RH 5.4 update: bind/named : network unreachable resolving (IPv6)
The new bind version 9.3.6 (Redhat 5.4) try to use IPv6 transport even if the server host does not have IPv6 connectivity, resulting in slower name resolution.
Sep 2 19:43:16 cpanel named[22767]: network unreachable resolving 'ns.isc.afilias-nst.info/A/IN': 2001:500:7::79#53
To fix this you have to start the bind damon with "-4" (IPv4 only). You can add the line "OPTIONS="-4"" to /etc/sysconfig/named to do so.
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DNS排错（1）
一、DNS解析IPV6错误
1、错误信息
在檢查 /var/log/messages会发现一大堆关于DNS启动过程中的错误，常會見到下列這樣的訊息：
May 20 17:14:42 postfix named[1925]: network unreachable resolving 'kernel.org/NS/IN': 2001:500:f::1#53
8 May 20 17:14:43 postfix named[1925]: network unreachable resolving 'kernel.org/NS/IN': 2001:500:e::1#53
9 May 20 17:14:43 postfix named[1925]: network unreachable resolving 'ns1.q.port80.se/A/IN': 2001:500:1::803f:235#53
10 May 20 17:14:43 postfix named[1925]: network unreachable resolving 'ns1.q.port80.se/AAAA/IN': 2001:500:1::803f:235#53
11 May 20 17:14:45 postfix named[1925]: network unreachable resolving 'ns1.q.port80.se/AAAA/IN': 2001:7fd::1#53
12 May 20 17:14:45 postfix named[1925]: network unreachable resolving 'ns1.q.port80.se/AAAA/IN': 2a01:6d0:1::1#53
May 20 20:53:47 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 192.31.80.30#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 192.41.162.30#5
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 78.16.49.15#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 69.65.5.106#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 75.102.60.46#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 75.102.59.82#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns2.no-ip.com/AAAA/IN': 69.72.255.6#53
May 20 20:53:48 postfix named[1925]: FORMERR resolving 'ns3.no-ip.com/AAAA/IN': 128.8.10.90#53
即：“AAAA" 或者"2001:500:f::1"，这就是IPV6，通常又不只一次，多了就会影响DNS查询的时间了，进而降低效能
2、解决方法
第一种：直接编译配置文件/etc/sysconfig/named，去除去IPv6的解析，只解析IPv4（这种方法方便，呵呵）
OPTIONS="whatever" 改为 OPTIONS="-4"，注意OPTIONS选项的值可以是：whatever、-4、-6中的一个
第二种：禁用IPv6也是可以的
（1）编辑配置文件/etc/sysconfig/network，然后将NETWORKING_IPV6=YES改为NETWORKING=no
（2）关闭ip6tables这个服务
（3）向/etc/modprobe.conf文件中，添加
alias ipv6 off
alias net-pf-10 off
#Dns Server