Secure Shell protocol is abbreviated as SSH. It is a secure and most commonly using protocol to access remote servers. This protocol uses encryption while transferring data between two hosts.
TCP Wrapper is a host-based networking ACL system, used to filter network access to Internet Protocol servers on (Unix-like) operating systems such as Linux or BSD.
It allows host or subnetwork IP addresses, names and/or ident query replies, to be used as tokens on which to filter for access control purposes.
I am going to explain how TCP wrappers help to protect server by controlling external connections. We can control the SSH connection to a server by using the following files:
/etc/hosts.allow and /etc/hosts.deny
How to allow an IP address for SSH connection?
Step 1 : SSH to server as root user.
Step 2 : Open /etc/hosts.allow file using your favorite text editor.
To allow one IP address to server:
sshd : IP-Address
To allow more than one IP addresses:
sshd : IP-Address1, IP-Address2, IP-Address3
To allow IP ranges:
sshd : 33.*.*.* sshd : 66.66.*.*
To allow all IP addresses for SSH:
sshd : ALL
ALL : ALL
We can also control the same from /etc/hosts.deny file by adding the keyword “ALLOW.” See the example pasted below:
# vi /etc/hosts.deny
sshd : ALL : ALLOW