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Tuesday, 6 March 2012

Punching holes into firewalls(在防火墙上打洞）


or "Why firewalls shouldn't be considered a ultimate weapon for network security"
or "Secure TCP-into-HTTP tunnelling guide"
This document is located at http://sebsauvage.net/punching/index.html
 This document explains how to securely tunnel any TCP-based protocol (SMTP, POP3, telnet...) in simple HTTP requests.

It is heavily inspired from http://proxytunnel.sourceforge.net/paper.php, although it does not use the same tools and does not cover outside to inside data flows (backward tunnels).

Introduction

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.
Figure 1 : Using POP3 to fetch mail

Trouble: there is a Big Bad firewall which blocks everything.
Figure 2 : The firewall blocks POP3
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:
Figure 3 : The whole 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.
The administrator of the HTTP proxy cannot see which protocol is used, which server is contacted (except the home computer), nor the nature of transmitted data.

Setting up the tunnel

To create the tunnel as in our example above:
On the home computer (server):
sshd (start the ssh server)
hts --forward-port localhost:22 80 (start the HTTP Tunnel server)
On the office computer (client):
htc --forward-port 900 --proxy HttpProxy:3128 HomeComputer:80 (start the HTTP Tunnel client)
ssh -L 800:pop3server:113 sshlogin@localhost -p 900 (start the ssh client)
Then read your email with your mail program at localhost:800
Notes:
  • If your proxy requires authentication, add --proxy-authorization login:password to the htc command line.
  • sshlogin is your ssh login name on the ssh server on the Home computer.
  • You can set up as many additionnal tunnels as you want with:
    ssh -L localport:destinationServer:destinationPort sshlogin@localhost -p 900
    (localport is the local port you want to map to a destination server outside the firewall (destinationServer:destinationPort)).
Drawbacks of this solution:
  • it does not work for UDP-based protocols (NFS, chat...).
  • it does not work for programs which act as server (most games, chat, peer-to-peer...)
  • HTTP encapsulations and proxy delays can add some latency.
Good point of this solution:
  • Setting up the server is easy.
  • By using ports above 1024, setting up the client does not require administratror (root) privileges.
  • Multiple users can use the server to create multiple tunnels to any destination. Each user has its own private tunnels.
  • This tunnel can secure communications even if the proxy does not accept to proxy HTTPS.
  • This tunnel does not require the HTTP proxy to accept the CONNECT command.
  • This tunnel can work on proxies which are not capable of - or forbid - proxying of HTTPS (port 443).
  • With Linux Live CDs like Knoppix this can be a great solution for cybercafés: Live Linux CD ensures there is no lurking keylogger or troyan, and the tunnel ensures that the cybercafé owner, a troyaned computer or the government cannot sniff your passwords, spy on your data or censor websites. I especially think of China here.

Conlusion

As you can see, setting up such tunnels does not requires advanced skills, especially with the recent Linux distributions which come with pre-installed and pre-configured ssh servers.
With a little more skills, it is possible to tunnel just about everything into everything. For example, it is possible to tunnel PPP into HTTP, providing a full IP-stack tunnelling, including ICMP (ping...), DNS and servers (backward tunnels).
Opensource and commercial VPN solutions also come into mind.
See references for programs and papers about firewall bypassing below.
Security is not only a matter of firewall configuration, it must be seen at a larger scale. Do not rely on the firewall alone.
Censorship bypassing should not be only considered as a terrorist or hacker weapon, but also as tools for privacy, free speech, democraty and human rights protection (Please read papers written by PGP-author Philip Zimmerman, they are very instructive).

References

Articles and software about tunnelling and firewall/proxy/censorship bypassing:
from http://sebsauvage.net/punching/
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