Reset the sequence numbers on Session disconnect to support reconnection

src/main/java/com/jcraft/jsch/Session.java

@@ -93,6 +93,7 @@ public class Session {
   private byte[] MACc2s;
   private byte[] MACs2c;
 
+  // RFC 4253 6.4. each direction must run independently hence we have an incoming and outgoing sequence
   private int seqi = 0;
   private int seqo = 0;
 
@@ -1998,6 +1999,12 @@ public void disconnect() {
     }
     io = null;
     socket = null;
+
+    // RFC 4253 6.4. the 'sequence_number' is never reset, even if keys/algorithms are renegotiated later. Hence we only
+    // reset these on session disconnect as the sequence has to start at zero for the first packet during (re)connect.
+    seqi = 0;
+    seqo = 0;
+
     // synchronized(jsch.pool){
     // jsch.pool.removeElement(this);
     // }

src/test/java/com/jcraft/jsch/SessionReconnectIT.java

@@ -0,0 +1,161 @@
+package com.jcraft.jsch;
+
+import com.github.valfirst.slf4jtest.LoggingEvent;
+import com.github.valfirst.slf4jtest.TestLogger;
+import com.github.valfirst.slf4jtest.TestLoggerFactory;
+import org.apache.commons.codec.digest.DigestUtils;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.testcontainers.containers.GenericContainer;
+import org.testcontainers.containers.output.Slf4jLogConsumer;
+import org.testcontainers.images.builder.ImageFromDockerfile;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Base64;
+import java.util.List;
+import java.util.Random;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.fail;
+
+@Testcontainers
+public class SessionReconnectIT {
+
+  private static final int timeout = 2000;
+  private static final DigestUtils sha256sum = new DigestUtils(DigestUtils.getSha256Digest());
+  private static final TestLogger jschLogger = TestLoggerFactory.getTestLogger(JSch.class);
+  private static final TestLogger sshdLogger =
+      TestLoggerFactory.getTestLogger(SessionReconnectIT.class);
+
+  @TempDir
+  public Path tmpDir;
+  private Path in;
+  private Path out;
+  private String hash;
+  private Slf4jLogConsumer sshdLogConsumer;
+
+  @Container
+  public GenericContainer<?> sshd = new GenericContainer<>(new ImageFromDockerfile()
+      .withFileFromClasspath("dropbear_rsa_host_key", "docker/dropbear_rsa_host_key")
+      .withFileFromClasspath("authorized_keys", "docker/authorized_keys")
+      .withFileFromClasspath("Dockerfile", "docker/Dockerfile.dropbear")).withExposedPorts(22);
+
+  @BeforeAll
+  public static void beforeAll() {
+    JSch.setLogger(new Slf4jLogger());
+  }
+
+  @BeforeEach
+  public void beforeEach() throws IOException {
+    if (sshdLogConsumer == null) {
+      sshdLogConsumer = new Slf4jLogConsumer(sshdLogger);
+      sshd.followOutput(sshdLogConsumer);
+    }
+
+    in = tmpDir.resolve("in");
+    out = tmpDir.resolve("out");
+    Files.createFile(in);
+    try (OutputStream os = Files.newOutputStream(in)) {
+      byte[] data = new byte[1024];
+      for (int i = 0; i < 1024 * 100; i += 1024) {
+        new Random().nextBytes(data);
+        os.write(data);
+      }
+    }
+    hash = sha256sum.digestAsHex(in);
+
+    jschLogger.clearAll();
+    sshdLogger.clearAll();
+  }
+
+  @AfterAll
+  public static void afterAll() {
+    JSch.setLogger(null);
+    jschLogger.clearAll();
+    sshdLogger.clearAll();
+  }
+
+  @Test
+  public void testReconnectWithExtraAlgorithms() throws Exception {
+    JSch ssh = createRSAIdentity();
+    Session session = createSession(ssh);
+    try {
+      doSftp(session, false);
+      fail("exception expected");
+    } catch (JSchAlgoNegoFailException e) {
+      // Dropbear does not support rsa-sha2-512/rsa-sha2-256, so add ssh-rsa
+      String serverHostKey = session.getConfig("server_host_key") + ",ssh-rsa";
+      String pubkeyAcceptedAlgorithms = session.getConfig("PubkeyAcceptedAlgorithms") + ",ssh-rsa";
+      session.setConfig("server_host_key", serverHostKey);
+      session.setConfig("PubkeyAcceptedAlgorithms", pubkeyAcceptedAlgorithms);
+      doSftp(session, true);
+    }
+  }
+
+  private JSch createRSAIdentity() throws Exception {
+    HostKey hostKey = readHostKey(getResourceFile("docker/ssh_host_rsa_key.pub"));
+    JSch ssh = new JSch();
+    ssh.addIdentity(getResourceFile("docker/id_rsa"), getResourceFile("docker/id_rsa.pub"), null);
+    ssh.getHostKeyRepository().add(hostKey, null);
+    return ssh;
+  }
+
+  private HostKey readHostKey(String fileName) throws Exception {
+    List<String> lines = Files.readAllLines(Paths.get(fileName), UTF_8);
+    String[] split = lines.get(0).split("\\s+");
+    String hostname = String.format("[%s]:%d", sshd.getHost(), sshd.getFirstMappedPort());
+    return new HostKey(hostname, Base64.getDecoder().decode(split[1]));
+  }
+
+  private Session createSession(JSch ssh) throws Exception {
+    Session session = ssh.getSession("root", sshd.getHost(), sshd.getFirstMappedPort());
+    session.setConfig("StrictHostKeyChecking", "yes");
+    session.setConfig("PreferredAuthentications", "publickey");
+    return session;
+  }
+
+  private void doSftp(Session session, boolean debugException) throws Exception {
+    try {
+      session.setTimeout(timeout);
+      session.connect();
+      ChannelSftp sftp = (ChannelSftp) session.openChannel("sftp");
+      sftp.connect(timeout);
+      sftp.put(in.toString(), "/root/test");
+      sftp.get("/root/test", out.toString());
+      sftp.disconnect();
+      session.disconnect();
+    } catch (Exception e) {
+      if (debugException) {
+        printInfo();
+      }
+      throw e;
+    }
+
+    assertEquals(1024L * 100L, Files.size(out));
+    assertEquals(hash, sha256sum.digestAsHex(out));
+  }
+
+  private void printInfo() {
+    jschLogger.getAllLoggingEvents().stream().map(LoggingEvent::getFormattedMessage)
+        .forEach(System.out::println);
+    sshdLogger.getAllLoggingEvents().stream().map(LoggingEvent::getFormattedMessage)
+        .forEach(System.out::println);
+    System.out.println("");
+    System.out.println("");
+    System.out.println("");
+  }
+
+  private String getResourceFile(String fileName) {
+    return ResourceUtil.getResourceFile(getClass(), fileName);
+  }
+}