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documentation.suse.com / SUSE Linux Enterprise Server Documentation / Security and Hardening Guide / Network Security / SSH: Secure Network Operations
Applies to SUSE Linux Enterprise Server 12 SP5

15 SSH: Secure Network Operations

In networked environments, it is often necessary to access hosts from a remote location. If a user sends login and password strings for authentication purposes as plain text, they could be intercepted and misused to gain access to that user account. This would open all the user's files to an attacker and the illegal account could be used to obtain administrator or root access, or to penetrate other systems. In the past, remote connections were established with telnet, rsh or rlogin, which offered no guards against eavesdropping in the form of encryption or other security mechanisms. There are other unprotected communication channels, like the traditional FTP protocol and some remote copying programs like rcp.

The SSH suite provides the necessary protection by encrypting the authentication strings (usually a login name and a password) and all the other data exchanged between the hosts. With SSH, the data flow could still be recorded by a third party, but the contents are encrypted and cannot be reverted to plain text unless the encryption key is known. So SSH enables secure communication over insecure networks, such as the Internet. The SSH implementation coming with SUSE Linux Enterprise Server is OpenSSH.

SUSE Linux Enterprise Server installs the OpenSSH package by default providing the commands ssh, scp, and sftp. In the default configuration, remote access of a SUSE Linux Enterprise Server system is only possible with the OpenSSH utilities, and only if the sshd is running and the firewall permits access.

SSH on SUSE Linux Enterprise Server uses cryptographic hardware acceleration if available. As a result, the transfer of large quantities of data through an SSH connection is considerably faster than without cryptographic hardware. As an additional benefit, the CPU will see a significant reduction in load.

15.1 ssh—Secure Shell

With ssh it is possible to log in to remote systems and to work interactively. To log in to the host sun as user tux enter one of the following commands:

ssh tux@sun
ssh -l tux sun

If the user name is the same on both machines, you can omit it. Using ssh sun is sufficient. The remote host prompts for the remote user's password. After a successful authentication, you can work on the remote command line or use interactive applications, such as YaST in text mode.

Furthermore, ssh offers the possibility to run non-interactive commands on remote systems using ssh HOST COMMAND. COMMAND needs to be properly quoted. Multiple commands can be concatenated as on a local shell.

ssh root@sun "dmesg -T | tail -n 25"
ssh root@sun "cat /etc/issue && uptime"

15.1.1 Starting X Applications on a Remote Host

SSH also simplifies the use of remote X applications. If you run ssh with the -X option, the DISPLAY variable is automatically set on the remote machine and all X output is exported to the local machine over the existing SSH connection. At the same time, X applications started remotely cannot be intercepted by unauthorized individuals.

15.1.2 Agent Forwarding

By adding the -A option, the ssh-agent authentication mechanism is carried over to the next machine. This way, you can work from different machines without having to enter a password, but only if you have distributed your public key to the destination hosts and properly saved it there. Refer to Section 15.5.2, “Copying an SSH Key” for details.

This mechanism is deactivated in the default settings, but can be permanently activated at any time in the systemwide configuration file /etc/ssh/sshd_config by setting AllowAgentForwarding yes.

15.2 scp—Secure Copy

scp copies files to or from a remote machine. If the user name on jupiter is different than the user name on sun, specify the latter using the USER_NAME@host format. If the file should be copied into a directory other than the remote user's home directory, specify it as sun:DIRECTORY. The following examples show how to copy a file from a local to a remote machine and vice versa.

# local -> remote
scp ~/MyLetter.tex tux@sun:/tmp
# remote -> local
scp tux@sun:/tmp/MyLetter.tex ~
Tip: The -l Option

With the ssh command, the option -l can be used to specify a remote user (as an alternative to the USER_NAME@host format). With scp the option -l is used to limit the bandwidth consumed by scp.

After the correct password is entered, scp starts the data transfer. It displays a progress bar and the time remaining for each file that is copied. Suppress all output with the -q option.

scp also provides a recursive copying feature for entire directories. The command

scp -r src/ sun:backup/

copies the entire contents of the directory src including all subdirectories to the ~/backup directory on the host sun. If this subdirectory does not exist, it is created automatically.

