Linux – file that associates a thread to its network namespace

You can start the OpenVPN link inside a namespace and then run every command you want to use that OpenVPN link inside the namespace. Details on how to do it are presented in Running an OpenVPN tunnel inside a network namespace, by Sebastian Thorarensen.

I tried it and it does work.  The idea is to provide a custom script to carry out the up and route-up phases of the OpenVPN connection inside a specific namespace instead of the global one.  Here is an answer based on the above source, but modified to add Google DNS to resolv.conf.

First create an --up script for OpenVPN.  This script will create the VPN tunnel interface inside a network namespace called vpn, instead of the default namespace.

$ cat > netns-up << 'EOF'
#!/bin/sh
case $script_type in
        up)
                ip netns add vpn
                ip netns exec vpn ip link set dev lo up
                mkdir -p /etc/netns/vpn
                echo "nameserver 8.8.8.8" > /etc/netns/vpn/resolv.conf
                ip link set dev "$1" up netns vpn mtu "$2"
                ip netns exec vpn ip addr add dev "$1" \
                        "$4/${ifconfig_netmask:-30}" \
                        ${ifconfig_broadcast:+broadcast "$ifconfig_broadcast"}
                test -n "$ifconfig_ipv6_local" && \
                        ip netns exec vpn ip addr add dev "$1" \
                                "$ifconfig_ipv6_local"/112
                ;;
        route-up)
                ip netns exec vpn ip route add default via "$route_vpn_gateway"
                test -n "$ifconfig_ipv6_remote" && \
                        ip netns exec vpn ip route add default via \
                                "$ifconfig_ipv6_remote"
                ;;
        down)
                ip netns delete vpn
                ;;
esac
EOF

Then start OpenVPN and tell it to use our --up script instead of executing ifconfig and route.

openvpn --ifconfig-noexec --route-noexec --up netns-up --route-up netns-up --down netns-up

Now you can start programs to be tunneled like this:

ip netns exec vpn command

The only catch is that you need to be root to invoke ip netns exec ... and maybe you do not want your application to run as root.  The solution is simple:

sudo ip netns exec vpn sudo -u $(whoami) command

Starting openvpn inside the network namespace is safer. I use the following script (fork of Schnouki's) to create namespace, configure firewall, DNS, test connectivity, start openvpn, and finally start a torrent client. I put TODOs in the script where you have to adjust it to your needs.

#!/bin/sh
# start openvpn tunnel and torrent client inside Linux network namespace
#
# this is a fork of schnouki's script, see original blog post
# https://schnouki.net/posts/2014/12/12/openvpn-for-a-single-application-on-linux/
#
# original script can be found here
# https://gist.github.com/Schnouki/fd171bcb2d8c556e8fdf

# ------------ adjust values below ------------
# network namespace
NS_NAME=myVPN
NS_EXEC="ip netns exec $NS_NAME"
# user for starting the torrent client
REGULAR_USER=heinzwurst
# ---------------------------------------------

# exit on unbound variable
set -u

# exit on error
set -e
set -o pipefail

# trace option
#set -x

if [ $USER != "root" ]; then
    echo "This must be run as root."
    exit 1
fi

start_vpn() {
    echo "Add network interface"

    # Create the network namespace
    ip netns add $NS_NAME

    # Start the loopback interface in the namespace
    $NS_EXEC ip addr add 127.0.0.1/8 dev lo
    $NS_EXEC ip link set lo up

    # Create virtual network interfaces that will let OpenVPN (in the
    # namespace) access the real network, and configure the interface in the
    # namespace (vpn1) to use the interface out of the namespace (vpn0) as its
    # default gateway
    ip link add vpn0 type veth peer name vpn1
    ip link set vpn0 up
    ip link set vpn1 netns $NS_NAME up

    ip addr add 10.200.200.1/24 dev vpn0
    $NS_EXEC ip addr add 10.200.200.2/24 dev vpn1
    $NS_EXEC ip link set dev vpn1 mtu 1492
    $NS_EXEC ip route add default via 10.200.200.1 dev vpn1

    # Configure the nameserver to use inside the namespace
    # TODO use VPN-provided DNS servers in order to prevent leaks
    mkdir -p /etc/netns/$NS_NAME
    cat >/etc/netns/$NS_NAME/resolv.conf <<EOF || exit 1
nameserver 8.8.8.8
nameserver 8.8.4.4
EOF

    # IPv4 NAT, you may need to adjust the interface name prefixes 'eth' 'wlan'
    iptables -t nat -A POSTROUTING -o eth+ -m mark --mark 0x29a -j MASQUERADE
    iptables -t nat -A POSTROUTING -o wlan+ -m mark --mark 0x29a -j MASQUERADE
    iptables -t mangle -A PREROUTING -i vpn0 -j MARK --set-xmark 0x29a/0xffffffff

    # TODO create firewall rules for your specific application (torrent)
    # or just comment the line below
    $NS_EXEC iptables-restore < /etc/iptables/iptables-$NS_NAME.rules

    # we should have full network access in the namespace
    $NS_EXEC ping -c 3 www.google.com

    # start OpenVPN in the namespace
    echo "Starting VPN"
    cd /etc/openvpn
    # TODO create openvpn configuration in /etc/openvpn/$NS_NAME.conf
    $NS_EXEC openvpn --config $NS_NAME.conf &

    # wait for the tunnel interface to come up
    while ! $NS_EXEC ip link show dev tun0 >/dev/null 2>&1 ; do sleep .5 ; done
}

stop_vpn() {
    echo "Stopping VPN"
    ip netns pids $NS_NAME | xargs -rd'\n' kill
    # TODO wait for terminate

    # clear NAT
    iptables -t nat -D POSTROUTING -o eth+ -m mark --mark 0x29a -j MASQUERADE
    iptables -t nat -D POSTROUTING -o wlan+ -m mark --mark 0x29a -j MASQUERADE
    iptables -t mangle -D PREROUTING -i vpn0 -j MARK --set-xmark 0x29a/0xffffffff

    echo "Delete network interface"
    rm -rf /etc/netns/$NS_NAME

    ip netns delete $NS_NAME
    ip link delete vpn0
}

# stop VPN on exit (even when error occured)
trap stop_vpn EXIT

start_vpn

# TODO start your favorite torrent client
$NS_EXEC sudo -u $REGULAR_USER transmission-gtk