when you really want to be someone else just for a second

run_with_gateway

#!/bin/bash

function cleanup {
    brctl delif 9net-bridge vethgw0
    #ip netns exec blue ip link del vethgw1 type veth
    ip link del vethgw0 type veth
    ip netns del blue
}
trap cleanup EXIT

gw=$1
shift

ip netns add blue
ip link add vethgw0 type veth peer name vethgw1
ip link set vethgw0 up
ip link set vethgw1 netns blue
ip netns exec blue ip addr add 172.31.0.238/16 dev vethgw1
ip netns exec blue ip link set vethgw1 up
ip netns exec blue ip r add default via $gw
brctl addif 9net-bridge vethgw0
ip netns exec blue "$@"