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Title: Demystifying SSH Date: 2019-06-17 Tags: ssh Slug: demystifying-ssh Description: Understanding, using and troubleshooting SSH for fun and not-for-profit Author: decentral1se Status: draft Category: fundamentals

Introduction

One of the more important and foundational skills needed for experimenting and maintaining servers is understanding, using and troubleshooting SSH.

SSH is all about remotely gaining access to the command-line interface of your server. Remotely accessing your server is probably one of the first things you'll want to achieve when starting to run a server from your home.

When your server has no screen attached, no desktop environment installed, you would like to access it when you are not at home or allow your friend to log in, you will need to use SSH.

Using SSH can be a frustrating experience. There are many moving parts to consider when dealing with user accounts, permissions, public key cryptography, protocols, clients, servers and agents. And yet despite so much to consider, SSH is praised as something easy to use and quite often presupposed knowledge between peers.

With this in mind, there is a need to demystify SSH. In this guide we aim to show how to setup an SSH client and server as well as discuss common issues and approaches for day to day use and troubleshooting.

Prerequisites

The SSH ecosystem is old and venerable. It is available on all modern GNU/Linux distributions. You can use your home server or if you don't have one yet you can use your own personal laptop to experiment (in this case, your laptop will play the role of both the server and client as explained later).

The commands shown in this guide were run on a Debian Stretch distribution but the actual tool names should be the same on other distributions.

SSH and OpenSSH

The term "SSH" can refer to a number of different but related things.

SSH stands for the Secure Shell which is a protocol for describing how to provide a secure channel of communication from a client to a server. However, it more typically refers to the OpenSSH suite of tools which are the programs you will install and use when dealing with SSH.

The OpenSSH tools are an implementation of the SSH protocol. This means that these tools follow the behaviour described in the documents of the protocol.

Installing the SSH server and client

It is important to understand the client/server architecture of SSH. If you are remotely connecting to your home server from your laptop, then your laptop is the client and the home server is the server.

There are two packages which contain all the tools that the OpenSSH tool suite provides. The openssh-server and openssh-client packages.

To install the SSH server on your home server, run:

$ sudo apt install -y openssh-server

And from your client (laptop or otherwise), run:

$ sudo apt install -y openssh-client

Creating a user account on the server

In order for your client to connect to the server, a user account must be created on the server. This can be done with the following command (where homebrewer is the new user account):

$ sudo useradd --create-home --shell /bin/bash homebrewer

You should then set a password for this new user:

$ sudo passwd homebrewer

This password will be used in the following step.

Logging in for the first time

You can now SSH into the server from your client. Assuming that you have a DNS entry which points to your home server (myhomebrewserver.com) then you can log in with:

$ ssh homebrewer@myhomebrewserver.com

On first connection to a new server, you will be shown a fingerprint and asked if you would like to continue connecting. For now, choose to continue without validating the fingerprint.

You should then be asked for the password that you entered when creating the account. If you have any issues at this point, please see the Troubleshooting SSH section.

Passwords, keys and security

As we have seen so far, connecting to an SSH server using password authorisation is relatively simple. However, password authorisation is typically recommended against due to security considerations.

Security is relative and you may not be concerned with defending against a brute-force attack. However, since other methods of authorisation are so commonly used and often the source of of problems when dealing with SSH connectivity, we will also cover this topic also.

The alternative to password authorisation is public key cryptography. The idea is to generate two keys, one public and the other private. The public key part can be shared publicly and will be registered with the SSH server. The private key part cannot be shared with anyone else and will be used on the client side for authentication.

For those not familiar with public key encryption, a brief and pragmatic introduction is provided by this ssd.eff.org guide.

Validating server host keys

When logging into your server you were asked to validate a key fingerprint:

The authenticity of host 'myhomebrewserver (666.666.666.666)' ed.
ECDSA key fingerprint is SHA256:lUhBmQXjkOL0zyoDNarUIbhd3RXo7X
Are you sure you want to continue connecting (yes/no)?

When an SSH server is installed, key pairs are generated on the server. The design of SSH is such that when you make a secure connection you should be sure that where you are connecting to is the server you expect.

