Understanding SSH

ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine.  It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP ports can also be forwarded over the secure channel.

ssh connects and logs into the specified hostname (with optional user name).The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used.

 If command is specified, it is executed on the remote host instead of a login shell.

The options are as follows:


     -1      Forces ssh to try protocol version 1 only.


     -2      Forces ssh to try protocol version 2 only.


     -4      Forces ssh to use IPv4 addresses only.


     -6      Forces ssh to use IPv6 addresses only.


-A      Enables forwarding of the authentication agent connection.  This can also be specified on a per-host basis in a configuration file.

Agent forwarding should be enabled with caution.  Users with the ability to bypass file permissions on the remote host (for the agents Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.

-a      Disables forwarding of the authentication agent connection.

-b       bind_address

Use bind_address on the local machine as the source address of the connection.  Only useful on systems with more than one address.

 -C      Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11 and TCP connections).The compression algorithm is the same used by gzip.Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks.The default value can be set on a host-by-host basis in the configuration files

-c cipher_spec

Selects the cipher specification for encrypting the session.

Protocol version 1 allows specification of a single cipher.  The supported values are des blowfish des 3des (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.  It is believed to be secure.  blowfish is a fast block cipher; it appears very secure and is much faster than 3des. 

Des is only supported in the ssh client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to cryptographic weaknesses. The default is â3desâ For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of preference. 

 -D [bind_address:]port

Specifies a local dynamic port on the local side, optionally bound to the specified bind_address.  Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server.  Only root can forward privileged ports.  Dynamic port forwarding’s can also be specified in the configuration file.


IPv6 addresses can be specified with an alternative syntax: [bind_address/]port or by enclosing the address in square brackets. Only the superuser can forward privileged ports.By default, the local port is bound in accordance with the Gateway Ports setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of localhost tening port be bound for local use only, while an empty address or â*â indicates that the port should be available from all interfaces.

-e escape_char

Sets the escape character for sessions with a pty (default: â~â).  The escape character is only recognized at the beginning of a line.The escape character followed by a dot (â.â) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once.  Setting the character to ânoneâ

-F configfile

Specifies an alternative per-user configuration file.  If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config) will be ignored.  The default for the per-user configuration file is ~/.ssh/config.

-f      Requests ssh to go to background just before command execution.  This is useful if ssh is going to ask for passwords or passphrases,but the user wants it in the background.  This implies -n.  The recommended way to start X11 programs at a remote site is with something like

ssh -f host xterm.

If the ExitOnForwardFailure configuration option is set to âyesâ-f will wait for all remote port forwards to be successfully established before placing itself in the background.

  -g      Allows remote hosts to connect to local forwarded ports.


-i identity_file

Selects a file from which the identity (private key) for RSA or DSA authentication is read.  The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol version 2.  Identity files may also be specified on a per-host basis in the configuration file.  It is possible to have multiple -i options (and multiple identities specified in configuration files).


-K      Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.

-k      Disables forwarding (delegation) of GSSAPI credentials to the server.

-L [bind_address:]port:host:hostport

Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side.This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address.

Whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. Port forwarding’s can also be specified in the configuration file. 

IPv6 addresses can be specified with an alternative syntax:[bind_address/]port/host/hostport or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the Gateway Ports setting.

However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of âlocalhostâ empty address or â*â indicates that the port should be available from all interfaces.

-l login_name

Specifies the user to log in as on the remote machine.  This also may be specified on a per-host basis in the configuration file.

-M      Places the ssh client into master-M options places ssh into master required before slave connections are accepted. 

-m mac_spec

Additionally, for protocol version 2 a comma-separated list of MAC (message authentication code) algorithms can be specified in order of preference.

-N      Do not execute a remote command.  This is useful for just forwarding ports (protocol version 2 only).

-n      Redirects stdin from /dev/null (actually, prevents reading from stdin).  This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background.

This does not work if ssh needs to ask for a password or passphrase; see also the -f option.

-O ctl_cmd

Control an active connection multiplexing master process.  When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: check exit


-o option

Can be used to give options in the format used in the configuration file.  This is useful for specifying options for which there is no separate command-line flag.  For full details of the options listed below, and their possible values,

-p port

Port to connect to on the remote host.  This can be specified on a per-host basis in the configuration file.

