Ssh Config Forward Agent

Using an ssh-agent, or how to type your ssh password once, safely.

On Mac and Linux, SSH agent forwarding is built into ssh, and the ssh-agent process is launched automatically. All you’ll have to do is make sure your keys are added to ssh-agent and configure ssh to use forwarding. Add Keys to ssh-agent You can use the utility ssh-add to add keys to your local agent. IdentitiesOnly Specifies that ssh(1) should only use the configured authentication identity and certificate files (either the default files, or those explicitly configured in the sshconfig files or passed on the ssh(1) command-line), even if ssh-agent(1) or a PKCS11Provider or SecurityKeyProvider offers more identities.

If you work a lot on linux and use ssh often, you quicklyrealize that typing your password every time you connect to a remotehost gets annoying.

Ssh-agent forwarding can be accomplished with ssh -A. Most references I have found state that the local machine must configure /.ssh/config to enable AgentForwarding with the following code: Host ForwardAgent yes Host. ForwardAgent no. SSH Agent Forwarding with SecureCRT ®. Configured to allow only public-key encryption by the use of the PasswordAuthentication no keyword in the sshdconfig file. Public Key Authentication with Agent Support. Now that we've taken the leap into public-key authentication, we'll take the next step to enable agent support.

Not only that, it is not the best solution in terms of security either:

Every time you type a password, a snooper has an extra chance to see it.
Every host you ssh to with which you use your password, well, has to know your password. Or a hash of your password. In any case, you probably have typed your password on that host once or twice in your life (even if just for passwd, for example).
If you are victim of a Man In The Middle attack, your password may get stolen. Sure, you can verify the fingerprint of every host you connect to, and disable authentication without challenge and response in your ssh config. But what if there was a way you didn't have to do that?

This is where key authentication comes into play: instead of using a passwordto log in a remote host, you can use a pair of keys, and well, ssh-agent.

Using ssh keys

All you have to do is: Fujitsu fi 6140z drivers for mac.

generate a pair of keys with ssh-keygen. This will create two files: a public key (normally .pub), and a private key. The private key is normally kept encrypted on disk. After all, it's well, supposed to be private. ssh-keygen will ask you to insert a password. Note that this password will be used to decrypt this file from your local disk, and never sent to anyone. And again, as the name suggest, you should never ever disclose your private key.
copy your public key into any system you need to have access to. You can use rsync, scp, type it manually, or well, use the tool provided with openssh: ssh-copy-id. Note that you could even publish your public key online: there is no (known) way to go from a public key to your private key and to get access to any of your systems. And if there was a way, well, public key encryption would be dead, and your bank account likely empty.

and .. done! That's it, really, just try it out:

So.. what are the advantages of using keys? There are many:

Your passphrase never leaves your local machine. Which generally makes it harder to steal.
You don't have a password to remember for each different host. Or..
.. you don't have the same password for all hosts you connect to (depending on your password management philosophies).
If somebody steals your passphrase, there's not much he can do without your private key.
If you fear somebody has seen your passphrase, you can change it easily. Once. And for all.
If there is a 'man in the middle', he may be able to hijack your session. Once (and well, feast on your machine, but that's another story). If a 'man in the middle' got hold of your password instead, he could enjoy your machine later, more stealthy, for longer, and may be able to use your password on other machines.
They just work. Transparently, most of the times. With git, rsync, scp, and all their friends.
You can use an agent to make your life happier and easier.

And if you're wondering what an agent is, you can go to the next section.

Your agent friend

Ok. So you have read this much of the article, and still we have not solved theproblem of having to type your password every freaking time, have we?

That's where an agent comes in handy. Think of it as a safe box you have tostart in the background that holds your keys, ready to be used.

You start an ssh-agent by running something like:

in your shell. You can then feed it keys, with ssh-add like:

or, if your key is in the default location, you can just:

ssh-add will ask your passphrase, and store your private key into thessh-agent you started earlier. ssh, and all its friends (including git,rsync, scp..) will just magically use your agent friend when you try tossh somewhere. Convenient, isn't it?

Assuming you added all the keys you need, you can now ssh to any host,as many times as you like, without ever ever having to retype your password.

Not only that, but you can exploit agent forwarding to jump from one hostto another seamlessly.

Let me give you an example:

Let's say you have to connect to a server at your office.
Let's say this server is firewalled. In order to ssh there, you first need to ssh into another gateway. Sounds familiar, doesn't it? This means you end up doing:

On this second ssh, what happens? Well, if you type your password, yourcleartext password is visible to the gateway. Yes, it is sent encrypted,decrypted, and then through the console driver fed to the ssh process.If a keylogger was running, your password would be lost.

Worst: we are back to our original problem, we have to type our passwordmultiple times!

We could, of course, store our private key on the company gateway and runan agent there. But that would not be a good idea, would it? Remember:your private key never leaves your private computer, you don't wantto store it on a remote server.

So, here's a fancy feature of ssh and ssh-agent: agent forwarding.

On many linux systems, it is enabled by default: but if you pass -A tothe first ssh command (or the second, or the third, ..), ssh willensure that your agent running on your local machine is usable from theremote machine as well.

For example:

The second ssh here, run from the company gateway, will not ask youfor a password. Instead, it will detect the presence of a remote agent,and use your private key instead, and ask for no password.

Sounds dangerous? Well, there are some risks associated with it, whichwe'll discuss in another article. But here is the beauty of the agent:

Your private key never leaves your local computer. That's right.By design, the agent never ever discloses your private key, itnever ever hands it over to a remote ssh or similar. Instead,ssh is designed such as when an agent is detected, the informationthat needs to be encrypted or verified through the agent is forwardedto the agent. That's why it is called agent forwarding, andthat's why it is considered a safer option.

