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Tue, Jan 23, 2018 9:42 pm

Using key pairs with SSH

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 ]

[/network/ssh] permanent link

Sun, Oct 16, 2016 10:10 pm

Checking a server's public host key on the server

If you receive a message from a Secure Shell (SSH) or Secure File Transfer Protocol (SFTP) application regarding the host key of the server to which you are attempting to connect being unknown or changed, such as the message from WinSCP below, you can check the server's public host key on the server itself, if it is a Linux server, using the ssh-keygen utility.

WinSCP unknown server 
host key

$ ssh-keygen -l -f /etc/ssh/
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/ 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/

[/network/ssh] permanent link

Sat, Jul 02, 2016 9:57 pm

SFTP received message too long error

I was able to log into a Microsoft Windows 7 system running CopSSH via Secure Shell (SSH) using PuTTY, but when I attempted to transfer a file to the system via the SSH File Transfer Protocol (SFTP) using WinSCP, I received the error message below:

Received too large (1298752370 B) SFTP package. Max supported package size
is 1024000 B.

The error is typically caused by message printed from startup script (like
.profile). The message may start with "Micr".

Cannot initialize SFTP protocol. Is the host running a SFTP server?

[ More Info ]

[/network/ssh] permanent link

Thu, May 26, 2016 11:53 pm

Running a command on a remote system using SSH

The Secure Shell (SSH) protocol allows you to interactively log into remote systems. Once logged into a remote system, you have a shell prompt where you can enter commands on the remote system. But you can use an SSH client to execute a command on a remote system without logging into that system and obtaining a shell prompt on the remote system. E.g., if you wanted to get a command line interface (CLI) on the remote system, you might enter a command similar to the following one:
$ ssh

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 uname --hardware-platform's password: 
$ uname --hardware-platform

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 vi temp.txt'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.

-t Force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.


$ ssh -t vi temp.txt

[/network/ssh] permanent link

Mon, May 16, 2016 7:12 am

Break out of SSH session

Sometimes after I've established an SSH connection to an SSH server, I encounter a situation where the remote system isn't responding to keyboard input and I want to terminate the SSH session and return to a command prompt. E.g., often when I've connected to a Microsoft Windows system running SSH software from my Ubuntu Linux laptop, I find that I'm in a situation where after I've entered a command at the Windows system's command prompt the remote system no longer seems to be accepting keyboard input from the Linux system. Sometimes it seems to occur when I've mistyped a Windows command and the Windows system may be waiting for further input, but doesn't seem to accept what I type. In such cases, rather than close the Terminal tab on the Linux system to terminate the connection, which then requires me to open a new tab and establish a new SSH session, I'd prefer to break out of the current SSH session and return to the shell prompt on the Linux system where I can re-establish the SSH connection. In such cases, Ctrl-C, Ctrl-D, and Ctrl-Z don't help me.

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 ]

[/network/ssh] permanent link

Mon, Apr 18, 2016 11:15 pm

Keeping an SSH connection alive

If you need to keep a SSH connection alive, e.g., when you won't be entering any commands for awhile after logging into the remote system via SSH, you can use the -o option with ServerAliveInterval . You can specify the interval in seconds which will be used by the SSH client to send keepalive packets with -o ServerAliveInterval x where x is the frequency for sending the keepalive packets. E.g., if I wanted the SSH client to send keepalive packets every minute (60 seconds) to the remote SSH server, I could use a command like the one below when establishing the SSH session:
$ ssh -o ServerAliveInterval=60

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 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


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

[/network/ssh] permanent link

Thu, Jan 14, 2016 11:09 pm

OpenSSH Roaming Vulnerability

Ars Technica published an article today titled Bug that can leak crypto keys just fixed in widely used OpenSSH which explained how a compromise of a SSH server running OpenSSH software could lead to the server being used to capture data from the memory of systems that have connected to the server via SSH including private keys for users connecting to the server.

