Autossh
=======

Autossh is a program to start a copy of ssh and monitor it, restarting it as necessary should it die or stop passing traffic. Here we use autossh to realize ``NAT traversal``.

Suppose we have three mechine:

A. HBC without public IP, ssh port 22
B. Server(like `aliyun <http://www.aliyun.com>`_) with public IP, ssh port 12318, another two free ports 5678 and 8765
C. PC which need to connect A


Connect without password for ``A->B``
---------------------------------------------
- Generate key in A:

.. code:: shell

    ssh-keygen -t rsa 

- Give public key of A to B

.. code:: shell

    cat .ssh/id_rsa.pub | ssh user_B@ip_B 'cat >> .ssh/authorized_keys'


- Check that you can connect from A to B without password, otherswise you should see `Others`_ .

Auto ssh from ``B->A``
----------------------
- Now you can create a tunnel from B to A

.. code:: shell

    autossh -M 5678 -fCNR 8765:localhost:22 user_B@ip_B -p12318

- In the file ``/etc/ssh/sshd_config``, you need to set

.. code:: shell

    GatewayPorts yes

- Finally, connection from C to A is OK

.. code:: shell

    ssh user_A@ip_B -p 8765



Others
--------
.. note::
    #. make sure that A can connect B     
    #. need to change access permission for .ssh and .ssh/* in B

    .. code:: shell
        
        chmod 700 .ssh
        chmod 600 .ssh/* 

.. error::

    you may still have wrong in connect B from A
    
    .. code:: shell

        sign_and_send_pubkey: signing failed: agent refused operation
    
    you should do this to add the key

    .. code:: shell

        ssh-add ~/.ssh/id_rsa