On the server, you can validate that this is fingerprint you expect:

$ sudo ssh-keygen -lf /etc/ssh/ssh_host_ecdsa_key
256 SHA256:lUhBmQXjkOL0zyoDNarUIbhd3RXo7X root@myhomebrewserver (ECDSA)

These values should match the ones you were first shown. Where the ECDSA in the first message corresponds to the key file name in /etc/ssh/. The SSH server will generate key pairs for each algorithm it supports.

As you have accepted your server host key fingerprint, the public key of the server will be placed in your $HOME/.ssh/known_hosts file on your client where it will be remembered for future connections.

If the host key ever changes, you will see something like:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
...

This simply means that that the host key you accepted for this host has changed and you should first figure out why this is the case before making a secure connection.

Dealing with SSH keys

Generating a public and private key pair

Now that we understand the idea of host keys, we should generate client keys. On your client, run the following:

$ ssh-keygen -t ed25519

This uses the currently preferred algorithm for generating keys. The key generation will ask you to enter a passphrase. It is typically recommended use a strong passphrase (see diceware) for the reason that if someone else gets your private key part, they still will still not be able to use it because they also need to know the passphrase.

You should now have two keys in your $HOME/.ssh folder. A id_ed25519 (private key part) and a id_ed25519.pub file (public key part).

Your public key part might look something like:

$ cat $HOME/.ssh/id_ed25519
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOTWIy+Em89QR/Xs6RUmr+0U0F3GVXY/UA2MXs9MoqdY myuser@myhost

You can then start an ssh-agent instance and register the private key part:

$ eval "$(ssh-agent -s)"
$ ssh-add $HOME/.ssh/id_ed25519

The ssh-agent should ask for your passphrase. The agent is useful because it will store the passphrase of your key and re-use it the next time you log into the server. This can be particularly useful later on when you need to use multiple SSH keys.

You can confirm that the ssh-agent has loaded this key by running:

$ ssh-add -L
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOTWIy+Em89QR/Xs6RUmr+0U0F3GVXY/UA2MXs9MoqdY myuser@myhost

And finally, you can then configure your client to use this key. Create a $HOME/.ssh/config file and put something like the following configuration into it (remember to replace with your server details):

Host myhomebrewserver
  Hostname myhomebrewserver.com
  User homebrewer
  Port 22
  IdentityFile ~/.ssh/id_ed25519

This is useful because it allows you to only type:

$ ssh myhomebrewserver

And the correct hostname, user, port and key will be chosen. However, before connecting, you must register your public key part on the server.

Register the public key with the server

On the server, you should register the public key. This is achieved by putting the public key part of the key into the /home/homebrewer/.ssh/authorized_keys file.

You can do this like so (replace the public key part with the one you generated):

$ sudo -su homebrewser
$ cd $HOME && mkdir .ssh
$ echo "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOTWIy+Em89QR/Xs6RUmr+0U0F3GVXY/UA2MXs9MoqdY myuser@myhost" > .ssh/authorized_keys

You must then ensure that your SSH server is accepting keys as a method of authentication. You should edit the /etc/ssh/sshd_config file which is the main configuration file for the SSH server.

You need to ensure the following is present:

PubkeyAuthentication yes

After editing the file, save your changes and then validate the configuration:

$ sudo sshd -t

No output should be shown if everything is validating correctly. This is important to do before restarting the SSH server in case you lock yourself out of the server.

You can restart the SSH server with:

$ sudo systemctl restart sshd

Now it should be possible to connect from your client with:

$ ssh myhomebrewserver

If you have any issues, please see the Troubleshooting SSH section.

Troubleshooting SSH

Despite our best intentions, we are often confronted with a failed login attempt:

$ ssh myhomebrewserver
Permission denied (publickey).

This is not the most helpful message. In order to go about solving this issue, we need to focus our detective work on the client/server architecture and try to discover which side is responsible for the problem. Hopefully the following tips can help you in this process.

On the server

Here are some questions to ask yourself:

Is your public key registered on the server in the $HOME/.ssh/authorized_keys folder?
Are the folder permissions correct? (see this SO thread)
Is the SSH server running? (sudo systemctl status sshd)
Is the /etc/ssh/ssd_config correct?
- Use the sshd_config as a reference. Don't forget to run sudo sshd -t to validate your changes. After validating, always restart the SSH server after making configuration changes

On the client