The /etc/sysctl.conf file

Securing and Optimizing Linux: RedHat Edition -A Hands on Guide
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# 9.6. The `/etc/sysctl.conf` file

In Red Hat Linux 6.2, many kernel options related to networking security such as dropping packets that come in over interfaces they shouldn't or ignoring `ping/broadcasts` request, etc. can be set in the new `/etc/sysctl.conf` file instead of the `/etc/rc.d/rc.local` file. One important consideration is the `IPv4` forwarding parameter which is now done via the sysctl program, as opposed to being controlled by the contents of the file in `/etc/sysconfig/network`. The sysctl settings are stored in `/etc/sysctl.conf`, and are loaded at each boot before the `/etc/rc.d/rc.local` file is loaded. We've already talked about all networking security parameters that we must set into the server in General System Security, and for this reason we'll focus only on the kernel option for `IPv4` forwarding.

To enable IPv4 forwarding on your RH 6.2 system, use the following command: Edit the `/etc/sysctl.conf` file and add the following line:

               # Enable packet forwarding
               net.ipv4.ip_forward = 1

You must restart your network for the change to take effect. The command to restart the network is the following:

               [root@deep] /# /etc/rc.d/init.d/network restart

               Setting network parameters	        [  OK  ]
               Bringing up interface lo	        [  OK  ]
               Bringing up interface eth0	        [  OK  ]
               Bringing up interface eth1	        [  OK  ]

> : Enabling IPv4 forwarding via the `sysctl.conf` file is only valid for Red Hat Linux 6.2 users. Users with version 6.1 of Red Hat must set this parameter into the `/etc/sysconfig/network` file as explained above.

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