[Contents] [Index]

About This Document >>     Installing SSH Tectia Server for IBM z/OS >>     Using SSH Tectia Server for IBM z/OS >>     Configuring the Server >>         Configuration Files >>         Subconfigurations >>         Ciphers and MACs         Compression         Configuring Root Logins         Restricting User Logins         Subsystems         Auditing >>         Securing the Server             Server Configuration     Configuring the Client >>     Authentication >>     Troubleshooting SSH Tectia Server for IBM z/OS >>     Examples of Use >>     Man Pages >>     Log Messages >>

Server Configuration

Disabling Tunneling

If you are sure you or your users do not need to create tunnels (possibly going around firewall restrictions or such), you can disable tunneling (port forwarding) altogether by adding the following to your sshd2_config:

AllowTcpForwarding no

If you need more fine-grained control, consider using AllowTcpForwardingForUsers (and related keywords DenyTcpForwardingForUsers, AllowTcpForwardingForGroups and DenyTcpForwardingForGroups). With ForwardACL you can even allow and deny tunnels based on originator and destination (based on the IP address and port).

Disabling Terminal Access

If you only want to enable file transfers or tunneling for users in group users, you can disable terminal access by adding the following to your sshd2_config:

Terminal.DenyGroups users

Other related keywords that can be used are: Terminal.AllowUsers, Terminal.DenyUsers and Terminal.AllowGroups.

It is recommended to deny also X11 forwarding and agent forwarding if Terminal Access is denied in sshd2_config.

AllowX11Forwarding no AllowAgentForwarding no

Note that the users will be able to use SFTP and other subsystems defined in the SSH Tectia Server configuration. Any other "exec" and "shell" requests will be denied for the users. This includes forced commands with public keys described in Section Forced Commands and the legacy style password changing when performed as forced command.

Disabling Root Login

Usually, allowing direct root logins from the network is a bad idea. It is better to use forced commands to automate tasks requiring privileges (described in Section Forced Commands below), and make people use su or sudo to elevate privileges otherwise.

In addition to AllowUsers and DenyUsers, you can easily disable root logins with passwords. Put the following to your sshd2_config:

PermitRootLogin nopwd

This way, jobs automated with forced commands will work. If you use PAM authentication, you may need to modify that configuration as well to disable root access via sshd2.

Forced Commands (with Public Keys)

If you have maintenance jobs requiring non-interactive access to your server, use public-key authentication and forced commands. This way, if the private key is compromised, the public key cannot be used to perform anything other than the predetermined command on the server. (This is, of course, also bad, but it would be worse if the malicious attacker would have unrestricted access to the machine.)

Do not use the root account for jobs where it is not absolutely necessary.

You can set up a forced command in the authorization file.

key backup-key.pub options command="tar zxvf - /usr/local"

This would, on a successful login with backup-key.pub, force a backup job to start.

You can also use the command that was given on the ssh2 command line:

key backup-key.pub options command="echo $SSH2_ORIGINAL_COMMAND"

Running ssh2:

% ssh2 localhost kukkuu
Authentication successful.
kukkuu
%

See the man page for ssh2 for a detailed description of public-key options (Appendix ssh2).

Note that if the user or the user's group has been denied terminal access (with the Terminal.DenyUsers or Terminal.DenyGroups keywords), also forced commands will be denied.