Redeemer - mass dns

I’m French, so excuse my poor English…
I’m connected through an openvpn (on a virtual machine) to the Redeemer machine. I can ping it without any problem.
When I type nmap command, I obtain this answer :

mass_dns: warning: Unable to determine any DNS servers. Reverse DNS is disabled. Try using --system-dns or specify valid servers with --dns-servers

Is anybody have an idea to help me ?
Thank’s and have a good day !