Chapter 1. Using secure communications between two systems with OpenSSH

Procedure

1. Start the sshd daemon in the current session and set it to start automatically at boot time:

   # systemctl start sshd
   # systemctl enable sshd

2. To specify different addresses than the default 0.0.0.0 (IPv4) or :: (IPv6) for the ListenAddress directive in the /etc/ssh/sshd_config configuration file and to use a slower dynamic network configuration, add the dependency on the network-online.target target unit to the sshd.service unit file. To achieve this, create the /etc/systemd/system/sshd.service.d/local.conf file with the following content:

   [Unit]
   Wants=network-online.target
   After=network-online.target

3. Review if OpenSSH server settings in the /etc/ssh/sshd_config configuration file meet the requirements of your scenario.

4. Optionally, change the welcome message that your OpenSSH server displays before a client authenticates by editing the /etc/issue file, for example:

   Welcome to ssh-server.example.com
   Warning: By accessing this server, you agree to the referenced terms and conditions.

   Ensure that the Banner option is not commented out in /etc/ssh/sshd_config and its value contains /etc/issue:

   # less /etc/ssh/sshd_config | grep Banner
   Banner /etc/issue

   Note that to change the message displayed after a successful login you have to edit the /etc/motd file on the server. See the pam_motd man page for more information.

5. Reload the systemd configuration and restart sshd to apply the changes:

   # systemctl daemon-reload
   # systemctl restart sshd

Verification

1. Check that the sshd daemon is running:

   # systemctl status sshd
   ● sshd.service - OpenSSH server daemon
      Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
      Active: active (running) since Mon 2019-11-18 14:59:58 CET; 6min ago
        Docs: man:sshd(8)
              man:sshd_config(5)
    Main PID: 1149 (sshd)
       Tasks: 1 (limit: 11491)
      Memory: 1.9M
      CGroup: /system.slice/sshd.service
              └─1149 /usr/sbin/sshd -D -oCiphers=aes128-ctr,aes256-ctr,aes128-cbc,aes256-cbc -oMACs=hmac-sha2-256,>
   
   Nov 18 14:59:58 ssh-server-example.com systemd[1]: Starting OpenSSH server daemon...
   Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on 0.0.0.0 port 22.
   Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on :: port 22.
   Nov 18 14:59:58 ssh-server-example.com systemd[1]: Started OpenSSH server daemon.

2. Connect to the SSH server with an SSH client.

   # ssh user@ssh-server-example.com
   ECDSA key fingerprint is SHA256:dXbaS0RG/UzlTTku8GtXSz0S1++lPegSy31v3L/FAEc.
   Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
   Warning: Permanently added 'ssh-server-example.com' (ECDSA) to the list of known hosts.
   
   user@ssh-server-example.com's password:

Procedure

1. Open the /etc/ssh/sshd_config configuration in a text editor, for example:

   # vi /etc/ssh/sshd_config

2. Change the PasswordAuthentication option to no:

   PasswordAuthentication no

   On a system other than a new default installation, check that PubkeyAuthentication no has not been set and the KbdInteractiveAuthentication directive is set to no. If you are connected remotely, not using console or out-of-band access, test the key-based login process before disabling password authentication.

3. To use key-based authentication with NFS-mounted home directories, enable the use_nfs_home_dirs SELinux boolean:

   # setsebool -P use_nfs_home_dirs 1

4. Reload the sshd daemon to apply the changes:

   # systemctl reload sshd

Procedure

1. To generate an ECDSA key pair for version 2 of the SSH protocol:

   $ ssh-keygen -t ecdsa
   Generating public/private ecdsa key pair.
   Enter file in which to save the key (/home/joesec/.ssh/id_ecdsa):
   Enter passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved in /home/joesec/.ssh/id_ecdsa.
   Your public key has been saved in /home/joesec/.ssh/id_ecdsa.pub.
   The key fingerprint is:
   SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI joesec@localhost.example.com
   The key's randomart image is:
   +---[ECDSA 256]---+
   |.oo..o=++        |
   |.. o .oo .       |
   |. .. o. o        |
   |....o.+...       |
   |o.oo.o +S .      |
   |.=.+.   .o       |
   |E.*+.  .  . .    |
   |.=..+ +..  o     |
   |  .  oo*+o.      |
   +----[SHA256]-----+

   You can also generate an RSA key pair by using the -t rsa option with the ssh-keygen command or an Ed25519 key pair by entering the ssh-keygen -t ed25519 command.

