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SSH has numerous uses beyond just logging into a remote system. In
particular, SSH allows you to forward ports from one machine to another,
tunnelling traffic through the secure SSH connection. This provides a
convenient means of accessing a service hosted behind a firewall, or one
blocked by an outgoing firewall.
However, forwarding an individual port still requires you to change
where your program connects, telling it to use a non-standard port on
|localhost| rather than the standard port on the remote machine, and it
requires a separate port forward for each machine you want to access.
Dynamic port forwarding via SOCKS
provides a more convenient alternative.
The examples in this article assume that you reside behind a restrictive
firewall which does not allow outgoing SMTP connections except to a
designated mail server. You want to connect to a different mail server,
|mail.example.net|, on port 25. You have an SSH account on a machine
|shell.example.org|, which does not reside within the restrictive
firewall and can thus access port 25 on |mail.example.net|.
With standard SSH port forwarding, you could enter the command:
ssh -L 2525:mail.example.net:25 shell.example.org
This will forward port 2525 on your machine to port 25 on
|mail.example.net|, by way of |shell.example.org|. You will then need to
configure your mailer to send mail to |localhost|, port 2525, and use
the authentication information for your mail account on
|mail.example.net|. For example, in Thunderbird^WIcedove, you could add
an additional outgoing mail server via Edit->Preferences, “Outgoing Mail
Server (SMTP)”, “Add…”, and either set it as the default or explicitly
set your mail account to use that server. You can then send your mail,
which will potentially (if you use secure authentication with
|mail.example.net|) give you a security warning about |localhost|
presenting a certificate for |mail.example.net|, and then prompt you for
your account password. After you have finished sending all the mails you
want to send, you can then change your outgoing mail server back to the
previous setting, and exit SSH.
To avoid all this hassle, SSH also supports dynamic port forwarding via
SOCKS. SOCKS defines a standard mechanism for a client to connect to a
server by way of a proxy. SSH can serve as the proxy, allowing you to
connect to |shell.example.org| and make connections from there to an
arbitrary server such as |mail.example.net|. Simply run:
ssh -D 1080 shell.example.org
to make the connection to |shell.example.org| and start a SOCKS proxy on
|localhost| port 1080.
In order to make use of the SOCKS proxy, you can either use applications
which can speak SOCKS natively, or you can use a socksifier program like
tsocks. tsocks provides a library used with |LD_PRELOAD|, which replaces
the standard sockets functions like |socket|, |connect|, and |sendto|
with functions that make use of a designated SOCKS proxy. The tsocks
script runs a program with this library loaded. The library will read
|/etc/tsocks.conf| to find out what SOCKS proxy to use. To configure
tsocks to work with an SSH SOCKS proxy on localhost, edit the default
|/etc/tsocks.conf|, change the server variable to 127.0.0.1, and comment
out the path example.
Now that you have tsocks configured, you can run the following whenever
you want to send mail via |mail.example.net|:
ssh -D 1080 shell.example.org
This will open the SSH-tunnelled SOCKS proxy to |shell.example.org| and
run thunderbird. You can then send mail normally, without changing the
outgoing server configuration, and without seeing any authentication