![]() ![]() This last point is extremely important to internalize. It will then apply any SSH options defined for the new section that have not already been defined by previous sections. If another definition is found that matches the current hostname given on the command line, it will consider the SSH options associated with the new section. SSH then moves down the file, checking to see if other Host definitions also match. The interpretation does not end here though. When the first matching Host definition is found, each of the associated SSH options are applied to the upcoming connection. It starts at the top and checks each Host definition to see if it matches the value given on the command line. With this in mind, we can now discuss the way in which SSH applies each configuration option as it moves down the file. This host allows us to connect as by typing this on the command line: ssh devel You can essentially use these definitions to set up aliases for hosts that can be used in lieu of the actual host name.įor example, consider this definition: Host devel This is a good time to point out that the patterns in the Host definition do not have to match the actual host that you will be connecting with. It will do this from the top of the file downwards, so order is incredibly important. SSH will match the hostname given on the command line with each of the Host headers that define configuration sections. This has large implications when using wildcards and the Host * generic host definition. It is very important to understand the way that SSH will interpret the file to apply the configuration values defined within. Here, we have four sections that will be applied on each connection attempt depending on whether the host in question matches. Host secondhost ANOTHER_OPTION custom_value The general format will look something like this: Host firsthost SSH_OPTION_1 custom_value SSH_OPTION_2 custom_value SSH_OPTION_3 custom_value This is not a hard requirement, but a useful convention that allows for easier interpretation at a glance. Typically, for organizational purposes and readability, the options being set for each host are indented. A section is defined from the Host header to the following Host header. Only items that differ from the default values need to be specified, as the host will inherit the defaults for any undefined items. The specific configuration items for that matching host are then defined below. Wildcards are also available to allow for options that should have a broader scope.Įach of the sections starts with a header defining the hosts that should match the configuration options that will follow. Each host definition can define connection options for the specific matching host. Often, this file is not created by default, so you may need to create it yourself: touch ~/.ssh/config The client-side configuration file is called config and it is located in your user’s home directory within the. The Location of the SSH Client Config File It is always possible to override the values defined in the configuration file at the time of the connection through normal flags to the ssh command. These can contain any options that you would use on the command line to specify connection parameters, allowing you to store your common connection items and process them automatically on connection. The SSH Config File Structure and Interpretation AlgorithmĮach user on your local system can maintain a client-side SSH configuration file. You may also wish to configure SSH key-based authentication for some of your users or hosts, at the very least for testing purposes. To complete this guide, you will need a working knowledge of SSH and some of the options that you can provide when connecting. In this guide, we’ll cover the basics of the SSH client configuration file, and go over some common options. This can help keep the different connection options you use for each host separated and organized, and can keep you from having to provide extensive options on the command line whenever you need to connect. These can be saved to a configuration file that can be used to define per-host values. Although the basics of connecting to a single host are often rather straight forward, this can become unwieldy and a much more complicated task when you begin working with a large number of remote systems.įortunately, OpenSSH allows you to provide customized client-side connection options. SSH, or secure shell, is the most common way of connecting to Linux hosts for remote administration.
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