3.2.2.3. Other Ways of Securing SSH

Even though the implementation of the SSH protocol supplied with Red Hat Enterprise Linux supports both the SSH-1 and SSH-2 versions of the protocol, only the latter should be used whenever possible. The SSH-2 version contains a number of improvements over the older SSH-1, and the majority of advanced configuration options is only available when using SSH-2.

Users are encouraged to make use of SSH-2 in order to maximize the extent to which the SSH protocol protects the authentication and communication for which it is used. The version or versions of the protocol supported by the sshd daemon can be specified using the Protocol configuration directive in the /etc/ssh/sshd_config file. The default setting is 2.

While the ssh-keygen command generates a pair of SSH-2 RSA keys by default, using the -t option, it can be instructed to generate DSA or ECDSA keys as well. The ECDSA (Elliptic Curve Digital Signature Algorithm) offers better performance at the same symmetric key length. It also generates shorter keys.

By default, the sshd daemon listens on the 22 network port. Changing the port reduces the exposure of the system to attacks based on automated network scanning, thus increasing security through obscurity. The port can be specified using the Port directive in the /etc/ssh/sshd_config configuration file. Note also that the default SELinux policy must be changed to allow for the use of a non-default port. You can do this by modifying the ssh_port_t SELinux type by typing the following command as root:

semanage -a -t ssh_port_t -p tcp port_number

~]# semanage -a -t ssh_port_t -p tcp port_number

In the above command, replace port_number with the new port number specified using the Port directive.

Provided that your particular use case does not require the possibility of logging in as the root user, you should consider setting the PermitRootLogin configuration directive to no in the /etc/ssh/sshd_config file. By disabling the possibility of logging in as the root user, the administrator can audit which user runs what privileged command after they log in as regular users and then gain root rights.