I have several services installed on my home server and some other units. I want to access such services both from Internet and from internal home LAN using the same domain name.
At this moment my home network has name <my_router>.ddns.net. All services are redirected to proper node by port mapping (NAT) in router (as always).
So I want install dnsmasq on my home server as DNS server for local network and forwarder to OpenDNS nameservers. It is planned that /etc/hosts file will have entry like this:

Question is. Will be all the traffic addressed to <my_router>.ddns.net be forwarded to address 192.168.0.120 when I am inside my LAN? Be patient that in both cases I use that same host and domain name.
Nest question is. Will be other (not local) addresses located in ddns.net domain available to?
I never used dnsmasq - so my questions.

Check out the man page and config file notes. You can identify exactly what names are local only, and you can set the name(s) of the external dns servers that dnsmasq will forward requests to. In my case, I just set dnsmasq to forward to google public dns, but you can set forwarding to your routers dns if you want.

It won't happen automatically, and your hosts entry you showed above won't have any effect on dnsmasq dns forwarding.

THX for Your response. But be so kind to explain me such situation. If I define IP address of my internal node as for example tv.ddns.net and I will query dnsmasq server form my internal LAN, it returns me address like 192.168.0.121, because tv.ddns.net is defined in /etc/hosts file
But if I will query server about lilith.ddns.net, not defined in /etc/hosts, will be such query forwarded to external DNS servers? Be patient that both nodes have the same domain that is ddns.net

In your /etc/dnsmasq.conf file you can put
In CentOS, the /etc/dnsmasq.conf file contains full explanations of all the settings you can use.