In Unix, ssh-agent is a background program that handles passwords for SSH private keys. The ssh-add command prompts the user for a private key password and a. Just use ssh-agent on your machine to. Enter the passphrase for your ssh private key once. All subsequent calls that need your ssh key will use the saved.
The Visual Studio Code Remote - SSH extension allows you to open a remote folder on any remote machine, virtual machine, or container with a running SSH server and take full advantage of VS Code's feature set. Once connected to a server, you can interact with files and folders anywhere on the remote filesystem.
Ssh User Agent
No source code needs to be on your local machine to gain these benefits since the extension runs commands and other extensions directly on the remote machine.
This lets VS Code provide a local-quality development experience — including full IntelliSense (completions), code navigation, and debugging — regardless of where your code is hosted.
Getting started
Note: After reviewing this topic, you can get started with the introductory SSH tutorial.
System requirements
Local: A supported OpenSSH compatible SSH client must also be installed.
Remote SSH host: A running SSH server on:
Ssh Use Agent Forwarding
- x86_64 Debian 8+, Ubuntu 16.04+, CentOS / RHEL 7+.
- ARMv7l (AArch32) Raspbian Stretch/9+ (32-bit).
- ARMv8l (AArch64) Ubuntu 18.04+ (64-bit).
- Windows 10 / Server 2016/2019 (1803+) using the official OpenSSH Server.
- macOS 10.14+ (Mojave) SSH hosts with Remote Login enabled.
Other glibc based Linux distributions for x86_64, ARMv7l (AArch32), and ARMv8l (AArch64) should work if they have the needed prerequisites. See the Remote Development with Linux article for information prerequisites and tips for getting community supported distributions up and running.
While ARMv7l (AArch32) and ARMv8l (AArch64) support is available, some extensions installed on these devices may not work due to the use of x86 native code in the extension.
Installation
To get started, you need to:
Install an OpenSSH compatible SSH client if one is not already present.
Install Visual Studio Code or Visual Studio Code Insiders.
Install the Remote Development extension pack.
SSH host setup
If you do not have an SSH host set up, follow the directions for Linux, Windows 10 / Server (1803+), or macOS SSH host or create a VM on Azure.
[Optional] If your Linux or macOS SSH host will be accessed by multiple users at the same time, consider enabling Remote.SSH: Remote Server Listen On Socket in VS Code User settings for improved security.
In the Settings editor:
See the Tips and Tricks article for details.
[Optional] While password-based authentication is supported, we recommend setting up key based authentication for your host. See the Tips and Tricks article for details.
Connect to a remote host
To connect to a remote host for the first time, follow these steps:
Verify you can connect to the SSH host by running the following command from a terminal / PowerShell window replacing
user@hostnameas appropriate.In VS Code, select Remote-SSH: Connect to Host... from the Command Palette (F1) and use the same
user@hostnameas in step 1.If VS Code cannot automatically detect the type of server you are connecting to, you will be asked to select the type manually.
Once you select a platform, it will be stored in VS Code settings under the
remote.SSH.remotePlatformproperty so you can change it at any time.After a moment, VS Code will connect to the SSH server and set itself up. VS Code will keep you up-to-date using a progress notification and you can see a detailed log in the
Remote - SSHoutput channel.Tip: Connection hanging or failing? See troubleshooting tips for information on resolving common problems.
If you see errors about SSH file permissions, see the section on Fixing SSH file permission errors.
After you are connected, you'll be in an empty window. You can always refer to the Status bar to see which host you are connected to.
Clicking on the Status bar item will provide a list of remote commands while you are connected.
You can then open any folder or workspace on the remote machine using File > Open... or File > Open Workspace... just as you would locally!
From here, install any extensions you want to use when connected to the host and start editing!
Note: On ARMv7l / ARMv8l glibc SSH hosts, some extensions may not work due to x86 compiled native code inside the extension.
Disconnect from a remote host
To close the connection when you finish editing files on the remote host, choose File > Close Remote Connection to disconnect from the host. The default configuration does not include a keyboard shortcut for this command. You can also simply exit VS Code to close the remote connection.
Remember hosts and advanced settings
If you have a set of hosts you use frequently or you need to connect to a host using some additional options, you can add them to a local file that follows the SSH config file format.
To make setup easy, the extension can guide you through adding a host without having to hand edit this file.
Start by selecting Remote-SSH: Add New SSH Host... from the Command Palette (F1) or clicking on the Add New icon in the SSH Remote Explorer in the Activity Bar.
