" and move all disconnected ESXi hosts into that folder which would help mask that away and disable any alarms for those hosts.

Some additional per-requisite checks that you can perform prior to the partial Virtual Machine migration:

Ensure that the destination vCenter Server is not configured with the same VC ID else you can potentially run into duplicate MAC Address conflicts. You can do this either manually through the vSphere Web/C# Client or leveraging our CLI/API to do so.

Here is an example using PowerCLI to retrieve the vCenter Server ID:

Get-AdvancedSetting -Entity $server -Name instance.id

Ensure no duplicate MAC Addresses exists by comparing the MAC Addresses of the Virtual Machines to be migrated to the Virtual Machines in the new environment. Again, you can either do this by hand (which I would not recommend) or leveraging our CLI/API to extract this information.

Here is an example using PowerCLI to retrieve the MAC Addresses for a Virtual Machine:

Get-VM | Select-Object -Property Name, PowerState, @{"Name"="MAC";"Expression"={($_ | Get-NetworkAdapter).MacAddress}}

If there are other scenarios or solutions that you have seen with Virtual Machine migrations between vCenter Serves, feel free to leave a comment. I am sure others can benefit from past experiences or any other lesson learns.

Does ESXi Support DDNS (Dynamic DNS)?

08.01.2013 by William Lam // 5 Comments

An interesting feature request that was raised internally was for ESXi to support DDNS (Dynamic DNS) which allows a host client to update it's DNS record when using a DHCP Server. In most environments, to assign a hostname from DHCP, a DHCP reservation is used and this is maintained by the DHCP Server versus DDNS, where it is maintained by the client. Thanks to my colleague Eric Wager who did some quick research and found that ESXi does in fact supports DDNS and has been since ESXi 5.0.

I have not worked with DDNS much in the past and I have only seen it used for free/paid online services targeted at consumers to provide a well known address when their public IP Address changes frequently as with most ISPs. If your DHCP Server supports DDNS, this can be a handy feature to have, especially as you add new hosts you no longer have to manually create individual DNS record before hand and great for a lab environment. I did a big more digging to have a better understanding of how DDNS works with ESXi.

To enable support for DDNS on your ESXi host, you just need to set the hostname for the following ESXi Advanced Setting:

/Misc/PreferredHostName

You can do this in a variety of ways using either the vSphere Web/C# Client or using the command-line with ESXCLI.

Here is the syntax for the command:

esxcli system settings advanced set -o /Misc/PreferredHostName -s vesxi04.primp-industries.com

Once you have configured the setting, for the changes to go into effect, you will need to restart the management network. The easiest way to do this is via DCUI which you can run remotely by just typing dcui if you have an SSH session to your ESXi host. If you are using scripted install such as Kickstart, this can easily be automated as part of the post-install and upon first boot, DDNS will be enabled and configured with the proper hostname.

To test this in my lab environment, before enabling DDNS, I performed a reverse lookup of the assigned IP Address of my ESXi host from my DHCP server. In this example, the host received the address 192.168.1.135.

As you can see from the screenshot, a hostname could not be resolved as I would expect. After our changes, if we perform the reverse lookup again, we should now see the hostname that we had configured.

Another useful tidbit is the DHCP Client on ESXi is an ISC BIND implementation and this means if you require advanced things such as authentication keys, you can configured these options in /etc/dhclient-vmkX.conf where X is the specific VMkernel interface. For most deployments, you should not have to edit this file. Also if you want to prevent your DHCP Server from overriding the hostname of your ESXi host, you can add the following entry to the dhclient-vmkX.conf configuration file:

interface vmk0 {
supersede host-name "vesxi04.primp-industries.com";
}

Just when I thought I knew about all the awesome features ESXi offers, it is a nice surprise to learn about another one!