Site-to-site VPN, How to resolve hostnames across OpenVPN site-to-site network

[elkesan]

Title: How to resolve hostnames across OpenVPN site-to-site network?

Message:

I have a site-to-site VPN setup with OpenVPN, and I need help resolving hostnames across different networks. Network setup:

• OpenVPN Server: Running on a VPS with a static IP.
• Client networks (all running OpenWrt routers):
  • Home network: 10.15.0.0/22 (OpenWrt router)
  • Work network: 10.16.0.0/22 (OpenWrt router)
  • City office network: 10.17.0.0/22 (OpenWrt router)
  • Additional clients: Laptops running OpenVPN client software
• VPN internal subnet: 10.8.0.0/24

The network works perfectly in terms of routing. I can ping and connect to devices using IP addresses between all locations. Printers and file sharing work across sites. The problem:

• I recently moved to Microsoft Business 365, and my computers are now Azure AD joined (Entra).
• Previously, I could connect via RDP using just an IP address.
• Now, RDP requires hostname-based authentication, using the computer name, Entra user credentials, and Network Level Authentication (NLA).
• Hostname-based connections only work within the same local network.
• From a remote network, I can ping my work PC via IP (e.g., ping 10.16.0.110), but ping JUKKA1 or nslookup JUKKA1 fails.

Potential solutions I considered:

1. Manually editing the Windows hosts file on every device (e.g., 10.16.0.110 JUKKA1). This works but is impractical for maintenance.
2. Using the OpenVPN server as a DNS server, but OpenVPN is designed to mask hostnames and does not seem to distribute them.
3. Setting up an external DNS server, but this seems overly complex.
4. Making each OpenWrt router act as a DNS server for its local network and allowing other networks to query them for hostnames.

What I need help with:

• How can I configure OpenVPN or OpenWrt routers to allow hostname resolution across VPN-connected sites?
• Is there a way to make OpenWrt routers share their local hostnames with other sites?
• What is the best approach to make RDP work with hostnames over VPN?

Any detailed guidance would be greatly appreciated.