Why does a DHCP-provided DNS IP address becomes a static one in Windows 10

Bravo on deciding to get rid of static IP assignments (except where absolutely necessary). I'd tell you to use the DHCP database as your "IP address list" documentation, too. Put in reservations for devices with static IP addresses assigned, as well. Make the DHCP database be the authoritative "IP address list" instead of having spreadsheets, etc, that fall out of date.

Here's some background re: DNS and DHCP in Windows. It sounds like you may not be aware that the client computer performs half of the registration (the "A" record), and can also perform the "PTR" record registration, too. This allows you to use virtually any DHCP server you want, so long as it hands out addresses of DNS servers that can accept dynamic registrations.

The Windows DHCP server performs backups of the DHCP database to the local hard disk drive periodcially. You can also export the database with "netsh" (W2K3 or newer) such that restore it to another server easily. Restoring an ISC DHCPd scope to another server is a matter of copying the relevant portions of the dhcpd.leases file and the dhcpd.conf file. Embedded DHCP servers may be more problematic in a restore scenario.

As stated above, your Cisco routers can hand out DHCP to Windows PCs, but the PCs can register their own "PTR" and "A" records. Have a look at the Group Policy setting "Register PTR Records" located under "Comptuer Settings", "Administrative Templates", "Network", and "DNS Client". The client will register the "A" record itself by default.

I wouldn't got with a roll-your-own Linux DNS deployment for this application. You're going to put a lot of time in it, and it will always be the source of "Gee, will this work with Windows Server 2029..." type of musings. If not for Active Directory, I might think differently. Since you've got AD in your environment, and since Microsoft tests AD on Microsoft DNS, I'd use Microsoft DNS.

Windows Server does not have the capability to sync DHCP server databases so that you can have multiple authoritative servers for the same subnet. This continues to be decidedly sub-optimal in Windows DHCP. This might be a "win" for ISC DHCPd on Linux. I haven't got any experience with this capability to share a DHCP lease database across multiple DHCP server, but it certainly sounds sweet. I am not aware of any capability in Cisco routers to do this. Again, you can have your PCs register themselves in DNS regardless of what DHCP server you use.

You could rig up active / passive DHCP on multiple Windows Server computers with some scripting and the native database export functionality, as well.

Personally, I'd go for the option of using Windows DNS everywhere, Windows DHCP everywhere where you can have a Windows DHCP server on the same LAN as the clients, and your Cisco routers handing out DHCP everywhere that you can't have a Windows Server computer. The Linux ISC DHCPd solution might be a "win", too, but I'd stick with Windows DNS.

The problem you're going to run into is that Active Directory uses DNS to tell client machines where to find various resources, so turning off DNS on the Windows server will eventually stop things that require Active Directory from working. It sounds like it worked for a number of hours because clients had it cached, but then the cache expired.

My suggestion would be to run bind on your Linux server, and make it act as a secondary to your Windows server, and then configure your DHCP server to give out the Linux server as the DNS server clients should be using. That way, your DNS queries are offloaded onto the Linux server whilst retaining the ability to use Active Directory.

You'll need a line in your named.conf (or such) a bit like this:-

zone "ad.internal.company.com"
{
  type slave;
  file "/etc/bind/db.ad.internal.company.com";
  masters { aaa.bbb.ccc.ddd; };
};

Not sure which version of SBS you're on, but for 2003, open up the dnsmgmt console, go to the properties for your active directory domain, and add your Linux server as a nameserver on the Name Servers tab. You'll also want to make sure Allow zone transfers is ticked on the Zone Transfers tab, along with Only to servers listed on the Name Servers tab. Additionally, you'll want to make sure that when you click Notify... (also on the Zone Transfers tab), that Automatically notify and Servers listed on the Name Servers tab are selected.

Reload (or restart) bind on your Linux server, and keep an eye on the logs, and you should see bind requesting a copy of the zonefile from the Windows server. To make sure everything's working, try making an addition to the zonefile on the Windows server and make it's propagated to bind on the Linux server.

Hope that helps!