RUOFF LARS wrote:
Hi,
i'm using BIND9 on an Ubuntu-8.10-server.
I'd like to configure the following:
For a given name (eg. vega.lab.ts), I'd like to forward the request to
two external DNS servers, *simultaneously*, and respond with the first
response that i get.
Is this possible?
Short answer: not possible with BIND currently, that I know of.
Longer answer: why would you even want to do this? Sounds like you're
barking up the wrong tree. Latency of DNS response may have little or
nothing to do with the latency of whatever real-time connection (HTTP,
LDAP, VoIP, video, audio, whatever) is being established using that DNS
information. Trying to equate DNS response latency to anything else that
a user would care about, is an exercise in futility, IMO.
Furthermore, have you considered caching? Once the answer is cached,
then a BIND nameserver won't try to fetch the information from other
servers *at*all*, until that cache entry expires.
If your goal is to optimize application performance by always directing
users to a "best" node, among a set or cluster of nodes, then put a
load-balancer in front of this resource: on the back-end, it can measure
latency or any other metric(s), which is most representative of the
"user experience" for this resource (depending on the
probing/measurement capabilities of the load-balancer
device/package/subsystem). On the front-end, the load-balancer responds
with whatever IP represents the "best" choice for that resource, at any
particular point in time. As with any DNS-based load-balancing scheme,
you might have to lower the TTLs of the relevant records to
ridiculously- (possibly anti-socially-)low values in order to provide
sufficiently-dynamic load balancing.
I didn't see how to do it directly, so i tried using a subdomain, (eg.
x.vega.lab.ts) and specifiying the two DNS for this subdomain:
Extract from the lab.ts zone file:
[...]
x.lab.ts. IN NS vega-a.x.lab.ts.
x.lab.ts. IN NS vega-b.x.lab.ts.
vega-a.x.lab.ts. IN A 172.25.32.252
vega-b.x.lab.ts. IN A 192.168.2.3
[...]
But this doesnt seem to work:
named-checkzone lab.ts /etc/bind/db.lab.ts says:
zone lab.ts/IN: x.lab.ts/NS 'vega-a.x.lab.ts' (out of zone) has no
addresses records (A or AAAA) zone lab.ts/IN: x.lab.ts/NS
'vega-b.x.lab.ts' (out of zone) has no addresses records (A or AAAA)
zone lab.ts/IN: loaded serial 2 OK
I just ran a quick test, and it appears that named-checkzone actually
goes out and tries to resolve glue records it encounters. Since you
haven't delegated the zone yet, it's not surprising that the glue
records don't resolve from the authoritative nameservers for the zone.
In this respect, I think named-checkzone is being more rigorous than
named itself would be, as it loads the zone. If these "no addresses
records [sic]" errors are the *only* ones being reported for the zone,
then I'd try to load it and see if those errors magically evaporate once
you do that.
I don't think delegation is the solution to your load-balancing
requirement, however. NS'es are tried, sequentially, according to
historical RTT statistics; _over_time_, faster-responding servers will
tend to get tried before slow-responding ones, but this adaptivity may
take time to kick in when the responsiveness of the target nameservers
changes, so it would probably not be dynamic enough to meet your
requirements, even _if_ the latency of DNS responses were reflective of
the performance of the underlying app (which I question above).
- Kevin
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