What is the dns client used for?

[rickardnobel]

On ESX you can set some dns settings, like dns server address and domain name, however I am not sure what these are used for? Is it just for name resolution for administrators working in the Service Console or do any vmkernel part demand dns resolution?

[a_p_]

If you run an ESX(i) host as a standalone system DNS may not be an issue, however a proper DNS name resulution is required for several services and functions in vCenter (HA, linked mode, ...).

Open the VMware vSphere Online Library and search for "DNS". You will find a lot of descriptions where DNS resolution is a prerequisite.

André

[rickardnobel]

Thank you for your reply!

In HA, is dns needed if using hostnames for isolation test addresses or for other reasons?

For iSCSI and NFS, can you use dns names for targets or must they be IP addresses?

[mcowger]

You can use hostnames for all those items.

--Matt

VCP, vExpert, Unix Geek, Storage Nerd

[rickardnobel]

Thanks! And if I only use IP addresses for HA isolation, iSCSI and NFS targets, and do not have vCenter Linked Mode, would I need dns then?

And if my DNS servers are virtualized, and I only use IP based storage for my VMs - is there not a risk that I do not get to access them after a host shutdown? Is there some recommendation regarding this?

[a_p_]

Did you read all the results of the link I provided, yet? I guess not :smileywink:

Thanks! And if I only use IP addresses for HA isolation, iSCSI and NFS targets, and do not have vCenter Linked Mode, would I need dns then?

from http://pubs.vmware.com/vsphere-esx-4-1/wwhelp/wwhimpl/js/html/wwhelp.htm#href=availability_guide/r_ha_checklist.html

All hosts in a VMware HA cluster must have DNS configured so that the short host name (without the domain suffix) of any host in the cluster can be resolved to the appropriate IP address from any other host in the cluster. Otherwise, the Configuring HA task could fail. If you add the host using the IP address, also enable reverse DNS lookup (the IP address should be resolvable to the short host name).

And if my DNS servers are virtualized, and I only use IP based storage for my VMs - is there not a risk that I do not get to access them after a host shutdown? Is there some recommendation regarding this?

from http://pubs.vmware.com/vsphere-esx-4-1/wwhelp/wwhimpl/js/html/wwhelp.htm#href=iscsi_san_config/t_set_up_dynamic_discovery.html#1_8_18_7_1

6 Enter the IP address or DNS name of the storage system and click OK.

Anyway, it's your responsibility to have DNS up and running. Usually you don't have only one DNS server, so the failure of one of them does not affect the environment.

André

[rickardnobel]

Did you read all the results of the link I provided, yet? I guess not :smileywink:

It returned lots of result, but I did actually read the parts that seemed to deal with dns. :smileyhappy: Anyway, it was interesting to see that dns is needed when configuring HA.

I have some vague memory of dns being necessary when connecting to a remote console (MKS) from the vSphere client.. but I guess that should only be from the client point of view?

Anyway, it's your responsibility to have DNS up and running. Usually you don't have only one DNS server, so the failure of one of them does not affect the environment.

Yes, it is of course common to have at least two dns servers, but IF there are two and both are running in VMs and they are needed for iSCSI to work - should that not require some design to not lose both of them if a host goes down? Perhaps some anti-affinity setting?

[a_p_]

... should that not require some design to not lose both of them if a host goes down? Perhaps some anti-affinity setting?

Exactly!

André

[amvmware]

I would recommend at least one of the DNS servers is left physical if you have only a few hosts and\or limited resilience and redundancy - usually this is also an AD domain controller.

If both go down the impact on the esx hosts, vCenter and the VM's that have applications and dependencies on DNS would result in a very bad day all round, they all rely on DNS for name resolution.

[rickardnobel]

I would recommend at least one of the DNS servers is left physical if you have only a few hosts and\or limited resilience and redundancy

That could be something to consider. There seems to be not much writings from Vmware about this, or it might be that I have missed it.

[amvmware]

There seems to be not much writings from Vmware about this, or it might be that I have missed it.

I think you are missing the point - whether you chose to virtualise a server or service requires thought and planning - just because you can virtualise the server or service does not necessarily mean it is a good idea in your Infrastructure.

You need to understand your services such as DNS and what dependencies other servers and applications have of that service. You then need to consider how you are going to make it resilient and redundant - asuming you want to. Vmware cannot provide a best practise recommendation for every scenario that customers may have to do with applications and servcies - this is up to you to perform this with the guides VMware provides as a starting point or engage with a VMware partner than can assist you in arriving at the correct decisions for your organisation

[rickardnobel]

I think you are missing the point - whether you chose to virtualise a server or service requires thought and planning - just because you can virtualise the server or service does not necessarily mean it is a good idea in your Infrastructure.

It is of course true that I will have to take the responsabilty for every service I choose to virtualize and that Vmware could not possible have a best practice for all 3rd party software. But... what I am asking for is the core functions of the ESX platform itself and that kind of dependencies I will expect Vmware to get me a best practice for.

If this is true: let us say we have a couple of hosts and we are only using IP based storage with hostnames as targets and both our DNS servers are virtualized. If I for some reason loose power on my hosts at the same time then the enviroment will not start again?

If that is correct then I think some kind of advice from Vmware could be good, as if one DNS server should be left physical.