The -p option tells scp to leave the time stamp of files unchanged. -C compresses the data transfer. This minimizes the data volume to transfer, but creates a heavier burden on the processors of both machines.

15.3 sftp—Secure File Transfer

15.3.1 Using sftp

If you want to copy several files from or to different locations, sftp is a convenient alternative to scp. It opens a shell with a set of commands similar to a regular FTP shell. Type help at the sftp-prompt to get a list of available commands. More details are available from the sftp man page.

sftp sun
Enter passphrase for key '/home/tux/.ssh/id_rsa':
Connected to sun.
sftp> help
Available commands:
bye                                Quit sftp
cd path                            Change remote directory to 'path'

15.3.2 Setting Permissions for File Uploads

As with a regular FTP server, a user cannot only download, but also upload files to a remote machine running an SFTP server by using the put command. By default the files will be uploaded to the remote host with the same permissions as on the local host. There are two options to automatically alter these permissions:

Setting a umask

A umask works as a filter against the permissions of the original file on the local host. It can only withdraw permissions:

Table 15.1:

permissions original


permissions uploaded










To apply a umask on an SFTP server, edit the file /etc/ssh/sshd_configuration. Search for the line beginning with Subsystem sftp and add the -u parameter with the desired setting, for example:

Subsystem sftp /usr/lib/ssh/sftp-server -u 0002
Explicitly Setting the Permissions

Explicitly setting the permissions sets the same permissions for all files uploaded via SFTP. Specify a three-digit pattern such as 600, 644, or 755 with -u. When both -m and -u are specified, -u is ignored.

To apply explicit permissions for uploaded files on an SFTP server, edit the file /etc/ssh/sshd_config. Search for the line beginning with Subsystem sftp and add the -m parameter with the desired setting, for example:

Subsystem sftp /usr/lib/ssh/sftp-server -m 600

15.4 The SSH Daemon (sshd)

To work with the SSH client programs ssh and scp, a server (the SSH daemon) must be running in the background, listening for connections on TCP/IP port 22. The daemon generates three key pairs when starting for the first time. Each key pair consists of a private and a public key. Therefore, this procedure is called public key-based. To guarantee the security of the communication via SSH, access to the private key files must be restricted to the system administrator. The file permissions are set accordingly by the default installation. The private keys are only required locally by the SSH daemon and must not be given to anyone else. The public key components (recognizable by the name extension .pub) are sent to the client requesting the connection. They are readable for all users.

A connection is initiated by the SSH client. The waiting SSH daemon and the requesting SSH client exchange identification data to compare the protocol and software versions, and to prevent connections through the wrong port. Because a child process of the original SSH daemon replies to the request, several SSH connections can be made simultaneously.

For the communication between SSH server and SSH client, OpenSSH supports versions 1 and 2 of the SSH protocol. Version 2 of the SSH protocol is used by default. Override this to use version 1 of protocol with the -1 option.

When using version 1 of SSH, the server sends its public host key and a server key, which is regenerated by the SSH daemon every hour. Both allow the SSH client to encrypt a freely chosen session key, which is sent to the SSH server. The SSH client also tells the server which encryption method (cipher) to use. Version 2 of the SSH protocol does not require a server key. Both sides use an algorithm according to Diffie-Hellman to exchange their keys.

The private host and server keys are absolutely required to decrypt the session key and cannot be derived from the public parts. Only the contacted SSH daemon can decrypt the session key using its private keys. This initial connection phase can be watched closely by turning on verbose debugging using the -v option of the SSH client.

Tip: Viewing the SSH Daemon Log File

To watch the log entries from the sshd use the following command:

tux > sudo journalctl -u sshd

15.4.1 Maintaining SSH Keys

It is recommended to back up the private and public keys stored in /etc/ssh/ in a secure, external location. In this way, key modifications can be detected or the old ones can be used again after having installed a new system.

Tip: Existing SSH Host Keys

If you install SUSE Linux Enterprise Server on a machine with existing Linux installations, the installation routine automatically imports the SSH host key with the most recent access time from an existing installation.

