Пинг ванильным методом и причины отказа по токину

- StatusSession: ответ на ванильный статус на том же порту. Первый байт потока различает наш протокол и гостей: R - наш кадр, иначе старый пинг (FE) или новый handshake + request + ping. Логин в старом стиле получает вежливый отказ словами. - Reader: первый байт, уже прочитанный вручную, теперь правильно исключается из головы - раньше счётчик дважды пропускал его. - TokenAuth: точные причины отказа (не подписано, другой сервер, не тот вид, истёк, не тот игрок) и Verdict для каждой сессии. - ClientSession: первое чтение смотрит на байт и отдаёт гостей статусу; остальное идёт через прежнее расщепление. - StatusTest: 5 проверок (json, modern, pong, legacy, отказ). Легаси-пинг читался как знаковый байт; исправлено на беззнаковый. - StatusRouteTest: 3 проверки через настоящий сервер и настоящие порты. - watch-game.ps1: удаляет устаревший dev-token перед стартом сервера.
2026-10-22 19:25:06 +03:00
parent 6019654188
commit d98cd83fe5
8 changed files with 1012 additions and 31 deletions
@@ -55,6 +55,8 @@ $suites = @(
    'net.rustnotmc.server.hitbox.HitboxTest',
    'net.rustnotmc.server.player.PlayerDataTest',
    'net.rustnotmc.server.combat.DamageTest',
+    'net.rustnotmc.server.status.StatusTest',
+    'net.rustnotmc.server.status.StatusRouteTest',
    'net.rustnotmc.server.PlayTest'
 )
 foreach ($suite in $suites) {
@@ -79,6 +79,8 @@ public final class ClientSession implements Runnable {
    private volatile int playerId = -1;
    private volatile int entityId = -1;
    private volatile String lastError;
+    private boolean firstLook = true;
+
    private volatile Player player;

    private byte[] nonce;
@@ -174,6 +176,20 @@ public final class ClientSession implements Runnable {
        }
    }

+    /**
+     * Whether the stream starts like our protocol.
+     *
+     * Our frames start with the magic "RSX1". The vanilla status question
+     * starts with something else, and the two are told apart by the first
+     * byte, so a port can serve both without waiting for the whole frame.
+     */
+    private static boolean looksLikeOurs(byte[] chunk, int read) {
+        if (read <= 0) {
+            return true;
+        }
+        return (chunk[0] & 0xFF) == (net.rustnotmc.protocol.Frame.MAGIC >>> 24);
+    }
+
    private void step() throws IOException {
        if (expired()) {
            throw new ProtocolException("handshake took too long");
@@ -186,10 +202,22 @@ public final class ClientSession implements Runnable {
                this.lastError = "peer closed the connection";
                close();
                return;
+            }
+                if (this.firstLook) {
+                this.firstLook = false;
+                System.out.println("[DBG] first read=" + read + " b0=" + (read > 0 ? Integer.toHexString(chunk[0] & 0xFF) : "-"));
+                if (!looksLikeOurs(chunk, read)) {
+                    // Not our protocol: the other side is asking about
+                    // the server, so the server answers that question.
+                    this.server.status().serve(this.socket, chunk, 0, read, this.address);
+                    close();
+                    return;
+                }
            }
            this.feed(chunk, 0, read);
        } catch (SocketTimeoutException nothingYet) {
            // expected: the loop wakes regularly to check the deadline
+            System.out.println("[DBG] step timeout");
        } catch (SocketException gone) {
            this.lastError = String.valueOf(gone.getMessage());
            close();
@@ -340,17 +368,17 @@ public final class ClientSession implements Runnable {
        long now = System.currentTimeMillis();
        TokenAuth checker = this.server.tokenAuth();
        if (checker != null) {
-            net.rustnotmc.auth.GameToken.Claims claims =
-                    checker.check(given.uuid, given.token, now);
-            if (claims == null) {
+            TokenAuth.Verdict verdict = checker.verdict(given.uuid, given.token, now);
+            if (!verdict.accepted()) {
                this.server.log("token of " + given.account + " refused: "
-                        + "it is not ours, it is for another server, or it has run out");
+                        + verdict.refusal.reason);
                this.server.throttle().failed(given.account, this.address);
                send(Packets.AUTH_ACK, Packets.AuthAck.refused(
-                        "the session is not valid, please sign in again"));
+                        "the session was refused: " + verdict.refusal.reason));
                close();
                return;
            }
+            net.rustnotmc.auth.GameToken.Claims claims = verdict.claims;

            // The name is inside the token, and the uuid has to be the same
            // one the token was made for.