You'll then be asked to enter the SSH connection information. You can either enter a host name:
Or the full ssh command you would use to connect to the host from the command line:
Finally, you'll be asked to pick a config file to use. You can also set the 'remote.SSH.configFile' property in your User settings.json file if you want to use a different config file than those listed. The extension takes care of the rest!
For example, entering ssh -i ~/.ssh/id_rsa-remote-ssh yourname@remotehost.yourcompany.com in the input box would generate this entry:
See Tips and Tricks for details on generating the key shown here. You can manually edit this file with anything the SSH config file format supports, so this is just one example.
From this point forward, the host will appear in the list of hosts when you select Remote-SSH: Connect to Host... from the Command Palette (F1) or in the SSH Targets section of the Remote Explorer.
The Remote Explorer allows you to both open a new empty window on the remote host or directly open a folder you previously opened. Expand the host and click on the Open Folder icon next to the folder you want to open on the host.
Managing extensions
VS Code runs extensions in one of two places: locally on the UI / client side, or remotely on the SSH host. While extensions that affect the VS Code UI, like themes and snippets, are installed locally, most extensions will reside on the SSH host. This ensures you have smooth experience and allows you to install any needed extensions for a given workspace on an SSH host from your local machine. This way, you can pick up exactly where you left off, from a different machine complete with your extensions.
If you install an extension from the Extensions view, it will automatically be installed in the correct location. Once installed, you can tell where an extension is installed based on the category grouping.
There will be a category for your remote SSH host:
And also a Local - Installed category:
Note: If you are an extension author and find that your extension is not working properly or installs in the wrong place, see Supporting Remote Development for details.
Local extensions that actually need to run remotely will appear dimmed and disabled in the Local - Installed category. Select Install to install an extension on your remote host.
You can also install all locally installed extensions on the SSH host by going to the Extensions view and selecting Install Local Extensions in SSH: [Hostname] using the cloud button at the right of the Local - Installed title bar. This will display a dropdown where you can select which locally installed extensions to install on your SSH host.
'Always installed' extensions
If there are extensions that you would like to always have installed on any SSH host, you can specify which ones using the remote.SSH.defaultExtensions property in settings.json. For example, if you wanted to install the GitLens and Resource Monitor extensions, specify their extension IDs as follows:
Advanced: Forcing an extension to run locally / remotely
Extensions are typically designed and tested to either run locally or remotely, not both. However, if an extension supports it, you can force it to run in a particular location in your settings.json file.
For example, the setting below will force the Docker extension to run locally and Debugger for Chrome extension to run remotely instead of their defaults:
A value of 'ui' instead of 'workspace' will force the extension to run on the local UI/client side instead. Typically, this should only be used for testing unless otherwise noted in the extension's documentation since it can break extensions. See the article on Supporting Remote Development for details.
Forwarding a port / creating SSH tunnel
Sometimes when developing, you may need to access a port on a remote machine that is not publicly exposed. There are two ways to do this using an SSH tunnel that 'forwards' the desired remote port to your local machine.
Temporarily forwarding a port
Once you are connected to a host, if you want to temporarily forward a new port for the duration of the session, select Forward a Port from the Command Palette (F1) or click on the Forward New Port icon in the Remote Explorer after selecting it from the Activity Bar.
You'll be asked to enter the port you would like to forward and you can give it a name.
A notification will tell you the localhost port you should use to access the remote port. For example, if you forwarded an HTTP server listening on port 3000, the notification may tell you that it was mapped to port 4123 on localhost since 3000 was already in use. You can then connect to this remote HTTP server using http://localhost:4123.
This same information is available in the Forwarded Ports section of the Remote Explorer if you need to access it later.
If you would like VS Code to remember any ports you have forwarded, check Remote: Restore Forwarded Ports in the Settings editor (⌘, (Windows, Linux Ctrl+,)) or set 'remote.restoreForwardedPorts': true in settings.json.
Change local port on tunnel
If you would like the local port of the tunnel to be different than the remote server's, you can change this via the Forwarded Ports panel.
Right-click the tunnel you want to modify, and select Change Local Port in the context menu.
Always forwarding a port
If you have ports that you always want to forward, you can use the LocalForward directive in the same SSH config file you use to remember hosts and advanced settings.
For example, if you wanted to forward ports 3000 and 27017, you could update the file as follows:
Opening a terminal on a remote host
Opening a terminal on the remote host from VS Code is simple. Once connected, any terminal window you open in VS Code (Terminal > New Terminal) will automatically run on the remote host rather than locally.
You can also use the code command line from this same terminal window to perform a number of operations such as opening a new file or folder on the remote host. Type code --help to see all the options available from the command line.
