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© Rakhesh Sasidharan

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Windows DNS server subnet prioritization and round-robin

Consider the following multiple A records for a DNS record proxy.mydomain.com:

  • proxy.mydomain.com IN A 192.168.10.5
  • proxy.mydomain.com IN A 10.136.53.5
  • proxy.mydomain.com IN A 10.136.52.5
  • proxy.mydomain.com IN A 10.136.33.5
  • proxy.mydomain.com IN A 192.168.15.5

These records are defined on a DNS server. When a client queries the DNS server for the address to proxy.mydomain.com, the DNS server returns all the addresses above. However, the order of answers returned keeps varying. The first client asking for answers could get them in the following order for instance:

  • proxy.mydomain.com IN A 192.168.10.5
  • proxy.mydomain.com IN A 10.136.53.5
  • proxy.mydomain.com IN A 10.136.52.5
  • proxy.mydomain.com IN A 10.136.33.5
  • proxy.mydomain.com IN A 192.168.15.5

The second client could get them in the following order:

  • proxy.mydomain.com IN A 10.136.53.5
  • proxy.mydomain.com IN A 10.136.52.5
  • proxy.mydomain.com IN A 10.136.33.5
  • proxy.mydomain.com IN A 192.168.15.5
  • proxy.mydomain.com IN A 192.168.10.5

The third client could get:

  • proxy.mydomain.com IN A 10.136.52.5
  • proxy.mydomain.com IN A 10.136.33.5
  • proxy.mydomain.com IN A 192.168.15.5
  • proxy.mydomain.com IN A 192.168.10.5
  • proxy.mydomain.com IN A 10.136.53.5

This is called round-robin. Basically it rotates between the various IP addresses. All IP addresses are offered as answers, but the order is rotated so that as long as clients choose the first answer in the list every client chooses a different IP address.

Notice I said clients choose the first answer in the list. This needn’t always be the case though. When I said clients above, I meant the client computer that is querying the DNS server for an answer. But that’s not really who’s querying the server. Instead, an application on the client (e.g. Chrome, Internet Explorer) or the client OS itself is the one looking for an answer. These ask the DNS resolver which is usually a part of the OS for an answer, and it’s the resolver that actually queries the server and gets the list of answers above.

The DNS resolver can then return the list as it is to the requesting application, or it can apply a re-ordering of its own. For instance, if the client is from the 192.168.10.0 network, the resolver may re-order the answers such that the 192.168.10.5 answer is always first. This is called Subnet prioritization. Basically, the resolver prioritizes answers that are from the same subnet as the client. The idea being that client applications would prefer reaching out to a server in their same subnet (it’s closer to them, no need to go over the WAN link for instance) than one on a different subnet.

Subnet prioritization can be disabled on the resolver side by adding a registry key PrioritizeRecordData (link) with value 0 (REG_DWORD) at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DnsCache\Parameters. By default this key does not exist and has a default value of 1 (subnet prioritization enabled).

Subnet prioritization can also be set on the server side so it orders the responses based on the client network. This is controlled by the registry key LocalNetPriority (link) under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters\ on the DNS server. By default this is 0, so the server doesn’t do any subnet prioritization. Change this to 1 and the server will order its responses according to the client subnet.

By default the server also does round-robin for the results it returns. This can be turned off via the DNS Management tool (under server properties > advanced tab). If round-robin is off the server returns records in the order they were added.

More on subnet prioritization at this link.

That’s is not the end though. :)

Consider a server who has round-robin and subnet prioritization enabled. Now consider the DNS records from above:

  • proxy.mydomain.com IN A 192.168.10.5
  • proxy.mydomain.com IN A 10.136.53.5
  • proxy.mydomain.com IN A 10.136.52.5
  • proxy.mydomain.com IN A 10.136.33.5
  • proxy.mydomain.com IN A 192.168.15.5

The first and last records are from class C networks. The other three are from Class A networks. In reality though thanks to CIDR these are all class C addresses.

Now say there’s a client with IP address 10.136.50.2/24 asking the server for answers. On the face of it the client network does not match any of the answer record networks so the server will simply return answers as per round-robin, without any re-ordering. But in reality though the client 10.136.50.2/24 is in the same network as 10.136.52.5/24 and both are part of a larger 10.136.48.0/20 network that’s simply been broken into multiple /24 networks (to denote clients, servers, etc). What can we do so the server correctly identifies the proxy record for this client?

This is where the LocalNetPriorityNetMask registry key under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters\ on the DNS server comes into play. This key – which does not exist by default – tells the server what subnet mask to assume when it’s trying to subnet prioritize. By default the server assumes a /24 subnet, but by tweaking this key we can tell the server to use a different subnet in its calculations and thus correctly return an answer.

The LocalNetPriorityNetMask key takes a REG_DWORD value in a hex format. Check out this KB article for more info, but a quick run through:

A netmask can be written as xxx.xxx.xxx.xxx. 4 pairs of numbers. The LocalNetPriorityNetMask key is of format 0xaabbccdd – again, 4 pairs of hex numbers. This is a mask that’s applied on the mask of 255.255.255.255 so to calculate this number you subtract the mask you want from 255.255.255.255 and convert the resulting numbers into hex.

For example: you want a /8 netmask. That is 255.0.0.0. Subtracting this from 255.255.255.255 leaves you with 0.255.255.255.255. What’s that in hex? 00ffffff. So LocalNetPriorityNetMask will be 0x00ffffff. Easy?

So in the example above I want a /20 netmask. That is, I am telling the server to assume the clients and the record IPs it has to be in a /20 network, so subnet prioritize accordingly. A /20 netmask is 255.255.240.0. Subtract from 255.255.255.255 to get 0.0.15.255. Which in hex is 00000fff (15 decimal is F hex). So all I have to do is put this value as LocalNetPriorityNetMask on the DNS server, restart the service, and now the server will correctly return subnet prioritized answers for my /20 network.

Update: Some more links as I did some more reading on this topic later.

  • Ace Fekay’s post – a must read!
  • A subnet calculator (also gives you the wildcard, which you can use for calculating the LocalNetPriorityNetMask key)
  • I am not very clear on what happens if you disable RoundRobin but there are multiple entries from the same subnet. What order are they returned in? Here’s a link to the RoundRobin setting, doesn’t explain much but just linking it in case it helps in the future.
  • More as a note to myself (and any others if they wonder the same) – the subnet mask you specify is applied on the client. That is to say if you client has an IP address of say 10.136.20.10, by default the DNS server will assume a subnet mask of /24 (Class C is the default) and assume the client is in a 10.136.20.0/24 network. So any records from that range are prioritized. If you want to include other records, you specify a larger subnet mask. Thus, for example, if you specify a /20 then the client is assumed to have an IP address 10.136.20.10/20, so its network range is considered to be 10.136.16.1 – 10.136.31.254 (don’t wrack your brain – use the subnet calculator for this). So any record in this range is prioritized over records not in this range.
  • The Windows calculator can be used to find the LocalNetPriorityNetMask key value. Say you want a subnet mask of /19. The subnet calculator will tell you this has a wildcard of 0.0.31.255 – i.e. 00011111.11111111. Put this (13 1’s) into the Windows calculator to get the hex value 3FFF.