sshuttle/sshuttle.md

% sshuttle(8) Sshuttle 0.46
% Avery Pennarun <apenwarr@gmail.com>
% 2011-01-25

# NAME

sshuttle - a transparent proxy-based VPN using ssh

# SYNOPSIS

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


# DESCRIPTION

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 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, sshuttle can forward traffic for your
entire subnet to the VPN.


# OPTIONS

\<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').

-l, --listen=*[ip:]port*
:   use this ip address and port number as the transparent
    proxy port.  By default 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 sshuttle on a router) try enabling IP Forwarding in
    your kernel, then using `--listen 0.0.0.0:0`. 

-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 sshuttle
    into more than one VPN at a time, it's impossible to
    use more than one DNS server at once anyway, but
    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.
    
-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.
    
--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.

-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.

-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.

-v, --verbose
:   print more information about the session.  This option
    can be used more than once for increased verbosity.  By
    default, sshuttle prints only error messages.
    
-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'`.

--seed-hosts
:   a comma-separated list of hostnames to use to
    initialize the `--auto-hosts` scan algorithm. 
    `--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.
    
--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, 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
    sshuttle seem slow in bandwidth benchmarks (benchmarks
    rarely test ping latency, which is what sshuttle is
    trying to control).  This option disables the latency
    control feature, maximizing bandwidth usage.  Use at
    your own risk.
    
-D, --daemon
:   automatically fork into the background after connecting
    to the remote server.  Implies `--syslog`.
    
--syslog
:   after connecting, send all log messages to the
    `syslog`(3) service instead of stderr.  This is
    implicit if you use `--daemon`.
    
--pidfile=*pidfilename*
:   when using `--daemon`, save sshuttle's pid to
    *pidfilename*.  The default is `sshuttle.pid` in the
    current directory.

--server
:   (internal use only) run the sshuttle server on
    stdin/stdout.  This is what the client runs on
    the remote end.

--firewall
:   (internal use only) run the firewall manager.  This is
    the only part of sshuttle that must run as root.  If
    you start sshuttle as a non-root user, it will
    automatically run `sudo` or `su` to start the firewall
    manager, but the core of sshuttle still runs as a
    normal user.
    
--hostwatch
:   (internal use only) run the hostwatch daemon.  This
    process runs on the server side and collects hostnames for
    the `--auto-hosts` option.  Using this option by itself
    makes it a lot easier to debug and test the `--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, 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, sshuttle uploads its
(python) source code to the remote end and executes it
there.  Thus, you don't need to install sshuttle on the
remote server, and there are never sshuttle version
conflicts between client and server.

Unlike most VPNs, sshuttle forwards sessions, not packets. 
That is, it uses kernel transparent proxying (`iptables
REDIRECT` rules on Linux, or `ipfw fwd` rules on BSD) 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.

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.


# BUGS

On MacOS 10.6 (at least up to 10.6.6), your network will
stop responding about 10 minutes after the first time you
start sshuttle, because of a MacOS kernel bug relating to
arp and the net.inet.ip.scopedroute sysctl.  To fix it,
just switch your wireless off and on. Sshuttle makes the
kernel setting it changes permanent, so this won't happen
again, even after a reboot.


# SEE ALSO

`ssh`(1), `python`(1)