@@ -372,11 +400,20 @@ public final class ClientSession implements Runnable {
        // launcher.
        net.rustnotmc.auth.TokenService.Token own =
                this.server.tokens().check(given.token);
-        if (own == null || own.expired(now)) {
-            this.server.log("token of " + given.account + " refused: it is not ours");
+        if (own == null) {
+            this.server.log("token of " + given.account + " refused: "
+                    + "this server did not give it out");
            this.server.throttle().failed(given.account, this.address);
            send(Packets.AUTH_ACK, Packets.AuthAck.refused(
-                    "the session is not valid, please sign in again"));
+                    "the session was refused: this server did not give out that token"));
+            close();
+            return;
+        }
+        if (own.expired(now)) {
+            this.server.log("token of " + given.account + " refused: it has run out");
+            this.server.throttle().failed(given.account, this.address);
+            send(Packets.AUTH_ACK, Packets.AuthAck.refused(
+                    "the session was refused: the token has run out"));
            close();
            return;
        }
@@ -31,7 +31,7 @@ import net.rustnotmc.server.world.World;
 * depend on the graphics card, and the whole point of moving the simulation to
 * our own server is that the client stops being trusted.
 */
-public final class Server {
+public final class Server implements net.rustnotmc.server.status.StatusSession.Info {

    /** Ticks per second. Fixed, and the same number the client counts. */
    public static final int TICKS_PER_SECOND = 20;
@@ -63,6 +63,10 @@ public final class Server {
            new net.rustnotmc.server.player.PlayerData(null);
    private volatile net.rustnotmc.server.world.LodGenerator lod;

+    /** The status of the server, in the way the game asks for it. */
+    private final net.rustnotmc.server.status.StatusSession status =
+            new net.rustnotmc.server.status.StatusSession(this);
+
    /**
     * The address the server listens on.
     *
@@ -275,6 +279,11 @@ public final class Server {
        this.bindAddress = address;
    }

+    /** The status of the server, in the way the game asks for it. */
+    public net.rustnotmc.server.status.StatusSession status() {
+        return this.status;
+    }
+
    /** The far view of the world. */
    public net.rustnotmc.server.world.LodGenerator lod() {
        return this.lod;
@@ -354,6 +363,17 @@ public final class Server {
        return this.name;
    }

+    @Override
+    public String name() {
+        return this.serverName();
+    }
+
+    @Override
+    public String version() {
+        return "RustNotMC 1.0";
+    }
+
+    @Override
    public String motd() {
        return this.motd;
    }
@@ -42,6 +42,83 @@ public final class TokenAuth {
        return this.audience;
    }

+    /** Why a token was not taken. */
+    public enum Refusal {
+
+        /** Nothing is wrong with the token. */
+        NONE(""),
+
+        /** The signature does not match: the token is not from this server. */
+        NOT_SIGNED("the token is not signed by this server"),
+
+        /** The token was made for another server. */
+        WRONG_AUDIENCE("the token is meant for another server"),
+
+        /** The token is a reload token, not a session token. */
+        WRONG_KIND("that token is not a session token"),
+
+        /** The token has run out. */
+        EXPIRED("the token has run out"),
+
+        /** The token was made for another player. */
+        WRONG_PLAYER("the token belongs to another player");
+
+        /** What to tell the player. */
+        public final String reason;
+
+        Refusal(String reason) {
+            this.reason = reason;
+        }
+    }
+
+    /** The answer to a token: what it says, or why it was refused. */
+    public static final class Verdict {
+
+        /** What the token says, or null when it was refused. */
+        public final GameToken.Claims claims;
+
+        /** Why the token was refused. */
+        public final Refusal refusal;
+
+        Verdict(GameToken.Claims claims, Refusal refusal) {
+            this.claims = claims;
+            this.refusal = refusal;
+        }
+
+        /** Whether the token was taken. */
+        public boolean accepted() {
+            return this.claims != null;
+        }
+    }
+
+    /**
+     * Checks a token and says what was wrong.
+     *
+     * The reason matters: "the token is not ours" tells a player nothing, and
+     * a player who cannot tell a stale token from a foreign one cannot fix
+     * anything either.
+     */
+    public Verdict verdict(String uuid, String token, long now) {
+        GameToken.Claims claims = this.tokens.read(token);
+        if (claims == null) {
+            return new Verdict(null, Refusal.NOT_SIGNED);
+        }
+        if (!GameToken.KIND_SESSION.equals(claims.kind)) {
+            return new Verdict(null, Refusal.WRONG_KIND);
+        }
+        if (!this.audience.equals(claims.audience)) {
+            return new Verdict(null, Refusal.WRONG_AUDIENCE);
+        }
+        if (claims.expiresAt <= now) {
+            return new Verdict(null, Refusal.EXPIRED);
+        }
+        if (uuid != null && !uuid.isEmpty() && claims.uuid != null
+                && !uuid.equals(claims.uuid)) {
+            return new Verdict(null, Refusal.WRONG_PLAYER);
+        }
+        return new Verdict(claims, Refusal.NONE);
+    }
+
    /**
     * Checks a token.
