What is a proxy?

The typical example is the corporate web proxy in a company. The corporate firewall is a boundary, usually very tightly closed, between the corporate network and the outside world (wild wild Internet).

To let the employees access all the nice web sites outside, the company sets up a web proxy, which is authorised to cross the boundary (firewall) on your behalf. The web browser asks the proxy for whatever it needs, and the proxy goes and fetches the requested stuff on the web.

Since the proxy sees the client requests, it can check if they fit the corporate browsing policy and decide if it will fetch the document for the requestor. It can also request authentication, and log the username with the request.

Transparent proxies mimic the actual service you asked for, and reply as if they were the actual service provider. Except that the client doesn't notice there is a proxy in between. Most transparent web proxies grab outgoing traffic on port 80. Some ISP do this to cache responses and spare their bandwidth.

Why do I need a proxy?

For example, if you need to modify network traffic, almost transparently, at a high level, you probably need Net::Proxy.

A basic Net::Proxy script

• The usual boilerplate:

      #!perl
      use strict;
      use warnings;
      use Net::Proxy;
      

• One or more proxies are created by calling "new()" with the appropriate parameters:

      my $proxy = Net::Proxy->new( ... );
      

• The individual proxies are registered with the Net::Proxy framework:

      $proxy->register();
      

• Some framework options are defined:

      Net::Proxy->set_verbosity(1);
      

  Note: The "set_verbosity()" method is available only since Net::Proxy version 0.04.

• The framework is started, sets up the listening sockets, and waits for connections to proxy around:

      Net::Proxy->mainloop();
      

The concepts behind Net::Proxy

Net::Proxy introduces the concept of "connectors". Connectors are used to represent the ends of the two connections that the proxy handles to create a single client-server connection.

                      +-------+
                      | proxy |
                      |       |
    "client" --->(xx)[in]  [out]---> "server"
                      +-------+

In the above ASCII diagram, "(xx)" represents the listening port number, and "[in]" (left) and "[out]" (right) the Net::Proxy connectors.

The "in" connector accepts incoming connections on a listening port. Once a connection with the client is established, the proxy uses the "out" connector to connect to the destination server.

The simplest connector is named Net::Proxy::Connector::tcp (we'll use "tcp" for short). When placed on the "in" side, it simply "listen()"s for incoming connections and them "accept()"s them. Then the "out" connector "connect()"s to the server.

Each connector accepts different parameters, which we'll see in the following examples.

Since the proxy must handle every item of data going through, it can look at it, and modify it. This is what other connectors do: they can insert or transform data on the fly, which provides us with an incredible amount of power on our network connections, which we will leverage throughout this document.

Contacting a SSH server through the corporate web proxy

In many companies, the corporate firewall doesn't let you connect outside with SSH. The only allowed access to the outside is via the web proxy.

Luckily, web proxies are designed to let certain types of TCP connection go through them without modifications: encrypted SSL connections, used in HTTPS. These connections are handled in the following way: the client sends a "CONNECT" connect to the proxy which (according to a policy based on the hostname, port and the user's credentials) actually connects to the remote host and transfers the data between the client and server, without looking at the encrypted data. The proxy doesn't even check that the traffic is actual SSL traffic.

So your SSH client could connect to a local proxy, which would send the "CONNECT" request to the web proxy, asking for a connection to your home SSH server. Thereafter, the local proxy would behave like a standard TCP proxy and simply pass the data around.

Here is a network diagram showing the network configuration in ASCII-art:

                                             '
                     (internal network)      '     (Internet)
                                             '
                  +-------+     +-------+    '     +-------+
                  | local |     |  web  |    '     |  ssh  |
   ssh            | proxy |     | proxy |    '     | server|
  client --->(22)[tcp]    |     |       |    '     |       |
                  |[connect]-->(8080)   |----'--->(22)     |
                  +-------+     +-------+    '     +-------+
                                             '
                                             '

Here's how to set up the local Net::Proxy instance:

    Net::Proxy->new(
        {   in => {
                type => 'tcp',
                host => 'localhost',
                port => 22,
            },
            out => {
                type => 'connect',
                host => 'home.example.com',
                port => 22,

                # proxy details
                proxy_host => 'proxy.mycompany.com',
                proxy_port => 8080,

                # proxy credentials
                proxy_user => 'me',
                proxy_pass => 's3kr3t',
            },
        }
    )->register();

Most of the time, corporate web proxies do not allow connections on other ports than 443, the standard HTTPS port. You just need to reconfigure your SSH server so that it also listens on port 443:

    # sshd configuration file
    Port 22
    Port 443

In the exemple above, you need to change the "out"/"port" from 22 to 443.

Many SSH clients (like PuTTY) already include configuration options to get through web proxies, so Net::Proxy probably isn't necessary any longer to handle this kind of traffic.

Running two services on the same TCP port

So you managed to get out of your corporate prison^Wnetwork by setting up your SSH server to listen on port 443. The problem is that you also run a HTTPS server; and if you want it to be accessible to anyone, it must run on port 443 (otherwise the corporate proxy won't let you pass through, and noone will find it anyway).

Therefore, the only option is to run both the SSL web server and the SSH server on the same port. How is that even possible? TCP clearly doesn't allow this (or we wouldn't need those long services files in our /etc directories).

