--server-relay Server relay. NON-STANDARD AND DANGEROUS OPTION. Only for an application that want to run a server on the relay endpoints. This option eliminates the IP permissions check on the packets incoming to the relay endpoints. That makes the system vulnerable to DOS attack, for example, among other bad things. The rule is: if you do not understand what is this option about and why would you need it, then you absolutely must NOT use it under any circumstances. See http://tools.ietf.org/search/rfc5766#section-17.2.3 .
https://github.com/coturn/coturn/wiki/turnserver