2 files changed, 221 insertions, 0 deletions
diff --git a/adapters/rcon/rcon.go b/adapters/rcon/rcon.go
new file mode 100644
index 0000000..4b9e8b6
--- /dev/null
+++ b/adapters/rcon/rcon.go
@@ -0,0 +1,130 @@
+// Package rcon is a minimal Source RCON client (the protocol Minecraft speaks),
+// enough to authenticate and run a command like "list".
+package rcon
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"io"
+	"net"
+	"time"
+)
+
+const (
+	typeResponse    int32 = 0
+	typeAuthResp    int32 = 2
+	typeExecCommand int32 = 2
+	typeAuth        int32 = 3
+
+	authRequestID int32 = 0
+	execRequestID int32 = 1
+
+	headerSize  = 8 // id (int32) + type (int32)
+	paddingSize = 2 // two trailing null bytes
+	maxBodySize = 4096
+
+	DefaultTimeout = 5 * time.Second
+)
+
+var (
+	ErrAuthFailed       = errors.New("rcon: authentication failed")
+	ErrResponseMismatch = errors.New("rcon: response id mismatch")
+	ErrPacketSize       = errors.New("rcon: invalid packet size")
+)
+
+type Client struct {
+	conn    net.Conn
+	timeout time.Duration
+}
+
+func Dial(address, password string, timeout time.Duration) (*Client, error) {
+	conn, err := net.DialTimeout("tcp", address, timeout)
+	if err != nil {
+		return nil, err
+	}
+
+	client := &Client{conn: conn, timeout: timeout}
+	if err := client.auth(password); err != nil {
+		conn.Close()
+		return nil, err
+	}
+	return client, nil
+}
+
+func (c *Client) Close() error { return c.conn.Close() }
+
+func (c *Client) auth(password string) error {
+	if err := c.write(authRequestID, typeAuth, password); err != nil {
+		return err
+	}
+
+	// Some servers send an empty SERVERDATA_RESPONSE_VALUE before the auth
+	// response; skip anything that isn't the auth response.
+	for {
+		id, typ, _, err := c.read()
+		if err != nil {
+			return err
+		}
+		if typ != typeAuthResp {
+			continue
+		}
+		if id == -1 {
+			return ErrAuthFailed
+		}
+		return nil
+	}
+}
+
+func (c *Client) Execute(command string) (string, error) {
+	if err := c.write(execRequestID, typeExecCommand, command); err != nil {
+		return "", err
+	}
+
+	id, _, body, err := c.read()
+	if err != nil {
+		return "", err
+	}
+	if id != execRequestID {
+		return "", ErrResponseMismatch
+	}
+	return body, nil
+}
+
+func (c *Client) write(id, typ int32, body string) error {
+	c.conn.SetWriteDeadline(time.Now().Add(c.timeout))
+
+	var buf bytes.Buffer
+	size := int32(headerSize + len(body) + paddingSize)
+	binary.Write(&buf, binary.LittleEndian, size)
+	binary.Write(&buf, binary.LittleEndian, id)
+	binary.Write(&buf, binary.LittleEndian, typ)
+	buf.WriteString(body)
+	buf.Write([]byte{0, 0})
+
+	_, err := c.conn.Write(buf.Bytes())
+	return err
+}
+
+func (c *Client) read() (id, typ int32, body string, err error) {
+	c.conn.SetReadDeadline(time.Now().Add(c.timeout))
+
+	var size int32
+	if err = binary.Read(c.conn, binary.LittleEndian, &size); err != nil {
+		return
+	}
+	if size < headerSize+paddingSize || size > headerSize+maxBodySize+paddingSize {
+		err = ErrPacketSize
+		return
+	}
+
+	payload := make([]byte, size)
+	if _, err = io.ReadFull(c.conn, payload); err != nil {
+		return
+	}
+
+	id = int32(binary.LittleEndian.Uint32(payload[0:4]))
+	typ = int32(binary.LittleEndian.Uint32(payload[4:8]))
+	body = string(payload[headerSize : size-paddingSize])
+	return
+}
diff --git a/adapters/rcon/rcon_test.go b/adapters/rcon/rcon_test.go
new file mode 100644
index 0000000..f712fa5
--- /dev/null
+++ b/adapters/rcon/rcon_test.go
@@ -0,0 +1,91 @@
+package rcon
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+	"net"
+	"testing"
+	"time"
+)
+
+func writeTestPacket(w io.Writer, id, typ int32, body string) {
+	binary.Write(w, binary.LittleEndian, int32(headerSize+len(body)+paddingSize))
+	binary.Write(w, binary.LittleEndian, id)
+	binary.Write(w, binary.LittleEndian, typ)
+	io.WriteString(w, body)
+	w.Write([]byte{0, 0})
+}
+
+func readTestPacket(r io.Reader) (id, typ int32, body string) {
+	var size int32
+	binary.Read(r, binary.LittleEndian, &size)
+	payload := make([]byte, size)
+	io.ReadFull(r, payload)
+	id = int32(binary.LittleEndian.Uint32(payload[0:4]))
+	typ = int32(binary.LittleEndian.Uint32(payload[4:8]))
+	return id, typ, string(payload[headerSize : size-paddingSize])
+}
+
+// fakeServer speaks just enough Source RCON to exercise the client.
+func fakeServer(t *testing.T, password string) string {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { ln.Close() })
+
+	go func() {
+		conn, err := ln.Accept()
+		if err != nil {
+			return
+		}
+		defer conn.Close()
+
+		id, typ, body := readTestPacket(conn)
+		if typ != typeAuth {
+			return
+		}
+		respID := id
+		if body != password {
+			respID = -1
+		}
+		writeTestPacket(conn, id, typeResponse, "") // empty value first
+		writeTestPacket(conn, respID, typeAuthResp, "")
+		if respID == -1 {
+			return
+		}
+
+		cid, _, cbody := readTestPacket(conn)
+		out := "unknown command"
+		if cbody == "list" {
+			out = "There are 1 of a max of 20 players online: Steve"
+		}
+		writeTestPacket(conn, cid, typeResponse, out)
+	}()
+
+	return ln.Addr().String()
+}
+
+func TestClientExecute(t *testing.T) {
+	client, err := Dial(fakeServer(t, "secret"), "secret", time.Second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	out, err := client.Execute("list")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "There are 1 of a max of 20 players online: Steve"; out != want {
+		t.Errorf("Execute = %q, want %q", out, want)
+	}
+}
+
+func TestClientAuthFailure(t *testing.T) {
+	_, err := Dial(fakeServer(t, "secret"), "wrong", time.Second)
+	if !errors.Is(err, ErrAuthFailed) {
+		t.Errorf("Dial err = %v, want ErrAuthFailed", err)
+	}
+}