Firewalls are heavily used to secure private networks (home or corporate). Usually, they are used to protect the network from:
- intrusions from outsiders
- misuse from insiders
In a TCP/IP environment, the typical corporate firewall configuration is to block everything (both incoming and outgoing), and give access to the internet only through a HTTP proxy. The proxy usually has filtering capabilities (censors URLs and file types), and access to the proxy often requires credentials (login/password). This gives greater contol to the network administrator over what and who is going in and out of the network.
Still, this should not considered a ultimate weapon, and network administrators should not rely on the firewalls only.
Encapsulation is the basis of networking. For example, HTTP is encapsulated by TCP, TCP is encapsulated by IP, and IP is often encapsulated in PPP or Ethernet.
Encapsulating protocols in an unsual way is often reffered as tunnelling.
As soon as you let a single protocol out, tunelling allows to let anything go through this protocol, and thus through the firewall.
This paper demonstrates how to encapsulate any TCP-based protocol (SMTP, POP3, NNTP, telnet…) into HTTP, thus bypassing the firewall protection/censorship (depending on your point of view)
A word of warning:
In many countries and corporate environments, bypassing a firewall is forbidden and exposes you to sanctions, redundancy, legal proceedings and – in some countries – death penalty.
You are warned.
Nevertheless, in some countries this kind of firewall/proxy bypassing is the only way to ensure free speech (such as China or United Arab Emirates where the government severly censors the internet and where firewall bypassing is a national sport.)
Now you known what you’re doing, let’s move on.
The problem
Say you want to fetch your mail from your ISP mail server. You usually simply connect to port 110 on the POP server of your ISP.
Trouble: there is a Big Bad firewall which blocks everything.
Well… it does not exactly block everything: it lets HTTP out through a proxy.
Let’s encapsulate our POP3 connection into HTTP.
The tools
We need:
- A computer on the internet which has unrestricted access to the internet, such as a home ADSL computer.
- GNU HTTP Tunnel (http://www.nocrew.org/software/httptunnel.html). It encapsulates TCP into HTTP requests.
- SSH is a secure shell (http://www.openssh.com). It provides secure (and compressed) channels between two hosts using SSL. Besides providing a shell (like telnet), it also provides file copy (scp) and TCP port forwarding (tunnelling). We will use the port forwarding feature.
Why not use GNU HTTP Tunnel alone ?
In principle, only HTTP Tunnel is necessary. But this is not desirable:
- the tunnel is public: anyone can use your tunnel. Your could be held liable for what anybody has done with your tunnel.
- the tunnel is cleartext: anyone can spy on your connection. Your passwords (SMTP, POP3, telnet…) are transmitted in clear text.
- the tunnel is not protected: anyone can alter the datastream.
- you have to run a new instance of the HTTP Tunnel client and the server for each new tunnel you want to set up.
This is where ssh come in. ssh provides:
- authentication (only authorised users can use the tunnel)
- privacy (no one can spy on what’s going through the tunnel)
- integrity (no one can tamper data going through the tunnel)
- easy tunnel set-up (you can create a new tunnel with a single ssh command on the client side).
These tools are available on Unix/Linux and Windows environments.
The whole chain
Let’s see how this works. Here is the full chain:
Technically speaking, once this chain is established, connecting to OfficeComputer:800 is identical to connecting to pop3server:110.
The mail client will not see the difference.
- On the office computer:
- TCP data sent to port 800 is encrypted by ssh, which forwards data to port 900.
- ssh stream sent to port 900 is chunked in individual HTTP requests by the HTTP Tunnel client and sent to the home computer through the proxy.
- On the home computer:
- the HTTP Tunnel server receives HTTP requests, decapsulates and re-assembles the ssh stream and forwards it to port 22 (to the ssh server).
- the ssh server decrypts the datastream and forwards it to the pop3server on port 110.
As TCP is a bi-directionnaly datastream, once established, the TCP connection can pass data back and forth through the HTTP proxy.
Lire la suite…