Enable IP forwarding under RHEL/CentOS

June 6, 2006

The regular way

Edit /etc/sysctl.conf

Edit the “net.ipv4.ip_forward” line and set it to 1

1. Kernel sysctl configuration file for Red Hat Linux
3. For binary values, 0 is disabled, 1 is enabled. See sysctl(8) and
4. sysctl.conf(5) for more details.

1. Controls IP packet forwarding

net.ipv4.ip_forward = 1

1. Controls source route verification

net.ipv4.conf.default.rp_filter = 1

1. Do not accept source routing

net.ipv4.conf.default.accept_source_route = 0

1. Controls the System Request debugging functionality of the kernel

kernel.sysrq = 0

1. Controls whether core dumps will append the PID to the core filename.
2. Useful for debugging multi-threaded applications.*

kernel.core_uses_pid = 1

When done type the following to validate the new setting :

sysctl -p

The manual way :

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

This wouldn’t be persistent though, so you should edit sysctl.conf anyway, or add the command in /etc/rc.local

A great guide : http://www.ducea.com/2006/08/01/how-to-enable-ip-forwarding-in-linux/