CS2910 Week 5, Class 2 Today DNS Muddy Points More HTTP Headers Review for Midterm Exam This coming Monday: Midterm Exam SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 1

DNS Muddy Points question ambiguity How do you know we are actually understanding any of this if you don't go over the answers we are supposed to get so we know if we got them right or not? Tracing the chain of DNS servers what does the nslookup command actually do? Who Who has the "authority" to change the root DNS servers?"authority" What is any of this for/what does this mean? No issues here... Makes cents to me Seemed pretty straight forward today I'm confused about the different DNS record names I don't understand the last exercise we did? I was not able to find the correct IP address for the msoe.edu mail server 2

DNS Video Questions -What layer of the internet stack do DNS services occur or do they occur outside of the internet stack? -Can you expand upon host aliasing and mail server aliasing? How does all of the DNS information you need locally get propogated globally? Why are the first two numbers of the IPv4 Address always ? Isn't mail serving alias and load distribution the same thing?They both just translate the same name to different IP addresses. What is an mx record used for? So just to clarify, load distribution is used to evenly spread out clients so one server is not overloaded? Also I was wondering what is a mail server and how is it different? What other DNS servers are used frequently aside from BIND? How does the rotation of DNS servers work in order to balance the load upon lookup of a Web site URL? 3

DNS Video Questions (2) I am confused on the topic of mail server aliasing and how they work. What does the mx mean? I don't really understand the mail server aliasing one because he said he would go into greater detail later. How does it work? Are the DNS servers ran by ISPs; if not, then who? If the ip address changes, how the dns server get the changed ip address? What would happen if the client used the second IP address given back It seems that, by using dnslookup, you can gain access to a lot of information about a network such as the IP address. How safe is this for the network? How exactly does BIND translate from DNS into an IP address and is it possible to reverse the process how often do servers typically shuffle / rotate when load sharing? SE-2811 Dr.Yoder 4

DNS Video Questions (3) As I figured out, Windows does not have 'dnslookup' command, as Linux and OS X do. Instead it uses 'ping' command (does it?). When I tried to 'ping google.com' multiple times I got the same server ip address all the time. Why do I get the same ip address? Because I'm pretty sure that they should have some kind of load distribution... I am confused about mail server aliasing; how and why does the name translate to a different IP address for the mail server? Why are we watching a video that looks as if someone took crayons to a powerpoint? What would the DNS code look like and how can the DNS be used in code to get a the IP address from the url of any general website? Why doesn't DNS just also save the website's default http header so that it doesn't have to be sent and received every time you load a site? Unless it already is... I think the video did a good job of explaining DNS. However, when the instructor talks about different kinds of DNS servers, I was wondering when and why you would use each. What does the BIND software do and will we use this software? 5

HTTP headers required for Lab 5 (HTTP Server) Date (See exercise/RFC) Connection Content-Type (See exercise/RFC) Content-Length (or optionally Transfer- Encoding) SE-2811 Dr.Yoder 6

SE-2811 Dr. Josiah Yoder 7

Acknowledgement This course is based on the text Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Where noted, slide material is copyright J.F Kurose and K.W. Ross, used by permission 8