     *
@@ -49,24 +126,7 @@ public final class TokenAuth {
     * caller tells the player what was wrong.
     */
    public GameToken.Claims check(String uuid, String token, long now) {
-        GameToken.Claims claims = this.tokens.read(token);
-        if (claims == null) {
-            return null;
-        }
-        if (!GameToken.KIND_SESSION.equals(claims.kind)) {
-            return null;
-        }
-        if (!this.audience.equals(claims.audience)) {
-            return null;
-        }
-        if (claims.expiresAt <= now) {
-            return null;
-        }
-        if (uuid != null && !uuid.isEmpty() && claims.uuid != null
-                && !uuid.equals(claims.uuid)) {
-            return null;
-        }
-        return claims;
+        return verdict(uuid, token, now).claims;
    }

    /**
@@ -0,0 +1,334 @@
+package net.rustnotmc.server.status;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.Socket;
+import java.nio.charset.StandardCharsets;
+
+/**
+ * The status of the server, in the way the game asks for it.
+ *
+ * The vanilla client checks a server before it lets anybody in: it sends a
+ * handshake, then a request, and waits for an answer that says how the server
+ * calls itself, how many players are on it and how many may come. The answer
+ * is a small piece of text in a shape everybody knows.
+ *
+ * Our own protocol has a magic of its own, so a port can tell at once that it
+ * is being spoken nonsense. But a server that cannot be seen is a server that
+ * does not exist for half of the tools around, so the port answers the
+ * vanilla question too: the first bytes say which of the two conversations
+ * this is.
+ *
+ * The vanilla client cannot join through this door: the game speaks our
+ * protocol, not the vanilla one. It is told so in words instead of being cut
+ * off in the middle of a handshake.
+ */
+public final class StatusSession {
+
+    /** The packet id of a handshake in the vanilla protocol. */
+    private static final int VANILLA_HANDSHAKE = 0x00;
+
+    /** The packet id of a status request in the vanilla protocol. */
+    private static final int VANILLA_REQUEST = 0x00;
+
+    /** The packet id of a ping in the vanilla protocol. */
+    private static final int VANILLA_PING = 0x01;
+
+    /** The byte a legacy ping starts with. */
+    private static final int LEGACY_PING = 0xFE;
+
+    /** The byte a legacy answer starts with. */
+    private static final int LEGACY_ANSWER = 0xFF;
+
+    /** The state of the vanilla protocol that means "the status is wanted". */
+    private static final int STATE_STATUS = 1;
+
+    /** The state of the vanilla protocol that means "somebody wants in". */
+    private static final int STATE_LOGIN = 2;
+
+    /** How long the answer may take. */
+    private static final int TIMEOUT_MILLIS = 4000;
+
+    /** What the server says about itself. */
+    public interface Info {
+
+        /** The name of the server. */
+        String name();
+
+        /** The line under the name. */
+        String motd();
+
+        /** How many players are on the server right now. */
+        int online();
+
+        /** How many players may come. */
+        int capacity();
+
+        /** The version the server calls itself. */
+        String version();
+    }
+
+    private final Info info;
+
+    public StatusSession(Info info) {
+        this.info = info;
+    }
+
+    /**
+     * Answers one status conversation.
+     *
+     * The bytes that were already read are handed over, so that nothing is
+     * lost between the first look and the rest of the stream.
+     */
+    public void serve(Socket socket, byte[] head, int offset, int length, String address) {
+        try {
+            socket.setSoTimeout(TIMEOUT_MILLIS);
+            InputStream in = socket.getInputStream();
+            OutputStream out = socket.getOutputStream();
+
+            // The first byte says which of the two conversations this is.
+            int first;
+            if (length > 0) {
+                first = head[offset] & 0xFF;
+            } else {
+                first = in.read();
+                if (first < 0) {
+                    return;
+                }
+            }
+
+            Reader reader = new Reader(head, offset, length, in, first);
+            if (first == LEGACY_PING) {
+                // the marker is read; the rest of the old packet is not
+                reader.skipPending();
+                legacy(reader, out);
+                return;
+            }
+            modern(reader, out);
+        } catch (IOException broken) {
+            // the other side went away; nothing to say about it
+        } catch (RuntimeException broken) {
+            // a broken status request must never take the server down
+        }
+    }
+
+    /** The old way of asking: one packet, one answer. */
+    private void legacy(Reader reader, OutputStream out) throws IOException {
+        reader.readUnsignedByte();   // 0x01
+        reader.readUnsignedByte();   // 0xFA
+        int length = reader.readUnsignedByte();   // length of the rest
+        for (int i = 0; i < length; i++) {
+            reader.readUnsignedByte();
+        }
+        String text = "§1\000" + vanillaProtocol() + "\000" + this.info.version()
+                + "\000" + this.info.motd() + "\000" + this.info.online()
+                + "\000" + this.info.capacity();
+        byte[] body = text.getBytes(StandardCharsets.UTF_8);
+        out.write(LEGACY_ANSWER);
+        out.write((body.length >> 8) & 0xFF);
+        out.write(body.length & 0xFF);
+        out.write(body);
+        out.flush();
+    }
+
+    /** The way the game asks today: a handshake, a request, a ping. */
+    private void modern(Reader reader, OutputStream out) throws IOException {
+        int packetId = reader.readUnsignedByte();
+        if (packetId != VANILLA_HANDSHAKE) {
+            return;
+        }
+        reader.readVarInt();          // the protocol version of the other side
+        reader.readString();          // the name the player typed
+        reader.readShort();           // the port
+        int state = reader.readVarInt();
+
+        if (state == STATE_LOGIN) {
+            // Somebody wants in through the vanilla door. They cannot come in
+            // that way, but they can be told why in words they will see.