-q      Quiet mode.  Causes most warning and diagnostic messages to be suppressed.

-R [bind_address:]port:host:hostport

Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side.  This works by allocating a socket to listen to port on the remote side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the local machine.

Port forwarding’s can also be specified in the configuration file.  Privileged ports can be forwarded only when logging in as root on the remote machine.IPv6 addresses can be specified by enclosing the address in square braces or using an alternative syntax [bind_address/]host/port/hostport.

By default, the listening socket on the server will be bound to the loopback interface only.  This may be overridden by specifying a bind_address. An empty bind_address, or the address *, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the servers Gateway Ports option is enabled

If the port argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time.


-S ctl_path

Specifies the location of a control socket for connection sharing. 

 -s      May be used to request invocation of a subsystem on the remote system.  Subsystems are a feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for other applications (eg. sftp(1)).  The subsystem is specified as the remote command.

-T      Disable pseudo-tty allocation.

-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.


-V      Display the version number and exit.

-v      Verbose mode.  Causes ssh to print debugging messages about its progress.  This is helpful in debugging connection, authentication, and configuration problems.  Multiple -v options increase the verbosity.The maximum is 3.

-w local_tun[:remote_tun]

Requests tunnel device forwarding with the specified tun devices between the client (local_tun) and the server (remote_tun).The devices may be specified by numerical ID or the keyword any remote_tun is not specified, it defaults to any Tunnel and Tunnel Device directives in ssh_config.If the Tunnel directive is unset, it is set to the default tunnel mode, which is point-to-point


-X      Enables X11 forwarding.  This can also be specified on a per-host basis in a configuration file.

X11 forwarding should be enabled with caution.  Users with the ability to bypass file permissions on the remote host (for the users X authorization database) can access the local X11 display through the forwarded connection.  An attacker may then be able to perform activities such as keystroke monitoring. For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default.

-x      Disables X11 forwarding.

-Y      Enables trusted X11 forwarding.  Trusted X11 forwarding’s are not subjected to the X11 SECURITY extension controls.

-y      Send log information using the syslog system module.  By default this information is sent to stderr.ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. 

ssh exits with the exit status of the remote command or with 255 if an error occurred.



The OpenSSH SSH client supports SSH protocols 1 and 2.Protocol 2 is the default, with ssh falling back to protocol 1 if it detects protocol 2 is unsupported. These settings may be altered using the Protocol option in ssh_config, or enforced using the -1 and -2 options protocols support similar authentication methods, but protocol 2 is preferred since it provides additional mechanisms for confidentiality

The traffic is encrypted using AES, 3DES, Blowfish and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).Protocol 1lacks a strong mechanism for ensuring the integrity of the connection.

The methods available for authentication are: GSSAPI-based authentication, host-based authentication, public key authentication, challenge-response authentication, and password authentication.  Authentication methods are tried in the order specified above, though protocol 2 has a configuration option to change the default order: PreferredAuthentications.

Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote machine, and the user names are the same on both sides, or if the files ~/.rhosts or ~/.shosts exist in the user’s home directory on the remote machine and contain a line containing the name of the client machine and the name of the user on that machine, the user is considered for login.

Additionally, the server must be able to verify the client’s host key (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing spoofing.

 Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.


Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and it is unfeasible to derive the decryption key from the encryption key. 

The idea is that each user creates a public/private key pair for authentication purposes.  The server knows the public key, and only the user knows the private key.ssh implements public key authentication protocol automatically, using either the RSA or DSA algorithms. Protocol 1 is restricted to using only RSA keys, but protocol 2 may use either. 

The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in.  When the user logs in, the ssh program tells the server which key pair it would like to use for authentication.  The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.

The user creates his/her key pair by running ssh-keygen(1).  This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user’s home directory.

The user should then copy the public key to ~/.ssh/authorized_keys in his/her home directory on the remote machine.  The authorized_keys file corresponds to the conventional ~/.rhosts file, and has one key per line, though the lines can be very long.  After this, the user can log in without giving the password.

The most convenient way to use public key authentication may be with an authentication agent. 

