tap-setup.sh from How to Build Your Own Digital DEC MicroVAX 3900 Running OpenVMS VAX VMS Operating System: SIMH on CentOS 7 Running OpenVMS/VAX 7.3: http://supratim-sanyal.blogspot.com/2016/10/how-to-build-your-own-digital-dec.html

tap-setup.sh

#!/bin/sh
#
# tap-setup.sh
#
# Sets up a bridge br0 to the physical ethernet adapter and a tap tap0 for connecting from SIMH
# Also creates additional taps tap1 and tap2 for use if needed by other programs
# Build your own SIMH Micro VAX 3900 running OpenVMS VAX 7.3:
# http:/supratim-sanyal.blogspot.com/2016/09/how-to-build-your-own-digital-dec.html
#
# Modified from simh documentation by Supratim Sanyal <supratim at riseup dot net>
#
# ====
# Change the DEVICE name below to your actual network interface name.
# Important: Make sure there is only one IP address assigned to the interface.
# ====
#

DEVICE="ens33"

# ----
# No more changes should be required from here

/bin/echo `/bin/date` ---- BEFORE BRIDGE AND TAP ----
/sbin/ip addr
/bin/echo `/bin/date` ROUTE
/sbin/ip route show
/bin/echo `/bin/date` -------------------------------

HOSTIPANDMASK=`ip addr show dev $DEVICE | grep inet | head -1 | cut -f 6 -d " "`
HOSTIP=`echo $HOSTIPANDMASK|cut -f 1 -d "/"`
HOSTNETMASK=`echo $HOSTIPANDMASK|cut -f 2 -d "/"`
HOSTBCASTADDR=`ip addr show dev $DEVICE | grep inet | head -1 | cut -f 8 -d " "`
HOSTDEFAULTGATEWAY=`/sbin/route -n | grep ^0.0.0.0 | gawk -- '{ print $2 }'`

/bin/echo `/bin/date` " "
/bin/echo `/bin/date` ---- GATHERED INFORMATION -----
/bin/echo `/bin/date` HOSTIP=$HOSTIP HOSTNETMASK=$HOSTNETMASK HOSTBCASTADDR=$HOSTBCASTADDR HOSTDEFAULTGATEWAY=$HOSTDEFAULTGATEWAY
/bin/echo `/bin/date` -------------------------------

# Create the TAP network devices
/sbin/ip tuntap add dev tap0 mode tap
/sbin/ip link set dev tap0 up

/sbin/ip tuntap add dev tap1 mode tap
/sbin/ip link set dev tap1 up

/sbin/ip tuntap add dev tap2 mode tap
/sbin/ip link set dev tap2 up

/bin/sleep 1

# Bridge $DEVICE to br0
/sbin/brctl addbr br0
/sbin/brctl addif br0 $DEVICE
/sbin/brctl setfd br0 0
/sbin/ip addr flush dev $DEVICE
/sbin/ip addr add $HOSTIPANDMASK broadcast $HOSTBCASTADDR dev br0
/sbin/ip link set dev br0 up

/bin/sleep 1

# bridge in the tap devices
/sbin/brctl addif br0 tap0
/sbin/ip addr flush dev tap0
/bin/sleep 1

/sbin/brctl addif br0 tap1
/sbin/ip addr flush dev tap1
/bin/sleep 1

/sbin/brctl addif br0 tap2
/sbin/ip addr flush dev tap2
/bin/sleep 1

# set the default route to the br0 interface
/sbin/ip route replace default via $HOSTDEFAULTGATEWAY dev br0
/bin/sleep 1

/bin/echo `/bin/date` " "
/bin/echo `/bin/date` ---- AFTER BRIDGE AND TAP ----
/sbin/ip addr
/bin/echo `/bin/date` ROUTE
/sbin/ip route show
/bin/echo `/bin/date` BRIDGE
/sbin/brctl show
/bin/echo `/bin/date` -------------------------------

# --
# We can now run SIMH VAX simulator and "attach xq tap:tap0" in vax.ini
# --

Typical working output of the script:

Sat Nov 12 20:29:35 UTC 2016 ---- BEFORE BRIDGE AND TAP ----
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:50:56:a4:55:80 brd ff:ff:ff:ff:ff:ff
    inet 64.137.228.122/24 brd 64.137.228.255 scope global ens33
       valid_lft forever preferred_lft forever
Sat Nov 12 20:29:35 UTC 2016 ROUTE
default via 64.137.228.1 dev ens33  proto static  metric 100
64.137.228.0/24 dev ens33  proto kernel  scope link  src 64.137.228.122  metric 100
Sat Nov 12 20:29:35 UTC 2016 -------------------------------
Sat Nov 12 20:29:37 UTC 2016
Sat Nov 12 20:29:37 UTC 2016 ---- GATHERED INFORMATION -----
Sat Nov 12 20:29:37 UTC 2016 HOSTIP=64.137.228.122 HOSTNETMASK=24 HOSTBCASTADDR=64.137.228.255 HOSTDEFAULTGATEWAY=64.137.228.1
Sat Nov 12 20:29:37 UTC 2016 -------------------------------
Sat Nov 12 20:29:44 UTC 2016
Sat Nov 12 20:29:44 UTC 2016 ---- AFTER BRIDGE AND TAP ----
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UP qlen 1000
    link/ether 00:50:56:a4:55:80 brd ff:ff:ff:ff:ff:ff
3: tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast master br0 state DOWN qlen 500
    link/ether 6a:0a:23:82:04:da brd ff:ff:ff:ff:ff:ff
4: tap1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast master br0 state DOWN qlen 500
    link/ether 42:00:fb:47:96:1f brd ff:ff:ff:ff:ff:ff
5: tap2: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast master br0 state DOWN qlen 500
    link/ether 72:af:de:c6:e6:19 brd ff:ff:ff:ff:ff:ff
6: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP
    link/ether 00:50:56:a4:55:80 brd ff:ff:ff:ff:ff:ff
    inet 64.137.228.122/24 brd 64.137.228.255 scope global br0
       valid_lft forever preferred_lft forever
Sat Nov 12 20:29:44 UTC 2016 ROUTE
default via 64.137.228.1 dev br0
64.137.228.0/24 dev br0  proto kernel  scope link  src 64.137.228.122
Sat Nov 12 20:29:44 UTC 2016 BRIDGE
bridge name     bridge id               STP enabled     interfaces
br0             8000.005056a45580       no              ens33
                                                        tap0
                                                        tap1
                                                        tap2
Sat Nov 12 20:29:44 UTC 2016 -------------------------------