Configuring all of this on your machine

So, let's summarize the steps:

Generate a set of keys, with ssh-keygen.
Install your keys on remote servers, with ssh-copy-id.
Start an ssh-agent to use on your machine, with eval ssh-agent.
ssh-add your key, type your password once.
Profit! You can now ssh to any host that has your public key without having to enter a password, and use ssh -A to forward your agent.

Easy, isn't it? Where people generally have problems is on how andwhere to start the ssh-agent, and when and how to start ssh-add.

The long running advice has been to start ssh-agent from your .bashrc,and run ssh-add similarly.

In today's world, most distributions (including Debian and derivatives),just start an ssh-agent when you first login. So, you really don't haveanything to do, except run ssh-add when you need your keys loaded,and be done with it.

Still many people have snippets to the extent of:

in their .bashrc, which basically says 'is there an ssh-agent already running? no? start one, and add my keys'.

This is still very annoying: for each console or each session you login into, youend up with a new ssh-agent. Worse: this agent will run forever with your privatekeys loaded! Even long after you logged out. Nothing and nobody will ever kill your agent.

So, your three lines of .bashrc snippet soon becomes 10 lines (to cache agents on disk),then it breaks the first time you use NFS or any other technology to share your homedirectory, and then.. more lines to load only some keys, some magic in .bash_logout tokill your agent, and your 4 lines of simple .bashrc get out of control

Conclusion

I promised myself to talk about the pitfalls of using an agent and common approachesto solving the most common problems in a dedicated article. My suggestion for now?

Use the ssh-agent tied with your session, and managed by your distro, when one is available (just try ssh-add and see if it works!).
Use -t to ssh-add and ssh-agent, so your private key is kept in the agent for a limited amount of time. One hour? 5 miutes? you pick. But at the end of that time, your key is gone.
Use something like ssh-ident, to automatically maintain one or more agents, and load ssh keys on demand, so you don't even have to worry about ssh-add.

For full disclosure, I wrote ssh-ident. Surprisingly, that still doesn't preventme from liking it.

Article version: GitHub AE

Ssh Config Localforward

Article version: GitHub AE

To simplify deploying to a server, you can set up SSH agent forwarding to securely use local SSH keys.

If you've already set up an SSH key to interact with GitHub AE, you're probably familiar with ssh-agent. It's a program that runs in the background and keeps your key loaded into memory, so that you don't need to enter your passphrase every time you need to use the key. The nifty thing is, you can choose to let servers access your local ssh-agent as if they were already running on the server. This is sort of like asking a friend to enter their password so that you can use their computer.

Check out Steve Friedl's Tech Tips guide for a more detailed explanation of SSH agent forwarding.

Setting up SSH agent forwarding

Ensure that your own SSH key is set up and working. You can use our guide on generating SSH keys if you've not done this yet.

You can test that your local key works by entering ssh -T git@hostname in the terminal:

We're off to a great start. Let's set up SSH to allow agent forwarding to your server.

Using your favorite text editor, open up the file at ~/.ssh/config. If this file doesn't exist, you can create it by entering touch ~/.ssh/config in the terminal.
Enter the following text into the file, replacing example.com with your server's domain name or IP:

Warning: You may be tempted to use a wildcard like Host * to just apply this setting to all SSH connections. That's not really a good idea, as you'd be sharing your local SSH keys with every server you SSH into. They won't have direct access to the keys, but they will be able to use them as you while the connection is established. You should only add servers you trust and that you intend to use with agent forwarding.

Testing SSH agent forwarding

To test that agent forwarding is working with your server, you can SSH into your server and run ssh -T git@hostname once more. If all is well, you'll get back the same prompt as you did locally.

If you're unsure if your local key is being used, you can also inspect the SSH_AUTH_SOCK variable on your server:

Ssh Agent Forwarding Linux

If the variable is not set, it means that agent forwarding is not working:

Troubleshooting SSH agent forwarding

Here are some things to look out for when troubleshooting SSH agent forwarding.

You must be using an SSH URL to check out code

SSH forwarding only works with SSH URLs, not HTTP(s) URLs. Check the .git/config file on your server and ensure the URL is an SSH-style URL like below:

Your SSH keys must work locally

Before you can make your keys work through agent forwarding, they must work locally first. Our guide on generating SSH keys can help you set up your SSH keys locally.

Your system must allow SSH agent forwarding

Sometimes, system configurations disallow SSH agent forwarding. You can check if a system configuration file is being used by entering the following command in the terminal:

In the example above, the file ~/.ssh/config is loaded first, then /etc/ssh_config is read. We can inspect that file to see if it's overriding our options by running the following commands:

In this example, our /etc/ssh_config file specifically says ForwardAgent no, which is a way to block agent forwarding. Deleting this line from the file should get agent forwarding working once more.

Your server must allow SSH agent forwarding on inbound connections

Agent forwarding may also be blocked on your server. You can check that agent forwarding is permitted by SSHing into the server and running sshd_config. The output from this command should indicate that AllowAgentForwarding is set.

Your local `ssh-agent` must be running

On most computers, the operating system automatically launches ssh-agent for you. On Windows, however, you need to do this manually. We have a guide on how to start ssh-agent whenever you open Git Bash.

To verify that ssh-agent is running on your computer, type the following command in the terminal:

Your key must be available to `ssh-agent`

You can check that your key is visible to ssh-agent by running the following command:

If the command says that no identity is available, you'll need to add your key:

On Mac OS X, ssh-agent will 'forget' this key, once it gets restarted during reboots. But you can import your SSH keys into Keychain using this command: