Hitchhiker's Guide to Openbsd

6.2.7 - Setting up your OpenBSD box as a forwarding gateway
This is the basic information you need to set up your OpenBSD box as a gateway (also called a router). 
If you are using OpenBSD as a router on the Internet, we suggest that you also read the Packet Filter 
setup instructions below to block potentially malicious traffic. Also, due to the low availability of 
IPv4
 
addresses from network service providers and regional registries, you may want to look at Network 
Address Translation for information on conserving your IP address space. 
The GENERIC kernel already has the ability to allow IP Forwarding, but needs to be turned on. You 
should do this using the 
sysctl(8)
 utility. To change this permanently you should edit the file 
/etc/sysctl.
http://www.openbsd.org/faq/faq6.html (7 of 33)9/4/2011 10:02:06 AM

---

6 - Networking
conf
to allow for IP Forwarding. To do so add this line in that configuration file. 
net.inet.ip.forwarding=1
To make this change without rebooting you would use the 
sysctl(8)
 utility directly. Remember though 
that this change will no longer exist after a reboot, and needs to be run as root. 
# sysctl net.inet.ip.forwarding=1
net.inet.ip.forwarding: 0 -> 1
Now modify the routes on the other hosts on both sides. This is often done with static route entries, but 
more advanced networks can make use of the rich suite of routing daemons included as part of 
OpenBSD to pass routes to other systems. These include 
OpenBGPD
, 
ospfd(8)
, 
ospf6d(8)
, 
ldpd(8)
and 
ripd(8)
. Additionally the ports collection includes software such as 
bird
, 
igmpproxy
 and 
quagga
. 
OpenBSD supports several T1, HSSI, ATM, FDDI and serial (PPP/SLIP) interfaces, and of course many 
Ethernet devices (including 10Gb).