Configuring Secure Shell (Tasks)

By default, host-based authentication and the use of both protocols are not enabled in Secure Shell. Changing these defaults requires administrative intervention. Also, for port forwarding to work requires administrative intervention.

Configuring Secure Shell (Task Map)

The following task map points to procedures for configuring Secure Shell.

How to Set Up Host-Based Authentication for Secure Shell

The following procedure sets up a public key system where the client's public key is used for authentication on the server. The user must also create a public/private key pair.

In the procedure, the terms client and local host refer to the machine where a user types the ssh command. The terms server and remote host refer to the machine that the client is trying to reach.

Before You Begin

You must be in the root role.

1. On the client, enable host-based authentication.

   In the client configuration file, /etc/ssh/ssh_config, type the following entry:

   HostbasedAuthentication yes

   For the syntax of the file, see the ssh_config(4) man page

2. On the server, enable host-based authentication.

   In the server configuration file, /etc/ssh/sshd_config, type the same entry:

   HostbasedAuthentication yes

   For the syntax of the file, see the sshd_config(4) man page

3. On the server, configure a file that enables the client to be recognized as a trusted host.

   For more information, see the FILES section of the sshd(1M) man page.

   • Add the client as an entry to the server's /etc/ssh/shosts.equiv file.

     client-host

   • Or, you can instruct users to add an entry for the client to their ~/.shosts file on the server.

     client-host

4. On the server, ensure that the sshd daemon can access the list of trusted hosts.

   Set IgnoreRhosts to no in the /etc/ssh/sshd_config file.

   ## sshd_config
   IgnoreRhosts no

5. Ensure that users of Secure Shell at your site have accounts on both hosts.
6. Do one of the following to put the client's public key on the server.
   • Modify the sshd_config file on the server, then instruct your users to add the client's public host keys to their ~/.ssh/known_hosts file.

     ## sshd_config
     IgnoreUserKnownHosts no

     For user instructions, see How to Generate a Public/Private Key Pair for Use With Secure Shell.

   • Copy the client's public key to the server.

     The host keys are stored in the /etc/ssh directory. The keys are typically generated by the sshd daemon on first boot.

     1. Add the key to the /etc/ssh/ssh_known_hosts file on the server.

        On the client, type the command on one line with no backslash.

        # cat /etc/ssh/ssh_host_dsa_key.pub | ssh RemoteHost \
        'cat >> /etc/ssh/ssh_known_hosts && echo "Host key copied"'

     2. When you are prompted, supply your login password.

        When the file is copied, the message “Host key copied” is displayed.

        Each line in the /etc/ssh/ssh_known_hosts file consists of fields that are separated by spaces:

        hostnames algorithm-name publickey comment

     3. Edit the /etc/ssh/ssh_known_hosts file and add RemoteHost as the first field in the copied entry.

        ## /etc/ssh/ssh_known_hosts File
        RemoteHost <copied entry>

Example 17-1 Setting Up Host-based Authentication

In the following example, each host is configured as a server and as a client. A user on either host can initiate an ssh connection to the other host. The following configuration makes each host a server and a client:

• On each host, the Secure Shell configuration files contain the following entries:

  ## /etc/ssh/ssh_config
  HostBasedAuthentication yes
  # 
  ## /etc/ssh/sshd_config
  HostBasedAuthentication yes
  IgnoreRhosts no

• On each host, the shosts.equiv file contains an entry for the other host:

  ## /etc/ssh/shosts.equiv on machine2
  machine1

  ## /etc/ssh/shosts.equiv on machine1
  machine2

• The public key for each host is in the /etc/ssh/ssh_known_hosts file on the other host:

  ## /etc/ssh/ssh_known_hosts on machine2
  … machine1

  ## /etc/ssh/ssh_known_hosts on machine1
  … machine2

• Users have an account on both hosts:

  ## /etc/passwd on machine1
  jdoe:x:3111:10:J Doe:/home/jdoe:/bin/sh

  ## /etc/passwd on machine2
  jdoe:x:3111:10:J Doe:/home/jdoe:/bin/sh

How to Configure Port Forwarding in Secure Shell

Port forwarding enables a local port be forwarded to a remote host. Effectively, a socket is allocated to listen to the port on the local side. Similarly, a port can be specified on the remote side.

Before You Begin

You must be in the root role.

1. Configure a Secure Shell setting on the remote server to allow port forwarding.

   Change the value of AllowTcpForwarding to yes in the /etc/ssh/sshd_config file.

   # Port forwarding
   AllowTcpForwarding yes

2. Restart the Secure Shell service.

   remoteHost# svcadm restart network/ssh:default

   For information about managing persistent services, see Chapter 6, Managing Services (Overview), in Oracle Solaris Administration: Common Tasks and the svcadm(1M) man page.

3. Verify that port forwarding can be used.

   remoteHost# /usr/bin/pgrep -lf sshd
    1296 ssh -L 2001:remoteHost:23 remoteHost

How to Create User and Host Exceptions to SSH System Defaults

This procedure adds a conditional Match block after the global section of the /etc/ssh/sshd_config file. Keyword-value pairs that follow the Match block specify exceptions for the user, group, host, or address that is specified as the match.

Before You Begin

You must be in the root role.

1. Edit the sshd_config file.
2. Configure a user, group, host, or address to use different SSH keyword settings from the default settings.

   Place the Match blocks after the global settings.

   ---

   Note - The global section of the file might or might not list the default settings. For the defaults, see the sshd_config(4) man page.

   ---

   You might have users who should not be allowed to use TCP forwarding. In the following example, any user in the group public, and any user name that begins with test cannot use TCP forwarding:

   ## sshd_config file
   ## Global settings
   
   # Example (reflects default settings):
   #
   # Host *
   #   ForwardAgent no
   #   ForwardX11 no
   #   PubkeyAuthentication yes
   #   PasswordAuthentication yes
   #   FallBackToRsh no
   #   UseRsh no
   #   BatchMode no
   #   CheckHostIP yes
   #   StrictHostKeyChecking ask
   #   EscapeChar ~
   Match Group public
    AllowTcpForwarding no
   Match User test*
    AllowTcpForwarding no

   For information about the syntax of the Match block, see the sshd_config(4) man page.