alknet/docs/architecture/server.md

---
status: reviewed
last_updated: 2026-06-07
---

# 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 authenticates connections through the `IdentityProvider` trait (ADR-029, [identity.md](identity.md)). `IdentityProvider` decouples the server from any specific identity storage — the server resolves an identity, it doesn't manage keys.

**Phase 1 implementation**: `ConfigIdentityProvider` (in alknet-core) reads from `ArcSwap<DynamicConfig.auth>` (ADR-030). Every authorized key gets a default scope set. No database required. This is the default for CLI and single-node deployments.

**Future implementation**: `StorageIdentityProvider` (in alknet-storage, not yet built) backed by SQLite `peer_credentials` and `api_keys` tables plus the ACL graph. The server doesn't need to know which implementation is active — it goes through the trait.

The server supports two auth presentation paths (ADR-023, [auth.md](auth.md)):

**SSH public key auth** (SSH transports):
1. `auth_publickey()` callback receives the presented key
2. Delegates to `IdentityProvider::resolve_from_fingerprint()` with the key fingerprint
3. Returns `Accept` (with `Identity` attached) or `Reject`

**Ed25519 + OpenSSH certificate authority** (ADR-012):
1. If no direct key match, validate the presented certificate against trusted cert-authorities
2. Check CA signature, expiry, and principal restrictions
3. Certificate options: `permit-port-forwarding`, `no-pty`, `source-address`

**Token auth** (non-SSH transports, WebTransport):
1. Extract token from URL path or `Authorization` header
2. Delegate to `IdentityProvider::resolve_from_token()`
3. Same verification: same authorized keys set, same `Identity` result (ADR-023)

**No password authentication over SSH channels.** 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).

**Forwarding policy check** — Before the proxy task is spawned for any non-reserved destination, the server evaluates `ForwardingPolicy` against the authenticated `Identity` (ADR-031, [configuration.md](configuration.md)). The policy check uses `Identity.id` and `Identity.scopes` from the identity resolved during auth. If the policy denies the destination, the channel open is rejected — no TCP connection is attempted. The default policy (`ForwardingPolicy::allow_all()`) preserves current behavior.

**Regular destination** — For targets that pass the forwarding policy check:

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`)**:
- Delegate to `IdentityProvider::resolve_from_fingerprint()` with the presented key fingerprint
- If identity resolved, return `Accept` with the `Identity` attached to the session
- If no identity, check certificate authority: validate CA signature, expiry, principals
- Return `Accept` or `Reject`

**Channel handling (`channel_open_direct_tcpip`)**:
- If the destination host starts with `alknet-`, route internally (control channel, ADR-018)
- Otherwise, evaluate `ForwardingPolicy` against the session's `Identity` (ADR-031)
- If denied, reject the channel open
- If allowed, 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

### Interface Abstraction

SSH is one interface at Layer 2 in the three-layer model (ADR-026, [interface.md](interface.md)). The current `ServerHandler` will be refactored into `SshInterface` — it manages SSH session concerns (handshake, auth delegation, channel multiplexing). Forwarding policy, operation routing, and call protocol handling are Layer 3 concerns that live outside the interface. This refactoring is the most invasive code change in Phase 1 (integration-plan, Phase 1.8).

### 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

Configuration sources (in priority order): CLI flags, environment variables, optional `--config` TOML file (ADR-030). The TOML config file is a convenience input for reproducible deployments; it does not replace `ServeOptions` (ADR-011).

Multi-transport listeners use `[[listeners]]` in the TOML config (ADR-030):

```toml
[[listeners]]
transport = "tls"
listen = "0.0.0.0:443"

[listeners.tls]
cert = "/etc/alknet/tls/cert.pem"
key = "/etc/alknet/tls/key.pem"

[[listeners]]
transport = "iroh"
```

Currently, the server binds to a single transport at a time. Multi-transport via `[[listeners]]` is coming per ADR-030.

```bash
# 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 currently binds to a single transport at a time. Multi-transport via `[[listeners]]` is coming per ADR-030.
- Forwarding policy is evaluated before every channel proxy spawn. Denied channels are rejected immediately (ADR-031).
- Auth resolves through `IdentityProvider` (ADR-029). Phase 1 uses `ConfigIdentityProvider` backed by `ArcSwap<DynamicConfig>` (ADR-030). `StorageIdentityProvider` (Phase 2+) replaces it for production deployments with SQLite.
- 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](decisions/001-pluggable-transport.md) | Pluggable transport | Transport trait, SSH consumes stream |
| [004](decisions/004-ssh-over-transport.md) | SSH over transport | SSH never touches network directly |
| [006](decisions/006-no-logging-of-tunnel-destinations.md) | No logging of destinations | Server logs auth and connections, not destinations |
| [008](decisions/008-acme-lets-encrypt.md) | ACME/Let's Encrypt | Auto-provision TLS certs, domain and IP paths |
| [012](decisions/012-auth-ed25519-and-cert-authority.md) | Key + cert-authority auth | No password auth; support OpenSSH cert-authority |
| [013](decisions/013-fail2ban-friendly-logging.md) | Fail2ban-friendly logging | Structured auth logs + built-in rate limiting |
| [017](decisions/017-stealth-mode-protocol-multiplexing.md) | Stealth mode | Protocol multiplexing on port 443 |
| [018](decisions/018-control-channel-for-pubsub.md) | Control channel | Reserved `alknet-control` destination for pubsub |
| [019](decisions/019-proxy-dual-semantics.md) | Proxy dual semantics | `--proxy` routes transport on client, data on server |
| [026](decisions/026-transport-interface-separation.md) | Three-layer model | SSH is Layer 2 interface, ServerHandler → SshInterface |
| [028](decisions/028-auth-irpc-service.md) | Auth as irpc service | IdentityProvider is the contract; irpc service is one backend |
| [029](decisions/029-identity-core-type.md) | Identity as core type | IdentityProvider trait in alknet-core |
| [030](decisions/030-static-dynamic-config-split.md) | Static/dynamic config split | ArcSwap for dynamic config, ConfigReloadHandle |
| [031](decisions/031-forwarding-policy.md) | Forwarding policy | Evaluated before channel proxy spawn |

## References

- [configuration.md](configuration.md) — DynamicConfig, ForwardingPolicy, ConfigReloadHandle
- [identity.md](identity.md) — IdentityProvider trait, Identity struct
- [auth.md](auth.md) — Unified auth, AuthPolicy, token auth
- [interface.md](interface.md) — Interface trait, SshInterface, three-layer model