[Simh] Using TAP/TUN

[Michael Bloom]

Here's a relatively simple shell script I cobbled together to automate 
setting up TAP/TUN on Linux:
---------- CUT HERE ------------
#! /bin/sh

USERNAME=mab

ETH_IF=wlan3
TUNNEL_IF=tap0
NET_BITS=24
SUB=0

LAN_NET=192.168.$SUB
TUNNEL_NET=192.168.$SUB

# Addresses for both sides of the tunnel
SIMH_HOST=$TUNNEL_NET.253
TUNNEL_GATE=$TUNNEL_NET.254

#################### END OF STUFF YOU CAN EDIT

     # If tunnel doesn't exist, create it.
ifconfig $TUNNEL_IF > /dev/null 2>&1 ||  tunctl -u $USERNAME

ifconfig $TUNNEL_IF $TUNNEL_GATE up

if [ x$LAN_NET =  x$TUNNEL_NET ]
then
     # The above ifconfig of tap0 created a default route through our 
tunnel,
     # superceding the default route via $ETH_IF to our router.  Remove it.

     route delete -net $TUNNEL_NET/$NET_BITS gw 0.0.0.0 dev $TUNNEL_IF

else
     echo Your router probably has at least a default route going to the 
internet,
     echo and a route for your lan. If your tunnel and lan are on 
different nets,
     echo you should make sure the router knows to try to send packets 
intended
     echo for $TUNNEL_NET through $LAN_NET.  Linux is supposed to 
automagically
     echo proxy arp for a whole subnet when forwarding, but there must
     echo still be a route to this host from your router for $TUNNEL_NET.
fi

echo 1 > /proc/sys/net/ipv4/conf/$TUNNEL_IF/proxy_arp

     # create a route to SIMH's system host through our tunnel

route add -host $SIMH_HOST dev $TUNNEL_IF

# prepare to answer arps for the simulated machine

arp -Ds $SIMH_HOST $ETH_IF pub

exit 0

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