Netcross is a little modular tool that might be useful in restricted network environments (read firewalled).
It's in pre-alpha developing status and it's quite unstable (actually it's only intended for testing and research).
Currently Netcross comes with the following modules:

• Tun/Tap over DNS
• TCP Redirector over DNS (unstable)
• HTTP Proxy over DNS (unstable)

Download:

• Sources: http://downloads.sourceforge.net/netcross/netcross-0.9pre-alpha.tar.gz

Requirements:

Compiling and running:

FAQ:

• What do you mean with the words "over DNS"?
  The NSLink module of netcross is capable to transport data across DNS queries/replies. Imagine, for instance:
  - A public server connected to the internet (we'll call it "S") with  NO DNS service (e.g Bind) running.
  - An unlucky client (we'll call it "C") with restricted acces only to the local network (no internet)
  - A Real DNS Server, e.g. bind (we'll call it "D") in the local network (the same of A) that is recursive and resolves external address.
  so what would happend if you'll try to type this on C?:
  # ping www.google.com
  PING www.l.google.com (66.249.85.104) 56(84) bytes of data.        ----------------------------------------------------------
  Obviously it will solve the A address for www.google.com (becuase of the presence of D) but no traffic is will pass through (maybe due to a firewall).
  Here comes Netcross.
  All you have to do is launch an instance of netcross on S (configuring it    in "server mode"... se later instructions for the .conf file) and another instance on the client C (configuring it in "client mode" and specifying the ip of D in the dns_server parameter).
  Then all you need is a domain with a DNS configuration like this:    network.domain.com.   IN NS magicbox.domain.com.
  magicbox.domain.com.  IN A  X.X.X.X (the ip address of S)
  
  So what's happening? Every time you make a request like foo.bar.network.domain.com from C it will be delivered to the local DNS server (D). It will find that the authoritative server for *.network.domain.com is magicbox.domain.com with IP address X.X.X.X and all queries to *.network.domain.com will be redirected to S.
  Then S will reply to D who will bounce the reply backward to C.
  This is the trick that would allow us to transport data from C to S through the unconscious (D) DNS server.
  So all netcross does is encoding and decoding data to let it pass through DNS servers.
  The application in "client mode" will act as a DNS client-like who makes a lot of TXT queries.
  The "Server mode" application will act as a fake DNS server who'll reply to the incoming TXT queries (with 0 TTL to avoid caching)
• What is the Tun/Tap Endpoint?
  If you select the Tun/Tap endpoint in the .conf file (with endpoint=tuntap) a virtual ethernet device will be created on both client and server tunN or tapN) and data between the client and server's tap will be transported through the DNS Link.
  So it realizes a virtual network cable between the two interfaces. More documentation about Tun/Tap is available on http://vtun.sourceforge.net/faq.html
  Both client and server must run as root (or you have to chown /dev/net/tun, see tun FAQ)
• What is the TCP Redirector Endpoint?
  If you choose, instead, the TCP Proxy endpoint (with endpoint=redirector) netcross will act as a tcp port proxy.
  You have to tell the server-side application what host and port do you want to connect to (with remote_host=x.x.x.x and remote_port=N params).Then you and have to choose a local port (local_port=N) to listen on. All data arriving on the local port will be sent to the server application and then relayed on the remote host choosen.
  I.E. C tells S to connect to 1.2.3.4 on port 80 and listens on local port 100.  All incoming data on C port 100 is sent to S and relayed to 1.2.3.4 : 80 (and backwards)
  Please note that this module is actually very unstable. I don't have tested it so much and sometimes it crashed... will be fixed in next releases.
• What is the HTTP Proxy Endpoint?
  HTTP Proxy Endpoint (endpoint=httpproxy) acts very similarly to the TCP Redirector.    The only difference is that it emulates an http proxy on a local port (choosen with local_port=N). All the incoming requests are sent to S and processed back.
  Please note that this module is actually very unstable (too). I don't have tested it so much and sometimes it crashed... will be fixed in next releases.

License:

Links:

• Netcross on SourceForge