Connect to a server by using SSH on Linux or Mac OS X

• Ssh To Server With Private Keyword
• Ssh Private Key File
• Ssh To Server With Private Key
• Ssh Command To Connect To Remote Server With Private Key
• Ssh Private Key Password

Authored by: Rackspace Support

This article provides steps for connecting to a cloud server froma computer running Linux® or MacOS® X by using Secure Shell (SSH).It also discusses generating an SSH key and adding a public key tothe server.

• $ ssh-keygen On execution, we are prompted to specify a file in which to save the private key, the default being /home/user/.ssh/idrsa; here idrsa is the name of our Private Key file. You can always specify a different path and name for the Private Key file. For our demonstration, we.
• If you use ssh to connect to the remote host, one way to ensure the security of the connection is the use o.

Introduction

A tutorial on configuring SSH Server to Authenticate with Private/Public Keys using Ubuntu Linux http://www.danscourses.com/Linux-Fundamentals/how-to-install-ssh-in-ubuntu-388.html. For now, let's move from your web server to your computer. You just connected from your Windows box to your web server over SSH with Public/Private Key.

SSH is a protocol through which you can access your cloud server and runshell commands. You can use SSH keys to identify trusted computers withoutthe need for passwords and to interact with your servers.

SSH is encrypted with Secure Sockets Layer (SSL), which makes it difficultfor these communications to be intercepted and read.

Note: Many of the commands in this article must be run on your localcomputer. The default commands listed are for the Linux command line orMacOS X Terminal. To make SSH connections from Windows®, you can use a clientsimilar to the free program, PuTTY.To generate keys, you can use a related program, PuTTYGen.

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Log in

Using the Internet Protocol (IP) address and password for your cloud server, log in byrunning the following ssh command with username@ipaddress as the argument:

The system prompts you to enter the password for the account to which you’reconnecting.

Remote host identification

If you rebuilt your cloud server, you might get the following message:

One of the security features of SSH is that when you log in to a cloudserver, the remote host has its own key that identifies it. When you tryto connect, your SSH client checks the server’s key against any keysthat it has saved from previous connections to that IP address. After yourebuild a cloud server, that remote host key changes, so your computerwarns you of possibly suspicious activity.

To ensure the security of your server, you canuse the web console in the Cloud Control Panel to verify your server’s new key.If you’re confident that you aren’t being spoofed, you can skip thatstep and delete the record of the old SSH host key as follows:

On your local computer, edit the SSH known_hosts file and remove anylines that start with your cloud server’s IP address.

Note: Use the editor of your choice, such as nano on Debian or theUbuntu operating systemor vi on RPM or CENTOS servers. For simplicity, this article just uses nano. If you prefer to use vi,substitute vi for nano in the edit commands.For more on using nano, see/support/how-to/modify-your-hosts-file/.

If you are not using Linux or MacOS X on your local computer, thelocation of the known_hosts file might differ. Refer to your OS forinformation about the file location. PuTTY on Windows gives you theoption to replace the saved host key.

Generate a new SSH key pair

You can secure SSH access to your cloud server against brute forcepassword attacks by using a public-private key pair. A public key is placed onthe server and a matching private key is placed on your local computer. If youconfigure SSH on your server to accept only connections using keys,then no one can log in by using just a password. Connecting clientsare required to use a private key that has a public key registered onthe server. For more on security, reviewLinux server security best practices.

Use the following steps to generate an SSH key pair:

1. Run the following command using your email address as a label.Substitute your email address for your_email@example.com inthe command.

   A message indicates that your public-private RSA key pair isbeing generated.

   At the prompt, press Enter to use the default location or entera file in which to save the key and press Enter.

2. If you want the additional security of a password for the key pair,enter a passphraseand press Enter. If you don’t want to use a passwordwith the key pair, press Enter to continue without setting one.

   Your key pair is generated, and the output looks similar to the following example:

3. Optionally, add your new key to the local ssh-agent file to enableSSH to find your key without the need to specify its location everytime that you connect:

   You can use an SSH configuration shortcut instead of the ssh-agent fileby following the instructions in the Shortcut configuration sectionlater in this article.

Add the public key to your cloud account

To make it easy to add your key to new cloud servers that you create,upload the public key to your cloud account by following these steps:

1. Log in to the Cloud Control Panel.

2. In the top navigation bar, click Select a Product > Rackspace Cloud.

3. Select Servers > SSH Keys.

4. Click Add Public Key.

5. Enter a key name, such as Work Laptop, to remind you which computer this key is for.

6. Select the region for which you want to store the public key. Tostore your key in multiple regions, repeat these steps foreach region. The key must reside in the same region as the server.

