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HTTP tunneling is a technique by which communications performed using various network protocols are encapsulated using the HTTP protocol, the network protocols in question usually belonging to the TCP/IP family of protocols. The HTTP protocol therefore acts as a wrapper for a channel that the network protocol being tunneled uses to communicate.
The HTTP stream with its covert channel is termed an HTTP tunnel.
HTTP tunnel software consists of client-server HTTP tunneling applications that integrate with existing application software, permitting them to be used in conditions of restricted network connectivity including firewalled networks, networks behind proxy servers, and network address translation.
An HTTP tunnel is used most often as a means for communication from network locations with restricted connectivity – most often behind NATs, firewalls, or proxy servers, and most often with applications that lack native support for communication in such conditions of restricted connectivity. Restricted connectivity in the form of blocked TCP/IP ports, blocking traffic initiated from outside the network, or blocking of all network protocols except a few is a commonly used method to lock down a network to secure it against internal and external threats.
The mediater server unwraps the actual data before forwarding it to the remote host in question. Symmetrically, when it receives data from the remote host, it wraps it in the HTTP protocol before sending it as part of an HTTP response to the application.
In this situation, the application plays the role of a tunneling client, while the remote host plays the role of the server being communicated with.
HTTP CONNECT tunneling
A variation of HTTP tunneling when behind an HTTP Proxy Server is to use the "CONNECT" HTTP method.
In this mechanism, the client, using the "CONNECT" HTTP method, asks an HTTP Proxy server to forward the TCP connection to the desired destination. The server then proceeds to make the connection on behalf of the client. Once the connection has been established by the server, the Proxy server continues to proxy the TCP stream to and from the client. Note that only the initial connection request is HTTP - after that, the server simply proxies the established TCP connection.
This mechanism is how a client behind an HTTP proxy can access websites using SSL (i.e. HTTPS).
Not all HTTP Proxy Servers support this feature, and even those that do, may limit the behaviour (for example only allowing connections to the default HTTPS port 443, or blocking traffic which doesn't appear to be SSL).
HTTP tunneling without using CONNECT
In some networks, the use of CONNECT method is restricted to some trusted sites. In such cases, an HTTP tunnel can still be implemented using only the usual HTTP methods as POST, GET, PUT and DELETE. This is similar to the approach used in Bidirectional-streams Over Synchronous HTTP (BOSH).
In this proof-of-concept program , the server runs outside the protected network and acts as a special HTTP server. The client program is run on a computer inside the protected network. Whenever any network traffic is passed to the client, it repackages it as an HTTP request and relays it to the outside server, which extracts and executes the original network request for the client. The response to the request, sent to the server, is then repackaged as an HTTP response and relayed back to the client. Since all traffic is encapsulated inside normal GET and POST requests and responses, this approach works through most proxies and firewalls.
HTTP tunnel clients
There are several free or open-source, and commercial HTTP tunneling client applications that allow even applications that lack native tunneling support to communicate from locations with restricted connectivity.
The free or open-source HTTP tunneling clients are usually packaged as a pair of applications, one of which performs the role of the mediator server, the other performing the role of the tunneling client. This requires the user to have access to their own server that they can run the mediator server software on.
The commercial HTTP tunneling client applications are provided by companies that run their own mediator server farms. They charge for the service provided, with various tiers of service that depend on the bandwidth provided.
Uses of HTTP tunnel clients
- To use applications (games/IM clients/browsers) from behind restrictive firewalls or proxy servers
- To access blocked sites
- To share some confidential resource over HTTP securely
- ICMP tunnel
- Tunnel broker
- Virtual private network
- Virtual extensible LAN
- Network virtualization using generic routing encapsulation