+            drain(reader, 512);
+            String reason = "{\"text\":\"This is a RustNotMC server. "
+                    + "It is played with the RustNotMC client, not with the vanilla one.\"}";
+            byte[] body = reason.getBytes(StandardCharsets.UTF_8);
+            out.write(0x00);
+            writeVarInt(out, body.length);
+            out.write(body);
+            out.flush();
+            return;
+        }
+        if (state != STATE_STATUS) {
+            return;
+        }
+
+        // The answer goes out right away: the client asks for the status and
+        // for the latency at the same time, and the latency is a ping that
+        // comes after the answer.
+        byte[] json = statusJson().getBytes(StandardCharsets.UTF_8);
+        out.write(VANILLA_REQUEST);
+        writeVarInt(out, json.length);
+        out.write(json);
+        out.flush();
+
+        // and now the ping, if it comes. The request itself is of no use
+        // here, so it is passed over.
+        try {
+            while (true) {
+                int next = reader.readUnsignedByte();
+                if (next != VANILLA_PING) {
+                    continue;
+                }
+                long stamp = 0L;
+                for (int i = 0; i < 8; i++) {
+                    stamp = (stamp << 8) | reader.readUnsignedByte();
+                }
+                out.write(VANILLA_PING);
+                for (int i = 0; i < 8; i++) {
+                    out.write((int) ((stamp >>> (8 * (7 - i))) & 0xFFL));
+                }
+                out.flush();
+                return;
+            }
+        } catch (IOException nothingMore) {
+            // the client did not want the latency; the status was enough
+        }
+    }
+
+    /** The status, in the shape the game expects. */
+    public String statusJson() {
+        StringBuilder out = new StringBuilder(256);
+        out.append("{\"version\":{\"name\":")
+                .append(net.rustnotmc.server.util.Json.quote(this.info.version()))
+                .append(",\"protocol\":").append(vanillaProtocol())
+                .append("},\"players\":{\"max\":").append(this.info.capacity())
+                .append(",\"online\":").append(this.info.online())
+                .append(",\"sample\":[]}")
+                .append(",\"description\":{\"text\":")
+                .append(net.rustnotmc.server.util.Json.quote(this.info.motd()))
+                .append("}}");
+        return out.toString();
+    }
+
+    /**
+     * The protocol number the game knows.
+     *
+     * The server does not speak the vanilla protocol, but the number is what
+     * the tools that list servers show next to the name.
+     */
+    private int vanillaProtocol() {
+        return 404;
+    }
+
+    /** Reads what is there and throws the rest away. */
+    private static void drain(Reader reader, int limit) {
+        try {
+            for (int i = 0; i < limit; i++) {
+                reader.readUnsignedByte();
+            }
+        } catch (IOException nothingMore) {
+            // there is nothing more to read
+        }
+    }
+
+    /** Writes a varint the way the game does: seven bits at a time. */
+    private static void writeVarInt(OutputStream out, int value) throws IOException {
+        int left = value;
+        while (true) {
+            int part = left & 0x7F;
+            left >>>= 7;
+            if (left != 0) {
+                part |= 0x80;
+            }
+            out.write(part);
+            if (left == 0) {
+                return;
+            }
+        }
+    }
+
+    /**
+     * Reads the little pieces of the vanilla protocol.
+     *
+     * The bytes that were already read from the socket come first, and the
+     * rest is taken from the stream.
+     */
+    private static final class Reader {
+
+        /** The bytes that were read already. */
+        private final byte[] head;
+
+        /** The next byte of those to read. */
+        private int pos;
+
+        /** Where the bytes that were read already end. */
+        private final int limit;
+
+        /** The rest of the stream. */
+        private final InputStream in;
+
+        /** A byte that was read before the reader was made. */
+        private int pending;
+
+        Reader(byte[] head, int offset, int length, InputStream in, int pending) {
+            this.head = head;
+            // When the first byte came out of the head, the head still holds a
+            // copy of it that was already counted in `pending` (it is read
+            // again from `pending`, not from the head).
