Linux route table maintenance

View Linux kernel route table

Use the following route command to view the Linux kernel route table.

# routeDestination     Gateway         Genmask         Flags Metric Ref    Use Iface192.168.0.0     *               255.255.255.0   U     0      0        0 eth0169.254.0.0     *               255.255.0.0     U     0      0        0 eth0default         192.168.0.1     0.0.0.0         UG    0      0        0 eth0

Route command output description

Three route types

Host Routing

Host Routing is a route record pointing to a single IP address or host name in the routing selection table. The Flags field of the host route is H. For example, in the following example, the local host uses the vro192.168.of IP address 192.168.1.1 to reach the host with IP address 10.0.0.10.

 Destination    Gateway       Genmask        Flags     Metric    Ref    Use    Iface-----------    -------     -------            -----     ------    ---    ---    -----10.0.0.10     192.168.1.1    255.255.255.255   UH       0    0      0    eth0

Network Routing

A network route is a network that can be accessed by a host. The Flags field of the network route is N. For example, in the following example, the local host forwards packets sent to 192.19.12 to the vro whose IP address is 192.168.1.1.

 Destination    Gateway       Genmask      Flags    Metric    Ref     Use    Iface-----------    -------     -------         -----    -----   ---    ---    -----192.19.12     192.168.1.1    255.255.255.0      UN      0       0     0    eth0

Default route

When the host cannot find the IP address or network route of the target host in the routing table, the packet is sent to the default route (Default Gateway. The Flags field of the default route is G. For example, in the following example, the default route is the vro with the IP address 192.168.1.1.

 Destination    Gateway       Genmask    Flags     Metric    Ref    Use    Iface-----------    -------     ------- -----      ------    ---    ---    -----default       192.168.1.1     0.0.0.0    UG       0        0     0    eth0

Configure Static Routing

Route command

You can use the route command to set and view route tables. The command format of the kernel route table is as follows:

# Route [add | del] [-net |-host] target [netmask Nm] [gw Gw] [[dev] If]

Where:

* Add: add a routing rule.

* Del: delete a routing rule.

* Net: the destination address is a network.

* Host: the destination address is a host.

* Target: target network or host

* Netmask: network mask of the destination address

* Gw: Gateway used to route data packets

* Dev: the network interface specified for the route

Route command example

Route entry added to the host

# Route add-host 192.168.1.2 dev eth0: 0

# Route add-host 10.20.30.148 gw 10.20.30.40

Route entry added to the network

# Route add-net 10.20.30.40 netmask 255.255.255.248 eth0

# Route add-net 10.20.30.48 netmask 255.255.255.248 gw 10.20.30.41

# Route add-net 192.168.1.0/24 eth1

Add default route

# Route add default gw 192.168.1.1

Delete A route

# Route del-host 192.168.1.2 dev eth0: 0

# Route del-host 10.20.30.148 gw 10.20.30.40

# Route del-net 10.20.30.40 netmask 255.255.255.248 eth0

# Route del-net 10.20.30.48 netmask 255.255.255.248 gw 10.20.30.41

# Route del-net 192.168.1.0/24 eth1

# Route del default gw 192.168.1.1

Configure packet forwarding

The default Kernel configuration in CentOS already contains the routing function, but it is not enabled by default when the system is started. To enable Linux routing, you can adjust the network parameters of the kernel. To configure and adjust kernel parameters, run the sysctl command. For example, to enable the packet forwarding function of the Linux kernel, run the following command.

# sysctl -w net.ipv4.ip_forward=1

After this setting, the current system will be able to forward packets, but the next time you start the computer, it will become invalid. To enable the system to remain valid the next time you start the computer, write the following lines to the configuration file/etc/sysctl. conf.

# vi /etc/sysctl.confnet.ipv4.ip_forward = 1

You can also run the following command to check whether the current system supports packet forwarding.

# sysctl net.ipv4.ip_forward