和DNS配置相关的文件
包括：/etc/defaultdomain、/etc/nsswitch.conf、/etc/resolv.conf 三个文件权限只需root可读即可。
1) /etc/defaultdomain
"/etc/defaultdomain"文件给出确省域名，该文件内容为域名名称，例如：
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nanjing.org.cn
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表示缺省域名为" nanjing.org.cn "，以后仅使用某个主机名时，系统假设为缺省域中的主机。例如，"ping www"将被理解为"ping www.nanjing.org.cn "。
2) /etc/nsswitch.conf
如果使用DNS，则必须在该文件的"HOST"一项中给出"DNS"。缺省为使用文件，即"FILES"，该选项可以保留。
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# /etc/nsswitch.files:
# An example file that could be copied over to /etc/nsswitch.conf; it
# does not use any naming service.
# "hosts:" and "services:" in this file are used only if the
# /etc/netconfig file has a "-" for nametoaddr_libs of "inet" transports.
passwd: files
group: files
hosts: files dns
networks: files
protocols: files
rpc: files
ethers: files
netmasks: files
bootparams: files
publickey: files
# At present there isn't a 'files' backend for netgroup; the system will
# figure it out pretty quickly, and won't use netgroups at all.
netgroup: files
automount: files
aliases: files
services: files
sendmailvars: files
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hosts: files dns 表示：将使用/etc/inet/hosts文件解析主机，如果无法解析主机名将使用DNS。
3) /etc/resolv.conf
该文件给出域名和域名服务器地址。
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; /etc/resolv.conf file
domain nanjing.org.cn ;域名
nameserver 61.155.107.131 ;域名服务器地址
;域名服务器地址可以给出多个
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