We need to have ssh installed to secure it, for debian based distro we can install it using the command apt-get, apt-get install ssh will install ssh on the server for you.
Some of the configuration files are as follows.
/etc/ssh$ ls -l total 160 -rw-r--r-- 1 root root 132839 2006-05-18 06:13 moduli -rw-r--r-- 1 root root 1423 2006-10-05 15:13 ssh_config -rw-r--r-- 1 root root 1909 2007-08-26 20:06 sshd_config -rw------- 1 root root 1192 2006-09-28 07:28 ssh_host_dsa_key -rw-r--r-- 1 root root 1119 2006-09-28 07:28 ssh_host_dsa_key.pub -rw------- 1 root root 1675 2006-09-28 07:28 ssh_host_rsa_key -rw-r--r-- 1 root root 399 2006-09-28 07:28 ssh_host_rsa_key.pub
The file sshd_config should be modified to increase the security.