Debugging on the SSH host
Once you are connected to a remote host, you can use VS Code's debugger in the same way you would when running the application locally. For example, if you select a launch configuration in launch.json and start debugging (F5), the application will start on remote host and attach the debugger to it.
See the debugging documentation for details on configuring VS Code's debugging features in .vscode/launch.json.
SSH host-specific settings
VS Code's local User settings are also reused when you are connected to an SSH host. While this keeps your user experience consistent, you may want to vary some of these settings between your local machine and each host. Fortunately, once you have connected to a host, you can also set host-specific settings by running the Preferences: Open Remote Settings command from the Command Palette (F1) or by selecting on the Remote tab in the Settings editor. These will override any User settings you have in place whenever you connect to the host. And Workspace settings will override Remote and User settings.
Working with local tools
The Remote - SSH extension does not provide direct support for sync'ing source code or using local tools with content on a remote host. However, there are two ways to do this using common tools that will work with most Linux hosts. Specifically, you can:
- Mount the remote filesystem using SSHFS.
- Sync files to/from the remote host to your local machine using
rsync.
SSHFS is the most convenient option and does not require any file sync'ing. However, performance will be significantly slower than working through VS Code, so it is best used for single file edits and uploading/downloading content. If you need to use an application that bulk reads/write to many files at once (like a local source control tool), rsync is a better choice.
Known limitations
Remote - SSH limitations
- Using key based authentication is strongly recommended. Passwords and other tokens entered for alternate authentication methods are not saved.
- Alpine Linux and non-glibc based Linux SSH hosts are not supported.
- Older (community supported) Linux distributions require workarounds to install the needed prerequisites.
- PuTTY is not supported on Windows.
- If you clone a Git repository using SSH and your SSH key has a passphrase, VS Code's pull and sync features may hang when running remotely. Either use an SSH key without a passphrase, clone using HTTPS, or run
git pushfrom the command line to work around the issue. - Local proxy settings are not reused on the remote host, which can prevent extensions from working unless the appropriate proxy information is configured on the remote host (for example global
HTTP_PROXYorHTTPS_PROXYenvironment variables with the appropriate proxy information). - See here for a list of active issues related to SSH.
Docker Extension limitations
While the Docker extension can run both remotely and locally, if it is already installed locally, you will be unable to install on a remote SSH host without first uninstalling it locally. We will address this problem in a future VS Code release.
Extension limitations
Many extensions will work on remote SSH hosts without modification. However, in some cases, certain features may require changes. If you run into an extension issue, there is a summary of common problems and solutions that you can mention to the extension author when reporting the issue.
In addition, some extensions installed on ARMv7l (AArch322) / ARMv8l (AArch64) devices may not work due to native modules or runtimes in the extension that only support x86_64. In these cases, the extensions would need to opt-in to supporting these platforms by compiling / including binaries for ARMv7l / ARMv8l.
Common questions
How do I set up an SSH client on ...?
See Installing a supported SSH client for details.
How do I set up an SSH server on ...?
See Installing a supported SSH server for details on setting up an SSH server for your host.
Can I sign in to my SSH server with another/additional authentication mechanism like a password?
Yes, you should be prompted to enter your token or password automatically. However, passwords are not saved, so using key based authentication is typically more convenient.
How do I fix SSH errors about 'bad permissions'?
See Fixing SSH file permission errors for details on resolving these types of errors.
What Linux packages / libraries need to be installed on remote SSH hosts?
Most Linux distributions will not require additional dependency installation steps. For SSH, Linux hosts need to have Bash (/bin/bash), tar, and either curl or wget installed and those utilities could be missing from certain stripped down distributions. Remote Development also requires kernel >= 3.10, glibc >=2.17, libstdc++ >= 3.4.18. Only glibc-based distributions are supported currently, so by extension Alpine Linux is not supported.
See Linux Prerequisites for details.
What are the connectivity requirements for the VS Code Server when it is running on a remote machine / VM?
Installation of VS Code Server requires that your local machine has outbound HTTPS (port 443) connectivity to:
update.code.visualstudio.comvscode.blob.core.windows.net*.vo.msecnd.net(Azure CDN)
By default, the Remote - SSH will attempt to download on the remote host, but if you enable remote.SSH.allowLocalServerDownload, the extension will fall back to downloading VS Code Server locally and transferring it remotely once a connection is established.
