A global or local configuration file for SSH client can create shortcuts for sshd server including advanced ssh client options.
Let use see some common OpenSSH config file examples.
System-wide OpenSSH config file client configuration
- /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 OpenSSH file client configuration
- ~/.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
For demonstration purpose my sample setup is as follows:
- Local desktop client – Apple OS X or Ubuntu Linux.
- Remote Unix server – OpenBSD server running latest OpenSSH server.
- Remote OpenSSH server ip/host: 184.108.40.206 (server1.cyberciti.biz)
- Remote OpenSSH server user: nixcraft
- Remote OpenSSH port: 4242
- 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 firstname.lastname@example.org
$ 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:
## edit file in $HOME dir vi ~/.ssh/config
## edit file in $HOME dir 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 in vi/vim by pressing Esc key, type :w and hit Enter key. 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 220.127.116.11 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@example.com 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 18.104.22.168:
ProxyCommand /usr/bin/nc -X connect -x 22.214.171.124:3128 %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.
Speed up ssh session
Multiplexing is nothing but send more than one ssh connection over a single connection. OpenSSH can reuse an existing TCP connection for multiple concurrent SSH sessions. This results into reduction of the overhead of creating new TCP connections. Update your ~/.ssh/config:
Host server1 HostName server1.cyberciti.biz ControlPath ~/.ssh/controlmasters/%r@%h:%p ControlMaster auto
See “Linux / Unix: OpenSSH Multiplexer To Speed Up OpenSSH Connections” for more info. In this example, I go through one host to reach another server i.e. jump host using ProxyCommand:
## ~/.ssh/config ## Host internal HostName 192.168.1.100 User vivek ProxyCommand ssh firstname.lastname@example.org -W %h:%p ControlPath ~/.ssh/controlmasters/%r@%h:%p ControlMaster auto
For more info see following tutorials:
- How To Reuse SSH Connection To Speed Up Remote Login Process Using Multiplexing
- How To Setup SSH Keys on a Linux / Unix System
A note about shell aliases (outdated method)
## create a new bash shell alias as follow ## alias server1="ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 email@example.com"
Then, to ssh into the server1, instead of typing full ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 firstname.lastname@example.org command, you would only have to type the command ‘server1’ and press the [ENTER] key:
- See ssh_config man page for more information on syntax and some of the other available options.
- Top 20 OpenSSH Server Best Security Practices
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