OpenSSH Config File Examples

by on October 4, 2013 · 1 comment· LAST UPDATED October 4, 2013

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How do I create and setup an OpenSSH config file to create shortcuts for servers I frequently access under Linux or Unix desktop operating systems?

A global or local configuration file for SSH client can create shortcuts for sshd server including advanced ssh client options.
Tutorial details
DifficultyIntermediate (rss)
Root privilegesYes
RequirementsOpenSSH client
Estimated completion time15m
You can configure your OpenSSH ssh client using various files as follows to save time and typing frequently used ssh client command line options such as port, user, hostname, identity-file and much more:

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 nixcraft@server1.cyberciti.biz
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 nixcraft@gateway.uk.cyberciti.biz 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 nixcraft@server1.cyberciti.biz"
 

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

References
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{ 1 comment… read it below or add one }

1 scott carlson October 13, 2013 at 8:30 pm

Nice examples…. here is what mine starts with, because I use the control master all the time. Which you can start in the background with “ssh -MNf host”.

I’ve found that attempting GSSApiAuth slows everything down, so I turn that off, and I’ve picked the order of the ciphers to be faster as well.

Also with newer versions of ssh, you don’t need netcat anymore for proxying, you can use this line in the host definition instead: “ProxyCommand ssh -W %h:%p”

Last comment, is that I recommend, using multiple aliases for the host, and include all possible ways you’ll refer to the box. So that if you cut and paste a name, you’ll still get the same settings. As an example, if your DNS search path has company.com, then you might do this: “host web webserver.priv.city webserver.priv.city.company.com ”

# ssh -Mnf starbuck sleep 30d
Host *
ControlPath ~/.ssh/%l-master-%r@%h:%p
ControlMaster auto
ServerAliveInterval 60
GSSAPIAuthentication no
Ciphers arcfour256,arcfour128,arcfour,blowfish-cbc,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour
ForwardAgent yes
LogLevel quiet

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