How to Enable IP Forwarding in Linux

docker WARNING: IPv4 forwarding is disabled. Networking will not work.

Posted by Dongyupu on October 22, 2017

Centos7 启动 docker镜像时报的错误,导致镜像中的web服务无法访问

I1023 10:58:31.075482 32368 exec.cpp:162] Version: 1.4.0
I1023 10:58:31.077666 32370 exec.cpp:237] Executor registered on agent 224f89b4-8c19-4909-a343-8f4380f4037a-S0
I1023 10:58:31.078027 32370 executor.cpp:120] Registered docker executor on 192.168.1.90
I1023 10:58:31.078045 32370 executor.cpp:160] Starting task nginx.0d04229c-b79e-11e7-9cd8-02423d8425a9

WARNING: IPv4 forwarding is disabled. Networking will not work.
  • IP-Forwarding IP转发。 一种路由协议。
  • IP转发是操作系统的一种选项，支持主机起到路由器的功能。在一个系统中含有两块以上的网卡，并将IP转发选项打开，这样该系统就可以作为路由器进行使用了。


开启IP-Forwarding

  • By default any modern Linux distributions will have IP Forwarding disabled. This is normally a good idea, as most peoples will not need IP Forwarding, but if we are setting up a Linux router/gateway or maybe a VPN server (pptp or ipsec) or just a plain dial-in server then we will need to enable forwarding. This can be done in several ways that I will present bellow.

Check if IP Forwarding is enabled

  • 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.

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.