Long Term Storage

Tiger warned me that IP fowarding was enabled this morning.  It’s behavior is definitely odd in terms of when it reports something is afoot.

NEW: --WARN-- [lin015w] The system has IP forwarding enabled


I digress.  Good article here on how to dis/enable IP Forwarding, but more importantly for my memory it also has the sysctl syntax which I forget on a regular basis.

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


There is some good stuff down in the comments too.
Thanks MDLog.