Certified Linux Administrator SSH


SSH (or Secure SHell) is a protocol which facilitates secure communications between two systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, rendering the connection difficult for intruders to collect unencrypted passwords.

SSH is designed to replace older, less secure terminal applications used to log into remote hosts, such as telnet or rsh. A related program called scp replaces older programs designed to copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the remote host.

Features of SSH

The SSH protocol provides the following safeguards:

  • After an initial connection, the client can verify that it is connecting to the same server it had connected to previously.

  • The client transmits its authentication information to the server using strong, 128-bit encryption.

  • All data sent and received during a session is transferred using 128-bit encryption, making intercepted transmissions extremely difficult to decrypt and read.

  • The client can forward X11 applications from the server. This technique, called X11 forwarding, provides a secure means to use graphical applications over a network.

Because the SSH protocol encrypts everything it sends and receives, it can be used to secure otherwise insecure protocols. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.

The OpenSSH server and client can also be configured to create a tunnel similar to a virtual private network for traffic between server and client machines.

Finally, OpenSSH servers and clients can be configured to authenticate using the GSSAPI implementation of the Kerberos network authentication protocol. 

Red Hat Enterprise Linux includes the general OpenSSH package (openssh) as well as the OpenSSH server (openssh-server) and client (openssh-clients) packages. Note, the OpenSSH packages require the OpenSSL package (openssl) which installs several important cryptographic libraries, enabling OpenSSH to provide encrypted communications.

Why Use SSH?

Nefarious computer users have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:

  • Interception of communication between two systems — In this scenario, the attacker can be somewhere on the network between the communicating parties, copying any information passed between them. The attacker may intercept and keep the information, or alter the information and send it on to the intended recipient.

    This attack can be mounted through the use of a packet sniffer — a common network utility.

  • Impersonation of a particular host — Using this strategy, an attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.

    This attack can be mounted through techniques known as DNS poisoning or IP spoofing.

Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous.

If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.


Event Sequence of an SSH Connection

The following series of events help protect the integrity of SSH communication between two hosts.

  1. A cryptographic handshake is made so that the client can verify that it is communicating with the correct server.

  2. The transport layer of the connection between the client and remote host is encrypted using a symmetric cipher.

  3. The client authenticates itself to the server.

  4. The remote client interacts with the remote host over the encrypted connection.

Transport Layer

The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of authentication and during subsequent communication. The transport layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also provides compression, speeding the transfer of information.

Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange:

  • Keys are exchanged

  • The public key encryption algorithm is determined

  • The symmetric encryption algorithm is determined

  • The message authentication algorithm is determined

  • The hash algorithm is determined

During the key exchange, the server identifies itself to the client with a unique host key. If the client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH gets around this problem by accepting the server's host key. This is done after the user is notified and has both accepted and verified the new host key. In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is indeed communicating with the intended server. If, in the future, the host key no longer matches, the user must remove the client's saved version before a connection can occur.


It is possible for an attacker to masquerade as an SSH server during the initial contact since the local system does not know the difference between the intended server and a false one set up by an attacker. To help prevent this, verify the integrity of a new SSH server by contacting the server administrator before connecting for the first time or in the event of a host key mismatch.

SSH is designed to work with almost any kind of public key algorithm or encoding format. After an initial key exchange creates a hash value used for exchanges and a shared secret value, the two systems immediately begin calculating new keys and algorithms to protect authentication and future data sent over the connection.

After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine the hash and shared secret value, this information is only useful for a limited period of time.


Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate itself to the server using one of these supported methods.

SSH servers and clients can be configured to allow different types of authentication, which gives each side the optimal amount of control. The server can decide which encryption methods it supports based on its security model, and the client can choose the order of authentication methods to attempt from the available options.


After a successful authentication over the SSH transport layer, multiple channels are opened via a technique called multiplexing. Each of these channels handles communication for different terminal sessions and for forwarded X11 sessions.

Both clients and servers can create a new channel. Each channel is then assigned a different number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be closed without disrupting the primary SSH connection.

Channels also support flow-control, which allows them to send and receive data in an orderly fashion. In this way, data is not sent over the channel until the client receives a message that the channel is open.

The client and server negotiate the characteristics of each channel automatically, depending on the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change the basic infrastructure of the protocol.


Configuring an OpenSSH Server

To run an OpenSSH server, you must first make sure that you have the proper RPM packages installed. The openssh-server package is required and is dependent on the openssh package.

The OpenSSH daemon uses the configuration file /etc/ssh/sshd_config. The default configuration file should be sufficient for most purposes. If you want to configure the daemon in ways not provided by the default sshd_config, read the sshd man page for a list of the keywords that can be defined in the configuration file.

To start the OpenSSH service, use the command /sbin/service sshd start. To stop the OpenSSH server, use the command /sbin/service sshd stop

If you reinstall, the reinstalled system creates a new set of identification keys. Any clients who had connected to the system with any of the OpenSSH tools before the reinstall will see the following message:

Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.

If you want to keep the host keys generated for the system, backup the /etc/ssh/ssh_host*key* files and restore them after the reinstall. This process retains the system's identity, and when clients try to connect to the system after the reinstall, they will not receive the warning message.

Requiring SSH for Remote Connections

For SSH to be truly effective, using insecure connection protocols, such as Telnet and FTP, should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet.

Some services to disable include:

  • telnet

  • rsh

  • rlogin

  • vsftpd

To disable insecure connection methods to the system, use the command line program chkconfig, the ncurses-based program /usr/sbin/ntsysv, or the Services Configuration Tool (system-config-services) graphical application. All of these tools require root level access.


OpenSSH Configuration Files

OpenSSH has two different sets of configuration files: one for client programs (ssh, scp, and sftp) and one for the server daemon (sshd).

System-wide SSH configuration information is stored in the /etc/ssh/ directory:

  • moduli — Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is critical for constructing a secure transport layer. When keys are exchanged at the beginning of an SSH session, a shared, secret value is created which cannot be determined by either party alone. This value is then used to provide host authentication.

  • ssh_config — The system-wide default SSH client configuration file. It is overridden if one is also present in the user's home directory (~/.ssh/config).

  • sshd_config — The configuration file for the sshd daemon.

  • ssh_host_dsa_key — The DSA private key used by the sshd daemon.

  • ssh_host_dsa_key.pub — The DSA public key used by the sshd daemon.

  • ssh_host_key — The RSA private key used by the sshd daemon for version 1 of the SSH protocol.

  • ssh_host_key.pub — The RSA public key used by the sshd daemon for version 1 of the SSH protocol.

  • ssh_host_rsa_key — The RSA private key used by the sshd daemon for version 2 of the SSH protocol.

  • ssh_host_rsa_key.pub — The RSA public key used by the sshd for version 2 of the SSH protocol.

User-specific SSH configuration information is stored in the user's home directory within the ~/.ssh/ directory:

  • authorized_keys — This file holds a list of authorized public keys for servers. When the client connects to a server, the server authenticates the client by checking its signed public key stored within this file.

  • id_dsa — Contains the DSA private key of the user.

  • id_dsa.pub — The DSA public key of the user.

  • id_rsa — The RSA private key used by ssh for version 2 of the SSH protocol.

  • id_rsa.pub — The RSA public key used by ssh for version 2 of the SSH protocol

  • identity — The RSA private key used by ssh for version 1 of the SSH protocol.

  • identity.pub — The RSA public key used by ssh for version 1 of the SSH protocol.

  • known_hosts — This file contains DSA host keys of SSH servers accessed by the user. This file is very important for ensuring that the SSH client is connecting the correct SSH server.


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