Interpretation algorithms and configuration principles for. ssh/config Files

Objective

SSH is the most common way to connect to a remote host, although the basic operation of connecting to a delayed host is straightforward, but when you start using a large number of remote systems, this becomes cumbersome and complex task.

Fortunately, OpenSSH allows you to provide custom client connection options. These options can be stored in a configuration file that can be used to define the configuration of each host. This helps keep the connection options for each host better independent and organized, and also lets you avoid writing cumbersome options on the command line when you need to connect.

In this article, we will cover the basics of the SSH client configuration file, understand how SSH interprets the configuration file, but will not describe the meaning and use of the SSH specific options.

The structure and interpretation algorithm of SSH file

Each user of the local system can maintain a client's SSH profile, which can include parameters that you use at the command line for SSH, or store common connection options and automatically process when connected. You can use SSH on the command to specify flag to override the options in the configuration file.

Location of the SSH client configuration file

The file name of the configuration file is config located under the. SSH folder in the user's home directory.

~/.ssh/config

Typically, the file is not created by default, so you might want to create it yourself.

Structure of the configuration file

The configuration files are organized by host, and each host definition item defines the connection options for the matching host. Wildcards can be used, in order to have a larger range of options.

The configuration file looks like this:

Host firsthost    SSH_OPTIONS_1 custom_value    SSH_OPTIONS_2 custom_value    SSH_OPTIONS_3 custom_valueHost secondhost    ANOTHER_OPTION custom_valueHost *host    ANOTHER_OPTION custom_valueHost *    CHANGE_DEFAULT custom_value

Interpreting algorithms

Only by understanding how SSH interprets the configuration file can you write a reasonable configuration file.

The SSH mission makes the hostname given in the row match the host name defined in the configuration file. It does this from the top of the file, so the order is important.

It is now a good time to point out that the schema in the host definition does not have to match the actual host you are connecting to. In fact, you can use these definitions to set aliases for hosts in place of the actual host names.

Look at an example:

Host dev1    HostName dev1.example.com    User tom

Now to connect to it [email protected] , you can enter the following command at the command line:

ssh dev1

With this in mind, we will now continue to discuss how SSH applies each configuration selected in the top down process. It starts at the top and checks to see if each host definition matches the one given on the command line. In the previous example, check the DEV1.

When the first matching host definition is found, each associated SSH option is applied to the incoming connection (for the sake of discussion below, we call this connection " connection a"). Nonetheless, the explanation is not over.

SSH continues to look down in the file to check for other matching Host definitions. If there is another host definition match, SSH will consider the configuration options under that host definition. If the new configuration option has an option that is not used when connecting a , add these options to connection a .

To summarize, SSH interprets each host definition that matches the hostname given on the command line in order. In this process, SSH always uses the first value given for each option. There is no way to overwrite the value given by the Host definition that was previously matched.

So it seems that a simple principle of writing a configuration file is. The more granular the Host definition, the more it is written on top. The generic Host definition should be written down below.

Look at an example:

Host firsthost    SSH_OPTIONS_1 custom_value1    SSH_OPTIONS_2 custom_value2    SSH_OPTIONS_3 custom_value3Host secondhost    ANOTHER_OPTION custom_value4Host *host    ANOTHER_OPTION custom_value5Host *    CHANGE_DEFAULT custom_value6

If so, the options used by SSH are,,,,,,,,,, and ssh firsthost SSH_OPTIONS_1 SSH_OPTIONS_2 SSH_OPTIONS_3 ANOTHER_OPTION CHANGE_DEFAULT custom_value1 custom_value2 custom_value3 custom_value5 custom_value6 .

If ssh secondhost so, the options used by SSH are, and the values used are, ANOTHER_OPTION CHANGE_DEFAULT respectively custom_value4 custom_value6 .

Basic connection Options

This section is directly explained by an example: To connect a user named Apollo to the name example.com, the host runs the SSH daemon on port 4567.

In general, we will write at the command line:

ssh -p 4567 [email protected]

If you use the full name of the option, this is the case:

ssh -o "User=apollo" -o "Port=4567" -o "HostName=example.com"

We can put the option that starts with uppercase in the second way into the configuration file. A man ssh_config complete list of available options can be obtained.

Now, use the configuration file to write the above example:

Host home    HostName example.com    User apollo    Port 4567

For each option, this is also possible (in port, for example):

Port = 4567Port=4567

Configure options for sharing

Continuing on to the last part of the example, I have two hosts, separate home and work, on both hosts, my user name is Apollo. Simply, modify the configuration file directly:

Host home    HostName example.com    User apollo    Port 4567Host work    HostName company.com    User apollo

There's no problem with writing this, but we can put the User up:

Host home    HostName example.com    Port 4567Host work    HostName company.comHost *    User apollo

Very good, but the new problem comes, you have a good friend Mars, there is a host sharing for you, but he likes to read your name backwards, then what to do?

Host home    HostName example.com    Port 4567Host work    HostName company.comHost friend-Mars    HostName abc.com    User ollopaHost *    User apollo

With the passage of time, more and more friends, they have a host for you to use, and all like to put your name upside down, what to do?

Host home    HostName example.com    Port 4567Host work    HostName company.comHost friend-Mars    HostName mars.comHost friend-Mercury    HostName mercury.comHost friend-Jupiter    HostName jupiter.comHost friend-*    User ollopaHost *    User apollo

The original text is the Configure Custom Connection Options for your SSH Client, this article on the basis of a slight modification.

Article Link: https://www.cnblogs.com/xjshi/p/9146296.html

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Interpretation algorithms and configuration principles for. ssh/config Files