If both servers are running directadmin you can setup multi-server settings to make the servers talk to each other and have them transfer DNS info about new domains to each other. With this you can have ns1 set to server1 and ns2 set to server 2 and just setup a domain on whichever server you want and tell the domain owner to use ns1/ns2 for the nameservers. Both servers will update the dns info so it will know which server to point to for the domain and would provide dns server distrobution as well.
If you still would rather use ns3/ns4 for it, setup the ns3/ns4.domain1.com DNS hosts at your registrar to point to the IPs of the new server and tell people to use them as their nameservers. Then on server1, go to the DNS admin area and go to domain1.com records. Add in A record for ns3/ns4 to point to IPs on second server you used at registrar then setup an A record for server2 to point to the main ip of the 2nd server and it should work fine.
Thats basically the 2nd method I mentioned except you cant make the A record for ns3/ns4 on the 2nd server because the domain itself is pointing to server 1 for the dns info so have to add in the A records for that domain there. For other domains that are using ns3/ns4 as their nameservers will have their records on the second server, its only the main domain that has to have the A records on server 1.
No the domains pointing to ns3/ns4 would still be up because their nameservers and dns records are on the second server. Only server2.domain1.com wouldnt resolve when typed into browser because that dns record is part of server1.
Sorry I guess I was a bit confusing in post, basically do this:
Registrar - setup ns3/ns4.domain.com to point to server2 IPs
Server1 - Setup DNS record to have ns3/ns4/server2.domain.com point to the server2 IPs (ns3/ns4 need to be same IPs you used at registrar)
Server2 - Setup zone for ns3/ns4.domain.com and have other domains to be hosted on this server use ns3/ns4.domain.com for their nameservers. (I am unsure if you need to do this as the domain information should be grabbed from server1 but I put it here as well just in case)
Domain.com is still hosted on server1 but server2.domain.com will point to server2 and all domains pointed at ns3/ns4.domain.com will go to server2 as well. I dont know if this is the "recommended" method but it's what I've done when transferring domains to new servers with one main domain used on both servers while transferring domains and it worked fine, both servers recognized with all sites going to the correct server.
You setup the zone files for the new domains being hosted at server2 on server2 since having the nameservers of those domains set to ns3/ns4 make them go straight to server2 to lookup the DNS records. Say you want domain2.com hosted on server2, you setup as follows:
At the registrar set its nameservers to ns3/ns4.domain1.com
On server2 setup the domain like normal so it creates the zone files for domain2.com on server2
Wait for domain to propogate and it should be pointing at server2 when you type it into browser.
For a domain on server1 like example.com you just set the nameservers to ns1/ns2.domain1.com and set it up on server1 like how he has been creating domains up to now. ns1/ns2 will point to server1 and ns3/ns4 will point to server2. Both nameservers are on the same physical server for each server. This is how he said he wanted it setup in his original post.
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