redundant servers
Kevin Darcy
kcd at daimlerchrysler.com
Wed Nov 19 00:10:10 UTC 2003
RYAN vAN GINNEKEN wrote:
>Ok here is my scenario i have 3 registered domains and two Freebsd boxes
>running bind 9 postfix 2 and apache 2. These servers are on separate
>networks server one is master for itself and slave for the remaining 2
>domains on the server 2. Server 2 is slave for server 1 and master for
>a couple of its own domains yet only has one ip address. This i seem to
>have figured out over the past couple of years what is confusing me
>right now is how do i get the servers to back each other up i mean if
>one goes down the other one takes over dns web page mail sever
>everything. seems like a great concept but how do i achieve this i
>think i got the dns stuff but what about the rest dns seems to be
>everything in these situations so i hope this is not to far off topic
>
Nameserver failover is automatic, as long as both servers are published
in the NS records of the zone and the delegation records.
Mail failover is also automatic with the use of MX records. For each
domain, you could have one of the servers be primary, and the other
failover, or you could (roughly) load-balance the mail traffic between
the servers. It all depends on how you set the "preference" fields on
the MX records.
For web failover, DNS is not a very good solution, because there is no
web equivalent of MX records, and no web clients yet understand SRV
records, which can be thought of as bringing MX-record-like failover and
loadbalancing capabilities to any record type. Even if SRV records were
understood by browsers, there would still be a timeout delay for
failover, which is not so critical in a store-and-forward system like
email, but *very* noticeable when a user is sitting and waiting for a
page or an image to load. Using DNS, you could assign the addresses of
both webservers to your website names, but you lose control of the
ordering of those records once they get cached by other nameservers on
the Internet, so you're still faced with the possibility of long
failover delays when one of the webservers is down. Another DNS-based
approach is to automatically change the A record for one of more
websites when the primary webserver goes down. However, caching gets in
the way here too, and you'd have to reduce the TTL (Time-to-Live) values
on your records to anti-social values in order for this "switcheroo" to
be dynamic enough for an average web user.
For this reason, many folks pay a lot of money for fancy standalone
load-balancing devices from Cisco and other vendors to enhance the
performance and/or availability of their websites...
- Kevin
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