+            this.pos = length > 0 ? offset + 1 : offset;
+            this.limit = offset + length;
+            this.in = in;
+            this.pending = pending;
+        }
+
+        /** Throws away a byte that was read before the reader was made. */
+        void skipPending() {
+            this.pending = -1;
+        }
+
+        int readUnsignedByte() throws IOException {
+            if (this.pending >= 0) {
+                int b = this.pending;
+                this.pending = -1;
+                return b;
+            }
+            if (this.pos < this.limit) {
+                return this.head[this.pos++] & 0xFF;
+            }
+            int b = this.in.read();
+            if (b < 0) {
+                throw new IOException("the other side closed the connection");
+            }
+            return b;
+        }
+
+        int readVarInt() throws IOException {
+            int result = 0;
+            int shift = 0;
+            while (true) {
+                int b = readUnsignedByte();
+                result |= (b & 0x7F) << shift;
+                if ((b & 0x80) == 0) {
+                    return result;
+                }
+                shift += 7;
+                if (shift > 35) {
+                    throw new IOException("the varint is too long");
+                }
+            }
+        }
+
+        String readString() throws IOException {
+            int length = readVarInt();
+            byte[] bytes = new byte[length];
+            for (int i = 0; i < length; i++) {
+                bytes[i] = (byte) readUnsignedByte();
+            }
+            return new String(bytes, StandardCharsets.UTF_8);
+        }
+
+        int readShort() throws IOException {
+            int a = readUnsignedByte();
+            int b = readUnsignedByte();
+            return (a << 8) | b;
+        }
+    }
+}
@@ -0,0 +1,220 @@
+package net.rustnotmc.server.status;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.Socket;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import net.rustnotmc.server.Server;
+import net.rustnotmc.server.world.ChunkStorage;
+import net.rustnotmc.server.world.World;
+
+/**
+ * Tests the status question the way it really arrives: the bytes go to a live
+ * server, a session takes them and hands them over to the status.
+ */
+public final class StatusRouteTest {
+
+    private static int checks;
+    private static int failed;
+    private static final java.util.List<String> failures = new java.util.ArrayList<String>();
+
+    public static void main(String[] args) throws Exception {
+        Path work = Files.createTempDirectory("status-route");
+        Server server = StatusRouteTest.boot(work);
+        try {
+            legacy(server.port());
+            modern(server.port());
+        } finally {
+            server.stop();
+        }
+
+        System.out.println();
+        for (int i = 0; i < failures.size(); i++) {
+            System.out.println("  FAIL " + failures.get(i));
+        }
+        System.out.println();
+        System.out.println("checks: " + checks + ", failed: " + failed);
+        if (failed != 0) {
+            System.out.println("FAILED");
+            System.exit(1);
+        }
+        System.out.println("OK");
+    }
+
+    /** A small server that only has to answer the question of the status. */
+    private static Server boot(Path work) throws Exception {
+        Server server = new Server("RustNotMC route", "hello", 200, 0,
+                new World(1234L, ChunkStorage.memoryOnly()));
+        Path accounts = work.resolve("route-accounts.txt");
+        server.loadAccounts(accounts);
+        server.start();
+        return server;
+    }
+
+    private interface Throwing {
+        void go() throws Exception;
+    }
+
+    private static void run(String name, Throwing body) {
+        checks++;
+        int before = failed;
+        try {
+            body.go();
+        } catch (Throwable t) {
+            failed++;
+            failures.add(name + ": threw " + t);
+            t.printStackTrace();
+            return;
+        }
+        if (failed == before) {
+            System.out.println("  ok  " + name);
+        } else {
+            System.out.println("  FAIL " + name);
+        }
+    }
+
+    private static void eq(Object what, Object expected, Object actual) {
+        if (expected == null ? actual != null : !expected.equals(actual)) {
+            failed++;
+            failures.add(what + ": expected " + expected + " but got " + actual);
+        }
+    }
+
+    private static void yes(String what, boolean condition) {
+        if (!condition) {
+            failed++;
+            failures.add(what);
+        }
+    }
+
+    private static void legacy(int port) throws Exception {
+        run("the old question is answered on a live port", new Throwing() {
+            public void go() throws Exception {
+                try (Socket socket = new Socket("127.0.0.1", port)) {
+                    socket.setSoTimeout(5000);
+                    DataOutputStream out = new DataOutputStream(
+                            socket.getOutputStream());
+                    out.writeByte(0xFE);
+                    out.writeByte(0x01);
+                    out.writeByte(0xFA);
+                    out.writeByte(0x01);
+                    out.writeByte(0x00);
+                    out.flush();
+
+                    DataInputStream in = new DataInputStream(
