If you want to prevent a Secure Shell (SSH) connection being dropped when it has been inactive for several minutes, you can specify a keepalive value. To specify a keepalive value directly from the command line, use the -o flag followed by the ServerAliveIntervaloption

Command Line Syntax

Run the following command to send a keepalive packet every 60 seconds:

ssh -o ServerAliveInterval=60 user@hostname

Advanced Keepalive Control

For more robust protection against drops, combine the interval with ServerAliveCountMax. This defines how many consecutive keepalive messages can go unanswered before the client disconnects.

ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 user@hostname

• ServerAliveInterval=60: Sends a packet every 60 seconds of inactivity.
• ServerAliveCountMax=3: Drops the dead connection if the server fails to respond 3 times in a row.

Making It Permanent (Optional)

If you get tired of typing this out every time, you can save it to your local configuration:

• Open your configuration file in a text editor, such as nano:

  nano ~/.ssh/config

• Add these lines to apply it to all hosts:

  Host *
   ServerAliveInterval 60
   ServerAliveCountMax 3

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To change the background color for a terminal session using the PuTTY ssh and telnet program, take the following steps:

1. If you wish to change the color for a saved session, load the session then click on "Colours," which is beneath "Window," in the left hand pane of the PuTTY Configuration window.

   

2. Click on "Default Background"; you should then see the default Red Green Blue (RGB) values with values of 0,0,0 representing black and 255, 255, 255 representing white. You can use an RGB Color Codes Chart, such as the one at RapidTables.com to select a color and then obtain its numerical RGB value. For instance, if I wanted the background to be a dark gray, I could use a value of 64 for red, green, and blue.

   

3. You can then click on the Open button to open a session using the new background color or click on "Session" and save the session with the new setting, if you wish to make the setting permanent before opening the session.

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The Secure Shell (SSH) protocol provides a means one can use for secure, encrypted connections between systems for logins or file transfers. One can use a username and password to login from an SSH client to an SSH server or one can use a public and private key combination where a public key for a user's account is stored on a remote SSH server while a corresponding private key is stored on the system from which the user will initiate the SSH or SFTP connection. On Linux systems, private keys are normally stored in the .ssh directory beneath the home directory for your account. If you haven't created any keys yet, the directory may only contain a known_hosts file that contains public keys for servers you've previously logged into via SSH.

[ More Info ]

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$ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key.pub
2048 96:f3:8b:03:13:06:13:4d:3c:7c:4b:fa:94:33:90:83   (RSA)
$

The -l option shows the fingerprint of a specified public key file. Private RSA1 keys are also supported. For RSA and DSA keys, ssh-keygen tries to find the matching public key file and prints its fingerprint. If the -l option is combined with -v, an ASCII art representation of the key is supplied with the fingerprint. The -f filename option allows you to specify the file name of the key file.

The ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key.pub command isn't showing the key itself, but instead shows the "fingerprint" for the key, which is a sequence of 32 hexadecimal digits. You can see the much larger key value itself by issuing the command cat /etc/ssh/ssh_host_rsa_key.pub.

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[ More Info ]

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$ ssh jdoe@example.com

But, if you just were logging in to enter one command, say you wanted to find the hardware platform of the remote system using the uname command uname --hardware-platform, you could simply append that command to the end of the above ssh command you would have used to log into the remote system. E.g.:

$ ssh jdoe@example.com uname --hardware-platform
jdoe@example.com's password: 
x86_64
$ uname --hardware-platform
i386

In the example above, issuing the same command on the local system, i.e., the one on which the SSH command is being issued shows that the result returned when the uname command was issued at the end of the ssh command line returned a result from the remote system.

You may even be able to use a text-based editor, such as the vi editor, though you may see error messages like the ones below:

$ ssh jdoe@example.com vi temp.txt
jdoe@example.com's password: 
Vim: Warning: Output is not to a terminal
Vim: Warning: Input is not from a terminal

When you enter an ssh command in the form ssh user@host the remote system allocates a pseudo-tty (PTY), a software abstraction used to handle keyboard input and screen output. However, if you request SSH to run a command on the remote server by appending that command after ssh user@host, then no interactive terminal session is required and a PTY is not allocated, so you see the error messages when you use a screen-based program intended for use with a terminal, such as the vi editor.

For such cases you should inclde the -t option to the SSH command.

E.g.:

$ ssh jdoe@example.com -t vi temp.txt

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But there is an escape sequence that will allow me to terminate the current SSH session. Hitting the three keys listed below will allow me to terminate the session.

↲ Enter, ~, .

[ More Info ]

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$ ssh -o ServerAliveInterval=60 jdoe@example.com

By using this option, you should be able to reduce the likelihood that your SSH connection will get dropped after a certain amount of time due to no activity for the session.

You can also use the ServerAliveCountMax parameter with ServerAliveInterval to drop the connection, if the SSH client hasn't received a response from the server to the prior "heartbeat" signal when the time comes to send another keepalive packet. E.g., ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=1 jdoe@example.com would result in the connection being dropped if the client was awaiting a response to even one outstanding keepalive packet.