You can install extensions manually without an internet connection using the Extensions: Install from VSIX... command, but if you use the extension panel to install extensions, your local machine and VS Code Server server will need outbound HTTPS (port 443) access to:
marketplace.visualstudio.com*.vo.msecnd.net(Azure CDN)*.gallerycdn.vsassets.io(Azure CDN)
Finally, some extensions (like C#) download secondary dependencies from download.microsoft.com or download.visualstudio.microsoft.com. Others (like Visual Studio Live Share) may have additional connectivity requirements. Consult the extension's documentation for details if you run into trouble.
All other communication between the server and the VS Code client is accomplished through the authenticated, secure SSH tunnel.
Allow Users Ssh
Can I use local tools on source code sitting on the remote SSH host?
Yes. Typically this is done using SSHFS or by using rsync to get a copy of the files on your local machine. SSHFS mounts the remote filesystem is ideal for scenarios where you need to edit individual files or browse the source tree and requires no sync step to use. However, it is not ideal for using something like a source control tool that bulk manages files. In this case, the rsync approach is better since you get a complete copy of the remote source code on your local machine. See Tips and Tricks for details.
Can I use VS Code when I only have SFTP/FTP filesystem access to my remote host (no shell access)?
Some cloud platforms only provide remote filesystem access for developers rather than direct shell access. VS Code Remote Development was not designed with this use case in mind since it negates the performance and user experience benefits.
However, this use case can typically be handled by combining extensions like SFTP with remote debugging features for Node.js, Python, C#, or others.
As an extension author, what do I need to do?
The VS Code extension API abstracts away local/remote details so most extensions will work without modification. However, given extensions can use any node module or runtime they want, there are situations where adjustments may need to be made. We recommend you test your extension to be sure that no updates are required. See Supporting Remote Development for details.
Questions or feedback
- See Tips and Tricks or the FAQ.
- Search on Stack Overflow.
- Add a feature request or report a problem.
- Contribute to our documentation or VS Code itself.
- See our CONTRIBUTING guide for details.
Introduction
The SSH agent is a central part of OpenSSH. In this post, I’ll explain what the agent is, how to use it, and how it works to keep your keys safe. I’ll also describe agent forwarding and how it works. I’ll help you reduce your risk when using agent forwarding, and I’ll share an alternative to agent forwarding that you can use when accessing your internal hosts through bastions.
What is the SSH agent?
ssh-agent is a key manager for SSH. It holds your keys and certificates in memory, unencrypted, and ready for use by ssh. It saves you from typing a passphrase every time you connect to a server. It runs in the background on your system, separately from ssh, and it usually starts up the first time you run ssh after a reboot.
Ssh Use Agent Key
The SSH agent keeps private keys safe because of what it doesn’t do:
- It doesn’t write any key material to disk.
- It doesn’t allow your private keys to be exported.
Private keys stored in the agent can only be used for one purpose: signing a message.
But if the agent can only sign messages, how does SSH encrypt and decrypt traffic?
When first learning about public and private SSH keys, it’s natural to assume that SSH uses these key pairs to encrypt and decrypt traffic. That’s what I thought. But it’s not the case. An SSH key pair is only used for authentication during the initial handshake.
For example, here’s how a user’s key is verified during the SSH handshake, from the server’s perspective:
- The client presents a public key to the server.
- The server generates and sends a brief, random message, asking the client to sign it using the private key.
- The client asks the SSH agent to sign the message and forwards the result back to the server.
- The server checks the signature using the client’s public key.
- The server now has proof that the client is in possession of their private key.
Later in the handshake process, a set of new, ephemeral and symmetric keys are generated and used to encrypt the SSH session traffic. These keys may not even last the entire session; a “rekey” event happens at regular intervals.
The agent protocol
SSH uses a Unix domain socket to talk to the agent via the SSH agent protocol. Most people use the ssh-agent that comes with OpenSSH, but there’s a variety of open-source alternatives.
The agent protocol is so simple that one could write a basic SSH agent in a day or two. It only has a few primary operations:
- Add a regular key pair (public and decrypted private keys)
- Add a constrained key pair (public and decrypted private keys)
- Add a key (regular or constrained) from a smart card (public key only)
- Remove a key
- List keys stored in the agent
- Sign a message with a key stored in the agent
- Lock or unlock the entire agent with a passphrase
🤔 What’s a constrained key? It’s usually a key that either has a limited lifetime or one that demands explicit user confirmation when it is used.
