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本机以太网卡eth0的IP地址为手动配置：
网关IP地址为192.168.1.1/24
#ifconfig eth0 192.168.1.100 netmask 255.255.255.0
#route add default dev eth0
//默认路由，将去往未知网络的数据包全部从接口eth0发出去
测试结果为ping外网失败；
[root@localhost ~]# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 0.0.0.0 0.0.0.0 U 0 0 0 eth0
[root@localhost ~]# ping
www.baidu.com
-c 5
PING
www.a.shifen.com
(61.135.169.105) 56(84) bytes of data.
From localhost (192.168.1.110) icmp_seq=2 Destination Host Unreachable
From localhost (192.168.1.110) icmp_seq=3 Destination Host Unreachable
From localhost (192.168.1.110) icmp_seq=4 Destination Host Unreachable
From localhost (192.168.1.110) icmp_seq=5 Destination Host Unreachable
—
www.a.shifen.com
ping statistics —
5 packets transmitted, 0 received, +4 errors, 100% packet loss, time 4001ms
pipe 3
但是，如果我不写默认路由出接口，而是直接下一跳，却可以ping通外网！
如下：
#route del default dev eth0
//删掉刚才配的静态路由
#route add default gw 192.168.1.1 //写默认路由的下一跳地址
现在ping外网却通了！
[root@localhost ~]# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
[root@localhost ~]# ping
www.baidu.com
-c 5
PING
www.a.shifen.com
(61.135.169.125) 56(84) bytes of data.
64 bytes from 61.135.169.125: icmp_req=1 ttl=51 time=305 ms
64 bytes from 61.135.169.125: icmp_req=2 ttl=51 time=145 ms
64 bytes from 61.135.169.125: icmp_req=3 ttl=51 time=98.3 ms
64 bytes from 61.135.169.125: icmp_req=4 ttl=51 time=75.5 ms
64 bytes from 61.135.169.125: icmp_req=5 ttl=51 time=342 ms
—
www.a.shifen.com
ping statistics —
5 packets transmitted, 5 received, 0% packet loss, time 4005ms
rtt min/avg/max/mdev = 75.570/193.664/342.982/109.636 ms
========================================================
那么为什么 route …dev eth0不行？而route …gw eth0却可以呢？
原理上这两条命令应该是等价的阿！
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