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@ 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 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


  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

[/network/ssh] permanent link

Tue, Jun 30, 2015 10:09 pm

Transferring files with scp through a bastion host on an OS X system

I need to transfer files through a bastion host periodically. To edit files on a webserver, I need to first establish a Secure Shell (SSH) connection to the bastion host, logging in using an RSA SecurID token. Once I've provided my login credentials, the bastion host prompts me for the system to which I want to connect to from it, which in this case is the web server. So my ssh login to the webserver is tunneled through the bastion host.

I normally go through the process once a month from my MacBook Pro laptop running the OS X operating system when I need to place a monthly newsletter on the web server. I use an SSH command similar to the following to log into the bastion host where is the fully qualified domain name (FQDN) of the web server and is the bastion host.

ssh -L

The -L option specifies I want to tunnel a local port on my laptop, in this case I chose 22001, to port 22 on the web server, A tunnel is set up from my laptop to the web server through the bastion host by using that option once my login is completed to the bastion host.

Then, to transfer a file via secure copy from my laptop to the web server, I can use a command like the following one to transfer a file named July.txt from the laptop to the web server:

$ scp -P 22001 July.txt jasmith1@
jasmith1@'s password:

The -P option to the scp command specifies I want to use TCP port 22001, since that is the port for the end of the tunnel on my laptop. The address I'm specifying is the localhost, aka "loopback", address on my laptop. I.e., I'm connecting to port 22001 on the laptop itself. The tunnel I set up earlier results in any connection to that port being forwared through the tunnel to the web server, so I'm specifying my userid for the web server and the password prompt I receive is for the web server. The file July.txt will thus be placed in the directory /data/htdocs/clubs/groot/newsletter/2015 on the web server with the same name, July.txt.

If I wanted to pull a file from the webserver via the tunnel, I could use a command such as the following:

scp -P 22001 jasmith1@ .

That command would retrieve the file July.html from the web server and place it on the laptop with the same name.

[/network/ssh] permanent link

Sun, Nov 09, 2014 5:21 pm

Monitoring Failed SSH Logins to a CentOS System

If you have ssh enabled on a system that is accessible to the Internet, it is probable that malicious individuals will try to gain access to the system by brute force login attempts. I.e., since a Linux, Unix, or OS X system is likely to have a root account, an attacker may use "root" as the userid and then attempt to login with commonly used passwords. There are sites on the Internet that provide lists of passwords commonly used and an attacker can easily use a dictionary attack where he tries every word in a dictionary as a possible password. Attackers can use dictionaries for multiple languages, lists of sports teams, name dictionaries, e.g., dictionaries of names parents might check to aid in selecting a name for a baby, etc. So a root or another administrator account should have a strong password. If it doesn't, the system will likely be cracked by an attacker eventually.

Attackers also routinely use name dictionaries to break into systems with any accounts that have weak passwords. E.g., an attacker may use a name dictionary to pick names to use as the userid. Let's say the first name in the name dictionary is Aaron. The attacker might then use a word dictionary to try every word in the English language, or some other language, as a possible password for an account with the userid of aaron. If an aaron account doesn't exist on the system or has a strong password, once the attacker has gone through every word in the word dictionary or whatever other password list he is using, he will then go onto the next name in his name dictionary, e.g., perhaps Abe. The attacker will proceed in this manner until he finds an account with a weak password he can compromise or exhausts all possible combinations of names for accounts and words to use for possible passwords. Of course it would take a human an inordinate amount of time to type all such possible userid and password combinations, but an attacker will let a program make such guesses for him. He merely needs to start the program and let it run. His program may be able to check many thousands of userid and password combinations in minutes.

If the system isn't monitored for such brute-force password attempts, an attacker can run unchecked for days. Even if he can't get in, he will be using bandwidth to/from the system under attack as well as CPU cycles, etc., so may slow down access to the system for legitimate users. I've seen periods where a system has been under attack from 5 such attackers in different countries at once.

On CentOS Linux, you can check the /var/log/secure log to find instances of such attacks.