What you need is a proxy that can guess what the client wants, but without contacting the server. If it manages to find out which server the client wants to connect to, it can then contact the expected server and do its usual proxy job.

Luckily, there is a fundamental difference of behaviour between a http/s client and a SSH client:

• during a HTTP(S) connection, the client "speaks" first
• during a SSH connection, the server sends a banner first

                 '
  (Internet)     '        (internal network)
                 '
                 '           +-------+
                 '           |reverse|
                 '           | proxy |
   SSL client ---'--->(      |    [tcp]---> SSL server
                 '    ((443)[dual]   |
   SSH client ---'--->(      |    [tcp]---> SSH server
                 '           +-------+
                 '

Net::Proxy's "dual" connector is able to detect between two such clients with the help of a timeout.

    Net::Proxy->new(
        {   in => {
                type         => 'dual',
                host         => '0.0.0.0',
                port         => 443,
                client_first => {
                    type => 'tcp',
                    port => 444,     # move the https server to another port
                },
                server_first => {
                    type => 'tcp',
                    port => 22,      # good old SSH
                },

                # wait during a 2 second timeout
                timeout => 2,
            },
            out => { type => 'dummy' },
        }
    )->register();

Hiding SSH connections going through the corporate proxy from IDS

The first technique we presented (using a CONNECT request to get out of the corporate network) is so well-known that many Intrusion Detection Systems (IDS) check the first packets of a connection to try and find hidden SSH connections crossing the corporate boundaries outwards.

The server banner looks like this:

    SSH-2.0-OpenSSH_3.9p1

while the client banner may look like this:

    SSH-2.0-OpenSSH_4.2p1 Debian-5

You want to deceive Intrusion Detection Systems (IDS) by modifying the cleartext part of your SSH connection. Since the detection code simply looks for the ""SSH-"" string, an "encryption" scheme as simple as ROT-13 is enough.

                                              '
                     (internal network)       '          (Internet)
                                              '
                  +-------+      +-------+    '          +-------+
                  | local |      |  web  |    '          |reverse|
   ssh            | proxy |      | proxy |    '          | proxy |
  client --->(22)[tcp]    |      |       |    '          |       |
                  |[connect]===>(8080)   |===='===>(443)[tcp][tcp]--->  ssh
                  +-------+      +-------+    '          +-------+     server
                                              '
    Traffic                \________ ________/'
    ---> ssh                        v         '
    ===> ssh + rot13         Traffic scanned  '
                               by the IDS     '
                                              '

The "hook" connector option accepts a callback that will be called for each chunk of data received, before sending it out. The callback must have the following signature:

    # Net::Proxy versions 0.06 and 0.07
    sub {
        my ( $dataref, $connector ) = @_;
        ...
    }

    # As from Net::Proxy version 0.08
    sub {
        my ( $dataref, $socket, $connector ) = @_;
        ...
    }

The ROT-13 routine is straightforward (and must be defined in both scripts):

    my $rot13 = sub { ${ $_[0] } =~ y/A-Za-z/N-ZA-Mn-za-m/ };

Client-side proxy:

    Net::Proxy->new(
        {   in => {
                type => 'tcp',
                host => '0.0.0.0',
                port => 22,
                hook => $rot13
            },
            out => {
                type => 'connect',
                host => 'home.example.com',
                port => 22,
                hook => $rot13,

                # proxy configuration
                proxy_host => 'proxy.mycompany.com',
                proxy_port => 8080,

                # proxy credentials
                proxy_user => 'me',
                proxy_pass => 's3kr3t',
            },
        }
    )->register();

Server-side proxy:

    Net::Proxy->new(
        {   in => {
                type => 'tcp',
                host => '0.0.0.0',
                port => 443,
                hook => $rot13
            },
            out => {
                type => 'tcp',
                port => 22,
                hook => $rot13
            }
        }
    )->register();

Hiding a SSH connection under SSL through a corporate proxy

Another option to hide what you are doing in your connection through the corporate proxy, is to actually use SSL to connect to your SSH server (à la stunnel). This is what the proxy expects, after all.

                                               '
                   (internal network)          '         (Internet)
                                               '
                +-----------+      +-------+   '         +-------+
                |   local   |      |  web  |   '         |reverse|
  ssh           |   proxy   |      | proxy |   '         | proxy |
 client -->(22)[tcp]        |      |       |   '         |       |
                |[connect_ssl]===>(8080)   |==='==>(443)[ssl][tcp]--->  ssh
                +-----------+      +-------+   '         +-------+     server
                                               '
   Traffic                    \_______ _______/'
   ---> ssh                           v        '
   ===> ssh over SSL           Traffic scanned '
                                 by the IDS    '
                                               '

Client-side proxy:

    Net::Proxy->new(
        {   in => {
                type => 'tcp',
                host => '0.0.0.0',
                port => 22,
            },
            out => {
                type => 'connect_ssl',
                host => 'home.example.com',
                port => 443,

                # proxy configuration
                proxy_host => 'proxy.mycompany.com',
                proxy_port => 8080,

                # proxy credentials
                proxy_user => 'me',
                proxy_pass => 's3kr3t',
            },
        }
    )->register();

Server-side proxy:

    Net::Proxy->new(
        {   in => {
                type => 'ssl',
                host => '0.0.0.0',
                port => 443,
            },
            out => {
                type => 'tcp',
                port => 22,
            }
        }
    )->register();