stunnel

Run the stunnel container

stunnel requires a configuration file to start. The default CMD passed to the entrypoint is /etc/stunnel/stunnel.conf. Running the container without that file causes stunnel to exit with a configuration error.

To display the stunnel version and compiled-in options:

docker run --rm --entrypoint /usr/bin/stunnel registry.ghost-prod.alphabravo.io/ghost-base/stunnel:<tag> -version

To display help:

docker run --rm --entrypoint /usr/bin/stunnel registry.ghost-prod.alphabravo.io/ghost-base/stunnel:<tag> -help

Configuration

stunnel has no exposed ports by default. All ports are defined in stunnel.conf using accept and connect directives. You must publish whatever port the accept directive listens on with -p or in your Compose file.

Minimal configuration example

The following stunnel.conf wraps a plaintext backend on 127.0.0.1:8080 with TLS, accepting connections on port 8443:

; Global options
foreground = yes

[https-wrapper]
accept  = 8443
connect = 127.0.0.1:8080
cert    = /etc/stunnel/certs/server.pem

foreground = yes is required when running inside a container so stunnel does not daemonize. The pid file is disabled implicitly because the runtime image runs as a nonroot user with no writable home; with foreground = yes stunnel does not need one.

Store your certificate and private key (concatenated) in a single PEM file, for example:

cat server.crt server.key > server.pem

Run with a volume-mounted configuration

docker run -d --name stunnel \
  -p 8443:8443 \
  -v /path/to/your/stunnel.conf:/etc/stunnel/stunnel.conf:ro \
  -v /path/to/your/certs:/etc/stunnel/certs:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/stunnel:<tag>

Docker Compose example

services:
  stunnel:
    image: registry.ghost-prod.alphabravo.io/ghost-base/stunnel:<tag>
    ports:
      - "8443:8443"
    volumes:
      - ./config/stunnel.conf:/etc/stunnel/stunnel.conf:ro
      - ./certs:/etc/stunnel/certs:ro
    restart: unless-stopped

Client mode: adding TLS to an outbound connection

stunnel can also act as a TLS client, useful when a local application needs to connect to a remote TLS endpoint but does not support TLS itself. The following example accepts plaintext connections on 127.0.0.1:5432 and forwards them as TLS to a remote PostgreSQL server:

foreground = yes

[pg-tls-client]
client  = yes
accept  = 127.0.0.1:5432
connect = db.example.com:5432

Point your PostgreSQL client at localhost:5432 and stunnel handles the TLS handshake transparently.

For the full configuration reference, see the stunnel manual page.

Image variants

Ghost hardened images come in different variants depending on their intended use.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as the nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

FIPS

The fips and fips-dev variants replace Debian's standard libssl3 with the DHI OpenSSL FIPS provider package (dhi/pkg-openssl:...-debian13-fips). The stunnel binary in all variants is compiled with --enable-fips, so it can load and use the FIPS provider when the library is present.

To opt into FIPS mode at runtime, add fips = yes to the global section of your stunnel.conf:

fips = yes
foreground = yes

[my-service]
accept  = 8443
connect = 127.0.0.1:8080
cert    = /etc/stunnel/certs/server.pem

With fips = yes, stunnel instructs OpenSSL to activate the FIPS provider. Any cipher or algorithm not permitted under FIPS 140 will be refused. Use a fips-tagged image alongside this setting; the standard runtime image does not include the FIPS-validated provider library.

These images are STIG-certified through the DHI OpenSSL FIPS include, which provides the validated cryptographic module.

Migrate from dockurr/stunnel

The dockurr/stunnel image is a third-party Docker Hub repackage and is not maintained by the stunnel upstream project. The DHI stunnel image is built directly from the official mtrojnar/stunnel source repository.

The stunnel configuration file format is identical between the two images. Migration requires only an image reference change. No configuration edits are needed.

Replace the image reference in your docker run command or Compose file:

# Before
image: dockurr/stunnel

# After
image: registry.ghost-prod.alphabravo.io/ghost-base/stunnel:<tag>

The following table summarises other differences to be aware of:

Migrate to a Ghost hardened image

To migrate your application to a Ghost hardened image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Ghost hardened image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Ghost hardened images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshooting migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Ghost hardened images is to use Docker Debug to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Ghost hardened images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Ghost hardened images and update your Dockerfile if necessary.