SUMMARY: packet forwarding on host connected to 2 nets

Date: Tue May 03 1994 - 06:09:55 CDT

Hello Managers, thanks for the quick reply.

To forward or not to forward ? ( sounds familiar )
My problem was how to make sure that a host connected to 2 networks
(in my case - one local and one to the internet)
won't forward tcp/ip packets between networks.

The answer: It depends on the kernel parameter IPFORWARDING (1=yes, -1=no)

Thanks to the quick replys from :
   Nate Itkin <>.
   Sharon Buller - Sys Adm Team (1723)" <>

Attached is the detailed explanation of Nate Itkin:

Look at /sys/netinet/in_proto.c. Possible values for IPFORWARDING are:

        ip_forwarding == -1 -- never forward; never change this value.
        ip_forwarding == 0 -- don't forward; set this value to 1 when two
                                or more interfaces are up.
        ip_forwarding == 1 -- always forward.

The Sun default is 0. To change the default to -1, edit your kernel config
file (/sys/`arch -k`/conf/MYSUN) and add the following line:

           options IPFORWARDING=-1

Rebuild your kernel. Or use adb on your existing kernel:

  mysun% echo "ip_forwarding/W -1" | adb -k -w /vmunix /dev/mem
  mysun% echo "ip_forwarding?W -1" | adb -k -w /vmunix /dev/mem

The first command modifies the running kernel and the second command
modifies the disk copy of vmunix so that when you reboot, the change
is saved. Enjoy.

- Nate Itkin
- Portland Technology Development, Intel Corporation      Aloha, Oregon
- E-mail:

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