ipv6地址是否存在

 ifconfig |grep inet6

没有默认inet6地址

ipv6模块是否存在

# modinfo -n ipv6
    /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6.ko
    # modinfo -n ipv6_lib
    /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6_lib.ko

系统是否加载IPv6相关模块

lsmod |grep ipv6
    # lsmod |grep ipv6
    ipv6_lib              341467  0

只有ipv6_lib模块，没有主模块ipv6

从上面信息得知，ipv6模块是存在的，只是加载出现了问题,由此想到可能是配置导致的。

# cat /lib/modules/`uname -r`/modules.dep |grep -w ipv6.ko:
    /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6.ko: /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6_lib.ko

没有问题

# cat /etc/modprobe.conf |grep -i ipv6
    alias sit0 ipv6

发现一个50-ipv6.conf文件，内容如下:

# cat /etc/modprobe.d/50-ipv6.conf 
    install ipv6 /bin/true

这句话是什么含义呢？通过modprobe.conf(5)文档，有如下内容：

install modulename command...
      This  is the most powerful primitive: it tells modprobe to run your command instead of inserting the module in the kernel as normal. 
      The command can be any shell command: this allows you to do any kind of complex processing you might wish. 
      For example, if the module "fred" works better with the module "barney" already  installed (but  it  doesn't  depend  on  it, so modprobe won't automatically load it), you could say "install fred /sbin/modprobe barney; /sbin/modprobe --ignore-install fred", which would do what you wanted. 
      Note the --ignore-install, which stops the second modprobe from running the same install command again. See also remove below.

      You can also use install to make up modules which don't otherwise exist. 
      For example: "install probe-ethernet /sbin/modprobe e100 || /sbin/modprobe  eepro100",  which
      will first try to load the e100 driver, and if it fails, then the eepro100 driver when you do "modprobe probe-ethernet".

      If  you  use  the  string  "$CMDLINE_OPTS" in the command, it will be replaced by any options specified on the modprobe command line. This can be useful because users expect "modprobe fred opt=1" to pass the "opt=1" arg to the module, even if there's an install command in  the  configuration  file.  So  our  above  example  becomes "install fred /sbin/modprobe barney; /sbin/modprobe --ignore-install fred $CMDLINE_OPTS"

比较长，关键的第一句我们来解释一下：
This is the most powerful primitive: it tells modprobe to run your command
instead of inserting the module in the kernel as normal.
这句话的意思是它让modprobe命令执行命令行里的command命令，而不是一般情况下去加载指定的内核模块。

该怎么理解这句话呢？我们通过两个命令的执行来说明：

 # modprobe -v -n ipv6
    insmod /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6_lib.ko 
    install /bin/true

-n --dry-run --show 表明不做真正的插入模块的操作
-v: 打印有关程序所做事情的信息
所以上述输出表明modprobe只做了加载ipv6_lib.ko模块和install /bin/true的动作，
而没有做加载ipv6.ko模块

# modprobe --show-depends ipv6
    insmod /lib/modules/3.0.101-0.47.90-default/kernel/net/ipv6/ipv6_lib.ko 
    install /bin/true

--show-depends 只列出模块依赖关系，以insmod开头；
install命令也不做实际的加载操作，只列出要做的加载动作
从以上信息可以看出，也不会做ipv6.ko模块加载。

去掉/bin/true之后，重新执行modprobe ipv6命令后检查模块加载情况：

# modprobe -v ipv6
    # lsmod |grep ipv6
    ipv6                  12758  1 
    ipv6_lib              341467  71 ipv6
    
    # ifconfig |grep inet6
      inet6 addr: fe80::9af5:37ff:fe00:9527/64 Scope:Link
      inet6 addr: fe80::9af5:37ff:fee3:3ac4/64 Scope:Link
      inet6 addr: ::1/128 Scope:Host

综上，可以认为是50-ipv6.conf文件的配置导致了ipv6加载不完整。