ssh-tunnel-gateway 0.4.3

SSH tunnel server + agent with reverse port forwarding

ssh-tunnel-gateway

Single Python package that ships both CLIs:

• ssh-tunnel-server (control plane + port allocation)
• ssh-tunnel-agent (reverse SSH tunnel on private host)

This project uses a public gateway as a bastion to reach private/internal servers over standard SSH.

Install

Install on both gateway and agent hosts:

pip install ssh-tunnel-gateway

SKILLS.md

Session skill guide for coding/release agents is in SKILLS.md.

Quick Start (Recommended)

1. Start gateway server:

API_KEY="change-me" ssh-tunnel-server

2. Start agent:

ssh-tunnel-agent --api-key change-me --endpoint http://<gateway_host>:12000

3. Read allocated port_b from agent session file:

cat ~/.ssh-tunnel/session.json

python3 -c 'import json, os; print(json.load(open(os.path.expanduser("~/.ssh-tunnel/session.json")))["port_b"])'

4. Add client SSH config (~/.ssh/config):

Host GWServer
    HostName <gateway_public_ip_or_dns>
    User <gateway_ssh_user>
    Port 22

Host AgentServer
    HostName localhost
    User <agent_ssh_user>
    Port <port_b>
    ProxyJump GWServer

5. Connect:

ssh AgentServer

Control Plane Modes

Standard mode

Agent calls HTTP API directly:

ssh-tunnel-agent --api-key change-me --endpoint http://<gateway_host>:12000

Gateway requirements:

• 22/tcp for SSH data plane
• 12000/tcp for HTTP control plane

--over-ssh mode

Agent reaches control plane through SSH local forwarding (no public 12000 required):

ssh-tunnel-agent --api-key change-me --endpoint http://127.0.0.1:12000 --over-ssh GWServer

SSH alias example:

Host GWServer
    HostName <gateway_public_ip_or_dns>
    User <gateway_ssh_user>
    Port 22

Important behavior:

• Alias match is strict against SSH config (~/.ssh/config by default, or SSH_CONFIG_PATH).
• If alias is missing, agent falls back to standard endpoint mode.
• In --over-ssh mode, API_URL host/IP is ignored; only the API port is used.
• Agent prefers binding the same local API port first; if unavailable, it picks a free local port.

Systemd Deployment

Use -d for systemd (run once and let systemd restart on failure).

Server unit (/etc/systemd/system/ssh-tunnel-server.service):

[Unit]
Description=ssh-tunnel-gateway server
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
User=gw-tunnel
WorkingDirectory=/home/gw-tunnel
EnvironmentFile=/etc/ssh-tunnel/server.env
ExecStart=/usr/local/bin/ssh-tunnel-server -d
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

Server env (/etc/ssh-tunnel/server.env):

# Required:
API_KEY=change-me

# Optional (use instead of API_KEY for multiple keys):
# API_KEYS=key1,key2,key3

# Optional overrides:
# STATE_DIR=/home/gw-tunnel/.ssh-tunnel/server
# SERVER_HOST=0.0.0.0
# SERVER_PORT=12000
# SSH_USER=gw-tunnel
# SSH_JUMP_USER=li
# SSH_PUBLIC_HOST=gateway.example.com
# AUTHORIZED_KEYS_PATH=/home/gw-tunnel/.ssh/authorized_keys
# AGENT_REVERSE_BIND_HOST=0.0.0.0

Agent unit (/etc/systemd/system/ssh-tunnel-agent.service):

[Unit]
Description=ssh-tunnel-gateway agent
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
User=li
WorkingDirectory=/home/li
EnvironmentFile=/etc/ssh-tunnel/agent.env
ExecStart=/usr/local/bin/ssh-tunnel-agent -d
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

Agent env (/etc/ssh-tunnel/agent.env):

# Required:
API_KEY=change-me
API_URL=http://<gateway_host>:12000

# Optional overrides:
# STATE_DIR=/home/li/.ssh-tunnel
# SSH_CONFIG_PATH=/home/li/.ssh/config
# SSH_HOST=<gateway_host>
# SSH_PORT=22
# LOCAL_TARGET_HOST=127.0.0.1
# LOCAL_TARGET_PORT=22
# REVERSE_BIND_HOST=0.0.0.0

# Optional control-plane over SSH (if enabled, API_URL host is ignored and port is used):
# OVER_SSH=GWServer
# API_URL=http://127.0.0.1:12000

Enable services:

sudo systemctl daemon-reload
sudo systemctl enable --now ssh-tunnel-server
sudo systemctl enable --now ssh-tunnel-agent

Systemd note:

• Use absolute paths in env files (/home/<user>/...) instead of ~.

