Run an SSH server

Illustration by Annie Ruygt of a terminal window, a computer and some numbers

A number of tools allow you to interact with your server over SSH. These tools are useful for tasks such as copying files (rsync, scp, sshfs), editing (emacs, vim, vscode), and deployment (ansible, github actions, and kamal).

One way to use these tools is to set up a wireguard VPN and issue a new SSH credential. This may be impractical for some use cases (example: github actions).

As an alternative, you can configure and deploy an SSH server on your machine(s). This guide will walk you through the process.

Before proceeding, a caution: unless you are certain that all of the clients will access this service through IPv6, you will need a dedicated IPv4 address.

Install and configure opensshd

Most Docker images are ultimately based on Debian, so the following will work. Adjust as necessary for other operating systems (example: Alpine).

RUN apt-get update \
 && apt-get install -y openssh-server \
 && cp /etc/ssh/sshd_config /etc/ssh/sshd_config-original \
 && sed -i 's/^#\s*Port.*/Port 2222/' /etc/ssh/sshd_config \
 && sed -i 's/^#\s*PasswordAuthentication yes/PasswordAuthentication no/' /etc/ssh/sshd_config \
 && mkdir -p /root/.ssh \
 && chmod 700 /root/.ssh \
 && mkdir /var/run/sshd \
 && chmod 755 /var/run/sshd \
 && rm -rf /var/lib/apt/lists /var/cache/apt/archives

Notes:

  • This runs sshd internally on port 2222. This is because by default sshd listens on all network interfaces, but fly.io machines already are listening on port 22 on the private network (ipv6) interface, so sshd will either complain or fail to start. Additionally, fly deploy will attempt to verify that your application is listening on the interfaces your application exports, and will incorrectly treat fly’s listening on port 22 as evidence that your application is up and running.
  • Password Authentication is disabled. This avoids unnecessary prompts. We will be using SSH keys instead.
  • The above assumes root user. If your application is running under a different user, replace /root with the home directory of that user (example: /home/rails).

Map internal port 2222 to external port 22

Add the following to fly.toml:

[[services]]
  internal_port = 2222
  protocol = "tcp"
  auto_stop_machines = true
  auto_start_machines = true
  [[services.ports]]
    port = 22

This section is needed even if the internal and external ports are the same.

Notes:

  • internal_port needs to match the port you selected in the previous step.
  • port can be any available port. 22 is the default port for SSH, and the one that most applications expect to be used.
  • Like with your web server port, your server can be configured to spin down when idle and restart when accessed. Feel free to adjust auto_stop_machines and auto_start_machines if your needs differ.

Start the openssh server

There are a number of ways to run multiple processes. The most straightforward way to start sshd before your application. Locate the ENTRYPOINT in your Dockerfile. If you don’t have one, create a script in your application directory, name that script as the ENTYRPOINT, and make it executable.

An example of such a script:

#!/bin/bash -e

/usr/sbin/sshd

exec "$@"

Note: the above needs to be run as root. If your Dockerfile specifies another USER, you can work around this by installing and configuring sudo and then removing sudo access before running your main process.

For example, if your userid is rails, you would add the following to your Dockerfile:

RUN apt-get install -y sudo && \
    echo "%rails ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers

And then update your entrypoint script:

sudo /usr/sbin/sshd
sudo sed -i "/^%rails/d" /etc/sudoers

The above commands will start sshd as root, then remove sudo access from your unprivileged userid.

Upload your SSH key

First, locate your SSH key. You can create a new key using ssh-keygen, Or you can use an existing one: look inside the .ssh folder in your home directory for a file with a name like id_rsa.pub.

Once located, there are multiple ways to proceed. Following you will find two ways. Depending on the framework your application uses, you may have other ways to store credentials or environment variables. As this step only deals with public keys, one need not take extraordinary measures to prevent leakage.

Alternative 1: Set and use a secret

fly secrets set "AUTHORIZED_KEYS=$(cat ~/.ssh/id_rsa.pub)"

Powershell users will want to use the following instead:

fly secrets set "AUTHORIZED_KEYS=$(Get-Content $HOME\.ssh\id_rsa.pub)"

Next, update your entrypoint script to contain the following:

echo $AUTHORIZED_KEYS > /root/.ssh/authorized_keys

If you are running with sudo, you would want to do the following instead:

echo $AUTHORIZED_KEYS | sudo tee /root/.ssh/authorized_keys > /dev/null

Alternative 2: Copy from a volume

If you have a volume and if you can arrange to upload the file there, you can directly copy it as a part of your entrypoint script.

cp /volume/.ssh/authorized_keys /root/.ssh

[RECOMMENDED] Make SSH host keys stable

The first time you SSH into a server you will be presented with a fingerprint for the server you are accessing. If you accept that fingerprint, it will be added to your known_hosts file in your .ssh directory. This key is generated when you install openssh-server.

The issue arises when you redeploy your application. If something changes (or your docker cache expires), the installation of openssh-server may be rerun, and new keys will be generated. To avoid these keys from being used, you can capture and restore the keys.

The following assumes that you have a volume mounted at /volume, and an entrypoint script.

mkdir -p /volume/.ssh

if [[ "$(ls /volume/.ssh/*_key)" = "" ]]; then
  cp /etc/ssh/*_key /volume/.ssh
else
  cp /volume/.ssh/*_key /etc/ssh
fi

Notes:

  • These keys are expected to be private, so if you go with an alternate route, make sure that these values are not committed to a public repository unencrypted.
  • If you are running with sudo, you need to add sudo before the mkdir, the ls and both of the cp statements.
  • If you have multiple machines, you may want all of them to share the same keys.

[OPTIONAL] Configure client user and aliases

Your full dnsname may be a mouthful, the user the application runs under may be different than the one you use on your laptop. the port you expose may be non-standard, or you may have multiple machines and a desire to be able to SSH into a specific one.

If any of these apply to you, you can create or update a file named config in your .ssh directory. Following is an example that illustrates addressing a number of the above cases: 

Host appname
  Hostname appname.fly.dev
  User rails
  Port 2222
Host *.appname
  HostName %h.internal
  ProxyJump rails@appname.fly.dev:2222
  User rails
  Port 2222

Additionally, if you want to avoid the fingerprint checking, you can add the following to each of the Host entries:

StrictHostKeyChecking no
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