7. Paste the contents of the id_rsa.pub file that you created intothe Public Key field. You can get the file contents by eitheropening the file in a text editor or by running the followingcommand:

8. Click Add Public Key.

If you want to add the key manually, instead of by using the Control Panel, reviewLinux server security best practicesand use the following command:

Create a new server by using a stored key

When you create a new cloud server, you can add a stored key to the newserver.

1. On the Create Server page, expand the Advanced Options section.

2. From the SSH Key menu, select your key from the list.

3. If you don’t see a stored key in the list, you can perform one of the following actions:

   • Switch the region for the new server to the region where you have stored the SSH key.
   • Repeat the steps in the preceding section, Add the public key to your cloud account,to add the key to the region in which you want to create the new server.

Add the key to an existing server

You can’t use the Cloud Control Panel to add a public key to anexisting server. Follow these steps to add the key manually:

1. On your cloud server, create a directory named .ssh in the homefolder of the user that you connect to by using SSH.

2. Create or edit the authorized_keys file and add your public key tothe list of authorized keys by using the following command:

   A key is all on one line, so ensure that the key isn’t broken byline breaks. You can have multiple keys in the authorized_keysfile, with one key per line.

3. Set the correct permissions on the key by using the following commands:

4. If you have any issues and need to fix permissions issues, run the following comand:

After you have added the public key to the authorized_keys, you can make an SSHconnection by using your key pair instead of the account password.

Shortcut configuration

Use the following instructions to set up a connection shortcut by creating a~/.ssh/config file on your local computer and adding your server and keydetails to it.

1. Using a text editor, add the following text to the ~/.ssh/config file, changing thevalues to match your server information:

   Each of the following entries describes a feature of the server:

   • Host: A shortcut name that you use to tell SSH to use thisconnection.
   • HostName: The address of the server to which you connect.
   • User: The name of the user account to connect to on theserver.
   • IdentityFile: The location of the private key file (id_rsa).

2. After you set up the config file, connect to the server by usingthe following command with your shortcut name:

Troubleshooting

If you have trouble making a new connection after you restart theserver, use the following steps to help you resolve the issue:

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• The best way to troubleshoot SSH or SFTP login issues is to attempt tologin through SSH while logged into the Emergency Console and to watch the log,which typically includes the reason for a failure. If no reason is given,it could be a firewall issue. For RPM servers, run the following command to watch the log:

  For Debian servers, run the following command to watch the log:

• If you get a connection timeout error, check the IP address thatyou used to ensure that it’s correct. You might also check theserver’s iptables to ensure that it isn’t blocking the port used by SSH.

• If you get a connection refused error, you might be trying to useSSH with the wrong port. If you changed your server to listen to aport other than 22, use the -p option with SSH to specifythe port.

• If your login is rejected, then you might have an issuewith your key. Change the sshd configuration to allow passwordconnections by setting PasswordAuthentication to yes. Restartthe server and try again. If you connect after these changes, thenthe issue is with the key and you must verify that the key is in theright place on the server.

• If all else fails, review your changes and restart the SSH daemon onthe server by running the following command:

  If you get a message that the SSH service is unknown, run thecommand with sshd as the service name instead.

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Ssh To Server With Private Keyword

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Every developer needs access to some servers for example to check the application logs.

Usually, this is done using public-private key encryption where each developer generates their own public-private key pair.The public keys of each developer are added to the authorized_keys file on each server they should have access to.

Ssh Private Key File

Painful manual changes

So far so good. However, what happens when one developer leaves the company?

In that case, the public keys of that developer should be removed from all servers. This could be quite a bit of work depending on how many servers they had access to.And even worse, if it’s done manually, there is quite some risk that the key is still forgotten on some server, so the access remains open.

Alternative solutions

There are some commercial and open source solutions out there which want to help out with this problem. The basic idea is that you add and maintain the keys and the access lists on that service and when you remove a key, they will remove it from all your servers.

Sounds good, but it has one very big disadvantage: it’s a potential single source of failure. If someone captures access to that service, they can gain access to all your servers. And if you lose access to that service, you also lose access to all your servers in the worst case.

The solution: signing keys

When I was facing this problem, I asked on HackerNews how others are tackling this problem.

There were some great suggestions and insights from the community and the best solution to the problem seems to be the signing of keys which I will present to you here in detail.

The rough idea

The rough idea is this: You still generate a public-private key pair for each developer. However, you don’t upload the public keys to your servers.

Instead, you sign the public keys with a so-called certificate authority (CA) key which you generate before. This signing simply generates a third certificate file which you give back to the developer and they put it inside of their .ssh/ folder next to the private and public key.

On the servers, you simply tell the server the public key of your CA and the server can detect if a user has a properly signed certificate and only allows access to the developers who have such a signed certificate.

