IntroductionIn recently weeks I have been involved with a few engagements where there has been a confusion with the understanding of how DNS within a Cloud at Customer environment works. This blog posting will be an attempt to clarify how it works at a reasonably high level.
DNS OverviewThe Domain Name System is provided to enable a mapping from a fairly human readable form to an actual IP address which computers can use to route traffic. There are essentially two components to the name, firstly the hostname of a machine and secondly a domain name. eg. donald.mycompany.com where donald represents the hostname and mycompany.com represents what is called the domain.
On a linux system the resolution of DNS is typically specified in a file /etc/resolv.conf it will look something like this.
# Generated by NetworkManager
The nameserver entry specifies the IP address of a DNS server which will be used to change names into IP addresses. We have two entries so that should the first one be unavailable for any reason then the second one will be used. We can then use a unix utility called nslookup (part of the bind-utils package if you want to install it) which will allow us to query the DNS server. Other utilities such as dig can also provide information from DNS.
$ nslookup oracle.com
Note that this response includes the text "Non-authorative answer". The reason for this is that all DNS servers will be in control of different domains and in our example we tried to lookup a domain called oracle.com. From my laptop the DNS server is from my ISP so it knows nothing about the oracle.com domain. In this situation the DNS server I talked to will forward the request on to a .com domain server which in turn is likely to delegate the request to the DNS server responsible (or authorative) for the oracle.com domain. This returns the IP address which flows back through the DNS servers to my ISP which caches the answer and gives me the IP address to use for a lookup on oracle.com.
DNS on Oracle Cloud@CustomerInside the control plane of an OCC environment there is a DNS server that runs. This server is authorative over two domains of interest to a customer. Specifically one in the format of <Subscription ID>.oraclecloudatcustomer.com and also one of the format <Account>/<Region>.internal. For example. S12345678.oraclecloudatcustomer.com and compute-500011223.gb1.internal.
cloudatcustomer.com is a domain that Oracle own and run on behalf of the customer and is used to provide the URLs that the customer will interact with. Utilise the UI for managing the rack or accessing the REST APIs for compute, PaaS, Storage etc. So, for example the underlying compute REST API of a C@C environment will be something like https://compute.<Region>.ocm.<Subscription ID>.oraclecloudatcustomer.com. So in our example above this would become https://compute.gb1.ocm.S12345678.oraclecloudatcustomer.com.
Clearly the domain <Subscription>.oraclecloudatcustomer.com is something that is not going to be known in any customer's DNS so normal DNS actions would take place and the customer's DNS would look up the domain chain to find a DNS server that is authorative for oraclecloudatcustomer.com. As these are not publicly accessible this lookup would fail. To fix this we need to change the customer's DNS to make it aware of the <Subscription ID>.oraclecloudatcustomer.com domain so it knows how to access a DNS service that will return an appropriate IP address and enable usage of the URL. This is done by adding a forwarding rule to your customer's DNS service which forwards any request for a C@C URL to the DNS running in the C@C environment.
Once the forwarding rule is in place then the customer is able to start using the UI and REST APIs of the C@C so they can manage the platform.
internal domainSo now the customer can access the cloud at customer management screens and can create virtual machines to run their enterprise. The next question is about what is the internal domain. This essentially relates to configuration of the VM itself. During the provisioning process there is a screen which requests the "DNS Hostname Prefix" as shown below.
The name used in the DNS Hostname Prefix becomes part of the registration into the OCC DNS server to represent the host portion. ie. When we create the VM it will have a DNS registration internally like df-web01.compute-500019369.gb1.internal which maps onto the private IP address allocated to the VM. This means if you are creating multiple VMs together or using a PaaS service that produces multipleVMs (like JCS) then there are URLs available to enable them to talk to each other without you having to know the IP address.
Once a VM has been created it is possible to see the internal DNS name for the VM from the details page as shown below.
This can be resolved from within the VM as shown below.
[opc@df-vm01 ~]$ nslookup df-vm01
[opc@df-vm01 ~]$ cat /etc/resolv.conf
; generated by /usr/sbin/dhclient-script
search compute-500019369.gb1.internal. compute-500019369.gb1.internal.
So we can see that the default behaviour of a VM created from one of the standard templates is to configure the VM to use the internal DNS server and the DNS Name of the VM is made available through the DNS lookup and will return the internal or private IP address of the VM.
So what about lookups on other domains. Provided the customer DNS allows lookups then the request will get forwarded on to the customer DNS which can provide the appropriate response.
[opc@df-vm01 ~]$ nslookup oracle.com
The next obvious question is regarding how a client sitting in the customer's datacentre would find the IP address of the VM that was created. There are no DNS records created for the "public" IP address which has been allocated (NAT) to the VM. The answer to this is that it is the same as if you were installing a new server in your environment. One of the steps would be to specify an IP address for the new machine and then register into the customer's DNS. Once you have created the VM and know which IP address is being used for it take that IP and add it to your DNS with an appriate fully qualified domain name. eg. donald.mycompany.com. Once registered any client would be able to lookup donald.mycompany.com and start using the services running on the VM instance.
With Oracle Cloud at Customer it is possible to use an IP Reservation which will reserve an IP address from the client IP pool/shared network and once reserved it is fully under the control of the cloud administrator. Thus the IP address can be registered in DNS before the VM is created if that suits the processes of your company better than registering the address after VM creation.