alknet/docs/architecture/server.md

status, last_updated

status	last_updated
draft	2026-06-01

Server

What

The wraith 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

┌──────────────────────────────────────────────────┐
│                   wraith 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                       │ │
│  └──────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────┘

Authentication

The server supports Ed25519 public key authentication by default:

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

Optional password authentication (not recommended, controlled by feature flag or CLI flag).

Channel Handling

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

1. ACL check — verify the client is allowed to connect to host:port (if ACLs are configured)
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.

Server Handler (russh)

struct WraithServerHandler {
    authorized_keys: HashSet<PublicKey>,
    proxy_config: Option<ProxyConfig>,
}

impl server::Handler for WraithServerHandler {
    type Error = anyhow::Error;

    async fn auth_publickey(&mut self, user: &str, key: &PublicKey) -> Auth {
        if self.authorized_keys.contains(key) {
            Auth::Accept
        } else {
            Auth::Reject { proceed_with_methods: None, partial_success: false }
        }
    }

    async fn channel_open_direct_tcpip(
        &mut self,
        channel: Channel<server::Msg>,
        host: &str,
        port: u32,
        originator_addr: &str,
        originator_port: u32,
        session: &mut server::Session,
    ) -> Result<Channel<server::Msg>, Self::Error> {
        // ACL check (if configured)
        // Connect to host:port (directly or via proxy)
        // Spawn bidirectional proxy task
        Ok(channel)
    }
}

Logging

• Log: Auth attempts (timestamp, source IP, user, key fingerprint, success/failure)
• Do not log: Channel open targets, DNS resolutions, bytes transferred, connection duration

This provides enough information for fail2ban integration without creating a privacy-sensitive audit trail.

CLI Interface

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

# With TLS on port 443
wraith serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport tls \
    --tls-cert /etc/ssl/cert.pem \
    --tls-key /etc/ssl/key.pem

# With TLS + stealth (fake nginx 404 to scanners)
wraith serve --key ~/.ssh/ssh_host_ed25519_key \
    --transport tls \
    --tls-cert /etc/ssl/cert.pem \
    --tls-key /etc/ssl/key.pem \
    --stealth

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

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

# All options
wraith serve \
  --key <path> \              # SSH host key path (required)
  --authorized-keys <path> \  # Authorized keys file (default: ~/.ssh/authorized_keys)
  --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 (required for tls transport)
  --tls-key <path> \          # TLS private key (required for tls transport)
  --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)

iroh Server Mode

When running with --transport iroh, the server:

1. Creates an iroh::Endpoint with the SSH ALPN
2. Prints its EndpointId (Ed25519 public key) — this is what clients use to connect
3. Uses iroh::protocol::Router to accept incoming connections
4. For each connection, accepts a open_bi() stream and passes it to server::run_stream()

No listening port is needed. The server connects outbound to the iroh relay and awaits connections from clients who know its EndpointId.

Constraints

• The server does not log tunnel destinations (ADR-006, pending)
• 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.

Open Questions

• OQ-07: Whether to support ACME/Let's Encrypt auto-provisioning for TLS certificates
• OQ-08: Connection limits and rate limiting configuration

Design Decisions

ADR	Decision	Summary
001	Pluggable transport	Transport trait, SSH consumes stream
004	SSH over transport	SSH never touches network directly