sshuttle/docs/manpage.rst

sshuttle
========


Synopsis
--------
**sshuttle** [*options*] [**-r** *[username@]sshserver[:port]*] \<*subnets* ...\>


Description
-----------
:program:`sshuttle` allows you to create a VPN connection from your
machine to any remote server that you can connect to via
ssh, as long as that server has python 2.3 or higher.

To work, you must have root access on the local machine,
but you can have a normal account on the server.

It's valid to run :program:`sshuttle` more than once simultaneously on
a single client machine, connecting to a different server
every time, so you can be on more than one VPN at once.

If run on a router, :program:`sshuttle` can forward traffic for your
entire subnet to the VPN.


Options
-------
.. program:: sshuttle

.. option:: subnets

    A list of subnets to route over the VPN, in the form
    ``a.b.c.d[/width]``.  Valid examples are 1.2.3.4 (a
    single IP address), 1.2.3.4/32 (equivalent to 1.2.3.4),
    1.2.3.0/24 (a 24-bit subnet, ie. with a 255.255.255.0
    netmask), and 0/0 ('just route everything through the
    VPN').

.. option:: -l, --listen=[ip:]port

    Use this ip address and port number as the transparent
    proxy port.  By default :program:`sshuttle` finds an available
    port automatically and listens on IP 127.0.0.1
    (localhost), so you don't need to override it, and
    connections are only proxied from the local machine,
    not from outside machines.  If you want to accept
    connections from other machines on your network (ie. to
    run :program:`sshuttle` on a router) try enabling IP Forwarding in
    your kernel, then using ``--listen 0.0.0.0:0``.

.. option:: -H, --auto-hosts

    Scan for remote hostnames and update the local /etc/hosts
    file with matching entries for as long as the VPN is
    open.  This is nicer than changing your system's DNS
    (/etc/resolv.conf) settings, for several reasons.  First,
    hostnames are added without domain names attached, so
    you can ``ssh thatserver`` without worrying if your local
    domain matches the remote one.  Second, if you :program:`sshuttle`
    into more than one VPN at a time, it's impossible to
    use more than one DNS server at once anyway, but
    :program:`sshuttle` correctly merges /etc/hosts entries between
    all running copies.  Third, if you're only routing a
    few subnets over the VPN, you probably would prefer to
    keep using your local DNS server for everything else.

.. option:: -N, --auto-nets

    In addition to the subnets provided on the command
    line, ask the server which subnets it thinks we should
    route, and route those automatically.  The suggestions
    are taken automatically from the server's routing
    table.

.. option:: --dns

    Capture local DNS requests and forward to the remote DNS
    server.

.. option:: --python

    Specify the name/path of the remote python interpreter.
    The default is just ``python``, which means to use the
    default python interpreter on the remote system's PATH.

.. option:: -r, --remote=[username@]sshserver[:port]

    The remote hostname and optional username and ssh
    port number to use for connecting to the remote server.
    For example, example.com, testuser@example.com,
    testuser@example.com:2222, or example.com:2244.

.. option:: -x, --exclude=subnet

    Explicitly exclude this subnet from forwarding.  The
    format of this option is the same as the ``<subnets>``
    option.  To exclude more than one subnet, specify the
    ``-x`` option more than once.  You can say something like
    ``0/0 -x 1.2.3.0/24`` to forward everything except the
    local subnet over the VPN, for example.

.. option:: -X, --exclude-from=file

    Exclude the subnets specified in a file, one subnet per
    line. Useful when you have lots of subnets to exclude.

.. option:: -v, --verbose

    Print more information about the session.  This option
    can be used more than once for increased verbosity.  By
    default, :program:`sshuttle` prints only error messages.

.. option:: -e, --ssh-cmd

    The command to use to connect to the remote server. The
    default is just ``ssh``.  Use this if your ssh client is
    in a non-standard location or you want to provide extra
    options to the ssh command, for example, ``-e 'ssh -v'``.

.. option:: --seed-hosts

    A comma-separated list of hostnames to use to
    initialize the :option:`--auto-hosts` scan algorithm.
    :option:`--auto-hosts` does things like poll local SMB servers
    for lists of local hostnames, but can speed things up
    if you use this option to give it a few names to start
    from.

.. option:: --no-latency-control

    Sacrifice latency to improve bandwidth benchmarks. ssh
    uses really big socket buffers, which can overload the
    connection if you start doing large file transfers,
    thus making all your other sessions inside the same
    tunnel go slowly. Normally, :program:`sshuttle` tries to avoid
    this problem using a "fullness check" that allows only
    a certain amount of outstanding data to be buffered at
    a time.  But on high-bandwidth links, this can leave a
    lot of your bandwidth underutilized.  It also makes
    :program:`sshuttle` seem slow in bandwidth benchmarks (benchmarks
    rarely test ping latency, which is what :program:`sshuttle` is
    trying to control).  This option disables the latency
    control feature, maximizing bandwidth usage.  Use at
    your own risk.

