alknet/docs/architecture/server.md

status, last_updated

status	last_updated
reviewed	2026-06-02

Server

What

The alknet server accepts SSH connections (via pluggable transport) and handles channel_open_direct_tcpip requests by connecting to the requested target — either directly or through an outbound proxy.

Why

The server is the tunnel endpoint. It receives SSH channels requesting TCP connections to specific hosts and ports, and makes those connections on behalf of the client. It's the same role as an SSH server with AllowTcpForwarding yes, but self-contained and transport-agnostic.

Architecture

Server Components

┌──────────────────────────────────────────────────┐
│                   alknet serve                     │
│                                                   │
│  ┌─────────────────────────────────────────────┐ │
│  │          SSH Server (russh)                  │ │
│  │   ServerHandler per connection               │ │
│  │   - auth_publickey() → Accept/Reject        │ │
│  │   - channel_open_direct_tcpip() → connect   │ │
│  │   - channel_open_forwarded_tcpip() → proxy  │ │
│  └──────────────────┬──────────────────────────┘ │
│                      │                            │
│  ┌──────────────────▼──────────────────────────┐ │
│  │         Transport Acceptor                   │ │
│  │   (TcpListener / TlsListener / IrohEndpoint) │ │
│  └──────────────────────────────────────────────┘ │
│                                                   │
│  ┌──────────────────────────────────────────────┐ │
│  │         Outbound Proxy (optional)            │ │
│  │   - Direct TCP                               │ │
│  │   - SOCKS5 proxy                            │ │
│  │   - HTTP CONNECT proxy                      │ │
│  └──────────────────────────────────────────────┘ │
│                                                   │
│  ┌──────────────────────────────────────────────┐ │
│  │         Rate Limiter                         │ │
│  │   - max-connections-per-ip                   │ │
│  │   - max-auth-attempts                        │ │
│  └──────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────┘

Authentication

The server supports Ed25519 public key authentication (default) and OpenSSH certificate authority authentication (ADR-012):

Ed25519 public key (default):

1. Load authorized keys from a specified path or in-memory data
2. auth_publickey() checks the presented key against the authorized set
3. Uses constant-time comparison to prevent timing attacks

OpenSSH certificate authority (ADR-012):

1. Load a trusted CA public key (--cert-authority <path>)
2. auth_publickey() validates the presented certificate: checks CA signature, expiry, and principal restrictions
3. Supports certificate options: permit-port-forwarding, no-pty, source-address

This enables multi-user deployments where adding one CA line to authorized_keys is simpler than managing individual keys for every user.

No password authentication over SSH. Keys and certificates are sufficient and more secure. If a local SOCKS5 proxy needs its own auth layer, that's a separate concern.

Key Material Format

Key inputs (--key, --authorized-keys, --cert-authority) accept either file paths or in-memory data (via library API or NAPI wrapper). The accepted format is OpenSSH key format throughout — private keys in OpenSSH format (-----BEGIN OPENSSH PRIVATE KEY-----), public keys in OpenSSH format (ssh-ed25519 AAAA... user@host), and authorized keys files in standard OpenSSH authorized_keys format. PEM-encoded keys (PKCS#1, PKCS#8) are not supported.

TLS Certificate Provisioning

The server supports three TLS certificate modes (ADR-008):

1. Manual certs (--tls-cert / --tls-key): User provides certificate and key files. For users with existing PKI.
2. Domain-based ACME (--acme-domain <domain>): Auto-provisions certificates from Let's Encrypt using HTTP-01 or TLS-ALPN-01 challenges. Certificate is domain-bound and auto-renews. Requires port 80 or DNS access for challenges.
3. IP-based ACME: Short-lived certificates via TLS-ALPN-01 challenge on port 443. No domain name needed, but certificates expire frequently. The ACME client runs continuously.

ACME support is feature-gated behind the acme feature flag to keep the base binary lean. Implementation uses rustls-acme or a similar pure-Rust ACME client to avoid an external certbot dependency.

