dn42 Auto-Peering SSH Service (AS4242420263)

A Python-based self-service SSH server that enables dn42 network maintainers to automatically establish and manage peering connections with AS4242420263.

Overview

This project implements a custom SSH server using Paramiko that authenticates users against the dn42 registry. It allows network operators to request peering sessions without manual intervention, storing all peering information in a SQLite database.

Quick Start

Prerequisites

• Debian >= 12
• Python 3

Installation

1. Clone this repository
2. Clone the dn42 registry
3. Install dependencies: apt install git python3-dnspython python3-packaging python3-paramiko python3-psutil python3-rich
4. Generate a SSH host key with ssh-keygen
5. Set up environment variables (see Configuration section)
6. Run the server:

$ python dn42-autopeer.py --server
2025-04-06 22:15:34,391:INFO:[SSHServerBase] Listening thread started

Usage

python dn42-autopeer.py --help
usage: dn42-sshd [-h] (--server | --genconfig | --gaming)

options:
  -h, --help   show this help message and exit
  --server     Start the auto peering server
  --genconfig  Generate the configuration from the peering db
  --gaming     Start the gaming server

Configuration

Environment variables

These environment variables can be set to configure the service

Variable	Description	Default
DN42_SSH_HOST_KEY	Path to SSH host key file	files/ssh-keys/ssh_host_rsa_key
DN42_SSH_LISTEN_ADDRESS	Listen address	::1
DN42_SSH_PORT	SSH port	4242
DN42_SERVER	Public domain name of the server	nl-ams2.flap42.eu
DN42_SSH_MOTD_PATH	Path to custom MOTD file	files/motd/$DN42_SERVER
DN42_DB_PATH	Path to SQLite database file	files/db/$DN42_SERVER
DN42_REGISTRY_DIRECTORY	Path to dn42 registry clone	files/registry
DN42_ASN	Your dn42 Autonomous System Number	4242420263
DN42_WG_PUB_KEY	WireGuard public key for tunnels	rj0SORruOE/hGV...
DN42_WG_PRIV_KEY	WireGuard private key used in --genconfig	**REPLACEME**
DN42_WG_LINK_LOCAL	IPv6 link-local base address (without last 4 bytes)	fe80:0263::
DN42_WG_BASE_PORT	Base WireGuard port	52000
DN42_RESERVED_NETWORK	Network where peering is restricted	None
DN42_BIRD_CONFIG_DIR	Place to generate the bird config files in --genconfig	files/bird
DN42_WG_CONFIG_DIR	Place to generate the wg config files in --genconfig	files/wireguard

Installation

I deploy this service on Debian using my own ansible role.

This is an example of a systemd service unit (/etc/systemd/system/dn42-sshd.service).

[Unit]
Description=DN42 SSHD Service
After=network.target

[Service]
Type=simple
User=dn42-sshd
Group=dn42-sshd
WorkingDirectory=/home/dn42-sshd/dn42-sshd
ExecStart=python3 dn42-sshd.py --peering
Restart=on-failure
RestartSec=5
StandardOutput=journal
StandardError=journal
Environment="DN42_SSH_HOST_KEY=/home/dn42-sshd/.ssh/id_rsa"
Environment="DN42_SSH_LISTEN_ADDRESS=::"
Environment="DN42_SSH_PORT=4242"
Environment="DN42_DB_PATH=/home/dn42-sshd/peering.db"
Environment="DN42_REGISTRY_DIRECTORY=/home/dn42-sshd/dn42-registry"
Environment="DN42_ASN=4242420263"
Environment="DN42_WG_PUB_KEY=C3Wlu6y+v84FN/vreuTqL6r5wEtGTMXX5rKgHkxDaTI="
Environment="DN42_WG_LINK_LOCAL=fe80:0263::"
Environment="DN42_WG_BASE_PORT=52000"
Environment="DN42_RESERVED_NETWORK=2001:0bc8:3feb::/48"

# Security hardening
ProtectSystem=full
PrivateTmp=true
NoNewPrivileges=true
ProtectControlGroups=true
ProtectKernelModules=true
ProtectKernelTunables=true
RestrictNamespaces=true

[Install]
WantedBy=multi-user.target

It assumes that:

• a local user dn42-sshd exists
• the present git repository is cloned in /home/dn42-sshd/dn42-sshd
• the dn42 registry is cloned in /home/dn42-sshd/dn42-registry
• the SQLite database file is /home/dn42-sshd/peering.db and is writeable

It can be enabled and started using the systemctl command:

$ systemctl enable --now dn42-sshd
Created symlink /etc/systemd/system/multi-user.target.wants/dn42-sshd.service → /etc/systemd/system/dn42-sshd.service

The logs are readable using the journalctl command:

$ journalctl -n 100 -f -u dn42-sshd.service
...
Apr 06 21:57:24 nl-ams2 systemd[1]: Started dn42-sshd.service - DN42 SSHD Service.
Apr 06 21:57:31 nl-ams2 python3[9780]: 2025-04-06 21:57:31,576:INFO:[SSHServerBase] Listening thread started
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,505:INFO:[SSHServerBase] Accepted connection from ('2a01:...', 48396, 0, 0)
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,509:INFO:Connected (version 2.0, client OpenSSH_9.9)
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,777:INFO:Auth rejected (none).
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,877:INFO:[AuthDn42] Authentication successful for hcartiaux
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,910:INFO:[AuthDn42] Authentication successful for hcartiaux
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,911:INFO:Auth granted (publickey).
Apr 06 22:12:28 nl-ams2 python3[9780]: 2025-04-06 22:12:28,942:INFO:[SSHServerBase] Started thread 139777270654656 for hcartiaux@('2a01:...', 48396, 0, 0)
Apr 06 22:14:51 nl-ams2 python3[9780]: 2025-04-06 22:14:51,924:INFO:[139777270654656][SSHServerShell] User hcartiaux disconnected

Internals

Authentication methods

Anonymous, class SSHServerAuthNone

Accept all connections

dn42 registry, class SSHServerAuthDn42

Accept connections based on the dn42 maintainer objects, using the defined public key(s)

• username: lowercase maintainer name, without the -MNT suffix
• public keys: supports ssh-rsa and ssh-ed25519 keys defined in the auth field

Specialized SSH servers

Common constructor parameters:

• server_interface - can be set to SSHServerAuthNone or SSHServerAuthDn42
• host_key_file - path of the ssh host key file

Piping, class SSHServerPipe(server_interface, cmd, host_key_file)

• cmd - command to be executed and piped to the user

Shell, class SSHServerShell(server_interface, shell_class, host_key_file)

• shell_class - name of the Cmd subclass to be used as shell instances.

Custom shell

The custom shell is implemented by the class ShellDn42(username, ...). It extends the Cmd class heavily.

Database management

The SQLite database is entirely managed using the class DatabaseManager(db_path). It contains only one table.

CREATE TABLE IF NOT EXISTS peering_links (
    id INTEGER PRIMARY KEY CHECK(id BETWEEN 1 AND 65535),
    as_num INTEGER UNIQUE NOT NULL,
    wg_pub_key TEXT NOT NULL,
    wg_endpoint_addr TEXT NOT NULL,
    wg_endpoint_port INTEGER NOT NULL CHECK(wg_endpoint_port BETWEEN 1 AND 65535)

External resources

• Ansible role for deployment
• PythonSSHServerTutorial (Used as a starting point)