MS Press Book Question Clarification

alharland

Hello all. Firstly just to introduce myself, I am currently studying for the 70-291 exam using the MS Press Book, CBT Nuggets, Transcender, Advice Forums, and the good old MS website. I am hoping to take this at the end of September, thus giving me a total of just under 3 months study in advance.

I was hoping that somebody could clarify one of the questions in the MS Press Book for me regarding DNS.


Q:
You discover that an administrator has adjusted the default TTL value for your company's primary DNS zone to 5 minutes. Which of the following is the most likely effect of this change?

A:
a. Resource records cached on the primary DNS server expire after 5 minutes.
b. DNS clients have to query the server more frequently to resolve names for which the server is authoritative.
c. Secondary servers initiate a zone transfer every 5 minutes.
d. DNS hosts reregister their records more frequently.


The correct answer is b, but could somebody please clarify why.

I am under the understanding that the TTL values are not relevant for the resource records within their authoritative zones, and that the TTL instead refers to the cache life of a resource record in nonauthoritative servers. Is this correct? If so, why is the above answer option b?

Please could somebody clear this up for me. It may be me being stupid, but I have been staring at this for ages and can't get my head around it.

Thanks,

Alfie.

CoryS

Hello, the question comes off a little garbled to me at least. With TTL values on secondary servers set lower when the TTL is reached the records are expired and the next request will have to be resolved from the primary server for resolution. So in this case the clients will need to hammer on the primary server more often in order to keep their records accurate.

Since the size of the query is like 256 bytes I dont think this is a problem really considering bandwidth these days, so if you host a business site or something thats super critical, setting this TTL lower would be a good idea (maybe not 5 minutes low but you know) since this will keep your records up to date and your customer/clients heading in the right direction in case of emergency modification to your dns.

The reason its not A is because on the primary server the records do not expire, and I dont think a full zone transfer is the case when TTL values are reached, D is just goofy IMO

If I slaughtered this explanation someone else please chime in and set me straight.

Thanks!

JdotQ

I'll give this explanation a shot...

alharland wrote:

a. Resource records cached on the primary DNS server expire after 5 minutes.

The TTL would not have any effect for this answer, as the TTL is attached to the resource record which it grabs from the zone that is hosting it (authoritative zone). For example, if company ABC Inc. has some records cached for Google -- even though ABC Inc has a DNS set for a TTL of 5 minutes, Google's TTL goes by what the Primary Zone from Google's server (default is 1hr). So Google could have a TTL of 1 hour, which it distributes with it's resource records, regardless of who has them cached.

alharland wrote:

b. DNS clients have to query the server more frequently to resolve names for which the server is authoritative.

Like I mentioned for answer A, the TTL is stamped from the authoritative zone that is hosting the resource record. So, DNS clients that grab the resource record from the authoritative zone will have that resource record cached locally with the TTL value of 5 minutes on it. Once that 5 minutes is expired, the next time they query for the record, it will no longer be in the local cache and they will have to send a query to the DNS server for the answer.

alharland wrote:

c. Secondary servers initiate a zone transfer every 5 minutes.

I'm not too sure of this explanation, but I believe zone transfer settings are independent of TTL values. So even though the TTL value is set to 5 minutes, the zone transfer could be set to 10 minutes for example, and would make this answer false. (someone please correct me if I'm incorrect)

alharland wrote:

d. DNS hosts reregister their records more frequently.

The only time DNS hosts register their records is when the DHCP lease is given (client registers A record, and the DHCP registers the PTR record). If they are statically assigned, they get reregistered on every reboot (both A & PTR records). The TTL value has nothing to do with the frequency of registering or reregistering the resource records.

Hope this helps! I'm also curious as to what page in the MS Press (2nd edition??) this question is on?

If there are any incorrect information above, someone please post the correct info -- I'm still learning all this myself

alharland

Thanks for the excellent replies. They have helped me to understand the answer a lot more clearly now. I guess when you spend hours staring at this stuff, then you can get a little confused at times.

For reference, the question is on page 5-43 of the second edition book.

Once again, thanks for the replies and the clarification.

Adam.