ssh-tunnel

Minimal, hardened, distroless SSH container for TCP port forwarding. Built on OpenSSH 10.4 with post-quantum cryptography (PQC) key exchange.

Features

• Distroless -- FROM scratch, no shell, no package manager (~8MB image)
• PQC key exchange -- ML-KEM-768 hybrid (FIPS 203) with X25519 fallback
• ED25519 only -- host keys and user authentication
• Tunnel-only -- local TCP forwarding, no shell, no SFTP, no SCP
• Hardened -- read-only filesystem, dropped capabilities, no-new-privileges

Quick Start

Generate a host key:

mkdir host_keys
ssh-keygen -t ed25519 -f host_keys/ssh_host_ed25519_key -N ""

Create an authorized_keys file with your public key(s):

cp ~/.ssh/id_ed25519.pub authorized_keys

Start the container:

docker compose up -d

Connect:

ssh -p 2222 -N -L 8080:internal-host:80 tunnel@gateway

Published image

Prebuilt images are published to the GitHub Container Registry, so you don't have to build locally:

docker pull ghcr.io/weaverant/ssh-tunnel:latest

Available tags:

Tag	Tracks
latest	Newest release
0.1.2	A specific pinned release
0.1	Latest patch within a major.minor line

To run the published image directly with the same hardening as docker-compose.yml:

docker run -d --name ssh-tunnel \
  -p 2222:2222 \
  --read-only --tmpfs /run --tmpfs /tmp \
  --security-opt no-new-privileges:true \
  --cap-drop ALL \
  --cap-add SETUID --cap-add SETGID --cap-add SYS_CHROOT --cap-add DAC_OVERRIDE \
  -v "$PWD/host_keys/ssh_host_ed25519_key:/etc/ssh/host_keys/ssh_host_ed25519_key:ro" \
  -v "$PWD/authorized_keys:/etc/ssh/authorized_keys:ro" \
  ghcr.io/weaverant/ssh-tunnel:latest

Or point docker-compose.yml at the published image by replacing build: . with image: ghcr.io/weaverant/ssh-tunnel:latest.

Configuration

All hardening is baked into the image. No environment variables, no runtime configuration.

The container expects two bind mounts:

Mount	Container Path	Mode
Host key	/etc/ssh/host_keys/ssh_host_ed25519_key	ro, 0600
Authorized keys	/etc/ssh/authorized_keys	ro, 0644

Security

sshd

Setting	Value
Authentication	Public key only (ED25519)
Key exchange	mlkem768x25519-sha256, sntrup761x25519-sha512, curve25519-sha256
Ciphers	chacha20-poly1305, aes256-gcm
Forwarding	Local TCP only
Shell access	None (ForceCommand /sbin/nologin, PermitTTY no)
SFTP/SCP	Disabled
Agent/X11 forwarding	Disabled

Container

Setting	Value
Filesystem	Read-only
Capabilities	All dropped, only SETUID/SETGID/SYS_CHROOT/DAC_OVERRIDE added
Privilege escalation	Blocked (no-new-privileges)
Base image	scratch (no shell, no package manager)

Building

docker build -t ssh-tunnel .

License

MIT