The ssh-add command is your gateway to the SSH agent. It performs all of these operations except for signing. When you run ssh-add without any parameters, it will scan your home directory for some standard keys and add them to your agent. By default, it looks for:
~/.ssh/id_rsa~/.ssh/id_ed25519~/.ssh/id_dsa~/.ssh/id_ecdsa
Once you add keys to the keychain, they will be used automatically by ssh.
ssh-agent and the macOS Keychain
The ssh-agent that ships with macOS can store the passphrase for keys in the macOS Keychain, which makes it even easier to re-add keys to the agent after a reboot. Depending on your Keychain settings, you still may need to unlock the keychain after a reboot. To store key passphrases in the Keychain, run ssh-add -K [key filename]. Passphrases are usually stored in the “Local Items” keychain. ssh-agent will use these stored passphrases automatically as needed.
What is agent forwarding?
SSH’s agent forwarding feature allows your local SSH agent to reachthrough an existing SSH connection and transparently authenticate on a more distant server. For example, say you SSH into an EC2 instance, and you want to clone a private GitHub repository from there. Without agent forwarding, you’d have to store a copy of your GitHub private key on the EC2 host. With agent forwarding, the SSH client on EC2 can use the keys on your local computer to authenticate to GitHub.
How agent forwarding works
First, a little background. SSH connections can have multiple channels. Here’s a common example: an interactive connection to a bastion host (jump box) runs on one channel. When agent forwarding is enabled for a connection (usually using ssh -A), a second channel is opened up in the background to forward any agent requests back to your local machine.
From ssh's perspective, there is no difference between a remote and a local ssh-agent. SSH always looks at the $SSH_AUTH_SOCK environment variable to find the Unix domain socket for the agent. When you connect to a remote host with agent forwarding enabled, SSHD will create a remote Unix domain socket linked to the agent forwarding channel, and export an $SSH_AUTH_SOCK pointing to it.
Agent forwarding comes with a risk
When you forward ssh-agent's Unix domain socket to a remote host, it creates a security risk: anyone with root access on the remote host can discreetly access your local SSH agent through the socket. They can use your keys to impersonate you on other machines on the network.
Here’s an example of how that might look:
How to reduce your risk when agent forwarding
Here are a few ways to make agent forwarding safer:
Don’t turn on
ForwardAgentby default.Many guides on agent forwarding will suggest turning on
ForwardAgentusing the following configuration:We suggest not doing that. Instead, only use agent forwarding in circumstances where you need it.
ssh -Aturns on agent forwarding for a single session.Lock your ssh agent when you use agent forwarding.
ssh-add -xlocks the agent with a password, andssh-add -Xunlocks it. When you’re connected to a remote host with agent forwarding, no one will be able to snake their way into your agent without the password.Or use an alternative SSH agent that prompts you when it’s being used. Sekey uses Touch ID on macOS to store keys in the MacBook Pro’s security enclave.
Or don’t use agent forwarding at all. If you’re trying to access internal hosts through a bastion,
ProxyJumpis a much safer alternative for this use case. (see below)
Use ProxyJump: a safer alternative
When you want to go through a bastion host (jumpbox), you really don’t need agent forwarding. A better approach is to use the ProxyJump directive.
Instead of forwarding the agent through a separate channel, ProxyJump forwards the standard input and output of your local SSH client through the bastion and on to the remote host. Here’s how that works:
- Run
ssh -J bastion.example.com cloud.computer.internalto connect tocloud.computer.internalvia yourbastion.example.combastion host.cloud.computer.internalis a hostname that can be looked up using DNS lookup onbastion.example.com. - Your SSH client uses keys from your agent to connect to
bastion.example.com. - Once connected, SSHD on the bastion connects to
cloud.computer.internaland hands that connection off to your local SSH client. - Your local SSH client runs through the handshake again, this time with
cloud.computer.internal.
You can think of it as SSHing within an SSH session; except the ssh program never runs on the bastion. Instead, sshd connects to cloud.computer.internal and gives control of that connection (standard in and out) back to your local SSH, which then performs a second handshake.
Setting up ProxyJump
Let’s say my bastion host is bastion.example.com. I could set up my ~/.ssh/config file like this:
Then I just run ssh cloud.computer.internal to connect to an internal destination through the bastion—without agent forwarding.
If ProxyJump doesn’t work…
Older versions of SSH and SSHD (prior to 7.2, released in 2016) don’t support ProxyJump. But you can do an equivalent operation using ProxyCommand and netcat. Here’s an example:
The magic here is that SSH itself is the proxy you’re using for SSH. The nc %h %p part simply opens up a raw socket connection to cloud.computer.internal on port 22. The standard I/O of the parent ssh command is piped right into the ProxyCommand so that the parent ssh can authenticate to the internal host through the proxy connection.
Questions? Comments?
We love talking about all things SSH. Hit us up on Twitter if you have questions or feedback!
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