There is also a TCPKeepAlive option in OpenSSH. That option is used to recognize when a connection is no longer active due to some problem such as the SSH client application crashing or a prolonged network outage. If the SSH server never recognizes that the client is no longer communicating with it, it will continue to allocate resources, such as memory, for the connection. The option is turned on by default in the OpenSSH configuration file /etc/ssh/sshd_config. You will see the following line in that file:

#TCPKeepAlive yes

You don't need to uncomment the line by removing the pound sign, since "yes" is the default value. The option causes Transmission Control Protocol (TCP) to periodically transmit keepalive messages. If it doesn't receive responses within the expected time, it returns an error to the sshd process, which will then shut down the connection. The purpose of this option is to prevent half-dead connections building up over time and consuming more and more system resources as the number grows. The keepalive interval is typically in the order of hours rather than minutes to minimize the network load for the server. If the keepalive period was made shorter, that would affect all TCP connections on the system, not just the SSH ones, potentially increasing the network load unnecessarily and also causing connections to be dropped even for transient issues, such as a short and temporary network issue.

The TCPKeepalive option is for dealing with longer term issues for a connection rather than the loss of connectivity due to firewall, proxying, or Network Address Translation (NAT) timeouts. You can specify the option on the command line at the SSH client end as follows:

$ ssh -o TCPKeepAlive=yes joe@example.com

References:

1. SSH, The Secure Shell: The Definitive Guide
   By: Daniel Barrett, Richard Silverman, Robert Byrnes
   

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The vulnerability resides only in the version end users use to connect to servers and not in versions used by servers. A maliciously configured server could exploit it to obtain the contents of the connecting computer's memory, including the private encryption key used for SSH connections. The bug is the result of code that enables an experimental roaming feature in OpenSSH versions 5.4 to 7.1

"The matching server code has never been shipped, but the client code was enabled by default and could be tricked by a malicious server into leaking client memory to the server, including private client user keys," OpenSSH officials wrote in an advisory published Thursday. "The authentication of the server host key prevents exploitation by a man-in-the-middle, so this information leak is restricted to connections to malicious or compromised servers."

The roaming feature was intended to allow users to resume broken SSH connections, even though the feature was disabled in OpenSSH server software years ago. E.g., when I connected to a server I have running OpenSSH server software, I saw the folowing:

$ ssh -v jdoe@127.0.0.1 2>&1 >/dev/null | grep -i 'roaming'
debug1: Roaming not allowed by server

The Red Hat article on the vulnerability OpenSSH: Information-leak vulnerability (CVE-2016-0777) notes:

Since version 5.4, the OpenSSH client supports an undocumented feature called roaming. If a connection to an SSH server breaks unexpectedly, and if the SSH server supports roaming as well, the client is able to reconnect to the server and resume the interrupted SSH session. The roaming feature is enabled by default in OpenSSH clients, even though no OpenSSH server version implements the roaming feature.

For affected products, the article also notes:

Red Hat Enterprise Linux 7 since version 7.1 has provided OpenSSH 6.6 for which the default configuration is not affected by this flaw. OpenSSH 6.6 is only vulnerable to this issue when used with certain non-default ProxyCommand settings. Security update RHSA-2016-0043 corrects this issue.

So CentOS 7 systems using a default OpenSSH configuration should be unaffected, since CentOS is derived from Red Hat Enterprise Linux.

On a Linux system, you can check the version of SSH installed with ssh -V.

$ ssh -V
OpenSSH_6.6.1p1, OpenSSL 1.0.1e-fips 11 Feb 2013

On a CentOS Linux system using the RPM Package Manager you can also use rpm -qi openssh | grep Version.

$ rpm -qi openssh | grep Version
Version     : 6.6.1p1

On a CentOS system, you can update the software from the command line, aka a shell prompt, using the command yum update openssh.

If you are using a vulnerable OpenSSH client, you can also specify the -oUseRoaming=no parameter on the command line to ensure that a malicious server can't take advantage of the vulnerability. E.g. ssh -oUseRoaming=no jdoe@example.com. Or the feature can be disabled for all users on a system by putting UseRoaming no in /etc/ssh/ssh_config or by an individual user for his account by adding the line to ~/.ssh/config.

echo 'UseRoaming no' >> /etc/ssh/ssh_config

References:

1. Bug that can leak crypto keys just fixed in widely used OpenSSH
   Date: January 14, 2016
   Ars Technica
2. Fixing The New OpenSSH Roaming Bug
   By: ScriptRock
   ScriptRock Blog
3. OpenSSH: Information-leak vulnerability (CVE-2016-0777)
   Updated: January 14, 2016
   Red Hat Customer Portal
4. Security OpenSSH Security Bug CVE-2016-0777 & CVE-2016-0778
   Date: January 4, 2016

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