Channel Handling

When a client opens a channel_open_direct_tcpip(host, port, originator_addr, originator_port):

Reserved destination — If host starts with alknet- (e.g., alknet-control), the server routes the channel internally instead of connecting to a TCP target. The primary reserved destination is alknet-control:0, which bridges the channel to the local pubsub event bus (ADR-018).

Regular destination — For all other targets:

1. Connection — connect to host:port, either directly or via the configured outbound proxy
2. Outbound connection — connect to the target, either directly or via the configured outbound proxy
3. Bidirectional proxy — tokio::io::copy_bidirectional between the SSH channel stream and the outbound TCP stream
4. Cleanup — close the channel and TCP stream when either side disconnects

Outbound Proxy Modes

Mode	CLI Flag	Behavior
Direct	(default)	TcpStream::connect(target)
SOCKS5	--proxy socks5://addr:port	Connect through SOCKS5 proxy
HTTP CONNECT	--proxy http://addr:port	Connect through HTTP CONNECT proxy

The proxy setting applies globally to all outbound connections from the server.

Stealth Mode

When --stealth is enabled on the server alongside TLS transport:

1. Non-SSH connections (normal web browsers, scanners) receive a fake nginx 404 response
2. The server detects whether the connecting client is speaking SSH or HTTP after the TLS handshake
3. If SSH: proceed with server::run_stream()
4. If HTTP: respond with HTTP/1.1 404 Not Found + Server: nginx headers, then close

This makes the server appear as an ordinary web server to port scanners and DPI systems.

Stealth mode requires TLS transport (--transport tls). It has no effect with TCP or iroh transports — in those cases, there is no TLS handshake to peek behind, and protocol multiplexing is impossible. The CLI should reject or warn if --stealth is used without --transport tls.

Server Handler Behavior

The server handler implements russh::server::Handler with two primary responsibilities:

Authentication (auth_publickey):

• Check the presented key against the configured authorized_keys set (constant-time comparison)
• If no direct match, check whether the key is a certificate signed by a trusted cert-authority
• Validate certificate signature, expiry, and principal restrictions (e.g., permit-port-forwarding, no-pty, source-address)
• Return Accept or Reject

Channel handling (channel_open_direct_tcpip):

• If the destination host starts with alknet-, route internally (control channel, ADR-018)
• Otherwise, connect to host:port (directly or via the configured outbound proxy)
• Spawn a bidirectional proxy task between the SSH channel and the outbound TCP stream
• Return the channel for data flow

Logging and Rate Limiting

Logging (for fail2ban integration on Linux):

• INFO level: auth attempts (remote_addr, user, key_fingerprint, accept/reject)
• INFO level: connection opened (remote_addr, transport kind)
• INFO level: connection closed (remote_addr, duration)
• Do NOT log: channel open targets, DNS resolutions, bytes transferred

This matches our production fail2ban setup which filters on source IP + failure indicators. Example log lines:

INFO auth attempt remote_addr=203.0.113.50 user=root key_fingerprint=SHA256:abc... result=reject
INFO connection opened remote_addr=203.0.113.50 transport=tls

Built-in rate limiting (platform-independent):

Flag	Default	Purpose
--max-connections-per-ip	0 (unlimited)	Reject new connections from IPs with N active connections
--max-auth-attempts	10	Disconnect after N failed auth attempts per connection

These provide abuse protection on platforms without fail2ban (macOS, Windows, BSD) and complement fail2ban on Linux.

CLI Interface

# Basic server (SSH on port 22)
alknet serve --key ~/.ssh/ssh_host_ed25519_key

# With TLS (manual certs)
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport tls \
    --tls-cert /etc/ssl/cert.pem \
    --tls-key /etc/ssl/key.pem

# With TLS (auto ACME, domain-based)
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport tls \
    --acme-domain example.com

# With TLS + stealth (fake nginx 404 to scanners)
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport tls \
    --acme-domain example.com \
    --stealth

# With iroh transport (no public IP needed)
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport iroh

# With outbound proxy
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --proxy socks5://127.0.0.1:9050

# With certificate authority authentication
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --cert-authority /etc/alknet/ca.pub

# With rate limiting
alknet serve --key ~/.ssh/ssh_host_ed25519_key \
    --max-connections-per-ip 5 \
    --max-auth-attempts 3