Operational Behavior

• Server API authentication supports API_KEY or API_KEYS (comma-separated).
• API endpoint is POST / with JSON action:
• register: allocate or reuse port_b, return encrypted key material.
• heartbeat: refresh lease.
• Agent keeps a stable local agent_id (UUID) unless explicitly overridden.
• Agent writes session state to ${STATE_DIR}/session.json (default ~/.ssh-tunnel/session.json).
• Server keeps agent_id -> port_b mapping and reuses the same port when possible.
• If reused port is busy, server tries reclaiming it; if reclaim fails, server allocates a new one.
• Lease timeout is 7 days by default (LEASE_TTL_DAYS); expired agents are cleaned from state and authorized_keys.
• Restarting ssh-tunnel-server does not restart existing tunnel processes in sshd; active tunnels stay up while their SSH sessions stay up.

SSHD Requirements (Gateway)

For default public reverse bind (AGENT_REVERSE_BIND_HOST=0.0.0.0):

AllowTcpForwarding remote
GatewayPorts clientspecified

For loopback-only reverse bind (AGENT_REVERSE_BIND_HOST=127.0.0.1):

• AllowTcpForwarding remote is still required.

Security Guidance

• Use a dedicated SSH tunnel user on gateway (for example gw-tunnel).
• Block shell/TTY for that user and allow only remote port forwarding.
• Keep client jump access (SSH_JUMP_USER) separate from tunnel login (SSH_USER) when needed.

Example sshd_config hardening:

Match User gw-tunnel
    PasswordAuthentication no
    PubkeyAuthentication yes
    PermitTTY no
    X11Forwarding no
    AllowAgentForwarding no
    PermitUserRC no
    AllowTcpForwarding remote

---

Agent Skills (From SKILLS.md)

Scope

• Single package: ssh-tunnel-gateway
• Two CLIs: ssh-tunnel-server, ssh-tunnel-agent
• Transport model: SSH data plane (22) + HTTP control plane (12000) or --over-ssh

Design Intent

• Keep SSH native (ProxyJump, standard ssh config, standard sshd settings).
• Keep control plane minimal (POST / with action=register|heartbeat).
• Keep agent identity stable (cached agent_id) and reuse port_b when possible.
• Prefer explicit and operationally safe defaults.

Runtime Defaults

• Agent state directory default: ~/.ssh-tunnel
• Agent session file default: ~/.ssh-tunnel/session.json
• Agent key file default: ~/.ssh-tunnel/agent.pem
• Agent id file default: ~/.ssh-tunnel/agent_id
• Lease cleanup default: 7 days (LEASE_TTL_DAYS)

--over-ssh Rules

• --over-ssh <alias> must match an alias in SSH config (~/.ssh/config or SSH_CONFIG_PATH).
• If alias is missing, fallback to standard endpoint mode.
• In --over-ssh mode, ignore API_URL host and use only its port.
• Use SSH alias directly as destination (do not force user@alias).

Systemd Rules

• Use -d for both CLIs under systemd.
• Prefer absolute paths in env files.
• Set User= explicitly so home expansion resolves as expected.
• Keep optional env vars commented in example env files.

Logging Expectations

• Agent logs register result with agent_id, port_b, ssh_user, jump_user, jump_host, and connect_hint.
• Server logs register/heartbeat with client IP and key operational fields.
• Foreground mode should provide enough info to debug without inspecting code.

Release Checklist

1. Update VERSION.
2. Verify README reflects current behavior and flags.
3. Build: make build.
4. Verify CLI versions: ssh-tunnel-server --version, ssh-tunnel-agent --version.
5. Upload: make upload.
6. If PyPI says file exists, bump version and rebuild.

Documentation Style

• Put usage and copy/paste commands before deep details.
• Keep server and agent examples separate and explicit.
• Document required env vars as active lines and optional ones as comments.