Skip to content

Latest commit

 

History

History
203 lines (138 loc) · 6.16 KB

tcp.md

File metadata and controls

203 lines (138 loc) · 6.16 KB
Raw

Make TCP connections

This guide describes how to establish and use raw TCP connections with the netcat (nc) command.

Legend

Parts of this guide are annotated with the following icons:

  • ❗ A task you MUST perform to complete the exercise.
  • ❓ An optional step that you may perform to make sure that everything is working correctly.
  • ⚠️ Critically important information about the exercise.
  • 💎 Tips on the exercise, reminders about previous exercises, or explanations about how this exercise differs from the previous one.
  • 👾 More advanced tips on how to save some time. Challenges.
  • 📚 Additional information about the exercise or the commands and tools used.
  • 🏁 The end of the exercise.
    • 🏛️ The architecture of what you deployed during the exercise.
  • 💥 Troubleshooting tips: how to fix common problems you might encounter.

❗ Communicate between servers

You need to find a partner for this part of the exercise, since the goal is to establish a TCP connection between two of the servers you have set up for the course.

Let's call you Bob and your partner Alice.

Alice will need to know the public IP address of Bob's server. We will refer to it as W.X.Y.Z.

❗ Bob: listen for TCP clients

Bob should run the nc command on his server to listen for TCP connections on port 3000:

$> nc -l 3000

📚 At this point, nc will start listening on port 3000 and take over the console, waiting for a TCP client to connect.

❗ Alice: connect to Bob's server

Alice should run the nc command on her server to connect to TCP port 3000 on Bob's server:

$> nc W.X.Y.Z 3000

📚 Here, nc is acting as a client, connecting to the listening nc process on Bob's server.

❗ Communicate!

Bob should type "Hello" and press Enter to send this text:

$> nc -l 3000
Hello

It should be immediately displayed in Alice's terminal:

$> nc W.X.Y.Z 3000
Hello

Similarly, if Alice types "World" after that and presses Enter:

$> nc W.X.Y.Z 3000
Hello
World

It should appear in Bob's terminal:

$> nc -l 3000
Hello
World

You have a two-way raw TCP connection running.

Once you're done, you can close the connection with Ctrl-C.

❗ Talk dirty HTTP to Google

Let's do something that your browser does every day: an HTTP request.

Find out Google's IP address (O.P.Q.R in this example) using the ping command:

$> ping -c 1 google.com
PING google.com (`O.P.Q.R`) 56(84) bytes of data.
64 bytes from google.com (O.P.Q.R): icmp_seq=1 ttl=53 time=0.890 ms
...

Open a TCP connection to the Google IP address you found:

$> nc O.P.Q.R 80

Now, talk to this Google server, but not in English or French, in the HTTP protocol:

GET / HTTP/1.1
Host: www.google.com

💎 Be sure to type exactly the text above. Your request must be a valid HTTP request or Google's server will not be able to interpret it correctly. If you have made a mistake, exit with Ctrl-C and start over.

📚 By sending this text over the TCP connection, you are communicating in the HTTP protocol, a text protocol: you are sending an HTTP request to retrieve (GET) the resource at path / of host www.google.com (the landing page of the Google website), using version 1.1 of the HTTP protocol.

Press Enter twice and you should receive the HTML for Google's home page.

💥 If you get a 400 Bad Request response, it means that your HTTP request is invalid. You probably did not type exactly the text above.

💥 If you don't get a response, it may be because you took too long to type the text, and the request has timed out. Try again a little faster.

Once you're done, you can close the connection with Ctrl-C.

If you open your browser, visit http://www.google.com and display the source code of the page, you should see the same result.

🏁 What have I done?

Contratulations!

Like the pioneers of the 1970s who developed the TCP/IP suite, you have established a TCP connection between two machines and exchanged some (hopefully nice) words with your classmate.

You have also spoken HTTP directly to Google's web server on a TCP connection, demonstrating the layering of the OSI model:

  • You have hand-written an HTTP request, a level 7 application protocol. As you've seen, this is simply text written in the correct format.
  • You have sent this request through a TCP connection, a level 4 transport protocol.
  • To reach the correct host, you have used an address of the Internet Protocol (IP), a level-3 network protocol.

📚 There are other protocols in other layers at work, but these are the ones that interest us in the context of this course.

You can also now fully appreciate what your browser does for you every day.

🏛️ Architecture

This is a simplified architecture of the main running processes and communication flow during this exercise:

Architecture diagram

Architecture diagram PDF.