Question

我正在寻找一个如何使用libssh2来设置ssh端口转发的示例。我查看了API，但在端口转发领域的文档方式很少。

例如，当使用PuTTY的plink时，有远程端口可以监听，但也有应该发送流量的本地端口。设置这个是开发人员的责任吗？有人可以举例说明如何做到这一点吗？

此外，将远程端口带到本地端口的示例将非常有用。我使用libssh2_channel_direct_tcpip_ex()吗？

如果需要的话，我愿意提供一些奖励。

Answer 1

使libssh2端口转发工作的关键是发现它基本上只是为您提供进入该端口的数据。您必须实际将数据发送到您打开的本地端口：

（注意，此代码尚未完成，没有错误检查，并且线程产生不正确，但它提供了如何完成此操作的大致概述。）

void reverse_port_forward(CMainDlg* dlg, addrinfo * hubaddr, std::string username, std::string password, int port)
{
  int iretval;
  unsigned long mode = 1;
  int last_socket_err = 0;
  int other_port = 0;
  fd_set read_set, write_set;

  SOCKET sshsock = socket(AF_INET, SOCK_STREAM, 0);
  iretval = connect(sshsock, hubaddr->ai_addr, hubaddr->ai_addrlen);
  if (iretval != 0)
    ::PostQuitMessage(0);

  LIBSSH2_SESSION * session = NULL;
  session = libssh2_session_init();

  iretval = libssh2_session_startup(session, sshsock);
  if (iretval)
    ::PostQuitMessage(0);

  iretval = libssh2_userauth_password(session, username.c_str(), password.c_str());

  dlg->m_track_status(dlg, 1, 0, "Authorized");

  LIBSSH2_LISTENER* listener = NULL;
  listener = libssh2_channel_forward_listen_ex(session, "127.0.0.1", port, &other_port, 1);
  if (!listener)
    ::PostQuitMessage(0);

  LIBSSH2_CHANNEL* channel = NULL;

  ioctlsocket(sshsock, FIONBIO, &mode);
  libssh2_session_set_blocking(session, 0); // non-blocking
  int err = LIBSSH2_ERROR_EAGAIN;
  while (err == LIBSSH2_ERROR_EAGAIN)
  {
    channel = libssh2_channel_forward_accept(listener);
    if (channel) break;
    err = libssh2_session_last_errno(session);
    boost::this_thread::yield();
  }

  if (channel)
  {
    char buf[MAX_BUF_LEN];
    char* chunk;
    long bytes_read = 0;
    long bytes_written = 0;
    int total_set = 0;
    timeval wait;
    wait.tv_sec = 0;
    wait.tv_usec = 2000;

    sockaddr_in localhost;
    localhost.sin_family = AF_INET;
    localhost.sin_addr.s_addr = inet_addr("127.0.0.1");
    localhost.sin_port = htons(5900);
    SOCKET local_sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    ioctlsocket(local_sock, FIONBIO, &mode);
    iretval = connect(local_sock, (sockaddr*) &localhost, sizeof(localhost) );
    if (iretval == SOCKET_ERROR)
      iretval = WSAGetLastError();

    while (1)
    {
      bytes_read = libssh2_channel_read(channel, buf, MAX_BUF_LEN);
      if (bytes_read >= 0){
        FD_ZERO(&read_set);
        FD_ZERO(&write_set);
        FD_SET(local_sock, &write_set);

        // wait until the socket can be written to
        while (select(0, &read_set, &write_set, NULL, &wait) < 1)
          boost::this_thread::yield();

        if (FD_ISSET(local_sock, &write_set))
        {
          FD_CLR(local_sock, &write_set);
          chunk = buf;

          // everything may not get written in this call because we're non blocking.  So
          // keep writing more data until we've emptied the buffer pointer.
          while ((bytes_written = send(local_sock, chunk, bytes_read, 0)) < bytes_read)
          {
            // if it couldn't write anything because the buffer is full, bytes_written
            // will be negative which won't help our pointer math much
            if (bytes_written > 0)
            {
              chunk = buf + bytes_written;
              bytes_read -= bytes_written;
              if (bytes_read == 0)
                break;
            }
            FD_ZERO(&read_set);
            FD_ZERO(&write_set);
            FD_SET(local_sock, &write_set);

            // wait until the socket can be written to
            while (select(0, &read_set, &write_set, NULL, &wait) < 1)
              boost::this_thread::yield();
          }

        }
      }

      FD_ZERO(&read_set);
      FD_ZERO(&write_set);
      FD_SET(local_sock, &read_set);
      select(0, &read_set, &write_set, NULL, &wait);
      if (FD_ISSET(local_sock, &read_set))
      {
        FD_CLR(local_sock, &read_set);
        bytes_read = recv(local_sock, buf, MAX_BUF_LEN, 0);
        if (bytes_read >= 0)
        {
          while ((bytes_written = libssh2_channel_write_ex(channel, 0, buf, bytes_read)) == LIBSSH2_ERROR_EAGAIN)
            boost::this_thread::yield();
        }
      }
      boost::this_thread::yield();
    } // while
  } // if channel
}

P.S。要完成这项工作，需要最新的libssh2 SVN版本。以前的版本中存在一些错误，导致端口转发无法使用。

Answer 2

自几年以来，libssh2源代码包含了一个direct_tcpip.c示例，该示例演示了如何创建direct-tcpip SSH通道，以及自上周以来的一个forward-tcpip.c示例，该示例演示了如何创建forward-tcpip SSH通道。

direct-tcpip是ssh -L使用的，而forward-tcpip是ssh -R使用的。

libssh2用户始终有责任处理实际数据。 libssh2负责SSH通道，没有别的。您可以从研究SSH RFC（特别是RFC 4254）中获益，以更多地了解每种渠道类型对您的承诺，以及您对libssh2的期望。

libssh2的示例代码用于端口转发

2 个答案: