Main question: is it safe, to open a port for an openssl server? 

e.g.:

server side - generate a self-signed cert.

time openssl req -x509 -nodes -days 365 -newkey rsa:8192 -keyout mycert.pem -out mycert.pem

openssl s_server -accept 52310 -cert mycert.pem

Is it secure? - it could be DOSed' [DenialofService] or could it be attacked in any way?

Are there any iptables rule for restricting connections to dyndns names?

e.g.: only allow connection from "asdfasdf.dyndns.com" and "asdfasdf2.dyndns.com" and "asdfasdf3.dyndns.com"?

How could i restrict the openssl server to only accept traffic from given clients? Please help me "think"..

Or are there any "production ready" methods, that can do authentication too? [+using ssl].

"openssl s_server" and "openssl s_client" would be perfect, but the problem is it doesn't has username/password auth :\

Thank you for any help.