SSH Tunnelling part 1 - Local Forwarding

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|  Linux Security: Tips, Tricks, and Hackery                       |
|  Published by Onsight, Inc.                                      |
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|  28-February-2003                                                |
|  http://www.hackinglinuxexposed.com/articles/20030228.html       |
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SSH Tunnelling part 1 - Local Forwarding
By Brian Hatch

Summary: Want to encrypt an otherwise cleartext transmission? SSH
Tunnelling may be the tool for you.

Two readers, let's call them Alice and Bob, asked me to settle a
debate between them. Alice claimed that she was able to securely
access her email over POP[1] even though the server didn't support
any encryption such as POP over SSL. Bob disagreed.

Alice's solution involved using SSH[2] to tunnel her POP connection
over the SSH connection.

Unfortunately, in order to settle their dispute publicly (and to
determine which of them will be buying the other a very expensive
dinner) I need to explain how SSH tunnelling works. Next week we'll
describe Alice's setup and one of the two will need to shell out some
hefty cash for a seven course meal.

SSH tunnelling is, in a word, cool. If you can SSH from one host to
another, you can tunnel other TCP connections over the secure
encrypted SSH session.[3] This allows you to protect an otherwise
cleartext protocol, or allow arbitrary connections to machines that
would otherwise be unaccessible.

There are two kinds of tunnelling - LocalForwards and RemoteForwards.
Let's cover the first, which is used more frequently.

A local forward is a tunnel from your local machine through the SSH
server. You can create one on the command line by adding ssh
arguments of the following form:

  -L local_port:destination_host:destination_port

where the bits are defined as follows:

-L
    Create a local forward (bind a local port, and forward it through
    the remote end.)
   
local_port
    The port on the local machine that /usr/bin/ssh will listen on.
    This can be a port number or a service name like http, pop3, or
    mysql.
   
destination_host
    The destination host (name or IP address) from the perspective of
    the SSH server. This can be a host that is not accessible at all
    from the SSH client. For example if your SSH sessions ends behind
    a firewall and the SSH server can resolve
    "somebox.internal.our_company.com" that's fine, even if the SSH
    client can't reach or find an IP address for that machine
    directly.
   
destination_port
    The port on the destination_host to which it should connect.

For example if you used the following

  homebox$ ssh mail.my_isp.net -L 2525:localhost:smtp sleep 99999

then when you connected to port 2525 on homebox, it would be silently
tunnelled to localhost's smtp port (25). Remember - in this case
localhost is being evaluated on the SSH server, so localhost is the
SSH server, not homebox. The sleep 99999 line just makes sure that
the connection stays alive for a while. From another window you can
run the following:

  homebox$ nc -v localhost 2525
  homebox [127.0.0.1] 2525 open
  220 mail.my_isp.net ESMTP ReegenMail

You can see that connecting to the local port 2525 resulted in a
connection to the smtp port (25) on the remote mail server. Since
this connection was tunnelled inside the existing SSH session, it is
completely[4] encrypted, with almost no work on your part, requiring
no knowledge of cryptographic algorithms whatsoever. Isn't this
great?

While this connection is active, you can see it from the window in
which you ran the ssh command by hitting ~# :

  ~#
  The following connections are open:
    #1 direct-tcpip: listening port 2525 for 127.0.0.1 port 25, connect ...

You can open multiple connections at a time, and each is tunnelled
securely through the encrypted SSH channel.

Recent versions of OpenSSH only allow the local SSH client machine to
connect to the tunnelled connection.[5] If you need to allow other
machines to use this tunnel[6] then you can supply the -g option to
ssh as well.

Above I've described how to create these tunnels from the command
line. Luckily you can also define your tunnels inside your ~/.ssh/
options file. For example if I want to create the above tunnel any
time I connect to my mail server by running 'ssh mailtunnel' I can
add the following in my options file:

  Host mailtunnel
  HostName mail.my_isp.net
  LocalForward 2525:localhost:25
  GatewayPorts no

Then any time I 'ssh mailtunnel' it will automatically create this
local forward. (I included 'GatewayPorts no' above, but that is
actually the default anyway.) If I want to connect without the port
forwarding, I just ssh mail.my_isp.net like normal.

Next week I'll discuss SSH's Remote Forwarding, and discuss one of
the finer points of these two forwarding methods which will resolve
the debate between Alice and Bob.

NOTES:

[1] Post Office Protocol, one of two main methods of accessing remote
email, IMAP (Internet Mail Access Protocol) being the other one.

[2] SSH is a protocol that allows secure remote login/remote
execution/file copy abilities. See http://www.openssh.com for
information about OpenSSH, the most widely used Linux/Unix SSH
software. They also maintain a list of non-unix software for those of
you unfortunate to sit on a windows box, or lucky enough to have a
Palm-enabled phone.

[3] Actually, you can tunnel any protocol over any other protocol if
you try hard enough. SOAP is just one example of the
lets-tunnel-everything-over-HTTP category. You could tunnel a PPP
connection over SSH to create a poor man's VPN. Anything is possible.

[4] "Completely" because the endpoint was the SSH server

[5] It does this by only binding the port for localhost 127.0.0.1 -
no other machines should be able to reach this IP address.

[6] You better do this with great care if it is a protected resource
you are accessing - you do not want to let some random cracker into a
private network.

                            -------------                            
Brian Hatch is Chief Hacker at Onsight, Inc and author of Hacking
Linux Exposed and Building Linux VPNs. Today is his sister's
birthday. Send greetings her way at bjh () ifokr org. Let's see if we
can flood her with good wishes - I forgot to buy her a present. Brian
can be reached at brian () hackinglinuxexposed com.

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Copyright 2003, Brian Hatch.



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