+                            socket.getInputStream());
+                    eq("the answer is a legacy one", Integer.valueOf(0xFF),
+                            Integer.valueOf(in.readUnsignedByte()));
+                    int length = in.readUnsignedShort();
+                    byte[] body = new byte[length];
+                    in.readFully(body);
+                    String text = new String(body,
+                            java.nio.charset.StandardCharsets.UTF_8);
+                    yes("the answer says the players", text.contains("2"));
+                    yes("the answer says the places", text.contains("200"));
+                }
+            }
+        });
+    }
+
+    private static void modern(int port) throws Exception {
+        run("the new question is answered on a live port", new Throwing() {
+            public void go() throws Exception {
+                try (Socket socket = new Socket("127.0.0.1", port)) {
+                    socket.setSoTimeout(5000);
+                    DataOutputStream out = new DataOutputStream(
+                            socket.getOutputStream());
+                    out.writeByte(0x00);       // the handshake packet
+                    writeVarInt(out, 404);   // the protocol version
+                    writeString(out, "127.0.0.1");
+                    out.writeShort(port);
+                    writeVarInt(out, 1);       // the status is wanted
+                    out.writeByte(0x00);       // the request
+                    out.flush();
+
+                    DataInputStream in = new DataInputStream(
+                            socket.getInputStream());
+                    eq("the answer is a status", Integer.valueOf(0x00),
+                            Integer.valueOf(in.readUnsignedByte()));
+                    String json = readString(in);
+                    yes("the answer names the server", json.contains("hello"));
+                    yes("the answer says the players", json.contains("\"online\":0"));
+
+                    long mark = 0x0102030405060708L;
+                    out.writeByte(0x01);
+                    out.writeLong(mark);
+                    out.flush();
+                    eq("the answer is a pong", Integer.valueOf(0x01),
+                            Integer.valueOf(in.readUnsignedByte()));
+                    eq("the mark comes back", Long.valueOf(mark),
+                            Long.valueOf(in.readLong()));
+                }
+            }
+        });
+
+        run("a game in vanilla clothes is turned away in words", new Throwing() {
+            public void go() throws Exception {
+                try (Socket socket = new Socket("127.0.0.1", port)) {
+                    socket.setSoTimeout(5000);
+                    DataOutputStream out = new DataOutputStream(
+                            socket.getOutputStream());
+                    out.writeByte(0x00);       // the handshake packet
+                    writeVarInt(out, 404);
+                    writeString(out, "127.0.0.1");
+                    out.writeShort(port);
+                    writeVarInt(out, 2);       // somebody wants in
+                    out.writeByte(0x00);       // the login packet
+                    writeString(out, "Player");
+                    out.flush();
+
+                    DataInputStream in = new DataInputStream(
+                            socket.getInputStream());
+                    eq("the answer is a disconnect", Integer.valueOf(0x00),
+                            Integer.valueOf(in.readUnsignedByte()));
+                    String reason = readString(in);
+                    yes("the reason says what to use", reason.contains("RustNotMC"));
+                }
+            }
+        });
+    }
+
+    private static void writeVarInt(DataOutputStream out, int value) throws IOException {
+        int left = value;
+        while (true) {
+            int part = left & 0x7F;
+            left >>>= 7;
+            if (left != 0) {
+                part |= 0x80;
+            }
+            out.writeByte(part);
+            if (left == 0) {
+                return;
+            }
+        }
+    }
+
+    private static void writeString(DataOutputStream out, String value) throws IOException {
+        byte[] bytes = value.getBytes(java.nio.charset.StandardCharsets.UTF_8);
+        writeVarInt(out, bytes.length);
+        out.write(bytes);
+    }
+
+    private static String readString(DataInputStream in) throws IOException {
+        int length = 0;
+        int shift = 0;
+        while (true) {
+            int b = in.readByte() & 0xFF;
+            length |= (b & 0x7F) << shift;
+            if ((b & 0x80) == 0) {
+                break;
+            }
+            shift += 7;
+        }
+        byte[] bytes = new byte[length];
+        in.readFully(bytes);
+        return new String(bytes, java.nio.charset.StandardCharsets.UTF_8);
+    }
+}
@@ -0,0 +1,300 @@
+package net.rustnotmc.server.status;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.ServerSocket;
+import java.net.Socket;
+
+/**
+ * Tests the answer to the vanilla status question.
+ *
+ * The game checks a server before it lets anybody in, and the tools that list
+ * servers do the same. The server has to answer that question on the same
+ * port it plays on, and the answer has to say how the server calls itself,
+ * how many players are on it and how many may come.
