/etc/rc.local

sysctl net.ipv4.conf.all.forwarding=1
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
while true ; do ifconfig tun0 inet 10.0.0.1 netmask 255.255.255.0 pointopoint 10.0.0.2 mtu 1500 up ; sleep 5 ; done &

[common]

S="1.2.3.4"

client

screen -S rssh
sudo ssh -w0:0 "root@$S" 'while true ; do echo `date`-$RANDOM ; sleep 5 ; done'

os x/bsd

screen -S rtun
sudo su -
ifconfig tun0 inet 10.0.0.2 netmask 255.255.255.0 pointopoint 10.0.0.1 mtu 1500 up
R=`netstat -nr | grep -i 'default[ ]*[1-9]' | awk '{ print $2 }'`
route add -host "$S" "$R"
route delete default "$R"
route add default 10.0.0.1
while true ; do c=`ifconfig tun0 2> /dev/null` ; if [ "$c" == "" ] ; then route add default "$R" ; break ; fi ; sleep 5 ; done