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Enable IP forwarding under RHEL/CentOS

The regular way

Edit /etc/sysctl.conf

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

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

# Controls IP packet forwarding
<strong>net.ipv4.ip_forward = 1</strong>

# Controls source route verification
net.ipv4.conf.default.rp_filter = 1

# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0

# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0

# Controls whether core dumps will append the PID to the core filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1</code>

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 :