DNS Round Robin
Stan
Posted this in NT DNS with no bites. Hope someone here can answer this."
Quick question. From what I've read, round robin dns is primarily associated
with clusters. My question is this:
If I have 2 Exchagne 5.5 servers both Exchange1 and Exchange2, both setup to
receive mail for mydomain.com, and have DNS mx and pointers setup to point
to both of these servers with equal priority, will this opffer any form of
tcp/ip load balancing? On the surface ot looks like it would. If Exchange1
was busy for whatever reason and couldn't handle the request, it seems that
Exchange2 would pick up the ball and or either server would have just as
equal a chance of being sent to.
Is my reasoning correct here?? Any comments appreciated.
Ryan Lambert
will share the workload equally.
Strangely, I have also done a form of round robin DNS by
assigning multiple A records to the same host. Mind you, I
did that on BIND, not Microsoft DNS... so the processing
of the record may be slightly different.
But if you ever have a need for that, it is a possible
option where priorities cannot be set.
Regards,
Ryan
>.
>
Ace Fekay [MVP]
"Virtual IP", where we set the resource name to that IP in DNS just once, so
we don;t need Round Robin for that. The cluster service will handle the load
balancing (if Active/Active).
Round Robin is basically used for when you have mutliple resources running
the same exact data that you want to split the requests between them. Say, a
Proxy Array, where you can enter in DNS:
proxyarray A 192.168.5.150
proxyarray A 192.168.5.151
proxyarray A 192.198.5.152
What will happen is that the first query that comes in will be given the
first IP. The next query that comes in will be given the second IP, the
third, gets the third, but the fourth will get the 1st, so it rotates. This
ability is what gives you load balancing.
MX records is based on a priority. The lower number is used first unless it
doesn;t respond, then it goes to the next higher up. If there are multiple
of the same Priority, then it takes a random pick. You can have the one MX
with a Priority of 10 point to say a record, but then the record is Round
Robin because there are mulitples. In the example below, there are two mailA
machines, but the MX only points to one. What will happen is that the
queries will get load balanced between 192.168.5.120 and 192.168.5.121. So
we are mixing and getting the best of both worlds, MX priorities and Round
Robin.
(same as parent) MX [10] maila.domain.com
(same as parent) MX [20] mailb.domain.com
maila A 192.168.5.120
maila A 192.168.5.121
mailB A 192.168.5.122
mailB A 192.168.5.123
Make sense?
Want a great real life example? Check out hotmail's and Microsoft's MX
records.
--
Regards,
Ace
Please direct all replies to the newsgroup so all can benefit.
Ace Fekay, MCSE 2000, MCSE+I, MCSA, MCT, MVP
Microsoft Windows MVP - Active Directory
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Stan
"Ace Fekay [MVP]" <PleaseSubstituteMyFirstName&LastNa...@hotmail.com>
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Ace Fekay [MVP]
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Regards,
Ace
Please direct all replies to the newsgroup so all can benefit.
Ace Fekay, MCSE 2000, MCSE+I, MCSA, MCT, MVP
Microsoft Windows MVP - Active Directory
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"Stan" <n...@no.com> wrote in message
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Jonathan de Boyne Pollard
AF> given the first IP. The next query that comes in will be given the
AF> second IP, the third, gets the third, but the fourth will get the
AF> 1st, so it rotates. This ability is what gives you load balancing.
No. It doesn't.
I've been meaning to write up the Frequently Given Answer on this for a
long time, because time and again people state that round robin cycling
of resource records within a set provides some form of useful service,
which it does not. When I get time, I will. In the meantime read this:
<URL:http://marc.theaimsgroup.com./?l=djbdns&m=98197912410576>
BIND may be the worst-documented of the major three DNS server softwares,
but even the ISC has a discussion of round-robin DNS pointing out (albeit
rather indirectly) that it doesn't achieve any of the goals that people
actually desire:
<URL:http://isc.org./products/BIND/docs/bind-load-bal.html>
AF> If there are multiple [MX records] of the same Priority [for a given
AF> domain], then [an SMTP client] takes a random pick.
No. RFC 974 does not mandate a random selection. SMTP clients are well
within their rights to sort the IP address list into proximity order
before using it, or to use the IP address list in the same order as they
obtained it from one's proxy DNS server. All that RFC 974 mandates is
that all IP addresses for one set of SMTP servers with the same preference
level be tried before falling back to the next, less preferable, set of
SMTP servers.
AF> In the example below, there are two mailA machines, but the MX only
AF> points to one. What will happen is that the queries will get load
AF> balanced between 192.168.5.120 and 192.168.5.121. [...]
AF>
AF> (same as parent) MX [10] maila.domain.com
AF> [...]
AF> maila A 192.168.5.120
AF> maila A 192.168.5.121
AF> [...]
False. First, load balancing is not the same as even distribution.
Second, the above does not even guarantee such an even distribution
across both IP addresses, for reasons already given.
Jonathan de Boyne Pollard
No. You have no guarantee that SMTP clients will try your two SMTP
servers in any particular order, or even about the distribution of which
server they will try first. And, furthermore, the connection pattern
will bear no relationship to server/network load.
Ace Fekay [MVP]
Ace
"Jonathan de Boyne Pollard" <J.deBoyn...@tesco.net> wrote in message
news:3E7230BE...@tesco.net...
Jonathan de Boyne Pollard
JdeBP> for a long time, because time and again people state that round
JdeBP> robin cycling of resource records within a set provides some form
JdeBP> of useful service, which it does not. When I get time, I will.
JdeBP> In the meantime read this:
JdeBP>
JdeBP> <URL:http://marc.theaimsgroup.com./?l=djbdns&m=98197912410576>
JdeBP>
JdeBP> BIND may be the worst-documented of the major three DNS server
JdeBP> softwares, but even the ISC has a discussion of round-robin DNS
JdeBP> pointing out (albeit rather indirectly) that it doesn't achieve
JdeBP> any of the goals that people actually desire:
JdeBP>
JdeBP> <URL:http://isc.org./products/BIND/docs/bind-load-bal.html>
MF> Thanks Jonathan for the links. Makes more sense now.
Here's another one for you, then. (-:
<URL:http://homepages.tesco.net./~J.deBoynePollard/FGA/dns-round-robin-is-useless.html>