Challenge-response authentication works as follows: The server sends an arbitrary “challenge” text, and prompts for a response. Protocol 2 allows multiple challenges and responses; protocol 1 is restricted to just one challenge/response

Finally, if other authentication methods fail, ssh prompts the user for a password.  The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network.

ssh automatically maintains and checks a database containing identification for all hosts it has ever been used with. Host keys are stored in ~/.ssh/known_hosts in the user’s home directory.  Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user’s file.

If a hostâs identification ever changes, ssh warns about this and disables password authentication to prevent server spoofing or man-in-the-middle attacks, which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option can be used to control logins to machines whose host key is not known or has changed.

When the user’s identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine.  All communication with the remote command or shell will be automatically encrypted.

If a pseudo-terminal has been allocated (normal login session), the user may use the escape characters noted below. If no pseudo-tty has been allocated, the session is transparent and can be used to reliably transfer binary data.  On most systems, setting the escape character to none

The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.



When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.

A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below.  The escape character must always follow a newline to be interpreted as special.  The escape character can be changed in configuration files using the EscapeChar configuration directive or on the command line by the -e option.

  The supported escapes (assuming the default â~â) are:

~.      Disconnect.


~^Z     Background ssh.


~#      List forwarded connections.


~&      Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.


~?      Display a list of escape characters.


~B      Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it).


~C      Open command line.  Currently this allows the addition of port forwardings using the -L, -R and -D options (see above).  It also allows the cancellation of existing remote port-forwarding’s  using –KR [bind_address:]port.  !command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config.


~R      Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it).



Forwarding of arbitrary TCP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another is going through firewalls.

In the example below, we look at encrypting communication between an IRC client and server, even though the IRC server does not directly support encrypted communications. This works as follows: the user connects to the remote host using ssh, specifying a port to be used to forward connections to the remote server.

After that it is possible to start the service which is to be encrypted on the client machine, connecting to the same local port, and ssh will encrypt and forward the connection. The following example tunnels an IRC session from client machine â127.0.0.1âserver.example.comâ


$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10

$ irc -c  #users -p 1234 pinky

This tunnels a connection to IRC server server.example.com #users pinky

Which port is used, as long as it’s greater than 1023 (remember, only root can open sockets on privileged ports) and doesn’t conflict with any ports already in use. The connection is forwarded to port 6667 on the remote server, since that’s the standard port for IRC services.

The -f option backgrounds ssh and the remote command sleep 10

service which is to be tunneled.  If no connections are made within the time specified, ssh will exit.


If the ForwardX11 variable is set to yes-X, -x, and -Y options above) and the user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY.  Forwarding of X11 connections can be configured on the command line or in configuration files.

The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is normal, and happens because ssh creates a proxy

will also automatically set up Xauthority data on the server machine.  For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection

is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).

If the ForwardAgent variable is set to yes-A and -a options above) and the user is using an authentication agent, the connection to the agent is automatically forwarded to the remote side.




When connecting to a server for the first time, a fingerprint of the servers public key is presented to the user (unless the option StrictHostKeyChecking has been disabled).  Fingerprints can be determined using ssh-keygen(1):

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

If the fingerprint is already known, it can be matched and the key can be accepted or rejected.  Because of the difficulty of comparing host keys just by looking at hex strings, there is also support to compare host keys visually, using random art.  By setting the VisualHostKey option to

yes pattern a known server produces, a user can easily find out that the host key has changed when a completely different pattern is displayed.

Because these patterns are not unambiguous however, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed proof.

To get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:

$ ssh-keygen -lv -f ~/.ssh/known_hosts

 If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS.An additional resource record (RR), SSHFP, is added to a zonefile and the connecting client is able to match the fingerprint with that of the key presented.

In this example, we are connecting a client to a server, host.example.com host.example.com:

$ ssh-keygen -r host.example.com.

The output lines will have to be added to the zonefile.  To check that the zone is answering fingerprint queries: $ dig -t SSHFP host.example.com

Finally the client connects:

$ ssh -o “VerifyHostKeyDNS ask” host.example.com


Matching host key fingerprint found in DNS.

Are you sure you want to continue connecting (yes/no)?



ssh contains support for Virtual Private Network (VPN) tunneling using the tun network pseudo-device, allowing two networks to be joined securely. The sshd_config configuration option PermitTunnel controls whether the server supports this, and at what level (layer 2 or 3 traffic).

