Setup a network namespace with Internet access

ns-inet.sh

#!/usr/bin/env bash

# set -x

if [[ $EUID -ne 0 ]]; then
    echo "You must be root to run this script"
    exit 1
fi

# Returns all available interfaces, except "lo" and "veth*".
available_interfaces()
{
   local ret=()

   local ifaces=$(ip li sh | cut -d " " -f 2 | tr "\n" " ")
   read -a arr <<< "$ifaces" 

   for each in "${arr[@]}"; do
      each=${each::-1}
      if [[ ${each} != "lo" && ${each} != veth* ]]; then
         ret+=( "$each" )
      fi
   done
   echo ${ret[@]}
}

IFACE="$1"
if [[ -z "$IFACE" ]]; then
   ifaces=($(available_interfaces))
   if [[ ${#ifaces[@]} -gt 0 ]]; then
      IFACE=${ifaces[0]}
      echo "Using interface $IFACE"
   else
      echo "Usage: ./ns-inet <IFACE>"
      exit 1
   fi
fi

NS="ns1"
VETH="veth1"
VPEER="vpeer1"
VETH_ADDR="10.200.1.1"
VPEER_ADDR="10.200.1.2"

trap cleanup EXIT

cleanup()
{
   ip li delete ${VETH} 2>/dev/null
}

# Remove namespace if it exists.
ip netns del $NS &>/dev/null

# Create namespace
ip netns add $NS

# Create veth link.
ip link add ${VETH} type veth peer name ${VPEER}

# Add peer-1 to NS.
ip link set ${VPEER} netns $NS

# Setup IP address of ${VETH}.
ip addr add ${VETH_ADDR}/24 dev ${VETH}
ip link set ${VETH} up

# Setup IP ${VPEER}.
ip netns exec $NS ip addr add ${VPEER_ADDR}/24 dev ${VPEER}
ip netns exec $NS ip link set ${VPEER} up
ip netns exec $NS ip link set lo up
ip netns exec $NS ip route add default via ${VETH_ADDR}

# Enable IP-forwarding.
echo 1 > /proc/sys/net/ipv4/ip_forward

# Flush forward rules.
iptables -P FORWARD DROP
iptables -F FORWARD
 
# Flush nat rules.
iptables -t nat -F

# Enable masquerading of 10.200.1.0.
iptables -t nat -A POSTROUTING -s ${VPEER_ADDR}/24 -o ${IFACE} -j MASQUERADE
 
iptables -A FORWARD -i ${IFACE} -o ${VETH} -j ACCEPT
iptables -A FORWARD -o ${IFACE} -i ${VETH} -j ACCEPT

# Get into namespace
ip netns exec ${NS} /bin/bash --rcfile <(echo "PS1=\"${NS}> \"")

I don't think using different private IP prefix block would cause trouble. @dpino

if the system uses a local dns server (like dnsmasqd on ubuntu ), a resolv.conf needs to be setup for the namespace, for example mkdir -p /etc/netns/ns1/ echo 'nameserver 8.8.8.8' > /etc/netns/ns1/resolv.conf

For anyone else, this resolved my name resolution issue on Ubuntu 22.04. Thanks @markododa.