When establishing a secure connection with a remote host for the first time, the client stores all public host keys in ~/.ssh/known_hosts. This prevents any man-in-the-middle attacks—attempts by foreign SSH servers to use spoofed names and IP addresses. Such attacks are detected either by a host key that is not included in ~/.ssh/known_hosts, or by the server's inability to decrypt the session key in the absence of an appropriate private counterpart.

If the public keys of a host have changed (that needs to be verified before connecting to such a server), the offending keys can be removed with ssh-keygen -r HOSTNAME.

15.4.2 Rotating Host Keys

As of version 6.8, OpenSSH comes with a protocol extension that supports host key rotation. It makes sense to replace keys, if you are still using weak keys such as 1024-bit RSA keys. It is strongly recommended to replace such a key and go for 2048-bit DSA keys or something even better. The client will then use the best host key.

Tip: Restarting sshd

After installing new host keys on the server, restart sshd.

This protocol extension can inform a client of all the new host keys on the server, if the user initiates a connection with ssh. Then, the software on the client updates ~/.ssh/known_hosts, and the user is not required to accept new keys of previously known and trusted hosts manually. The local known_hosts file will contain all the host keys of the remote hosts, in addition to the one that authenticated the host during this session.

Once the administrator of the server knows that all the clients have fetched the new keys, they can remove the old keys. The protocol extension ensures that the obsolete keys will be removed from the client's configuration, too. The key removal occurs while initiating an ssh session.

For more information, see:

15.5 SSH Authentication Mechanisms

In its simplest form, authentication is done by entering the user's password just as if logging in locally. However, having to memorize passwords of several users on remote machines is inefficient. What is more, these passwords may change. On the other hand—when granting root access—an administrator needs to be able to quickly revoke such a permission without having to change the root password.

To accomplish a login that does not require to enter the remote user's password, SSH uses another key pair, which needs to be generated by the user. It consists of a public (id_rsa.pub or id_dsa.pub) and a private key (id_rsa or id_dsa).

To be able to log in without having to specify the remote user's password, the public key of the SSH user must be in ~/.ssh/authorized_keys. This approach also ensures that the remote user has got full control: adding the key requires the remote user's password and removing the key revokes the permission to log in from remote.

For maximum security such a key should be protected by a passphrase which needs to be entered every time you use ssh, scp, or sftp. Contrary to the simple authentication, this passphrase is independent from the remote user and therefore always the same.

An alternative to the key-based authentication described above, SSH also offers a host-based authentication. With host-based authentication, users on a trusted host can log in to another host on which this feature is enabled using the same user name. SUSE Linux Enterprise Server is set up for using key-based authentication, covering setting up host-based authentication on SUSE Linux Enterprise Server is beyond the scope of this manual.

Note: File Permissions for Host-Based Authentication

If the host-based authentication is to be used, the file /usr/lib/ssh/ssh-keysign (32-bit systems) or /usr/lib64/ssh/ssh-keysign (64-bit systems) should have the setuid bit set, which is not the default setting in SUSE Linux Enterprise Server. In such case, set the file permissions manually. You should use /etc/permissions.local for this purpose, to make sure that the setuid bit is preserved after security updates of openssh.

15.5.1 Generating an SSH Key

  1. To generate a key with default parameters (RSA, 2048 bits), enter the command ssh-keygen.

  2. Accept the default location to store the key (~/.ssh/id_rsa) by pressing Enter (strongly recommended) or enter an alternative location.

  3. Enter a passphrase consisting of 10 to 30 characters. The same rules as for creating safe passwords apply. It is strongly advised to refrain from specifying no passphrase.

You should make absolutely sure that the private key is not accessible by anyone other than yourself (always set its permissions to 0600). The private key must never fall into the hands of another person.

To change the password of an existing key pair, use the command ssh-keygen -p.

