命令输入：
查询：
We have to query the sysctl kernel value net.ipv4.ip_forward to see if forwarding is enabled or not: Using sysctl:
sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0
or just checking out the value in the /proc system:
cat /proc/sys/net/ipv4/ip_forward
0
As we can see in both the above examples this was disabled (as show by the value 0).
Enable IP Forwarding on the fly
As with any sysctl kernel parameters we can change the value of net.ipv4.ip_forward on the fly (without rebooting the system):
sysctl -w net.ipv4.ip_forward=1
or
echo 1 > /proc/sys/net/ipv4/ip_forward
the setting is changed instantly; the result will not be preserved after rebooting the system.
永久修改：Permanent setting using /etc/sysctl.conf
If we want to make this configuration permanent the best way to do it is using the file /etc/sysctl.conf where we can add a line containing net.ipv4.ip_forward = 1
/etc/sysctl.conf:
net.ipv4.ip_forward = 1
if you already have an entry net.ipv4.ip_forward with the value 0 you can change that 1.
To enable the changes made in sysctl.conf you will need to run the command:
sysctl -p /etc/sysctl.conf
On RedHat based systems this is also enabled when restarting the network service:
service network restart
and on Debian/Ubuntu systems this can be also done restarting the procps service:
/etc/init.d/procps.sh restart
Using distribution specific init scripts
Although the methods presented above should work just fine and you would not need any other method of doing this, I just wanted to note that there are also other methods to enable IP Forwarding specific to some Linux distributions. For example Debian based distributions might use the setting:
/etc/network/options:
ip_forward=no
set it to yes and restart the network service. Also RedHat distributions might set this using:
/etc/sysconfig/network:
FORWARD_IPV4=true
and again restart the network service.
Regardless the method you have used once you have completed this you can check it out using the same method shown above:
sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1
cat /proc/sys/net/ipv4/ip_forward
1
If the result is 1 then the Linux system will start forwarding IP packets even if they are not destined to any of its own network interfaces.