Assignment: Networking

This assignment describes how to use basic networking with a focus on concepts most useful to robotics.

Background Information

When you enter a command in a shell, it executes a program. These programs read from a stream, known as “standard input” and write to two output streams, “standard output” and “standard error”. When you print in python, it writes its output to standard output. In another language, such as C, you use other functions, such as printf to write to standard output.

In addition to writing to standard output, a program can read from standard input. The program cat, short for concatentate, reads from standard input and writes the result to standard output.

Problem 1: Standard Output (10 points)

1. Write a python program that prints “Hello world” to standard output. Save the program as hello1.py and submit it.

2. Write a python program that prints “Hello world” to standard output using sys.stdout. Save the program as hello2.py and submit it.

3. Write a bash script that prints “Hello World” to standard output. Save the script as hello.sh and submit it.

Problem 2: Standard Input (10 points)

1. Run cat with no arguments. Why does cat seem like it is hanging?

2. When you run cat, type a message into your terminal, and press Control-D. Describe what cat does. Make sure to include which streams are being used, and for what purpose.

3. Write a python program my_cat.py that reads a message from standard input and prints to standard output, just as cat does. Submit this file.

Problem 3: Pipes (20 points)

Pipes are used to redirect standard input, standard output, and standard error. First, > is used to redirect standard output to a file. For example, echo "Hello World" > test.txt will write the string Hello World to test.txt.

1. Create files one.txt, two.txt and three.txt that contain the strings 1, 2, and 3, respectively using echo and output redirect. Write the commands you used to create these files in the corresponding section of answers.txt.

2. By convention, almost all shell programs read input from standard input, and write their output to standard output. Any error messages are printed to standard error. You can chain shell programs together by using |. For example, the program ls writes the contents of a directory to standard output. The program sort reads from standard input, sorts what it reads, and writes the sorted content to standard output. So you can use ls | sort to print out a sorted directory list. Read the man page for sort (man sort) to learn how to sort in reverse order. What is the bash script (using |) that prints the contents of a directory in reverse alphabetical order? Write the script in the corresponding section of answers.txt.

3. Use cat, | and echo to print hello world. Do not write to any files and use both commands one time. Write your answer in answers.txt.

4. This is not the simplest way to print hello world. Can you suggest a simpler way? (We asked you to do it the more complicated way to practice with pipes.) Write your answer in answers.txt.

5. Write a python script that reads from standard input, sorts lines in reverse alphabetical order, and prints the result. It should behave like sort -r. Submit your script in a file called my_reverse_sort.py.

Problem 4: Standard Error (10 points)

In addition to standard input and standard output, there is a third stream, standard error. If there is an error in a chain of pipes, it will be printed to the terminal rather than buried in the input to the next program.

1. Recall that ls -a | sort > sorted.txt puts all the names of files in a directory sorted in alphabetical order into the file sorted.txt. If you modify the command to be ls -a -hippo | sort > sorted.txt, what text is in sorted.txt, what is outputted as standard error, and why?

2. Create a python script that, in addition printing sorted inputs to standard out, prints status reports to standard error. Use it to sort ls -a instead of sort. Submit the file containing the script as my_sort_status.py.

Problem 5: Networking (20 points)

The command nc is short for “netcat” and is similar to cat but works over network connections. It reads from standard input and writes its contents not to standard output, but to a specified server. Write your answers in the corresponding sections of answers.txt.

1. Point nc to google.com as follows: nc www.google.com 80 When you first connect, it will be silent. Then type any arbitrary text and press enter. What is the error number?

2. Now type some valid http into nc: GET / HTTP/1.1. What is the output?

3. Now use nc to make a server. In one window, type nc -l 12345. This will cause nc to listen on port 12345. In another terminal on the same machine, type nc localhost 12345. You can type a message in one window and it will appear in the other window (and vice versa). This trick can be very useful to test basic internet connectivity - if the client and server can send packets at all. No answer is required for this question.

4. By convention, roscore listens on port 11311. Try using nc to connect to port 11311 on a machine where roscore is running, such as the Pi on your drone. What protocol is roscore using to communicate?

5. Another useful tool is nmap, which scans through a range of ports (and optionally, through a range of IP addresses) and reports information. Run nmap localhost on your Pi. What ports are open? Look up each port and submit what it does.

6. Run nmap with and without the nc -l 1234 command running from above. What is the difference? Why?

7. Run nmap with roscore. Does nmap report roscore? Why or why not? Use man nmap to find command line options for nmap that report the ROS port 11311.

8. Portscan google.com. List each open port and its purpose.

Problem 6: Talking to Your Robot (10 points)

Now we are going to talk about the public internet. So far we’ve been doing our work using mostly localhost, the local machine we are connected to, and google.com. The most common situation is that your base station machine (your laptop or desktop) and your robot are connected over TCP/IP to the same local network. Then you can look up your machine’s IP address (ifconfig in Unix; other ways in other OSes), and your robot’s IP address, and connect them. How can you find your robot’s IP address? Well it’s a chicken-and-egg problem. If you knew the IP address, you can connect to the robot and run ifconfig and find the IP address, but you don’t know the IP address!

What to do? There are several solutions.

