前言
默认情况下，Linux偏好通过IPv6联网。对于没有IPv6的云服务器、机房环境来说没有什么太大的问题，偶尔可能解析出IPv6地址返回网络不可达的话修改DNS即可，但是对于具有IPv6地址的系统来说，如果碰上了V6地址不靠谱的情况，相当浪费时间。虽然网络上建议优先这样，但是DNS返回结果不靠谱不得不修改。
禁用IPV6
计算机操作系统、浏览器之类的纷纷彻底的优先IPv6了，虽然有v4兜底回退，对于浏览器来说不是太大的问题，Windows操作系统也没那么“硬”，至于国内软件，分成两类：一类是运营商、大互联网厂商，具有稳定成熟的v6网络，软件、网页能够顺利流畅通过v6地址加载了；而另外一类就类似百度，百度竟然放弃了ipv6？也许是机房接入的繁琐。
早些时候某些大厂的v6地址访问延迟极大，其实本质就是目标集群所在的机房并没有提供V6带宽接入，通过其他地方“自建路由”变相提供的V6访问，并不是简单的网络没优化好。
至于IPV6在Linux，如果不可靠，基本就是导致缓慢需要等待超时重试IPV4这一种结局。毕竟大多数都是基础软件，软件开发者们所处的网络环境都是比较优异的，一般没有做处理，并不会像浏览器那样，v6地址响应缓慢立刻同时跟踪v4地址。所以可以禁用，当然我不是很建议禁用，可能有不少软件在这个状态下会无法运行，哪怕你的环境没有IPv6，软件也无法在本地禁用v6的情况下运行。
echo "1" > /proc/sys/net/ipv6/conf/all/disable_ipv6通过这个方法就可以简单的禁用ipv6，试试看有没有软件运行出现问题吧。
echo "0" > /proc/sys/net/ipv6/conf/all/disable_ipv6如果有问题，输入上面的方式就可以关闭了。基本上RH DEB系都是通过这个虚拟文件临时禁用v6。重启就恢复了。
而有一些软件，临时禁用不会出现问题，但是永久禁用，就会无法运行。
IPv4优先
一个更靠谱的方式，就是修改系统的偏好。这方法，“很官方”。
echo 'precedence ::ffff:0:0/96 100' > /etc/gai.conf这是相当“官方”的方法，出现问题的时候我就是用这个方法解决的。 ::ffff:0:0/96是IPv4在IPv6方法的兼容地址。
gai.conf，getaddrinfo，此文件用于管理员操作设定网络IP优先级。已CentOS 7或者类似的RH体验复刻系统，具体可以参见/usr/share/doc/glibc-common-2.17位置。debian类系统默认存在/etc/gai.conf，默认状态都是注释解释性的信息，而RH系默认并没有放置，所以直接覆盖此文件。
文件内就有具体的建议，如何进行一些设定。例如这一段；
# For sites which prefer IPv4 connections change the last line to
#
#precedence ::ffff:0:0/96 100
下面是原文：
# Configuration for getaddrinfo(3).
#
# So far only configuration for the destination address sorting is needed.
# RFC 3484 governs the sorting. But the RFC also says that system
# administrators should be able to overwrite the defaults. This can be
# achieved here.
#
# All lines have an initial identifier specifying the option followed by
# up to two values. Information specified in this file replaces the
# default information. Complete absence of data of one kind causes the
# appropriate default information to be used. The supported commands include:
#
# reload <yes|no>
# If set to yes, each getaddrinfo(3) call will check whether this file
# changed and if necessary reload. This option should not really be
# used. There are possible runtime problems. The default is no.
#
# label <mask> <value>
# Add another rule to the RFC 3484 label table. See section 2.1 in
# RFC 3484. The default is:
#
#label ::1/128 0
#label ::/0 1
#label 2002::/16 2
#label ::/96 3
#label ::ffff:0:0/96 4
#label fec0::/10 5
#label fc00::/7 6
#label 2001:0::/32 7
#
# This default differs from the tables given in RFC 3484 by handling
# (now obsolete) site-local IPv6 addresses and Unique Local Addresses.
# The reason for this difference is that these addresses are never
# NATed while IPv4 site-local addresses most probably are. Given
# the precedence of IPv6 over IPv4 (see below) on machines having only
# site-local IPv4 and IPv6 addresses a lookup for a global address would
# see the IPv6 be preferred. The result is a long delay because the
# site-local IPv6 addresses cannot be used while the IPv4 address is
# (at least for the foreseeable future) NATed. We also treat Teredo
# tunnels special.
#
# precedence <mask> <value>
# Add another rule to the RFC 3484 precedence table. See section 2.1
# and 10.3 in RFC 3484. The default is:
#
#precedence ::1/128 50
#precedence ::/0 40
#precedence 2002::/16 30
#precedence ::/96 20
#precedence ::ffff:0:0/96 10
#
# For sites which prefer IPv4 connections change the last line to
#
#precedence ::ffff:0:0/96 100
#
# scopev4 <mask> <value>
# Add another rule to the RFC 6724 scope table for IPv4 addresses.
# By default the scope IDs described in section 3.2 in RFC 6724 are
# used. Changing these defaults should hardly ever be necessary.
# The defaults are equivalent to:
#
#scopev4 ::ffff:169.254.0.0/112 2
#scopev4 ::ffff:127.0.0.0/104 2
#scopev4 ::ffff:0.0.0.0/96 14