使用Bind配置DNS Load Balancing

1. 负载均衡

最原始的负载平衡，可以在 DNS 里使用多个 A 纪录来达到效果。假设，如果你有三台 WWW 服务器，它的 IP 地址分别为 10.0.0.1，10.0.0.2 以及 10.0.0.3，使用以下的设定方式，每台服务器都会被分到 1/3 的联机。

Name	TTL	CLASS	TYPE	Resource Record (RR) Data
WWW	600	IN	A	192.168.1.1
	600	IN	A	192.168.1.96
	600	IN	A	192.168.1.99

当有客户端寻问这三笔纪录时，BIND 会使用不同的顺序轮流回复这三个 IP 地址。如上面这个例子，客户端将会接到任意的顺序，如 1,2,3 ，2,3,1 以及 3,1,2。大部份的客户端会使用第一个顺序，并且忽略其余的。

2.rrset-order 功能

rrset-order {
[ class class_name ]
[ type type_name ]
[ name "domain_name"]
order ordering }; //ordering 可為 fixed / random / cyclic
　

范例 :
rrset-order {class IN type A name "sip.com" order random;};

rrset-order 多笔 FQDN 时响应的方法，也就是当您有 Round Robin 的数据(一个名称对应到多个 RDATA)时，其响应的方式:

random: 随机选取，原来之系统默认值
cyclic: 循环式回答，即有三笔资料的话，即依 1->2->3->1->2…回答

3.Bind配置

这里以sip.com为范例:

/etc/named.conf

...

options {
listen-on port 53 { 127.0.0.1; };
listen-on-v6 port 53 { ::1; };
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
memstatistics-file "/var/named/data/named_mem_stats.txt";
allow-query { any; };
recursion yes;
rrset-order {
class IN type A name "sip.com" order random;
};

}

...

zone "sip.com" IN {
type master;
file "named.sip.zone";
};

/var/named/named.sip.zone

; BIND db file for sip.com

$TTL 86400

@ IN SOA primary.sip.com. admin.sip.com. (
2008090301; serial number YYMMDDNN
28800 ; Refresh
7200 ; Retry
864000 ; Expire
86400 ; Min TTL
)

NS primary.sip.com.
NS secondary.sip.com.

MX 10 mail.sip.com.
MX 20 mail-spool.sip.com.
_sip._udpSRV0 1 9 primary.sip.com.
SRV0 3 9 secondary.sip.com.
primaryINA10.30.32.30
secondaryINA10.30.32.31
mailINA10.30.32.32
mail-spoolINA10.30.32.34
wwwIN A192.168.1.96
wwwIN A192.168.1.99
wwwIN A192.168.1.1
$ORIGIN sip.com.

这里www.sip.com配置了3个ip地址，分别为192.168.1.1 , 192.168.1.96, 192.168.1.99.

4.Test

重新加载配置:

/etc/init.d/named reload

#第一次查询

dig www.sip.com

; <<>> DiG 9.5.0-P1 <<>> www.sip.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32738
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;www.sip.com. IN A

;; ANSWER SECTION:
www.sip.com. 86400 IN A 192.168.1.1
www.sip.com. 86400 IN A 192.168.1.96
www.sip.com. 86400 IN A 192.168.1.99

;; AUTHORITY SECTION:
sip.com. 86400 IN NS primary.sip.com.
sip.com. 86400 IN NS secondary.sip.com.

;; ADDITIONAL SECTION:
primary.sip.com. 86400 IN A 10.30.32.30
secondary.sip.com. 86400 IN A 10.30.32.31

;; Query time: 9 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Sep 4 00:01:39 2008
;; MSG SIZE rcvd: 155

# 第二次查询

dig www.sip.com

; <<>> DiG 9.5.0-P1 <<>> www.sip.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28609
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;www.sip.com. IN A

;; ANSWER SECTION:
www.sip.com. 86400 IN A 192.168.1.99
www.sip.com. 86400 IN A 192.168.1.1
www.sip.com. 86400 IN A 192.168.1.96

;; AUTHORITY SECTION:
sip.com. 86400 IN NS secondary.sip.com.
sip.com. 86400 IN NS primary.sip.com.

;; ADDITIONAL SECTION:
primary.sip.com. 86400 IN A 10.30.32.30
secondary.sip.com. 86400 IN A 10.30.32.31

;; Query time: 5 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Sep 4 00:01:41 2008
;; MSG SIZE rcvd: 155

注意红色部分，www.sip.com返回的地址顺序已经改变了！

5. Reference

BIND 9 Administrator Reference Manual