1. Brainstorm how you can solve the chicken-and-egg program to connect to your robot.


2. So how does the internet work? A computer typically uses the DHCP protocol to request an IP address from a server that manages the local network. At your house, it's likely to be your cable modem or home router. At Brown, CIS manages the routers that keep the network up. Once you have an IP address, you are on the internet. When I was a student at MIT, the internet connection in my dorm room gave me an 18.*.*.* address that was on the public internet with no firewall. I could run any server that I wanted, and many students did (and do!). However there are serious security concerns with this setup. People could serve bad stuff, like SPAM, or they could get hacked by bad actors who would *use* the connection to serve SPAM. So it's safer to not give people public IP addresses and most people don't. There aren't a lot of them either - one of the things you pay your home ISP for is a public IP address, and you usually only get one. To try out DHCP, connect to Brown, Brown_Guest, and RLAB. REport back your IP address each time using your operating system. Then connect again. Do you get the same address or a different address?


3. How can we have more than one device connected to the Internet? The usual answer is a protocol called Network Address Translation. This remaps the IP address space so that you can have one public IP address that usually connects to a router. Then the router has a public (WAN or wide-area-network) side with the public IP address) and a private (LAN or local-area network) with multiple connections. The IP addresses on the private side are not full-fledged IP addresses because they cannot act as servers. You can't listen on a port from the private side and connect to it from the public internet. However you can do private-to-private connections, and many people do, e.g., for games or robots! You can also selectively open a connection to the public internet on many routers using port forwarding. This can be configured on the router; most routers offer a web-based API to configure these kinds of remappings. You can say port 11311 on the public side maps to a particular IP address and port on the private side, for example. Under a typical NAT setting, the robot and the base station will typically both connect to the router via DHCP to obtain an IP address. Their IP address will be in the 192.168.*.* range, or the 10.*.*.* range, both by convention used for private local networks. The router's public IP address will be whatever it is, and both machines will have internet access through NAT. However neither machine will be a server to the public internet. But that's okay - they only need to be servers to each other. So they can listen on ports and server request using their local (192.168 or 10.0.0.*) IP addresses. Connect to the Brown_Guest, RLAB, and Brown networks. For each network, answer the following questions:
   1. What IP address do you have on each network?
   2. What is the router's IP?
   3. What ports are open on the router?
   4. Use `nmap` to identify the machines on each network. How many are there?

Problem 7: Look Ma, No Internet! (10 points)

But what about if there is no public internet connection? What if you want to fly your drone in the wilderness? Well, there does exist cellular modems and sattellite connections, but you can also tell your drone to act as a Wifi Hotspot. It can create a network and run a DHCP server. You can configure this on your drone using the file /etc/hostapd/hostapd.conf.

Then you can connect your laptop’s base station using the SSID and passphrase specified in that file, and connect to the drone.

Alternatively you can set up your laptop as the Wifi base station and configure the drone to connect to its network. The details will vary depending on your laptop OS and settings.

Your Pi is configured to be a Wireless AP Master by default. Connect to it with your base station.

1. Which machine is acting as the DHCP server?
2. What is the Pi's IP address? What is yours?
3. What is the ping time between you and the Pi when you are close to the Pi
4. How far away can you get from the Pi before it starts disconnecting?
5. What is the ping time when you are far away from the Pi?

Problem 8: Design Scenario 1 (5 points)

You want to fly your drone autonomously for 10 miles while viewing the camera output continuously from your base station. Design a solution that will allow you to do this. What hardware, software, and network access providers do you need? Report the weight, CPU computation, and cost this capability will add to your drone.

Problem 9: Design Scenario 2 (5 points)

You want to connect to the drone over Wifi but also be able to connect to the internet in the CIT using the RLAB network. Design a solution that will allow you to do this. What hardware, software, and access providers do you need? Report the weight, CPU computation, and cost this capability will add to your drone.

Problem 10: Environment Variables (30 points)

GNU/Linux uses environment variables to store configuration information about a variety of things. You can use env to view the environment variables in your shell on the Rasberry Pi. In bash (and most shells), environment variables are local to your bash session, so they are often set in configuration files that are run every time your shell starts, such as .bashrc.

1. Log into your Rasperry Pi. Use X=3 to set the value of an environment variable named X to the value 3. Use echo $X to display the variable. Note that you must prepend $ to the variable name when reading it, but not when setting it.

2. Log into your drone again in a separate SSH session. Use echo $X to see the value of the environment variable X. What happens? Does this work? Why or why not?

3. Use env to see all the environment variables set in your shell. Pick one. Research the one that you picked. Describe 1) What program sets the environment variable and 2) What the variable controls. For example, the EDITOR environment variable is set in the .bashrc file when you log in.

4. Start screen in one of your SSH sessions. Our setup.sh script sets the ROS_MASTER_URI and ROS_HOSTNAME or ROS_IP environment variables in your session. In a second SSH session in which you have not run screen (so just after you log in), assess the value of the environment variables. Are they set to the correct values? What is setting ROS_MASTER_URI? What is setting ROS_IP or ROS_HOSTNAME? How did you figure this out? (You might find the grep command useful. Use man grep to find out how to use it.)

Handin

When you are done, use this link to create your Networking Github Repo. Commit and push the relevant files (networking.pdf, and any scripts you wrote throughout the assignment) to this Github Repo before the deadline.