The following example would connect client network with remote network using a point-to-point connection from to, provided that the SSH server running on the gateway to the remote network, at, allows it.

On the client:

# ssh -f -w 0:1 true

# ifconfig tun0 netmask

# route add

On the server:

# ifconfig tun1 netmask

# route add


Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. The following entry would permit connections on tun(4) device 1 from user jane john PermitRootLogin is set to forced-commands-only

tunnel=”1″,command=”sh /etc/netstart tun1″ ssh-rsa … jane

tunnel=”2″,command=”sh /etc/netstart tun2″ ssh-rsa … john



ssh will normally set the following environment variables:

DISPLAY               The DISPLAY variable indicates the location of the X11 server.  It is automatically set by ssh to point to a value of the form hostname:n hostname ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).

HOME                  Set to the path of the userâs home directory.

LOGNAME          Synonym for USER; set for compatibility with systems that use this variable.

MAIL                  Set to the path of the userâs mailbox.

PATH                  Set to the default PATH, as specified when compiling ssh.

SSH_ASKPASS           If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal.If ssh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase.This is particularly useful when calling ssh from a .xsession or related script. 

Note that on some machines it may be necessary to redirect the input from /dev/null to make this work.

SSH_AUTH_SOCK                          Identifies the path of a UNIX-domain socket used to communicate with the agent.

SSH_CONNECTION                       Identifies the client and server ends of the connection.  The variable contains four space-separated values: client IP address, client port number, server IP address, and server port number.

SSH_ORIGINAL_COMMAND     This variable contains the original command line if a forced command is executed.  It can be used to extract the original arguments.

SSH_TTY                                          This is set to the name of the tty (path to the device) associated with the current shell or command.  If the current session has no tty, this variable is not set.

TZ                                                       This variable is set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections).

USER                                                 Set to the name of the user logging in.

Additionally, ssh reads ~/.ssh/environment, and adds lines of the format VARNAME=value

allowed to change their environment. 



This file is used for host-based authentication (see above).  On some machines this file may need to be world-readable if the userâs home directory is on an NFS partition, because sshd reads it as root. 

Additionally, this file must be owned by the user, and must not have write permissions for anyone else.  The recommended permission for most machines is read/write for the user, and not accessible by others.


This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.


This directory is the default location for all user-specific configuration and authentication information. There is no general requirement to keep the entire contents of this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others.


Lists the public keys (RSA/DSA) that can be used for logging in as this user. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.


This is the per-user configuration file.  The file format and configuration options are described in ssh_config.  Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not accessible by others.


Contains additional definitions for environment variables; see ENVIRONMENT, above.




Contains the private key for authentication.  These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute).ssh will simply ignore a private key file if it is accessible by others.  It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using 3DES.




Contains the public key for authentication.  These files are not sensitive and can (but need not) be readable by anyone.


Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. 


Commands in this file are executed by ssh when the user logs in, just before the userâs shell (or command) is started. 


This file is for host-based authentication (see above).  It should only be writable by root.


This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.


Systemwide configuration file. 




These three files contain the private parts of the host keys and are used for host-based authentication.  If protocol version 1 is used,ssh must be setuid root, since the host key is readable only by root.For protocol version 2, ssh uses ssh-keysign to access the host keys, eliminating the requirement that ssh be setuid root when host-based authentication is used.By default ssh is not setuid root.


System wide list of known host keys.  This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It should be world-readable. 



About Manish Jha

Hi All I am Manish Kumar Jha aka Alex Hunt. I am currently working in Iono,Inc as Cloud Solutions Architect. I have around 8 Years of IT experience and have exposure on VMware vSphere, vCloud Director, RHEL and modern data center technologies like Cisco UCS and Cisco Nexus 1000v and NSX. If you find any post informational to you please press like and share it across social media and leave your comments if you want to discuss further on any post. Disclaimer: All the information on this website is published in good faith and for general information purpose only. I don’t make any warranties about the completeness, reliability and accuracy of this information. Any action you take upon the information you find on this blog is strictly at your own risk. The Views and opinions published on this blog are my own and not the opinions of my employer or any of the vendors of the product discussed.
This entry was posted in Linux/CentOS. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s