norecurse
In the previous section, you ran dig without changing the default options. This causes dig to perform a recursive lookup if the DNS server being queried supports it. In this part, you will trace the intermediate steps involved in a performing recursive query by beginning at a root server and manually going through the DNS hierarchy to resolve a host name. You can obtain a list of all the root servers by running the command dig . NS
Task 6
TASK 6: Use dig to query the c DNS root server for the IP address of lirone.csail.mit.edu without using recursion. What is the command that you use to do this?
Task 7
TASK 7: Go through the DNS hierarchy from the root until you have found the IP address of lirone.csail.mit.edu.
You should disable recursion and follow the referrals manually. Take note of all the commands that you use, and addresses that you found.
DNS Caching
Task 8
TASK 8: Without using recursion, query your default (local) DNS server for information about www.dmoz.org. Answer the following questions for your own practice:
- What is the command that you used?
- Did your default server have the answer in its cache? How did you know?
- How long did the query take?
Task 9
TASK 9: Query your default DNS server for information about the host in the previous question, using the recursion option this time. How long did the query take? (is it faster or slower than the previous task?)
Task 10
TASK 10: Query your default DNS server for information about the same host without using recursion. How long did the query take?
Has the cache served its purpose? Think about the reason(s) why.
Task 11
TASK 11: Try to query about www.singtel.com as such: dig www.singtel.com +nocmd +noall +answer. Take note of the output. Then, wait a few seconds and type the query again.
Repeat the query a few times after waiting for a few seconds. Also, search what each query option does.
You might have such output:
Do you observe a pattern in the TTL field? (e.g: TTL value is reducing or increasing, or wrapped around).