TCP Wrappers

::

# blank lines and lines starting with '#' ignored
<daemon list> : <client list> [: <option> : <option> : ...]

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Security_Guide/sect-Security_Guide-TCP_Wrappers_and_xinetd.html
https://jamalahmed.wordpress.com/2010/03/19/using-etchosts-allow-and-etchosts-deny-to-secure-unix/
https://debian-administration.org/article/87/Keeping_SSH_access_secure