nsgw

#!/bin/sh
#
#                          NETWORK LAYOUT
#
#               +-------------------------------------+
#               |                 GW                  |
#               |  +-------------+   +-------------+  |
#               |  |   veth-c1   |   |   veth-c2   |  |
#               |  | 192.168.1.1 |   | 192.168.2.1 |  |
#               |  +-------------+   +-------------+  |
#               +-----------|-------------|-----------+
#                           |             |
#   +-------------------+   |             |   +-------------------+
#   |      client1      |   |             |   |      client2      |
#   |  +-------------+  |   |             |   |  +-------------+  |
#   |  |    veth0    |__|___/             \___|__|    veth0    |  |
#   |  | 192.168.1.2 |  |                     |  | 192.168.2.2 |  |
#   |  +-------------+  |                     |  +-------------+  |
#   +-------------------+                     +-------------------+
#
#
#     do an inter-ns ping with:
#     # tcpdump -i veth-c1 icmp &
#     # ip netns exec client1 ping -q 192.168.2.2
#
#     start a shell in a netns with:
#     # ip netns exec client1 bash
#

setupns() {
    ip netns add client$1
    ip -b - <<-EOF
        link add name veth-c$1 type veth peer netns client$1
        addr add 192.168.$1.1/24 dev veth-c$1
        link set veth-c$1 up
    EOF
    ip -n client$1 -b - <<-EOF
        addr add 192.168.$1.2/24 dev veth0
        link set veth0 up
        route add default via 192.168.$1.1
    EOF
}

setupns 1
setupns 2

sysctl -qw net.ipv4.ip_forward=1