Attach an ENI (Elastic Network Interface) to an instance

attach-elastic-network-interface.sh

#!/usr/bin/env bash

# This script assigns Elastic Network Interface (ENI) passed as argument to current instance.
# The goal of this script is to attach an ENI to a single instance running in an ASG for example.

export PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/local/bin:/root/bin

PROG_NAME=$(basename $0)
AWSCLI=$(which aws)
JQ=$(which jq)
CURL=$(which curl)
TR=$(which tr)
DHCLIENT=$(which dhclient)
IFCONFIG=$(which ifconfig)
GREP=$(which grep)
CUT=$(which cut)
AWK=$(which awk)
SED=$(which sed)
NIC="eth1"

ENI_ID=$1

usage() {
    cat<<EOF
NAME
    ${PROG_NAME} -- Assign an Elastic Network Interface to this instance

SYNOPSIS
    ${PROG_NAME} eni-id

EXIT STATUS
     The ${PROG_NAME} utility exits 0 on success, and > 0 if an error occurs.
EOF

    exit 0
}

parse_cmdline() {

  if [ $# -eq 0 ]; then
    usage
  else
    while [ $# -gt 0 ]; do
      case "$1" in
        -*)
          usage
          ;;
        *)
          break
          ;;
      esac
      shift
    done
  fi
}

get_eni_info () {
    local ENI_ID=$1

    ${AWSCLI} ec2 describe-network-interfaces --filters Name=network-interface-id,Values=${ENI_ID}
}
attach_eni () {
    local ENI_ID=$1
    local INSTANCE_ID=$2

    ${AWSCLI} ec2 attach-network-interface --network-interface-id ${ENI_ID} --instance-id ${INSTANCE_ID} --device-index 1
}

dettach_eni () {
    local ENI_ATTACHMENT_ID=$1

    ${AWSCLI} ec2 detach-network-interface --attachment-id ${ENI_ATTACHMENT_ID} --force
}

main() {
ENI_INFO=$(get_eni_info ${ENI_ID})
ENI_IP=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .PrivateIpAddresses[] .PrivateIpAddress' | ${SED} 's/"//g')
ENI_STATUS=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Status' | ${TR} '[A-Z]' '[a-z]' | ${SED} 's/"//g')
INSTANCE_ID=$(${CURL} -s http://169.254.169.254/latest/meta-data/instance-id )

echo "Configuring ENI ${ENI_ID}"
if [ "${ENI_STATUS}" == "available" ]; then
    echo "ENI ${ENI_ID} is available, attaching it to ${INSTANCE_ID}"
    while [ "${ENI_STATUS}" == "available" ]; do
        # Attach ENI to current instance
        attach_eni ${ENI_ID} ${INSTANCE_ID}
        ENI_INFO=$(get_eni_info ${ENI_ID})
        ENI_STATUS=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Status' | ${TR} '[A-Z]' '[a-z]' | ${SED} 's/"//g')

        sleep 1
    done
    echo "ENI ${ENI_ID} attached to ${INSTANCE_ID}"
    echo "Configuring ${NIC} with IP ${ENI_IP}"
    # Configure network interface
    ${IFCONFIG} ${NIC} up
    ${DHCLIENT} ${NIC}
else
    ENI_INSTANCE_ID=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Attachment .InstanceId' | ${SED} 's/"//g')

    if [ "${INSTANCE_ID}" == "${ENI_INSTANCE_ID}" ]; then
        # Check if the IP is configured properly
        NIC_IP=$(${IFCONFIG} ${NIC} | ${GREP} 'inet addr:' | ${CUT} -d: -f2 | ${AWK} '{ print $1}')
        if ! [ "${NIC_IP}" == "${ENI_IP}" ]; then
            echo "Configuring NIC ${NIC}..."
            # Configure network interface
            ${IFCONFIG} ${NIC} up
            ${DHCLIENT} ${NIC}
        fi
    else
        echo "ENI ${ENI_ID} is attached to ${ENI_INSTANCE_ID}, dettaching..."

        while [ "${ENI_STATUS}" == "in-use" ]; do
            # Dettach ENI
            ENI_ATTACHMENT_ID=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Attachment .AttachmentId' | ${SED} 's/"//g')
            dettach_eni ${ENI_ATTACHMENT_ID}
            ENI_INFO=$(get_eni_info ${ENI_ID})
            ENI_STATUS=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Status' | ${TR} '[A-Z]' '[a-z]' | ${SED} 's/"//g')

            sleep 1
        done

        echo "ENI ${ENI_ID} is available, attaching it to ${INSTANCE_ID}"
        while [ "${ENI_STATUS}" == "available" ]; do
            # Attach ENI to current instance
            attach_eni ${ENI_ID} ${INSTANCE_ID}
            ENI_INFO=$(get_eni_info ${ENI_ID})
            ENI_STATUS=$(echo ${ENI_INFO} | ${JQ} '.NetworkInterfaces[] .Status' | ${TR} '[A-Z]' '[a-z]' | ${SED} 's/"//g')

            sleep 1
        done
        echo "ENI ${ENI_ID} attached to ${INSTANCE_ID}"
        echo "Configuring ${NIC} with IP ${ENI_IP}"
        # Configure network interface
        ${IFCONFIG} ${NIC} up
        ${DHCLIENT} ${NIC}
    fi
fi
}

parse_cmdline $@
main

exit 0

Required IAM policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1346888659253",
            "Action": [
                "ec2:AttachNetworkInterface",
                "ec2:DescribeInstances",
                "ec2:DescribeInstanceStatus",
                "ec2:DescribeNetworkInterfaces",
                "ec2:DetachNetworkInterface",
                "ec2:DescribeNetworkInterfaceAttribute"
            ],
            "Effect": "Allow",
            "Resource": [
                "*"
            ]
        }
    ]
}