TTL Question

[Doug]

Hello All:

Quick question. Have a very low traffic website and need a cheap failover
solution. Wanted to know if anyone can see a problem with this.

Have 2 seperate ISP's. Was going to dual home my web server, DNS entires for
both IP's 200.x.x.x and 201.x.x.x. Was going to set TTL on the records to
very low, say 5 minutes or lower. In the event of a failure on one of these
T1's, the longest I would be down would be whatever I have the TTL set for
on the DNS record(s) correct??

I know this will generate alot of DNS traffic but as I said, very low demand
on this web server.

This sounds like it would work. Am I missing anything here?

Thanks

 

[Kevin D. Goodknecht Sr. [MVP]]

Hello All:

Quick question. Have a very low traffic website and need a cheap
failover solution. Wanted to know if anyone can see a problem with
this.

Have 2 seperate ISP's. Was going to dual home my web server, DNS
entires for both IP's 200.x.x.x and 201.x.x.x. Was going to set TTL
on the records to very low, say 5 minutes or lower. In the event of a
failure on one of these T1's, the longest I would be down would be
whatever I have the TTL set for on the DNS record(s) correct??

I know this will generate alot of DNS traffic but as I said, very low
demand on this web server.

This sounds like it would work. Am I missing anything here?

You will be down from the time the link goes down until the time you change
the record plus the TTL if the record is in cache.

 

[Doug]

Thanks - let's say both records are in DNS simultaneously. So anyone trying
to get to www.mywebserver.com would have a 50-50 chance of grabbing either
IP address. So, theoretically as you mentioned, if I just cached the lookup,
link goes downs and TTL is 5 minutes, it would be down for 10 minutes in tis
example....correct?

 

[Kevin D. Goodknecht Sr. [MVP]]

Thanks - let's say both records are in DNS simultaneously. So anyone
trying to get to www.mywebserver.com would have a 50-50 chance of
grabbing either IP address. So, theoretically as you mentioned, if I
just cached the lookup, link goes downs and TTL is 5 minutes, it
would be down for 10 minutes in tis example....correct?

It sounds like you are assuming the next time the party resolves the name,
they would receive the other IP address. The way round robin works there is
no way to assume this, because you are betting that nobody else is using the
DNS server to resolve the name.
You could conceivably get that same IP address on every query, if you are
not the only one resolving the name from DNS.

 

[Doug]

Yeah, I guess you're right about that....I guess to remedy that, in addition
to the short TTL's, if one of the lines went down, I'd have to pull that
entry out of DNS.....I see now that this is what you were referring to in
your first response. This wold ensure that the dead IP would not go out....

Thanks Kevin.