System-wide SSH client configuration files

1. /etc/ssh/ssh_config : This files set the default configuration for all users of OpenSSH clients on that desktop/laptop and it must be readable by all users on the system.

User-specific SSH client configuration files

1. ~/.ssh/config or $HOME/.ssh/config : This is user’s own configuration file which, overrides the settings in the global client configuration file, /etc/ssh/ssh_config.

~/.ssh/config file rules

The rules are as follows to create an ssh config file:
• You need to edit ~/.ssh/config with a text editor such as vi.
• One config parameter per line is allowed in the configuration file with the parameter name followed by its value or values. The syntax is:

  config value
  config1 value1 value2

• You can use an equal sign (=) instead of whitespace between the parameter name and the values.

  config=value
  config1=value1 value2

• All empty lines are ignored.
• All lines starting with the hash (#) are ignored.
• All values are case-sensitive, but parameter names are not.

Tip : If this is a brand new Linux, Apple OS X/Unix box, or if you have never used ssh before create the ~/.ssh/ directory first using the following syntax:
mkdir -p $HOME/.ssh
chmod 0700 $HOME/.ssh

Examples

For demonstration purpose my sample setup is as follows:

1. Local desktop client – Apple OS X or Ubuntu Linux.
2. Remote Unix server – OpenBSD server running latest OpenSSH server.
3. Remote OpenSSH server ip/host: 75.126.153.206 (server1.cyberciti.biz)
4. Remote OpenSSH server user: nixcraft
5. Remote OpenSSH port: 4242
6. Local ssh private key file path : /nfs/shared/users/nixcraft/keys/server1/id_rsa

Based upon the above information my ssh command is as follows:
$ ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 [email protected]
OR
$ ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 -l nixcraft server1.cyberciti.biz

You can avoid typing all of the ssh command parameters while logging into a remote machine and/or for executing commands on a remote machine. All you have to do is create an ssh config file. Open the Terminal application and create your config file by typing the following command:

vi ~/.ssh/config

OR

vi $HOME/.ssh/config

Add/Append the following config option for a shortcut to server1 as per our sample setup:

Host server1
     HostName server1.cyberciti.biz
     User nixcraft
     Port 4242
     IdentityFile /nfs/shared/users/nixcraft/keys/server1/id_rsa

Save and close the file. To open your new SSH session to server1.cyberciti.biz by typing the following command:
$ ssh server1

Adding another host

Append the following to your ~/.ssh/config file:

Host nas01
     HostName 192.168.1.100
     User root
     IdentityFile ~/.ssh/nas01.key

You can simply type:
$ ssh nas01

Putting it all together

Here is my sample ~/.ssh/config file that explains and create, design, and evaluate different needs for remote access using ssh client:

### default for all ##
Host *
     ForwardAgent no
     ForwardX11 no
     ForwardX11Trusted yes
     User nixcraft
     Port 22
     Protocol 2
     ServerAliveInterval 60
     ServerAliveCountMax 30
 
## override as per host ##
Host server1
     HostName server1.cyberciti.biz
     User nixcraft
     Port 4242
     IdentityFile /nfs/shared/users/nixcraft/keys/server1/id_rsa
 
## Home nas server ##
Host nas01
     HostName 192.168.1.100
     User root
     IdentityFile ~/.ssh/nas01.key
 
## Login AWS Cloud ##
Host aws.apache
     HostName 1.2.3.4
     User wwwdata
     IdentityFile ~/.ssh/aws.apache.key
 
## Login to internal lan server at 192.168.0.251 via our public uk office ssh based gateway using ##
## $ ssh uk.gw.lan ##
Host uk.gw.lan uk.lan
     HostName 192.168.0.251
     User nixcraft
     ProxyCommand  ssh [email protected] nc %h %p 2> /dev/null
 
## Our Us Proxy Server ##
## Forward all local port 3128 traffic to port 3128 on the remote vps1.cyberciti.biz server ## 
## $ ssh -f -N  proxyus ##
Host proxyus
    HostName vps1.cyberciti.biz
    User breakfree
    IdentityFile ~/.ssh/vps1.cyberciti.biz.key
    LocalForward 3128 127.0.0.1:3128

Understanding ~/.ssh/config entries

• Host : Defines for which host or hosts the configuration section applies. The section ends with a new Host section or the end of the file. A single * as a pattern can be used to provide global defaults for all hosts.
• HostName : Specifies the real host name to log into. Numeric IP addresses are also permitted.
• User : Defines the username for the SSH connection.
• IdentityFile : Specifies a file from which the user’s DSA, ECDSA or DSA authentication identity is read. The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and ~/.ssh/id_rsa for protocol version 2.
• ProxyCommand : Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed with the user’s shell. In the command string, any occurrence of %h will be substituted by the host name to connect, %p by the port, and %r by the remote user name. The command can be basically anything, and should read from its standard input and write to its standard output. This directive is useful in conjunction with nc(1) and its proxy support. For example, the following directive would connect via an HTTP proxy at 192.1.0.253:
  ProxyCommand /usr/bin/nc -X connect -x 192.1.0.253:3128 %h %p
• LocalForward : Specifies that a TCP port on the local machine be forwarded over the secure channel to the specified host and port from the remote machine. The first argument must be [bind_address:]port and the second argument must be host:hostport.
• Port : Specifies the port number to connect on the remote host.
• Protocol : Specifies the protocol versions ssh(1) should support in order of preference. The possible values are 1 and 2.
• ServerAliveInterval : Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server. See blogpost “Open SSH Server connection drops out after few or N minutes of inactivity” for more information.
• ServerAliveCountMax : Sets the number of server alive messages which may be sent without ssh(1) receiving any messages back from the server. If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminating the session.

A note about shell aliases (outdated method)

WARNING! This bash shell aliased based setup may work out for you. However, I recommend that you use ~/.ssh/config file for better results in a long run. SSH config file is more advanced and elegant solutions. The alias command only used here for demo purpose and it is here due to historical reasons.

An alias is nothing but shortcut to commands and you can create the alias use the following syntax in your ~/.bashrc file:

alias server1="ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 [email protected]"

Then, to ssh into the server1, instead of typing full ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 [email protected] command, you would only have to type the command ‘server1’ and press the [ENTER] key:
$ server1

References

• See ssh_config(5) for more information on syntax and some of the other available options.
• Top 20 OpenSSH Server Best Security Practices