initial commit: standalone SoftEther VPN client in Go

Built-in DHCP (raw Ethernet frames through tunnel), automatic reconnection,
host route management, classless static routes (option 121/249), DNS config.
Single static binary, Linux only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/structure.md

@@ -0,0 +1,57 @@
+# Project structure
+
+```
+softether-go/
+├── cmd/softether-go/
+│   └── main.go              CLI entry point
+├── pkg/
+│   ├── client/
+│   │   ├── client.go        SoftEther handshake and session
+│   │   ├── tunnel.go        TCP block framing and keepalive
+│   │   └── crypto.go        SHA-0 and password hashing
+│   ├── protocol/
+│   │   ├── http.go          HTTP transport layer
+│   │   └── pack.go          Pack binary serialization
+│   ├── dhcp/
+│   │   └── dhcp.go          DHCP client (raw Ethernet frames)
+│   └── tap/
+│       └── tap.go           Linux TAP device management
+├── docs/                    Documentation
+├── vendor/                  Vendored Go dependencies
+├── flake.nix                Nix build definition
+├── go.mod
+└── go.sum
+```
+
+## Package details
+
+### `cmd/softether-go`
+
+CLI entry point. Handles flag parsing, signal handling, and the reconnection loop. On each session:
+- Connects to the server
+- Runs DHCP through the tunnel
+- Configures the TAP interface (IP, routes, DNS)
+- Bridges Ethernet frames between the TAP device and the VPN tunnel
+- Cleans up on disconnect and retries
+
+### `pkg/client`
+
+**`client.go`** — implements the SoftEther handshake: TLS connect, signature upload, hello/auth/welcome pack exchange. Exports `Connect(Config) (*Session, error)` and the `Config`/`Session` types.
+
+**`tunnel.go`** — TCP block framing after the HTTP handshake. `ReadFrames()` reads batches of Ethernet frames from the server. `WriteFrames()` sends batches. `StartKeepalive()` sends periodic keepalive packets (every 3s) to prevent server timeout.
+
+**`crypto.go`** — SHA-0 implementation (differs from SHA-1 only in the message schedule — no left-rotate). `HashPassword()` produces `SHA0(password)`. `SecurePassword()` produces `SHA0(hashed + serverRandom)`.
+
+### `pkg/protocol`
+
+**`http.go`** — HTTP transport layer. `DialTLS()` establishes the TLS connection. `UploadSignature()` sends the protocol signature. `SendPack()` and `RecvPack()` exchange binary Packs as HTTP POST request/response bodies.
+
+**`pack.go`** — SoftEther Pack binary serialization. A Pack is a list of named Elements, each containing typed Values (int, string, data, ip4). Handles the BufStr wire format (`uint32(strlen+1)` then `strlen` bytes) and the `pencore` random padding element.
+
+### `pkg/dhcp`
+
+**`dhcp.go`** — DHCP client that constructs complete Ethernet/IP/UDP/DHCP frames. The full DHCP exchange (DISCOVER → OFFER → REQUEST → ACK) runs through the VPN tunnel's frame transport. Parses lease information including classless static routes (option 121/249, RFC 3442).
+
+### `pkg/tap`
+
+**`tap.go`** — Linux TAP (Layer 2) device management via `/dev/net/tun`. Opens TAP devices with `IFF_TAP | IFF_NO_PI`, reads/writes raw Ethernet frames. Provides `MAC()` to get the hardware address and `SetUp()` to bring the interface up.