demystifying

#!/usr/bin/env bash

# create veth
VETH_HOST=veth_host
VETH_NETNS=veth_netns
ip link add ${VETH_HOST} type veth peer name ${VETH_NETNS}

# put one of the veth interfaces into the new network namespace
NETNS=$(basename ${CNI_NETNS})
ip link set ${VETH_NETNS} netns ${NETNS}

# assign IP to veth interface inside the new network namespace
IP_VETH_NETNS=10.244.0.20
CIDR_VETH_NETNS=${IP_VETH_NETNS}/32
ip -n ${NETNS} addr add ${CIDR_VETH_NETNS} dev ${VETH_NETNS}

# assign IP to veth interface on the host
IP_VETH_HOST=10.244.0.101
CIDR_VETH_HOST=${IP_VETH_HOST}/32
ip addr add ${CIDR_VETH_HOST} dev ${VETH_HOST}

# rename veth interface inside the new network namespace
ip -n ${NETNS} link set ${VETH_NETNS} name ${CNI_IFNAME}

# ensure all interfaces are up
ip link set ${VETH_HOST} up
ip -n ${NETNS} link set ${CNI_IFNAME} up

# add routes inside the new network namespace so that it knows how to get to the host
ip -n ${NETNS} route add ${IP_VETH_HOST} dev eth0
ip -n ${NETNS} route add default via ${IP_VETH_HOST} dev eth0

# add route on the host to let it know how to reach the new network namespace
ip route add ${IP_VETH_NETNS}/32 dev ${VETH_HOST} scope host

# return a JSON via stdout
RETURN_TEMPLATE='
{
  "cniVersion": "1.0.0",
  "interfaces": [
    {
      "name": "%s",
      "mac": "%s"
    },
    {
      "name": "%s",
      "mac": "%s",
      "sandbox": "%s"
    }
  ],
  "ips": [
    {
      "address": "%s",
      "interface": 1
    }
  ]
}'

MAC_HOST_VETH=$(ip link show ${VETH_HOST} | grep link | awk '{print$2}')
MAC_NETNS_VETH=$(ip -netns $nsname link show ${CNI_IFNAME} | grep link | awk '{print$2}')

RETURN=$(printf "${RETURN_TEMPLATE}" "${VETH_HOST}" "${MAC_HOST_VETH}" "${CNI_IFNAME}" "${mac_netns_veth}" "${CNI_NETNS}" "${CIDR_VETH_NETNS}")
echo ${RETURN}