2. To copy the public key to a remote machine:

   $ ssh-copy-id joesec@ssh-server-example.com
   /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
   joesec@ssh-server-example.com's password:
   ...
   Number of key(s) added: 1
   
   Now try logging into the machine, with: "ssh 'joesec@ssh-server-example.com'" and check to make sure that only the key(s) you wanted were added.

   If you do not use the ssh-agent program in your session, the previous command copies the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. To specify another public-key file or to prioritize keys in files over keys cached in memory by ssh-agent, use the ssh-copy-id command with the -i option.

Verification

1. Log in to the OpenSSH server without providing any password:

   $ ssh joesec@ssh-server-example.com
   Welcome message.
   ...
   Last login: Mon Nov 18 18:28:42 2019 from ::1

Procedure

1. List all keys provided by the OpenSC PKCS #11 module including their PKCS #11 URIs and save the output to the keys.pub file:

   $ ssh-keygen -D pkcs11: > keys.pub
   $ ssh-keygen -D pkcs11:
   ssh-rsa AAAAB3NzaC1yc2E...KKZMzcQZzx pkcs11:id=%02;object=SIGN%20pubkey;token=SSH%20key;manufacturer=piv_II?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so
   ecdsa-sha2-nistp256 AAA...J0hkYnnsM= pkcs11:id=%01;object=PIV%20AUTH%20pubkey;token=SSH%20key;manufacturer=piv_II?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so

2. To enable authentication using a smart card on a remote server (example.com), transfer the public key to the remote server. Use the ssh-copy-id command with keys.pub created in the previous step:

   $ ssh-copy-id -f -i keys.pub username@example.com

3. To connect to example.com using the ECDSA key from the output of the ssh-keygen -D command in step 1, you can use just a subset of the URI, which uniquely references your key, for example:

   $ ssh -i "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" example.com
   Enter PIN for 'SSH key':
   [example.com] $

4. You can use the same URI string in the ~/.ssh/config file to make the configuration permanent:

   $ cat ~/.ssh/config
   IdentityFile "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so"
   $ ssh example.com
   Enter PIN for 'SSH key':
   [example.com] $

   Because OpenSSH uses the p11-kit-proxy wrapper and the OpenSC PKCS #11 module is registered to PKCS#11 Kit, you can simplify the previous commands:

   $ ssh -i "pkcs11:id=%01" example.com
   Enter PIN for 'SSH key':
   [example.com] $

Procedure

1. Define the jump host by editing the ~/.ssh/config file on your local system, for example:

   Host jump-server1
     HostName jump1.example.com

   • The Host parameter defines a name or alias for the host you can use in ssh commands. The value can match the real host name, but can also be any string.
   • The HostName parameter sets the actual host name or IP address of the jump host.

2. Add the remote server jump configuration with the ProxyJump directive to ~/.ssh/config file on your local system, for example:

   Host remote-server
     HostName remote1.example.com
     ProxyJump jump-server1

3. Use your local system to connect to the remote server through the jump server:

   $ ssh remote-server

   The previous command is equivalent to the ssh -J jump-server1 remote-server command if you omit the configuration steps 1 and 2.

Procedure

1. Optional: Verify you can use the key to authenticate to the remote host:

   1. Connect to the remote host using SSH:

      $ ssh example.user1@198.51.100.1 hostname

   2. Enter the passphrase you set while creating the key to grant access to the private key.

      $ ssh example.user1@198.51.100.1 hostname
       host.example.com

2. Start the ssh-agent.

   $ eval $(ssh-agent)
   Agent pid 20062

3. Add the key to ssh-agent.

   $ ssh-add ~/.ssh/id_rsa
   Enter passphrase for ~/.ssh/id_rsa:
   Identity added: ~/.ssh/id_rsa (example.user0@198.51.100.12)