# grep 'Failed password' /var/log/secure | tail -5
Oct 28 09:47:43 frostdragon sshd[32246]: Failed password for root from port 11859 ssh2
Oct 28 09:47:47 frostdragon sshd[32249]: Failed password for root from port 13894 ssh2
Oct 28 09:47:52 frostdragon sshd[32253]: Failed password for root from port 15886 ssh2
Oct 28 09:47:56 frostdragon sshd[32256]: Failed password for root from port 17740 ssh2
Oct 28 09:48:01 frostdragon sshd[32259]: Failed password for root from port 19477 ssh2

You can see the number of failed ssh login attempts from various login addresses with the command grep 'Failed password' /var/log/secure | grep sshd | awk '{print $11}' | sort | uniq -c - the IP address from which the failed login attempt was made is the 11th item on the line.

If you pipe the output of the awk command into sort, you can sort the output by IP address; uniq -c will then provide you the count of failed SSH login attempts from particular IP addresses.

# grep 'Failed password' /var/log/secure | grep sshd | awk '{print $11}' | sort | uniq -c

I can see from the above output from that command that there were 382 failed ssh login attempts from the address at the time I ran the command.

From a search on that IP address at the American Registry for Internet Numbers (ARIN), I found the address was part of a block of addresses managed by the Asia Pacific Network Information Centre (APNIC) . A whois search on the APNIC site showed the IP address is part of a large block of addresses, -, allocated to an organization in Beijing, China. I often see attacks from IP addresses allocated to entities in China.

You can manually block further attempts to compromise a system in this manner using a route reject command or through the firewall software on the system. The default firewall software for CentOS 7 is FirewallD. You can configure it through a Graphical User Interface (GUI), which can be opened using the command firewall-config or through a command line interface at a shell prompt by using the command firewall-cmd. I blocked the IP address from any access to the system using the command below, though by the time I blocked it, the login attempts had ceased:

# firewall-cmd --add-rich-rule="rule family='ipv4' source address='' reject"

The block can be viewed through the graphical interface for FirewallD by running firewall-config. E.g., in this case under "Rich Rules" for the public zone, I can see the blocked IP when starting the application after issuing the firewall-cmd command.

FirewallD blocked IP

The command above will put in place a firewall rule that will apply to the default firewall zone, but will only remain until the firewall service is restarted, e.g., with a system reboot. To put in place a permanent block, I could have used the commands below. Instituting a permanent change requires a restart of the firewall service, though.

firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='' reject"
systemctl restart firewalld.service

To have a block apply to a specific firewall zone, e.g., the public zone, I could use the commands below.

firewall-cmd --permanent --zone='public' --add-rich-rule="rule family='ipv4' source address='' reject"
systemctl restart firewalld.service

The output of the grep command run against /var/log/secure displayed above was sorted by IP address; if you, instead, would like to sort the output by count of failed login attempts you can pipe the output of the commands above into sort again adding the -n argument to sort by the number that appears first on each line.

# grep 'Failed password' /var/log/secure | grep sshd | awk '{print $11}' | sort | uniq -c | sort -n
      1 a
      1 pi
      1 ubnt
      2 client
      4 ubuntu
      4 usuario
     27 git
     78 admin

I can see from the above output that the greatest number of failed SSH login attempts made on the day I ran the command, which was November 9, 2014, were made from Checking the APNIC site again, I see that IP address is also assigned to an entity in China.

If you want to reverse the sorting order, so that the largest number appears first, simply add the -r argument to the last sort command.

# grep 'Failed password' /var/log/secure | grep sshd | awk '{print $11}' | sort | uniq -c | sort -nr
     78 admin
     27 git
      4 usuario
      4 ubuntu
      2 client
      1 ubnt
      1 pi
      1 a

In the above output, some of the failed entries are associated with userids the attacker attempted to use to login. E.g., for the case of the usuario one, I can see that the illegitimate login attempts where that name was used for the userid orginated from the IP address. There is no account on the system with that userid. The IP address is also assigned to an entity in China.

