enable NAT and IP forwarding on Mac OS X (10.10)

# enable IP forwarding and firewall in the kernel
sudo sysctl -w net.inet.ip.forwarding=1
sudo sysctl -w net.inet.ip.fw.enable=1

#flush all FW rules 
sudo pfctl -F all # or -F nat, for just the nat rules

cat ./nat-rules 
nat on en0 from 192.168.1.0/24 to any -> ozelmacpro #put this line in a text file

# en0 is the interface pointing to the network with internet access
# ozelmacpro is the local hostname or ip associated with the network that has internet access
# 192.168.1.0/24 is a separate network that shall get internet via ozelmacpro on interface en0
# final hint on this via https://discussions.apple.com/thread/6757798?start=0&tstart=0

# load NAT rules from file
sudo pfctl -f nat-rules -e

# list all FW config
sudo pfctl -s all

Thanks for this @ozel! :-)

FYI to anyone visiting: if you want to route internet to a specific IP address it might be problematic to give it the hostname, as this will route with a round-robin approach on multiple addresses, which will cause issues in some cases, as not all these addresses are guaranteed to provide internet. I put in the IP associated with the network that has internet access, which worked while the hostname did not.