The Domain Name Service, or DNS, allows people to use easily recognizable names in place of IP addresses. DNS works as a network service that operates over UDP port 53. Every time you connect to a network, that network provides your computer with the IP address of a local DNS server that it can use to lookup IP addresses. Learn how DNS functions.
- [Narrator] Computers use IP addresses to communicate over the network, but those addresses are very difficult for people to remember. Just imagine if you had to memorize the IP address of every single web server that you need to access in a given day. The domain name service, or DNS, allows people to use easily recognizable names in place of IP addresses and DNS servers translate those names. A DNS server can translate a name that you're more familiar with, such as www.lynda.com into the IP addresses that computers use to communicate such as 184.108.40.206.
DNS works as a network service that operates over UDP port 53. Every time you connect to a network that network provides your computer with the IP address of a local DNS server that it can use to look up IP addresses. Then, whenever you type the domain name of a website into your browser, your computer sends a request to the local DNS server, asking it for the IP address associated with that name. If the server knows the answer to your question it simply responds to the request with the IP address and then your web browser can go ahead and connect to the website using its IP address.
If the local DNS server doesn't know the answer to your question it contacts other DNS servers to determine the correct answer, and then responds to you. DNS is a hierarchical system, and organizations who own domain names designate the DNS servers that are the authoritative sources of information about hosts using that domain name. When a local DNS server needs to perform a DNS lookup it asks a series of questions that eventually lead it to the definitive answer from the DNS server that is authoritative for a domain.
I used the Dig tool to perform DNS look-ups in this video because it's the tool of choice for network administrators using Linux and Mac systems. Dig is not available by default on a Windows system. Windows users have the choice of either installing Dig as a free download from the Internet System Consortium website, or using an alternative tool such as Nslookup. Most of the time DNS queries happen behind the scenes and computer users never see them. However, we can use a tool called Dig to perform DNS look-ups manually.
Here I am in the terminal window on a Mac. I'm going to go ahead and use the Dig command line tool. Let's first just go ahead and type dig www.lynda.com, the example we were talking about earlier. I'm going to go ahead and hit enter and then I see some cryptic results here, but there's some important information buried in it. First, in the QUESTION SECTION here, we can see that I asked for the IP address for www.lynda.com.
Then the DNS server provided this ANSWER SECTION and down here you can see the IP address for lynda.com, 220.127.116.11. One other piece of information that you can see here is the IP address of the server that was used to perform this lookup. In this case, the IP address is 18.104.22.168. This is actually a very well known web server. It's a web server belonging to the Google public DNS service.
And anybody can use this server to perform DNS look-ups. One last quick note. Some content filtering tools alter DNS look-ups, to prevent users from accessing undesirable websites. Those filters work by simply providing incorrect answers to DNS queries for those undesirable sites. If your organization uses this content filtering approach you also need to make sure that you block outbound DNS requests sent to other organizations DNS servers.
The Google public DNS server that I used in this example is just one of many publicly available DNS servers. If I were on a network that used DNS based content filtering, but didn't block outbound DNS, I could simply change the DNS server in my network settings and avoid the content filtering system entirely.
Learn about communication and networking best practices, including TCP/IP networking, network security devices, and secure network design and management. Instructor and cybersecurity expert Mike Chapple also includes coverage of converged protocols, network encryption, and wireless networking. You can find Mike's companion study books for this series at the Sybex test prep site and review the complete CISSP Body of Knowledge at https://www.isc2.org/cissp-domains/default.aspx.
- IP addressing
- Switches and routers
- Content distribution networks
- Designing secure networks
- Specialized networking
- Managing secure networks
- Working with virtualized networks like SDNs
- Detecting and preventing network attaches
- Transport encryption
- Wireless networking
- Host security