/etc/hosts.allow

File /etc/hosts.allow is one of the configuration files of TCP Wrappers, effective either to servers launched via a superserver such as inetd, or to servers that use libwrap, such as the OpenSSH server (when compiled with --with-libwrap, which is usually the default).

The general syntax of the file is:

server: hosts

The hosts is a comma separated list, that can include full IP addresses, partial IP addresses, IP subnets in CIDR notation, FQDNs, or domains. For example, the following line limits access to the sshd server to a single host specified by IP, and to a subnet:

sshd: 8.8.8.1 10.1.0.0/24

Note that in order for any definitions in /etc/hosts.allow to be effective, the rest of the hosts need to be prohibited in /etc/hosts.deny. Since /etc/hosts.allow takes precedence, it is safe to disallow every host in /etc/hosts.deny and only selectively allow specific access to the server in /etc/hosts.allow. To deny all hosts except those specified in /etc/hosts.allow, add in /etc/hosts.deny:

sshd: ALL