Although seemingly contrary to conventional wisdom of closing ports from hackers, this port, which is used for ident requests, should be opened.
Port 113 initially was used as an authentication port, and later defined as an identification port (see RFC 1413). Some servers may still use this port to help in identifying users or other servers and establish a connection. Because port 113 receives a lot of unsolicited traffic, many routers, including on the FortiGate unit, close this port.
The issue arises in that unsolicited requests are stopped by the FortiGate unit, which will send a response saying that the port is closed. In doing so, it also lets the requesting server know there is a device at the given address, and thus announcing its presence. By enabling traffic on port 113, requests will travel to this port, and will most likely, be ignored and never responded to.
By default, the ident port is closed. To open it, use the following CLI commands:
config system interface
set ident_accept enable
You could also further use port forwarding to send the traffic to a non-existent IP address and thus never have a response packet sent.