+ */
+public final class StatusTest {
+
+    private static int checks;
+    private static int failed;
+    private static final java.util.List<String> failures = new java.util.ArrayList<String>();
+
+    public static void main(String[] args) throws Exception {
+        run("the status says who the server is", new Throwing() {
+            public void go() throws Exception { testJson(); }
+        });
+        run("the vanilla question is answered", new Throwing() {
+            public void go() throws Exception { testModern(); }
+        });
+        run("the latency is measured", new Throwing() {
+            public void go() throws Exception { testPing(); }
+        });
+        run("the old way of asking is answered too", new Throwing() {
+            public void go() throws Exception { testLegacy(); }
+        });
+        run("a vanilla login is turned away in words", new Throwing() {
+            public void go() throws Exception { testLogin(); }
+        });
+
+        System.out.println();
+        for (int i = 0; i < failures.size(); i++) {
+            System.out.println("  FAIL " + failures.get(i));
+        }
+        System.out.println();
+        System.out.println("checks: " + checks + ", failed: " + failed);
+        if (failed != 0) {
+            System.out.println("FAILED");
+            System.exit(1);
+        }
+        System.out.println("OK");
+    }
+
+    private interface Throwing {
+        void go() throws Exception;
+    }
+
+    private static void run(String name, Throwing body) {
+        checks++;
+        int before = failed;
+        try {
+            body.go();
+        } catch (Throwable t) {
+            failed++;
+            failures.add(name + ": threw " + t);
+            t.printStackTrace();
+            return;
+        }
+        if (failed == before) {
+            System.out.println("  ok  " + name);
+        } else {
+            System.out.println("  FAIL " + name);
+        }
+    }
+
+    private static void eq(Object what, Object expected, Object actual) {
+        if (expected == null ? actual != null : !expected.equals(actual)) {
+            failed++;
+            failures.add(what + ": expected " + expected + " but got " + actual);
+        }
+    }
+
+    private static void yes(String what, boolean condition) {
+        if (!condition) {
+            failed++;
+            failures.add(what);
+        }
+    }
+
+    /** A server that calls itself one thing and holds two players. */
+    private static final class Fake implements StatusSession.Info {
+        public String name() {
+            return "RustNotMC";
+        }
+
+        public String motd() {
+            return "an island in the sea";
+        }
+
+        public int online() {
+            return 2;
+        }
+
+        public int capacity() {
+            return 200;
+        }
+
+        public String version() {
+            return "RustNotMC 1.0";
+        }
+    }
+
+    private static void testJson() {
+        String json = new StatusSession(new Fake()).statusJson();
+        yes("the name is in there", json.contains("\"RustNotMC 1.0\""));
+        yes("the line under the name is in there", json.contains("an island in the sea"));
+        yes("the players are in there", json.contains("\"online\":2"));
+        yes("the places are in there", json.contains("\"max\":200"));
+        yes("it is one piece of text", json.startsWith("{") && json.endsWith("}"));
+    }
+
+    private static void testModern() throws Exception {
+        try (ServerSocket listener = new ServerSocket(0, 1, InetAddress.getByName("127.0.0.1"))) {
+            Thread server = new Thread(() -> {
+                try (Socket socket = listener.accept()) {
+                    new StatusSession(new Fake()).serve(socket,
+                            new byte[0], 0, 0, "127.0.0.1");
+                } catch (IOException ignored) {
+                    // the test is over anyway
+                }
+            });
+            server.start();
+
+            try (Socket socket = new Socket("127.0.0.1", listener.getLocalPort())) {
+                socket.setSoTimeout(5000);
+                DataOutputStream out = new DataOutputStream(socket.getOutputStream());
+                out.writeByte(0x00);       // the handshake
+                writeVarInt(out, 404);     // the protocol version
+                writeString(out, "127.0.0.1");
+                out.writeShort(listener.getLocalPort());
+                writeVarInt(out, 1);       // the status is wanted
+                out.writeByte(0x00);       // the request
+                out.flush();
+
+                DataInputStream in = new DataInputStream(socket.getInputStream());
+                eq("the answer is a status", Integer.valueOf(0x00), Integer.valueOf(in.readByte()));
+                String json = readString(in);
+                yes("the answer names the server", json.contains("RustNotMC 1.0"));
+                yes("the answer says how many players", json.contains("\"online\":2"));
+            }
+            server.join(5000);
+        }
+    }
+
+    private static void testPing() throws Exception {
+        try (ServerSocket listener = new ServerSocket(0, 1, InetAddress.getByName("127.0.0.1"))) {
+            Thread server = new Thread(() -> {
+                try (Socket socket = listener.accept()) {
+                    new StatusSession(new Fake()).serve(socket,
+                            new byte[0], 0, 0, "127.0.0.1");
+                } catch (IOException ignored) {
+                    // the test is over anyway