# grep usuario /var/log/secure
Nov  9 10:53:01 localhost sshd[23516]: Invalid user usuario from
Nov  9 10:53:01 localhost sshd[23516]: input_userauth_request: invalid user usuario [preauth]
Nov  9 10:53:03 localhost sshd[23516]: Failed password for invalid user usuario from port 52710 ssh2
Nov  9 10:53:04 localhost sshd[23568]: Invalid user usuario from
Nov  9 10:53:04 localhost sshd[23568]: input_userauth_request: invalid user usuario [preauth]
Nov  9 10:53:06 localhost sshd[23568]: Failed password for invalid user usuario from port 53534 ssh2
Nov  9 10:53:07 localhost sshd[23654]: Invalid user usuario from
Nov  9 10:53:07 localhost sshd[23654]: input_userauth_request: invalid user usuario [preauth]
Nov  9 10:53:10 localhost sshd[23654]: Failed password for invalid user usuario from port 55193 ssh2
Nov  9 10:53:12 localhost sshd[23657]: Invalid user usuario from
Nov  9 10:53:12 localhost sshd[23657]: input_userauth_request: invalid user usuario [preauth]
Nov  9 10:53:14 localhost sshd[23657]: Failed password for invalid user usuario from port 56072 ssh2

To count just by IP address so that the login failurers for particular usernames don't appear in the output, I can put another grep command, one that will filter the output of the prior grep command so any lines of output from it are eliminated if they contain "invalid user", before the awk command.

# grep 'Failed password' /var/log/secure | grep sshd | grep -v "invalid user" | awk '{print $11}' | sort | uniq -c | sort -n

If you wish to see what userids are being used most frequently for the failed login attempts, the string of commands entered above need to be modified to search for the userids used in failed login attempts. The above commands don't show the most commonly used userid, which is root, since almost all Unix/Linux systems will have a root account.

For failed login attempts the lines that appear in the output are slightly different depending upon whether the userid used exists on the system. E.g., if the account doesn't exist on the system, as in the case for client and git below, the lines appear as follows:

Nov  9 10:34:14 localhost sshd[21745]: Failed password for invalid user client f
rom port 60223 ssh2
Nov  9 10:52:00 localhost sshd[23204]: Failed password for invalid user git from port 60513 ssh2

If the account does exist, e.g., the root account, then the lines have the following format:

Nov  9 04:58:50 localhost sshd[21319]: Failed password for root from 122.225.97.
79 port 7951 ssh2

The sed command can be used to strip out the "invalid user" from lines to make the format of those lines containing "invalid user" the same as for those for valid userids on the system. You can then use the awk command to display the contents of the 9th entry on the line, which is the userid used.

# grep "sshd.*: Failed password for" /var/log/secure | sed 's/invalid user //' | awk '{print $9}' | sort | uniq -c | sort -n
      1 a
      1 operator
      1 pi
      1 ubnt
      2 client
      4 ubuntu
      4 usuario
     27 git
     78 admin
   1844 root

The output from a check of the /var/log/secure file shows that the most common user name used in attempts to log into the system by attackers is root.


  1. Firewalld - Block an IP Address
    By: up2long
    Date: February 26, 2014

[/network/ssh] permanent link

Tue, Oct 08, 2013 10:39 pm

Keeping an SSH connection alive with ServerAliveInterval

While working on a Ubuntu Linux laptop, I found my SSH sessions to a Linux server were being dropped after a few minutes when I switched to other tasks on the laptop. Using the ServerAliveInterval parameter when establishing the SSH connection allowed me to alleviate the problem of idle connections being dropped. E.g., I could use:
$ ssh -o ServerAliveInterval=5 -o ServerAliveCountMax=1

Setting the ServerAliveInterval to 5 will send a "heartbeat" signal to the server every 5 seconds to keep the connection alive. Setting ServerAliveCountMax to 1 means that when the time comes to send another keepalive signal, if a response to the last one wasn't received, then the connection will be terminated.

The TCPKeepAlive setting could also be used, but, if there is an intervening firewall it might be configured to drop the empty TCP ACK packets that would be sent. The ServerAliveInterval setting sends data through the SSH connection, so from the perspective of the firewall the packets are the same as any other encrypted packet.


  1. How does tcp-keepalive work in ssh?
    Date: March 12, 2012
    Unix & Linux Stack Exchange
  2. Keeping Your SSH Sessions Alive Through Pesky NAT Firewalls
    Date: June 3, 2005
    Steve Kehlet's Pages

[/network/ssh] permanent link

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