The advantages

When you sign a certificate, you can determine how long that signing is valid. So if you sign it with a validity of 3 months and the developer leaves the company, then after 3 months they won’t have access to any of the servers for sure.

Now you say: well, but I don’t want to sign keys of everyone every 3 months which is a fair point.

One possibility is to automate the process for example by building a service where a user can automatically get a signed certificate when they authorize with their company e-mail and password, but that is beyond the scope of this article.

The simple alternative is that you issue certificates that are valid longer and then if someone leaves the company, you can revoke the certificate, i.e. invalidate it. You can put a list of invalid certificates on your servers and they will not accept the user any more.This could for example be done by having this list on AWS S3 or some other storage and a cronjob on each server that regularly pulls this.

Show me how to do this

Glad that you asked!

It’s actually super simple once you know the drill.

First, you generate a certificate authority public-private key pair of which you should keep the private key very secure:

Then on your server you specify that all users signed by your CA are allowed to access the server:

1. Upload the public key of your CA on your server, e.g. at /etc/ssh/ca.pub
2. Tell the server to allow access to users signed by it by adding a line to /etc/ssh/sshd_config:

To make the changes effective, you should reload the ssh service: sudo service ssh reload.

Now if a developer generated their public-private key pair (e.g. ssh-keygen -t ecdsa -b 521), they simply send you their public key (note that you never need to send any private keys around!).Then you sign their public key to generate their certificate:

Quick explanation for the different parts:

• -s ca - you want to use your CA to sign
• -I USER_ID - the id of your user / the username
• -V +12w - how long before the certificate expires - here valid for 12 weeks
• -z 1 - the serial number of this certificate - can be used to make this particular certificate invalid later, should be unique
• id_ecdsa.pub: the public key of the developer which you want to sign

It will generate the certificate id_ecdsa-cert.pub which you can send to the developer and they put it into their ~/.ssh folder next to their public-private key pair.

It gets even better

Sounds cool, right? But you can do even better!

You probably have developers with different experience and different teams and roles and not everyone accessing the same servers.

So let’s add roles into the signing process!

That way, on the server you specify which roles are allowed to access the server and during the signing process you specify the roles of the developer you are signing.

Then, that developer can access all servers matching to their roles.

When you on board a new developer, you only need to generate that one certificate and boom they have access to all relevant servers without adding anything on those servers.

Here is how this looks like schematically:

This is how you configure roles on a server:

First, create the folder to configure access: sudo mkdir /etc/ssh/auth_principalsInside that folder, you can create files with the name of the server user that someone could login as. For example to grant root access to some roles, add the file /etc/ssh/auth_principals/root.

Inside /etc/ssh/auth_principals/root you simply list all roles which should be able to login as root with one role per line:

Finally, configure on the server to use roles by again adding a line to /etc/ssh/sshd_config:

To make the changes effective, you should reload the ssh service: sudo service ssh reload.

This is how you sign a key with roles (they are added to the certificate):

It’s the same as before, but with the -n ROLE1,ROLE2 flag. Important: there can’t be spaces between the comma for different roles!

Now, that developer could log onto any server where ROLE1 or ROLE2 are in an auth_principals file for a user name they try to login as.

Revoking keys

Ssh To Server With Private Key

Finally, if you want to invalidate a certificate, you can do that by the user name or the serial number of the certicate (-z flag). It’s recommended to make a list of generated certificates in an Excel spreadsheet or have a database depending on the number of your peeps.

This is when you already have a revoked-keys list and want to update it (-u flag). For the initial generation, use it without the update flag.

The list-to-revoke needs to consist of usernames (ids) or serial numbers (-z flag during generation) like this:

This would revoke access to the certificate with serial 1 and all certificates with id test.user.

To make the server respect revoked keys, you need to add the generated / updated revoked keys file to /etc/ssh/revoked-keys and configure it again in /etc/ssh/sshd_config:

Warning: make sure that the revoked-keys file is accessable and readable, otherwise you might lose access to your server

Summary: good ssh key management

In my opinion, this solution is as good as it gets. You have the option to manage the access to your servers via ssh based on roles. You only need to configure your servers once (which roles are allowed to access it). For each new developer, you only need to generate a signed certificate and they immediately have access to all relevant machines matching their role / experience.And when they leave the company, you can revoke their access also in a simple way.

Ssh Command To Connect To Remote Server With Private Key

Morrowind steam. And even if a mishap occurs and a developer leaves without having their access revoked, their certificate will expire after some time, so they also lose access automatically.

For small teams, you can do these steps manually as they are very fast to do; then as you grow, you can automate the certificate signing with a login service based on company authentication details.

Ssh Private Key Password

Happy ssh-ing!