[NCSA-discuss] DNS Questions

[Jeff The Riffer]

On Tue, February 21, 2006 2:36 pm, Iztok Umek said:
>> If I was
>> running a nameserver, I wouldn't accept short TTLs. When I
>> tried it on my changeover a few days ahead, none of the
>> machines accepted the short TTL (yes I updated the serial
>> number).
>>
>> You might have trouble getting some already uncooperative
>> nameserver hoster to change your TTLs anyhow.
> The issues are when you are not following the TTL directive. Some people use
> low or 0s TTL for SPECIFIC servers on purpose to achieve network load
> balancing over multiple ISPs in multihomed environment.

Actually there's another issue, which is common confusion about the two
different TTL values specified in a DNS zone. There's the default TTL setting
that you can specify:

$TTL           14400;

which is the default time-to-live all records will use unless an explicit TTL
is specified on each line. There is also a TTL value specified in the SOA
record for a domain. That is NOT the default time-to-live value but the
Negative Acknowledgement TTL value! It determines how long a nameserver will
cache a negative response to a query (i.e. how long a nameserver will
rememeber "Nameserver bob told me there was no such record as
gumbo.hotsheep.com")

Many folks mistakenly change the SOA value. I think this is because earlier
versions of BIND did in fact use that value for a default TTL on actual
records, but I'm too lazy to check that.

Oh, here's a good link:

http://www.netadmintools.com/art232.html



>
> With this it is possible to achieve inbound load balancing based on DNS
> resolution. There are many commercial products on the market including
> (shameless plug here) Radware who pioneered this approach to network load
> balancing across multiple ISPs.
>
> This way not only you can get away with BGP (as an end customer) but truly
> get better load balancing and redundancy for your network, not to mention
> you can have less then C class assigned to you by ISP and still get it
> working just fine. You can even use bunch of cheaper but less reliable
> sources, such as cable modem or DSL to augment your T1, T3 ... lines :)
>
>
>
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> ncsa-discussion mailing list
> ncsa-discussion at ncsysadmin.org
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>


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