.. option:: -D, --daemon

    Automatically fork into the background after connecting
    to the remote server.  Implies :option:`--syslog`.

.. option:: --syslog

    after connecting, send all log messages to the
    :manpage:`syslog(3)` service instead of stderr.  This is
    implicit if you use :option:`--daemon`.

.. option:: --pidfile=pidfilename

    when using :option:`--daemon`, save :program:`sshuttle`'s pid to
    *pidfilename*.  The default is ``sshuttle.pid`` in the
    current directory.

.. option:: --firewall

    (internal use only) run the firewall manager.  This is
    the only part of :program:`sshuttle` that must run as root.  If
    you start :program:`sshuttle` as a non-root user, it will
    automatically run ``sudo`` or ``su`` to start the firewall
    manager, but the core of :program:`sshuttle` still runs as a
    normal user.

.. option:: --hostwatch

    (internal use only) run the hostwatch daemon.  This
    process runs on the server side and collects hostnames for
    the :option:`--auto-hosts` option.  Using this option by itself
    makes it a lot easier to debug and test the :option:`--auto-hosts`
    feature.


Examples
--------
Test locally by proxying all local connections, without using ssh::

    $ sshuttle -v 0/0

    Starting sshuttle proxy.
    Listening on ('0.0.0.0', 12300).
    [local sudo] Password:
    firewall manager ready.
    c : connecting to server...
     s: available routes:
     s:   192.168.42.0/24
    c : connected.
    firewall manager: starting transproxy.
    c : Accept: 192.168.42.106:50035 -> 192.168.42.121:139.
    c : Accept: 192.168.42.121:47523 -> 77.141.99.22:443.
        ...etc...
    ^C
    firewall manager: undoing changes.
    KeyboardInterrupt
    c : Keyboard interrupt: exiting.
    c : SW#8:192.168.42.121:47523: deleting
    c : SW#6:192.168.42.106:50035: deleting

Test connection to a remote server, with automatic hostname
and subnet guessing::

    $ sshuttle -vNHr example.org

    Starting sshuttle proxy.
    Listening on ('0.0.0.0', 12300).
    firewall manager ready.
    c : connecting to server...
     s: available routes:
     s:   77.141.99.0/24
    c : connected.
    c : seed_hosts: []
    firewall manager: starting transproxy.
    hostwatch: Found: testbox1: 1.2.3.4
    hostwatch: Found: mytest2: 5.6.7.8
    hostwatch: Found: domaincontroller: 99.1.2.3
    c : Accept: 192.168.42.121:60554 -> 77.141.99.22:22.
    ^C
    firewall manager: undoing changes.
    c : Keyboard interrupt: exiting.
    c : SW#6:192.168.42.121:60554: deleting


Discussion
----------
When it starts, :program:`sshuttle` creates an ssh session to the
server specified by the ``-r`` option.  If ``-r`` is omitted,
it will start both its client and server locally, which is
sometimes useful for testing.

After connecting to the remote server, :program:`sshuttle` uploads its
(python) source code to the remote end and executes it
there.  Thus, you don't need to install :program:`sshuttle` on the
remote server, and there are never :program:`sshuttle` version
conflicts between client and server.

Unlike most VPNs, :program:`sshuttle` forwards sessions, not packets.
That is, it uses kernel transparent proxying (`iptables
REDIRECT` rules on Linux) to
capture outgoing TCP sessions, then creates entirely
separate TCP sessions out to the original destination at
the other end of the tunnel.

Packet-level forwarding (eg. using the tun/tap devices on
Linux) seems elegant at first, but it results in
several problems, notably the 'tcp over tcp' problem.  The
tcp protocol depends fundamentally on packets being dropped
in order to implement its congestion control agorithm; if
you pass tcp packets through a tcp-based tunnel (such as
ssh), the inner tcp packets will never be dropped, and so
the inner tcp stream's congestion control will be
completely broken, and performance will be terrible.  Thus,
packet-based VPNs (such as IPsec and openvpn) cannot use
tcp-based encrypted streams like ssh or ssl, and have to
implement their own encryption from scratch, which is very
complex and error prone.

:program:`sshuttle`'s simplicity comes from the fact that it can
safely use the existing ssh encrypted tunnel without
incurring a performance penalty.  It does this by letting
the client-side kernel manage the incoming tcp stream, and
the server-side kernel manage the outgoing tcp stream;
there is no need for congestion control to be shared
between the two separate streams, so a tcp-based tunnel is
fine.


See Also
--------
:manpage:`ssh(1)`, :manpage:`python(1)`