Round Robin Load Distribution (DNS and BIND, 4th Edition)

10.7. Round Robin Load Distribution

Name servers released since BIND 4.9 have formalized some load distribution functionality that has existed in patches to BIND for some time. Bryan Beecher wrote patches to BIND 4.8.3 to implement what he called "shuffle address records." These were address records of a special type that the name server rotated between responses. For example, if the domain name foo.bar.baz had three "shuffled" IP addresses, 192.168.1.1, 192.168.1.2, and 192.1.168.3, an appropriately patched name server would give them out first in the order:

192.168.1.1 192.168.1.2 192.168.1.3

then in the order:

192.168.1.2 192.168.1.3 192.168.1.1

and then in the order:

192.168.1.3 192.168.1.1 192.168.1.2

before starting all over with the first order and repeating the rotation ad infinitum.

The functionality is enormously useful if you have a number of equivalent network resources, such as mirrored FTP servers, web servers, or terminal servers, and you'd like to spread the load among them. You establish one domain name that refers to the group of resources and configure clients to access that domain name, and the name server distributes requests among the IP addresses you list.

BIND 4.9 and later versions do away with the shuffle address record as a separate record type, subject to special handling. Instead, a modern name server rotates addresses for any domain name that has more than one A record. (In fact, the name server will rotate any type of record as long as a given domain name has more than one of them.[75] ) So the records:

[75]Actually, until BIND 9, PTR records weren't rotated. BIND 9 rotates all record types.

foo.bar.baz.    60    IN    A    192.168.1.1
foo.bar.baz.    60    IN    A    192.168.1.2
foo.bar.baz.    60    IN    A    192.168.1.3

accomplish on a 4.9 or later name server just what the shuffle address records did on a patched 4.8.3 server. The BIND documentation calls this process round robin.

It's a good idea to reduce the records' time to live, too, as we did in this example. This ensures that if the addresses are cached on an intermediate name server that doesn't support round robin, they'll time out of the cache quickly. If the intermediate name server looks up the name again, your authoritative name server can round robin the addresses again.

Note that this is really load distribution, not load balancing, since the name server gives out the addresses in a completely deterministic way without regard to the actual load or capacity of the servers servicing the requests. In our example, the server at address 192.168.1.3 could be a 486DX33 running Linux and the other two servers HP9000 Superdomes, and the Linux box would still get a third of the queries. Listing a higher-capacity server's address multiple times won't help because BIND eliminates duplicate records.

10.7.1. Multiple CNAMEs

Back in the heyday of BIND 4 name servers, some folks set up round robin using multiple CNAME records instead of multiple address records:

foo1.bar.baz.   60   IN   A   192.168.1.1
foo2.bar.baz.   60   IN   A   192.168.1.2
foo3.bar.baz.   60   IN   A   192.168.1.3
foo.bar.baz.    60   IN   CNAME   foo1.bar.baz.
foo.bar.baz.    60   IN   CNAME   foo2.bar.baz.
foo.bar.baz.    60   IN   CNAME   foo3.bar.baz.

This probably looks odd to those of you who are used to our harping on the evils of mixing anything with a CNAME record. But BIND 4 name servers didn't recognize this as the configuration error it is and simply returned the CNAME records for foo.bar.baz in round robin order.

BIND 8 name servers, on the other hand, are more vigilant and catch this error. You can, however, explicitly configure them to allow multiple CNAME records for a single domain name with:

options {
	multiple-cnames yes;
};

BIND 9 name servers don't notice the multiple CNAME problem until 9.1.0. BIND 9.1.0 detects the problem but doesn't support the multiple-cnames statement.

10.7.2. The rrset-order Substatement

There are certain times when you'd rather the name server didn't use round robin. For example, maybe you'd like to designate one web server as a backup to another. To do this, the name server should always return the backup's address after the primary web server's address. But you can't do that with round robin; it'll just rotate the order of the addresses in successive responses.

BIND 8.2 and later name servers -- but not BIND 9 name servers, as of 9.1.0 -- allow you to turn off round robin for certain domain names and types of records. For example, if we wanted to ensure that the address records for www.movie.edu were always returned in the same order, we could use this rrset-order substatement:

options {
	rrset-order {
		class IN type A name "www.movie.edu" order fixed;
	};
};

We should probably lower the TTL on www.movie.edu's address records, too, so a name server that cached the records wouldn't round robin them for long.

The class, type, and name settings determine which records the specified order applies to. The class defaults to IN, type to ANY, and name to * -- in other words, any records. So the statement:

options {
	rrset-order {
		order random;
	};
};

applies a random order to all records returned by the name server. The name setting may contain a wildcard as its leftmost label, as in:

options {
	rrset-order {
		type A name "*.movie.edu" order cyclic;
	};
};

Only one rrset-order substatement is permitted, but it can contain multiple order specifications. The first order specification to match a set of records in a response applies.

rrset-order supports three (count 'em, three!) different orders:

fixed: Always return matching records in the same order

random: Return matching records in random order

cyclic: Return matching records in cyclic (round robin) order

The default behavior is:

options {
	rrset-order {
		class IN type ANY name "*" order cyclic;
	};
};

Configuring rrset-order is far from a complete solution, unfortunately, because resolver and name server caching can interfere with its operation. A better long-term solution is the SRV record, which we'll discuss in Chapter 16, "Miscellaneous".