Purpose of the `HOSTNAME` argument in the `cfssl selfsign` command

[borjapazr]

Hi! 👋

What is the purpose of the HOSTNAME argument of the cfssl selfsign command?

~ ❯ cfssl selfsign --help
	cfssl selfsign -- generate a new self-signed key and signed certificate

Usage of gencert:
        cfssl selfsign HOSTNAME CSRJSON

WARNING: this should ONLY be used for testing. This should never be
used in production.

WARNING: self-signed certificates are insecure; they do not provide
the authentication required for secure systems. Use these at your own
risk.

Arguments:
        HOSTNAME:   Hostname for the cert
        CSRJSON:    JSON file containing the request, use '-' for reading JSON from stdin

Flags:
  -config="": path to configuration file
  -loglevel=1: Log level (0 = DEBUG, 5 = FATAL)

In the help of this command it appears as a mandatory argument before CSRJSON, but checking the created certificate I don't see that it affects it at all. I have been reviewing the source code for this particular command and I don't see anything done with the HOSTNAMEparameter. Why is it necessary to specify it in the command?

Thank you very much!

Best regards