15.5.2 Copying an SSH Key

To copy a public SSH key to ~/.ssh/authorized_keys of a user on a remote machine, use the command ssh-copy-id. To copy your personal key stored under ~/.ssh/id_rsa.pub you may use the short form. To copy DSA keys or keys of other users, you need to specify the path:

# ~/.ssh/id_rsa.pub
ssh-copy-id -i tux@sun

# ~/.ssh/id_dsa.pub
ssh-copy-id -i ~/.ssh/id_dsa.pub  tux@sun

# ~notme/.ssh/id_rsa.pub
ssh-copy-id -i ~notme/.ssh/id_rsa.pub  tux@sun

To successfully copy the key, you need to enter the remote user's password. To remove an existing key, manually edit ~/.ssh/authorized_keys.

15.5.3 Using the ssh-agent

When doing lots of secure shell operations it is cumbersome to type the SSH passphrase for each such operation. Therefore, the SSH package provides another tool, ssh-agent, which retains the private keys for the duration of an X or terminal session. All other windows or programs are started as clients to the ssh-agent. By starting the agent, a set of environment variables is set, which will be used by ssh, scp, or sftp to locate the agent for automatic login. See the ssh-agent man page for details.

After the ssh-agent is started, you need to add your keys by using ssh-add. It will prompt for the passphrase. After the password has been provided once, you can use the secure shell commands within the running session without having to authenticate again. Using ssh-agent in an X Session

On SUSE Linux Enterprise Server, the ssh-agent is automatically started by the GNOME display manager. To also invoke ssh-add to add your keys to the agent at the beginning of an X session, do the following:

  1. Log in as the desired user and check whether the file ~/.xinitrc exists.

  2. If it does not exist, use an existing template or copy it from /etc/skel:

    if [ -f ~/.xinitrc.template ]; then mv ~/.xinitrc.template ~/.xinitrc; \
    else cp /etc/skel/.xinitrc.template ~/.xinitrc; fi
  3. If you have copied the template, search for the following lines and uncomment them. If ~/.xinitrc already existed, add the following lines (without comment signs).

    # if test -S "$SSH_AUTH_SOCK" -a -x "$SSH_ASKPASS"; then
    #       ssh-add < /dev/null
    # fi
  4. When starting a new X session, you will be prompted for your SSH passphrase. Using ssh-agent in a Terminal Session

In a terminal session you need to manually start the ssh-agent and then call ssh-add afterward. There are two ways to start the agent. The first example given below starts a new Bash shell on top of your existing shell. The second example starts the agent in the existing shell and modifies the environment as needed.

ssh-agent -s /bin/bash
eval $(ssh-agent)

After the agent has been started, run ssh-add to provide the agent with your keys.

15.6 Port Forwarding

ssh can also be used to redirect TCP/IP connections. This feature, also called SSH tunneling, redirects TCP connections to a certain port to another machine via an encrypted channel.

With the following command, any connection directed to jupiter port 25 (SMTP) is redirected to the SMTP port on sun. This is especially useful for those using SMTP servers without SMTP-AUTH or POP-before-SMTP features. From any arbitrary location connected to a network, e-mail can be transferred to the home mail server for delivery.

ssh -L 25:sun:25 jupiter

Similarly, all POP3 requests (port 110) on jupiter can be forwarded to the POP3 port of sun with this command:

ssh -L 110:sun:110 jupiter

Both commands must be executed as root, because the connection is made to privileged local ports. E-mail is sent and retrieved by normal users in an existing SSH connection. The SMTP and POP3 host must be set to localhost for this to work. Additional information can be found in the manual pages for each of the programs described above and in the OpenSSH package documentation under /usr/share/doc/packages/openssh.

15.7 For More Information


The home page of OpenSSH


The OpenSSH Wikibook

man sshd

The man page of the OpenSSH daemon

man ssh_config

The man page of the OpenSSH SSH client configuration files

man scp , man sftp , man slogin , man ssh , man ssh-add , man ssh-agent , man ssh-copy-id , man ssh-keyconvert , man ssh-keygen , man ssh-keyscan

Man pages of several binary files to securely copy files (scp, sftp), to log in (slogin, ssh), and to manage keys.

/usr/share/doc/packages/openssh/README.SUSE , /usr/share/doc/packages/openssh/README.FIPS

SUSE package specific documentation; changes in defaults with respect to upstream, notes on FIPS mode etc.