linux 路由表功能解析

linux 路由表功能解析

 

       本文,简单描述一下路由的作用.简单来说,路由器就是一个公交站台,公交站台上面有很多路公交.每条线路公交车又有很多站台(路由器),你就会选择最近的一条公交出行方案,当然啦,站牌上面的路线是会经常更新,更新的事,有当地部门更新.这就是路由.一台电脑主机有路由表,路由器也有路由表,一般来说,一台主机电脑如果只有一个网卡的话,应该会有最少两条路由信息,一条是公网路由,一条是局域网路由(它是不会经过路由器的路由表的,因为它走的是链路层,所以可以解析我们在局域网内传数据,速度是很快的).

 

测试环境:

主机(linux):                ip:192.168.168.1.119     netmask:255.255.0.0

另一台主机(xp):    ip:192.168.1.133   netmask:255.255.0.0

 

网关:   ip:192.168.0.3  netmask: 255.255.248.0

 

 

 

一, 我们可以查看一下路由表.(route 命令)

[user:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0                      //公网路由信息           //U – Up, G – Gateway, H – Host

192.168.0.0     *               255.255.0.0     U     0      0        0 eth0                      //局域网路由信息.
[user:dvr_linux]

Flags  可能出现的标志有：
              U (route is up) 路由正常
              H (target is a host) 主机路由
              G (use gateway) 使用网关的间接路由
              R (reinstate route for dynamic routing) 为动态选路恢复路由
              D (dynamically installed by daemon or redirect) 该路由由选路进程或重定向动态创建
              M (modified from routing daemon or rederict) 该路由已由选路进程或重定向修改
              ! (reject route) 阻塞路由

 

 

1.1 测试公网路由(切换到root用户下):

[root:dvr_linux] route    //打印路由表
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] ping 8.8.8.8           // 8.8.8.8是google公司的dns.
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_req=2 ttl=48 time=10.4 ms
64 bytes from 8.8.8.8: icmp_req=3 ttl=48 time=10.0 ms     //正常
^C
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 2 received, 33% packet loss, time 2013ms
rtt min/avg/max/mdev = 10.024/10.222/10.421/0.222 ms
[root:dvr_linux] route del default dev eth0     //删除默认路由,也就公网路由,因为有网关.
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] ping 8.8.8.8
connect: Network is unreachable          //ping 不通,没找到路由信息.数据包出不去.
[root:dvr_linux] route add default dev eth0    //增加默认路由,没加网关.
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         *               0.0.0.0         U     0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] ping 8.8.8.8  
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
From 192.168.1.119 icmp_seq=1 Destination Host Unreachable         //ping 不通,找到路由信息.但是找不到去8.8.8.8胡线路,数据包出不去.
From 192.168.1.119 icmp_seq=2 Destination Host Unreachable
From 192.168.1.119 icmp_seq=3 Destination Host Unreachable
^C
--- 8.8.8.8 ping statistics ---
5 packets transmitted, 0 received, +3 errors, 100% packet loss, time 4010ms
pipe 3
[root:dvr_linux] route del default dev eth0
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] route add default gw 192.168.0.3 dev eth0
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_req=1 ttl=48 time=11.0 ms      //重新添加带网关的路由信息后,又能ping 通了.
64 bytes from 8.8.8.8: icmp_req=3 ttl=48 time=10.0 ms
64 bytes from 8.8.8.8: icmp_req=4 ttl=48 time=10.3 ms
^C
--- 8.8.8.8 ping statistics ---
4 packets transmitted, 3 received, 25% packet loss, time 3015ms
rtt min/avg/max/mdev = 10.037/10.496/11.071/0.445 ms
[root:dvr_linux]

 

 

1.2 测试局域网路由(切换到root用户下):

[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux] ping 192.168.1.133
PING 192.168.1.133 (192.168.1.133) 56(84) bytes of data.
64 bytes from 192.168.1.133: icmp_req=1 ttl=64 time=2.13 ms
64 bytes from 192.168.1.133: icmp_req=2 ttl=64 time=0.450 ms   //正常通过.
^C
--- 192.168.1.133 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1003ms
rtt min/avg/max/mdev = 0.450/1.290/2.131/0.841 ms
[root:dvr_linux] route del -net 192.168.0.0/16 dev eth0              //删除局域网路由
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
[root:dvr_linux] ping 192.168.1.133
PING 192.168.1.133 (192.168.1.133) 56(84) bytes of data.
64 bytes from 192.168.1.133: icmp_req=1 ttl=64 time=5.06 ms
From 192.168.0.3: icmp_seq=1 Redirect Host(New nexthop: 192.168.1.133)     //数据包通过了默认路由信息发到了路由器,再查路由器的路由表找到了192.168.1.133的路由.再由网关发回来给192.168.1.133了.
64 bytes from 192.168.1.133: icmp_req=2 ttl=64 time=0.477 ms
64 bytes from 192.168.1.133: icmp_req=3 ttl=64 time=0.302 ms
64 bytes from 192.168.1.133: icmp_req=4 ttl=64 time=0.285 ms
64 bytes from 192.168.1.133: icmp_req=5 ttl=64 time=0.478 ms
64 bytes from 192.168.1.133: icmp_req=6 ttl=64 time=0.408 ms
^C
--- 192.168.1.133 ping statistics ---
6 packets transmitted, 6 received, 0% packet loss, time 5002ms
rtt min/avg/max/mdev = 0.285/1.169/5.066/1.744 ms
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
[root:dvr_linux] route del default  eth0                            //把默认路由都删掉的话,这就完全找不到路由信息了,也就是没网络了.
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
[root:dvr_linux] ping 192.168.1.133
connect: Network is unreachable
[root:dvr_linux]

在这个时候,192.168.1.133的xp主机去ping 192.168.1.119的话.也就ping 不通的.

C:\Documents and Settings\Administrator>ping 192.168.1.119 -t

Pinging 192.168.1.119 with 32 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.

 

[root:dvr_linux] service networking restart            //重启一下网络.
[warn] Running /etc/init.d/networking restart is deprecated because it may not re-enable some interfaces ... (warning).
[....] Reconfiguring network interfaces...RTNETLINK answers: No such process
done.
[root:dvr_linux]
[root:dvr_linux] route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.0.3     0.0.0.0         UG    0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 eth0
[root:dvr_linux]

 

在这个时候,192.168.1.133的xp主机去ping 192.168.1.119的话.网络恢复了.

C:\Documents and Settings\Administrator>ping 192.168.1.119 -t

Pinging 192.168.1.119 with 32 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Reply from 192.168.1.119: bytes=32 time=2ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time=6ms TTL=63
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64
Reply from 192.168.1.119: bytes=32 time<1ms TTL=64