We can set up a global or local configuration file for SSH clients can create shortcuts for sshd servers, including advanced ssh client options.
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You can configure your OpenSSH ssh client to save typing time for frequently used ssh client command-line options such as port number, user name, hostname/IP address, identity file, and much more. In addition to that it will increase your productivity from Linux/macOS or Unix desktop.
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 and lines starting with the hash (#) are ignored are ignored.
- Please note that all values are case-sensitive, but parameter names are not.
Tip : If this is a brand new Linux, macOS/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 macOS/OS X/Ubuntu Linux.
- Remote Unix server – OpenBSD server running latest OpenSSH server.
- OpenSSH remote server ip/host: 126.96.36.199 (server1.cyberciti.biz)
- Remote OpenSSH server user: nixcraft
- OpenSSH dest 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 email@example.com
$ ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 -l nixcraft server1.cyberciti.biz
See how much I need to type. I need to remember the remote hostname/IP, port number, the path to ssh key, username, etc. Too much typing and is not increasing my productivity. But fear not, there is an easy way out.
Using the ssh config file
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
Understanding Host Patterns
A pattern for Host directive is nothing but IP address, DNS hostname, or combination of special wildcard characters. For example, ? wildcard that matches exactly one character. On the other hand, * wildcard matches zero or more characters. It allows us to define the usage pattern. For instance, to specify and allow login from laptop.sweet.home, desktop.sweet.home, rpi.sweet.home, and corerouter.sweet.home, I could use the following pattern:
Host *.sweet.home Hostname 192.168.2.17 User vivek IdentityFile ~/.ssh/id_ed25519.pub
The following pattern would match any host in the 192.168.2.[0-9] network range:
Host 192.168.2.? Hostname 192.168.2.18 User admin IdentityFile ~/.ssh/id_ed25519.pub
We can also set a pattern list. It is a comma-separated list of patterns. Patterns within pattern lists may be negated by preceding them with an exclamation mark (!) in your authorized_keys. Here is an example from ~/.ssh/authorized_keys file on the remote server. First, login to the remote box:
$ ssh firstname.lastname@example.org
Now edit the file, run:
$ vim ~/.ssh/authorized_keys
Update it as follows:
# Allow login from 192.168.2.0/24 subnet but not from 192.168.2.25 from="!192.168.2.25,192.168.2.*" ssh-ed25519 my_random_pub_key_here vivek@nixcraft # Allow login from *.sweet.home but not from router.sweet.home from="!router.sweet.home,*.sweet.home" ssh-ed25519 my_random_pub_key_here vivek@nixcraft
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 188.8.131.52 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 184.108.40.206:
ProxyCommand /usr/bin/nc -X connect -x 220.127.116.11:3128 %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
How to override ssh config file option
The ssh command reads its configuration in the following order:
- ssh command line-option
- ~/.ssh/config option
- /etc/ssh/ssh_config options
Say you have the following options set in ~/.ssh/config:
Host ln.openvpn-sg-vpn1 ln.wireguard-sg-vpn1 Hostname 172.16.0.1 User vivek port 22 IdentityFile ~/.ssh/id_ed25519.pub StrictHostKeyChecking no
Now want to use all other options from ~/.ssh/config but to connect using admin user instead of vivek, then:
$ ssh -o "User=admin" ln.openvpn-sg-vpn1
We can specifies an alternative per-user configuration file such as /dev/null to disable ~/.ssh/config too by passing the -F:
$ ssh -F /dev/null email@example.com
$ ssh -F /dev/null firstname.lastname@example.org
$ ssh -F /dev/null -i ~/.ssh/aws/id_ed25519.pub email@example.com
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.
## create a new bash shell alias as follow ## alias server1="ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 firstname.lastname@example.org"
Then, to ssh into the server1, instead of typing full ssh -i /nfs/shared/users/nixcraft/keys/server1/id_rsa -p 4242 email@example.com command, you would only have to type the command ‘server1’ and press the [ENTER] key:
This page explained the ssh client configuration file syntax and examples to increase your productivity at Linux, macOS, or Unix shell. See the following resources:
- See ssh_config man page
- Top OpenSSH Server Best Security Practices
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