# All options
alknet serve \
  --key <path-or-buffer> \       # SSH host key (required)
  --authorized-keys <path> \     # Authorized keys file
  --cert-authority <path> \      # CA public key for cert-auth
  --transport tcp|tls|iroh \     # Transport mode
  --listen <addr:port> \         # Listen address for TCP/TLS (default: 0.0.0.0:22)
  --tls-cert <path> \            # TLS certificate (manual)
  --tls-key <path> \            # TLS private key (manual)
  --acme-domain <domain> \      # ACME auto-cert domain
  --stealth \                    # Serve fake nginx 404 to non-SSH connections
  --proxy <url> \                # Outbound proxy URL (socks5:// or http://)
  --iroh-relay <url> \           # iroh relay server URL (default: n0 relay)
  --max-connections-per-ip <n> \ # Max concurrent connections per IP (default: unlimited)
  --max-auth-attempts <n>        # Max auth failures before disconnect (default: 10)

iroh Server Mode

When running with --transport iroh, the server:

1. Creates an iroh endpoint with ALPN value b"alknet-ssh"
2. Prints its endpoint ID (base58-encoded Ed25519 public key) — this is what clients use as the --peer value
3. Accepts incoming connections on the endpoint
4. For each connection, accepts a bidirectional stream and passes it to server::run_stream()

No listening port is needed. The server connects outbound to the iroh relay (default: n0, override with --iroh-relay) and awaits connections from clients who know its endpoint ID (base58-encoded, printed on startup).

Constraints

• The server does not log tunnel destinations (ADR-006). Auth events and connection events are logged for fail2ban integration (ADR-013).
• Destination strings beginning with alknet- are reserved for internal use (ADR-018). The server must not attempt TCP connections to alknet-* destinations — these are intercepted for control channel routing.
• One ServerHandler instance per connection. Handler state is not shared between connections (unless explicitly configured via Arc shared state for things like connection limits).
• The server binds to a single transport at a time. Running multiple transports (e.g., TCP + iroh) simultaneously requires separate processes or a future multiplexing feature.
• ACME support requires the acme feature flag. Without it, only manual TLS certs are supported.
• No password authentication over SSH channels. Key-based and cert-authority only (ADR-012).
• Stealth mode (--stealth) requires TLS transport. It has no effect on TCP or iroh transports (ADR-017).

Graceful Shutdown

On SIGTERM or SIGINT:

1. Stop accepting new connections on the transport listener
2. Send SSH disconnect messages to all active sessions
3. Wait for in-flight channel data to drain (brief timeout, ~2 seconds per session)
4. Close all transport listeners
5. Exit

The server does not wait indefinitely for idle connections to close. After the drain timeout, remaining connections are forcibly terminated. This prevents a slow or stuck client from blocking shutdown indefinitely.

Error Handling

Error handling follows the project's layered pattern (see overview.md):

• Transport errors: Cause connection rejection. The listener remains active — a failed TLS handshake or iroh connection attempt does not affect other incoming connections.
• Auth errors: Result in connection rejection with a logged auth failure event (for fail2ban, ADR-013). Repeated failures from one connection trigger disconnect after --max-auth-attempts.
• Channel-level errors: Individual channel failures (target unreachable, proxy failure) close that channel without affecting the SSH session or other channels. The client receives a channel open failure message.
• CLI errors: Reported to stderr with a non-zero exit code. Fatal errors (invalid flags, key file not found, bind failure) exit immediately.

Open Questions

None — all resolved.

Design Decisions

ADR	Decision	Summary
001	Pluggable transport	Transport trait, SSH consumes stream
004	SSH over transport	SSH never touches network directly
006	No logging of destinations	Server logs auth and connections, not destinations
008	ACME/Let's Encrypt	Auto-provision TLS certs, domain and IP paths
012	Key + cert-authority auth	No password auth; support OpenSSH cert-authority
013	Fail2ban-friendly logging	Structured auth logs + built-in rate limiting
017	Stealth mode	Protocol multiplexing on port 443
018	Control channel	Reserved alknet-control destination for pubsub
019	Proxy dual semantics	--proxy routes transport on client, data on server