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|NAT with OpenBSD Quick and Dirty |
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Edit /etc/rc.conf and make the following changes:
pf=NO
change to
pf=YES

Edit /etc/sysctl.conf and uncomment the following:
#net.inet.ip.forwarding=1 
change to
net.inet.ip.forwarding=1

Setup /etc/pf.conf:
ext_if="sis0"
int_if="sis1"
nat on $ext_if from 192.168.0.0/24 to any -> $ext_if

Substitute sis0 and sis1 for your interfaces. 
Substitute 192.168.0.0/24 with your internal network block.

Now configure your interfaces, your internal interface will be the gateway (192.168.0.1) 
your external interface will have the IP assigned by your ISP. Create the following files to
configure your IP's:
# touch /etc/hostname.sis0
# touch /etc/hostname.sis1

Substitute sis0 and sis1 for your interface names.

Edit the files to include your IP's
# vi /etc/hostname.sis0
inet <ip assigned from service provider> <subnet mask> NONE

#vi /etc/hostname.sis1
inet 192.168.0.1 255.255.255.0 NONE

Reboot:
# halt