/etc/hosts
hosts（the static table lookup for host name） 主机名查询静态表。
/etc/hosts 文件用于系统快速解析IP地址与域名/主机名间对应关系。系统按照先DNS缓存在 /etc/hosts 文件最后DNS服务的顺序对域名进行解析，所以在hosts文件中配置了的域名就不再需要耗时较久的通过网络域名解析协议，能快速获取域名网络地址。再有，如果系统裁剪了DNS服务或DNS服务器地址没有正确配置，而又想通过域名来访问，也可以通过hosts文件文件来实现。
hosts文件包含了IP地址与域名/主机名之间的映射，还包括主机的别名。通过将常用的域名和IP地址映射加入到hosts文件中，实现快速方便的访问。
hosts 文件格式
ip地址 主机名/域名 （主机别名）
ip地址 主机名
每台主机都有一个 IP 地址，它区分开每台主机，并可以根据 ip 进行通讯。但 IP 地址不方便记忆，所以在一个局域网中，每台机器都设置一个不同主机名，用于区分主机，便于相互访问。扩展到互联网，因为主机数太多，就增加了域名来指定公网内的主机。
- 主机名通常在局域网内使用，通过 hosts 文件，主机名就被解析到对应的 ip。
- 域名通常在 internet 上使用，但是优先级低于 hosts 文件中内容，因此如果你不想使用 internet 上的域名解析，可以更改自己的 hosts 文件，加入自己的域名解析。
实例
默认情况下 /etc/hosts 文件中只有一条配置 127.0.0.1 localhost ，所以我们可以用 localhost 来替代自回环地址 127.0.0.1 来访问本机。我们也可以换个名字或者加个别名来访问。对 /etc/hosts 文件的修改，无需重启即可生效，也说明系统在每次解析主机名或域名时都要查询一次该文件。
[root@sylixos2:/root]# cat /etc/hosts
127.0.0.1 localhost
[root@sylixos2:/root]# ping localhost
Execute a DNS query...
Pinging localhost [127.0.0.1]
Pinging 127.0.0.1
Reply from 127.0.0.1: bytes=32 time=0.054ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Ping statistics for 127.0.0.1:
Packets: Send = 4, Received = 4, Lost = 0(0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0.016ms, Maximum = 0.054ms, Average = 0.025ms
[root@sylixos2:/root]#
[root@sylixos2:/root]# vi /etc/hosts
[root@sylixos2:/root]# cat /etc/hosts
127.0.0.1 localhost2 inet
[root@sylixos2:/root]# ping localhost2
Execute a DNS query...
Pinging localhost2 [127.0.0.1]
Pinging 127.0.0.1
Reply from 127.0.0.1: bytes=32 time=0.051ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.015ms TTL=255
Ping statistics for 127.0.0.1:
Packets: Send = 4, Received = 4, Lost = 0(0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0.015ms, Maximum = 0.051ms, Average = 0.024ms
[root@sylixos2:/root]# ping inet
Execute a DNS query...
Pinging inet [127.0.0.1]
Pinging 127.0.0.1
Reply from 127.0.0.1: bytes=32 time=0.890ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.044ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Reply from 127.0.0.1: bytes=32 time=0.016ms TTL=255
Ping statistics for 127.0.0.1:
Packets: Send = 4, Received = 4, Lost = 0(0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0.016ms, Maximum = 0.890ms, Average = 0.241ms
[root@sylixos2:/root]#
不配置DNS服务器，设备是无法访问到 www.baidu.com ，此时可以配置 /etc/hosts 文件为百度配置域名和别名地址，即可正常访问。
[root@sylixos2:/root]#
[root@sylixos2:/root]# ifconfig
en1 Link encap: Ethernet HWaddr: f6:4f:aa:4c:69:51
Dev: e1000_0 Ifidx: 2 DHCP: D4 D6 Spd: 1000 Mbps
inet addr: 192.168.196.89 netmask: 255.255.255.0
gateway: 192.168.196.1 broadcast: 192.168.196.255
inet6 addr: fe80::f44f:aaff:fe4c:6951 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
collisions:0 txqueue:0 tcpaf:2 tcpwnd:65535
RX ucast packets:13 nucast packets:571087 dropped:0
TX ucast packets:10 nucast packets:8 dropped:0
RX bytes:38021006 (36.2 MB) TX bytes:1296 (1.2 KB)
lo0 Link encap: Local Loopback
Dev: N/A Ifidx: 1 DHCP: D4 D6 Spd: N/A
inet addr: 127.0.0.1 netmask: 255.0.0.0
P-to-P: 127.0.0.1 broadcast: N/A
inet6 addr: ::1 Scope:Loopback
UP LOOPBACK RUNNING MTU:0 Metric:1
collisions:0 txqueue:0 tcpaf:2 tcpwnd:262140
RX ucast packets:75 nucast packets:0 dropped:0
TX ucast packets:75 nucast packets:0 dropped:0
RX bytes:4488 (4.3 KB) TX bytes:4488 (4.3 KB)
dns0: 0.0.0.0
dns1: 0.0.0.0
dns2: 0.0.0.0
dns3: 0.0.0.0
default device is: en1
list net interface: 2
[root@sylixos2:/root]# ping www.baidu.com
Execute a DNS query...
Pinging request could not find host www.baidu.com .Please check the name and try again.
parameter(s) error.
[root@sylixos2:/root]#
[root@sylixos2:/root]# vi /etc/hosts
[root@sylixos2:/root]# cat /etc/hosts
127.0.0.1 localhost2 inet
180.101.50.188 www.baidu.com baidu
[root@sylixos2:/root]# ping www.baidu.com
Execute a DNS query...
Pinging www.baidu.com [180.101.50.188]
Pinging 180.101.50.188
Reply from 180.101.50.188: bytes=32 time=19.369ms TTL=48
Reply from 180.101.50.188: bytes=32 time=19.327ms TTL=48
Reply from 180.101.50.188: bytes=32 time=19.059ms TTL=48
Reply from 180.101.50.188: bytes=32 time=18.921ms TTL=48
Ping statistics for 180.101.50.188:
Packets: Send = 4, Received = 4, Lost = 0(0% loss),
Approximate round trip times in milli-seconds:
Minimum = 18.921ms, Maximum = 19.369ms, Average = 19.169ms
[root@sylixos2:/root]# ping baidu
Execute a DNS query...
Pinging baidu [180.101.50.188]
Pinging 180.101.50.188
Reply from 180.101.50.188: bytes=32 time=18.952ms TTL=48
Reply from 180.101.50.188: bytes=32 time=19.110ms TTL=48
Reply from 180.101.50.188: bytes=32 time=18.973ms TTL=48
Reply from 180.101.50.188: bytes=32 time=18.931ms TTL=48
Ping statistics for 180.101.50.188:
Packets: Send = 4, Received = 4, Lost = 0(0% loss),
Approximate round trip times in milli-seconds:
Minimum = 18.931ms, Maximum = 19.110ms, Average = 18.991ms
[root@sylixos2:/root]#
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