How to enable IP forwarding on Ubuntu

IP addresses are divided into public IP addresses and private IP addresses. Public Address is managed by INIC (internet network information center). These IP addresses are assigned to organizations that register and apply to INIC. Private Address is a non-registered address and is exclusively used within the organization. Private Address cannot be used directly to communicate with WAN. Either frames are used to communicate (FRE frame relay, HDLC, PPP), or the forwarding (nat) function of routing is required to convert the private address into a public address.

For security reasons, Linux systems prohibit data packet forwarding by default. The so-called forwarding means that when the host has more than one network card, one of them receives the data packet and sends the data packet to another network card of the local machine according to the destination IP address of the data packet. The network card continues to send the data packet according to the routing table. This is usually what routers are supposed to do.

1. Determine whether forwarding is enabled by accessing the kernel ipv4.ip_forward of sysctl.

Note: Now some networks have popularized ipv6, so the following command can be changed from ipv4 to ipv6

Use sysctl:

sysctl net.ipv4.ip_forward net.ipv4.ip_forward = 0

Or check the files under /proc:

cat /proc/sys/net/ipv4/ip_forward 0

ipv4 forwarding is not enabled (value is 0)

2. Start IP forwarding

You can enable the forwarding function of ipv4 through sysctl (no need to restart):

sysctl -w net.ipv4.ip_forward=1

or

echo 1 > /proc/sys/net/ipv4/ip_forward

This setting is only temporary, and its effect will expire when the computer is restarted.

By setting parameters in /etc/sysctl.conf:

If you want to make IP forwarding permanent, please modify /etc/sysctl.conf, where you can add a net.ipv4.ip_forward = 1

/etc/sysctl.conf: net.ipv4.ip_forward = 1

If the ipv4 forwarding entry is already set to 0 then you only need to change it to 1.

3. For the changes to take effect, you need to execute the following instructions:

sysctl -p /etc/sysctl.conf

On Red Hat series distributions, you can make it take effect by restarting the network service:

service network restart

In Debian/Ubuntu series distributions, use this command:

#Early version

/etc/init.d/procps.sh restart

#The latest version of

/etc/init.d/procps restart

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