create a net namespace and set up relevant addresses and routes

netns.sh

#! /bin/bash

if [ $# -ne 3 ]
then
    echo Usage: $0 NS HOST_IP GUEST_IP
    exit 1
fi

NS=$1
HOST_IP=$2
GUEST_IP=$3
HOST_IF=veth-$HOST_IP
GUEST_IF=veth-$GUEST_IP

# create net namespace
ip netns add $NS

# set up local loopback
ip netns exec $NS ip link set dev lo up

# create a pair of veth to link host and guest
ip link add $HOST_IF type veth peer name $GUEST_IF
ip link set $GUEST_IF netns $NS

# set up host interface
ip link set dev $HOST_IF up
ip addr add $HOST_IP/24 dev $HOST_IF
ip route add $GUEST_IP dev $HOST_IF

# set up guest interface
ip netns exec $NS ip link set dev $GUEST_IF up
ip netns exec $NS ip addr add $GUEST_IP/24 dev $GUEST_IF
ip netns exec $NS ip route add $HOST_IP dev $GUEST_IF

# set guest's default route via host
ip netns exec $NS ip route add default via $HOST_IP