Build a DNS Server in Scratch

In this step, you’re going to have a go at building your own DNS server in Scratch.

First, go to scratch.mit.edu and create a new project called “DNS model”.

A DNS stores a list of domain names against their IP address. To simulate this, you’re going to create two lists in Scratch: one called “Domain Names” and another called “IP Addresses”.

Next, you’ll need to populate each list. About 1000 entries in each should be enough.

Now, this would take you ages to do by hand, but instead, you can download premade lists of Domain Names and IP Addresses from bournetocode.com.

Once you’ve downloaded each file to your computer, right-click the Domain Name box in Scratch and import the domain names into your list. Then do the same thing for IP addresses.

You should end up with something that looks like this.

When you type a URL into the address bar of your web browser, your computer consults the DNS server to find the IP address. The DNS server looks up the domain name and finds the matching IP address. To do this, you can create a linear search algorithm.

Linear search is a method of going through a list like the one we have here, and checking each item one by one until it finds the one it’s looking for.

Start by adding in a “When green flag clicked” block… then ask the user for the domain name they’re looking for.

Next, you’ll need to index the items in each list. This means that when the server locates item 572 in the domain name list, it can match it to item 572 in the list of IP addresses.

To do this, create two variables, one called “domain” and one called “index”.

Then, set the domain to be the answer of the ask block, and the index to be the integer 1.

Since your algorithm may need to search to the end of the list, you’ll need to use a loop that keeps going until the index is at the last item in the list.

Currently, the index is 1. So the algorithm needs to check whether the first item in the list matches the domain name the user is looking for. To do this, add a conditional if/else inside the repeat until loop.

If there is a match, then we know that the index of the IP address list will contain the IP address we’re looking for. So add a “say”, “join” and index blocks to display the result of the search.

If the domain name doesn’t match, we want the index to become 2, and the algorithm to go back to the start of the loop. So under your else, add a “change index by 1”.

Now try out your code and see if it works. Pick out a random domain name from the list and use your algorithm to search for the IP address.

www.google.co.uk

216.239.32.20

Thoughts

• How would you use this activity in a lesson with your students?
• How could you change this code to return a “not found” message if the search term doesn’t exist in the list?