Scenario
Currently client access to CIFS shares is achieved by
accessing the share over a DFS Namespace, and the DNS entry in the DFS path to
target folder is simply an A-record of a NetApp 7-Mode Filer/vFiler.
As part of a migration to Clustered ONTAP, we’re going to
implement external DNS load balancing across a number of data LIFs owned by the
SVM (Storage Virtual Machine.) Also, we’ve going to implement a GSLB (Global
Site Load Balancing) mechanism to control site failover.
Note that the
detail is intentionally light here.
In this post, we set out to simulate how the DNS side of
it works simply using Windows DNS (time permitting; I might post about the
client access, SnapMirror setup, failover, and behaviour during failover, at a
later stage). First a little more explanation...
Client Access
to the SVM
The following diagram hopefully demonstrates what we’re
trying to achieve. The client accesses the share over a DFS Namespace. The DFS
Namespace maps to a server share. The server name in the DFS path to folder
target is a CNAME. This CNAME maps to an A-Record. And the A-Record contains
multiple IP addresses (or more accurately - multiple A-Records with the same IP
address.) And these IP address in the DNS A-Record relate 1-1 to Data LIFs on
the SVM.
Image: Client
access over DFS, showing how DNS works to reach our destination - the SVM
Site Failover
Site failover is achieved simply by updating the
A-Records’ IP Addresses in the GSLB mechanism to point to the DR SVM and its
LIFs.
Image: A-Records
before failover pointing to the production SVMs Data LIFs
Image: A-Records
after failover pointing to the DR SVMs Data LIFs
Simulating This
in the Lab
I have a Windows 2008R2 AD Integrated DNS server (Domain
Controller with DNS on), and a standalone, non-domain joined, Windows 2003 DNS
server to simulate my GSLB appliance.
On the Windows 2008R2 AD DNS box, I create a CNAME for
fileserver.lab.domain and this points to fileserver.gslb.domain:
Image: CNAME
mapping fileserver.lab.domain to fileserver.gslb.domain
On the Windows 2003 DNS box, I create four DNS A-records
each with a different IP address.
Image: DNSMGMT - 4
x A-Records for fileserver.gslb.domain (Production site IPs)
From the Windows 2003 DNS box, we can test our “load
balancing” by pinging fileserver.gslb.domain:
ping fileserver.gslb.domain
Then do an:
ipconfig
/flushdns
Then ping again and you’ll see the IP address changes.
Now, back on the Windows 2008R2 AD DNS box, we want to
configure it to know what to do with DNS requests for the gslb.domain. Here we
configure a conditional forward to the gslb.domain and point it to the Windows
2003 DNS box.
Image: New
Conditional Forward to gslb.domain
Testing from the Windows 2008R2 AD DNS box, and we can now
ping fileserver.gslb.domain. And if we do the ‘ipconfig /flushdns’ and ping fileserver.gslb.domain
again, we are pinging another IP address.
And if we ping fileserver.lab.domain, it similarly works
as desired, as we see from the abridged output below:
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [10.10.10.10] with 32 bytes of data:
Reply
from 10.10.10.101: bytes=32 time<1ms ttl="254<o:p">
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [10.10.10.102] with 32 bytes of data:
Reply
from 10.10.10.102: bytes=32 time=1ms TTL=254
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [10.10.10.103] with 32 bytes of data:
Reply
from 10.10.10.103: bytes=32 time=1ms TTL=254
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [10.10.10.104] with 32 bytes of data:
Reply
from 10.10.10.104: bytes=32 time<1ms ttl="254<o:p">
Note: The only
reason why we’re doing ipconfig /flushdns here is because we’re testing from
just one client. If we had four clients and pinged fileserver.lab.domain in
turn, we’d get four different IP Addresses.
So, that’s great! Now what happens when we update the
A-records on the Windows 2003 DNS box as below:
Image: DNSMGMT - 4
x A-Records for fileserver.gslb.domain (DR site IPs)
After clearing the cache on the Windows 2008R2 AD DNS box
-
Image: DNS Manager
- Clear Cache
- and using ‘ipconfig /flushdns’ and pinging
fileserver.lab.domain to test, we get our 4 DR site IP Addresses which is
exactly what we wanted. Our site has failed over successfully in DNS!
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [20.20.20.10] with 32 bytes of data:
Reply
from 20.20.20.101: bytes=32 time<1ms ttl="254<o:p">
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [20.20.20.102] with 32 bytes of data:
Reply
from 20.20.20.102: bytes=32 time=1ms TTL=254
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [20.20.20.103] with 32 bytes of data:
Reply
from 20.20.20.103: bytes=32 time=1ms TTL=254
C:\Users\Administrator>ipconfig
/flushdns
Successfully
flushed the DNS Resolver Cache
C:\Users\Administrator>ping
fileserver.lab.domain
Pinging
fileserver.gslb.domain [20.20.20.104] with 32 bytes of data:
Reply
from 20.20.20.104: bytes=32 time<1ms ttl="254<o:p">
Simulation successful!
THE END
Not quite ... Continued here http://www.cosonok.com/2014/04/simulating-gslb-dns-load-balancing-and_16.html
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