computer science

HTTP tunnel

HTTP tunnel is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand HTTP tunnel rather than just read about it. In short: HTTP tunneling is used to create a network link between two computers in conditions of restricted network connectivity including firewalls, NATs and ACLs, among other restrictions. The tunnel is created by an intermediary called a proxy server which is usually located in a DMZ.

Key takeaways

HTTP tunnel belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HTTP tunnel to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HTTP tunnel from memory before moving on to harder problems.

Reference excerpt

HTTP tunneling is used to create a network link between two computers in conditions of restricted network connectivity including firewalls, NATs and ACLs, among other restrictions. The tunnel is created by an intermediary called a proxy server which is usually located in a DMZ. Tunneling can also allow communication using a protocol that normally wouldn’t be supported on the restricted network.

HTTP CONNECT method The most common form of HTTP tunneling is the standardized HTTP CONNECT method. In this mechanism, the client 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. 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 or TLS (i.e. HTTPS). Proxy servers may also limit connections by only allowing connections to the default HTTPS port 443, whitelisting hosts, or blocking traffic which doesn't appear to be SSL. The HTTP CONNECT method can not forward UDP connection, unless the MASQUE method is used for QUIC.

Example negotiation The client connects to the proxy server and requests tunneling by specifying the port and the host computer to which it would like to connect. The port is used to indicate the protocol being requested.

If the connection was allowed and the proxy has connected to the specified host then the proxy will return a 2XX success response.

The client is now being proxied to the remote host. Any data sent to the proxy server is now forwarded, unmodified, to the remote host and the client can communicate using any protocol accepted by the remote host. In the example below, the client is starting SSH communications, as hinted at by the port number in the initial CONNECT request.

SSH-2.0-OpenSSH_4.3\r\n ...

HTTP tunneling without using CONNECT A HTTP tunnel can also 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). A special HTTP server runs outside the protected network and a client program is run on a computer inside the protected network. Whenever any network traffic is passed from the client, the client repackages the traffic data as a HTTP request and relays the data 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.

See also ICMP tunnel Pseudo-wire Tunnel broker Virtual private network (VPN) Virtual extensible LAN Network virtualization using generic routing encapsulation

Notes

References

Worked examples

Example 1 — a first encounter with HTTP tunnel

Start with the simplest possible case. Write down what HTTP tunnel claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to HTTP tunnel before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about HTTP tunnel ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of HTTP tunnel

In research: HTTP tunnel appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses HTTP tunnel in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: HTTP tunnel is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer security, Hypertext Transfer Protocol, Network protocols, so understanding it makes those chapters shorter.
In everyday life: Look for HTTP tunnel outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study HTTP tunnel in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HTTP tunnel means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain HTTP tunnel out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HTTP tunnel in simple terms?

Why does HTTP tunnel matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study HTTP tunnel?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on HTTP tunnel.