gnutls-serv --http --x509cafile x509-ca-rsa.pem --x509keyfile x509-server-key-rsa-sign.pem --x509certfile x509-server-rsa-sign.pem
gnutls-serv --echo --udp --mtu 1500 --x509cafile x509-ca-rsa.pem --x509keyfile x509-server-key-rsa-sign.pem --x509certfile x509-server-rsa-sign.pem
Further information in GnuTLS documentation at https://www.gnutls.org/documentation.html see "7.2. Invoking gnutls-serv", section titled "gnutls-serv Examples" if you want to generate your own keys and certificates.