Do the following steps as:

1.vi /etc/hosts.deny

dovecot:ALL EXCEPT .example.com, .cracker.org

We can secure the services using tcp_wrappers. There are main two files, /etc/hosts.allow and /etc/hosts.deny.

There will be three stage access checking

- Is access explicitly permitted? Means permitted from /etc/hosts.allow?

- Otherwise, Is access explicitly denied? Means denied from /etc/hosts.deny?

- Otherwise, by default permit access if neither condition matched.

To deny the services we can configure /etc/hosts.deny file using ALL and EXCEPT operation. Pattern of /etc/hosts.allow and /etc/hosts.deny file is:

Demon_list:client_list:options

In Client list can be either domain name or IP address.