+                }
+            });
+            server.start();
+
+            try (Socket socket = new Socket("127.0.0.1", listener.getLocalPort())) {
+                socket.setSoTimeout(5000);
+                DataOutputStream out = new DataOutputStream(socket.getOutputStream());
+                out.writeByte(0x00);
+                writeVarInt(out, 404);
+                writeString(out, "127.0.0.1");
+                out.writeShort(listener.getLocalPort());
+                writeVarInt(out, 1);
+                out.writeByte(0x00);
+                out.flush();
+
+                DataInputStream in = new DataInputStream(socket.getInputStream());
+                in.readByte();
+                readString(in);
+
+                // and now the ping, with a mark of its own
+                long mark = 0x0102030405060708L;
+                out.writeByte(0x01);
+                out.writeLong(mark);
+                out.flush();
+                eq("the answer is a pong", Integer.valueOf(0x01), Integer.valueOf(in.readByte()));
+                eq("the mark comes back", Long.valueOf(mark), Long.valueOf(in.readLong()));
+            }
+            server.join(5000);
+        }
+    }
+
+    private static void testLegacy() throws Exception {
+        try (ServerSocket listener = new ServerSocket(0, 1, InetAddress.getByName("127.0.0.1"))) {
+            Thread server = new Thread(() -> {
+                try (Socket socket = listener.accept()) {
+                    new StatusSession(new Fake()).serve(socket,
+                            new byte[0], 0, 0, "127.0.0.1");
+                } catch (IOException ignored) {
+                    ignored.printStackTrace();
+                }
+            });
+            server.start();
+
+            try (Socket socket = new Socket("127.0.0.1", listener.getLocalPort())) {
+                socket.setSoTimeout(5000);
+                DataOutputStream out = new DataOutputStream(socket.getOutputStream());
+                out.writeByte(0xFE);
+                out.writeByte(0x01);
+                out.writeByte(0xFA);
+                out.writeByte(0x01);
+                out.writeByte(0x00);
+                out.flush();
+
+                DataInputStream in = new DataInputStream(socket.getInputStream());
+                eq("the answer is a legacy one", Integer.valueOf(0xFF),
+                        Integer.valueOf(in.readUnsignedByte()));
+                int length = in.readUnsignedShort();
+                byte[] body = new byte[length];
+                in.readFully(body);
+                String text = new String(body, java.nio.charset.StandardCharsets.UTF_8);
+                yes("the answer says how many players", text.contains("2"));
+                yes("the answer says how many places", text.contains("200"));
+            }
+            server.join(5000);
+        }
+    }
+
+    private static void testLogin() throws Exception {
+        try (ServerSocket listener = new ServerSocket(0, 1, InetAddress.getByName("127.0.0.1"))) {
+            Thread server = new Thread(() -> {
+                try (Socket socket = listener.accept()) {
+                    new StatusSession(new Fake()).serve(socket,
+                            new byte[0], 0, 0, "127.0.0.1");
+                } catch (IOException ignored) {
+                    // the test is over anyway
+                }
+            });
+            server.start();
+
+            try (Socket socket = new Socket("127.0.0.1", listener.getLocalPort())) {
+                socket.setSoTimeout(5000);
+                DataOutputStream out = new DataOutputStream(socket.getOutputStream());
+                out.writeByte(0x00);
+                writeVarInt(out, 404);
+                writeString(out, "127.0.0.1");
+                out.writeShort(listener.getLocalPort());
+                writeVarInt(out, 2);       // somebody wants in
+                out.writeByte(0x00);       // the login
+                writeString(out, "Player");
+                out.flush();
+
+                DataInputStream in = new DataInputStream(socket.getInputStream());
+                eq("the answer is a disconnect", Integer.valueOf(0x00),
+                        Integer.valueOf(in.readByte()));
+                String reason = readString(in);
+                yes("the reason says what to use", reason.contains("RustNotMC"));
+            }
+            server.join(5000);
+        }
+    }
+
+    private static void writeVarInt(DataOutputStream out, int value) throws IOException {
+        int left = value;
+        while (true) {
+            int part = left & 0x7F;
+            left >>>= 7;
+            if (left != 0) {
+                part |= 0x80;
+            }
+            out.writeByte(part);
+            if (left == 0) {
+                return;
+            }
+        }
+    }
+
+    private static void writeString(DataOutputStream out, String value) throws IOException {
+        byte[] bytes = value.getBytes(java.nio.charset.StandardCharsets.UTF_8);
+        writeVarInt(out, bytes.length);
+        out.write(bytes);
+    }
+
+    private static String readString(DataInputStream in) throws IOException {
+        int length = 0;
+        int shift = 0;
+        while (true) {
+            int b = in.readByte() & 0xFF;
+            length |= (b & 0x7F) << shift;
+            if ((b & 0x80) == 0) {
+                break;
+            }
+            shift += 7;
+        }
+        byte[] bytes = new byte[length];
+        in.readFully(bytes);
+        return new String(bytes, java.nio.charset.StandardCharsets.UTF_8);
+    }
+}