Configure the HTTP traffic replicator [gor](https://github.com/buger/gor) to run on an AWS Elastic Beanstalk instance as a daemon. Store this file as `./ebextensions/10gor.config`

10gor.config.yaml

# File: .ebextensions/10gor.config
# Config Gor to copy a sample of Prod http traffice to staging
files:
  # Utility for daemonizing binaries such as gor; see http://libslack.org/daemon
  /opt/daemon.rpm:
    source: "https://s3-eu-west-1.amazonaws.com/elasticbeanstalk-eu-west-1-<our id>/our_fileserver/daemon-0.6.4-1.x86_64.rpm"
    authentication: S3Access # See AWS::CloudFormation::Authentication below
    owner: root
    group: root

  # daemon config so that we don't need to repeat these command line options and
  # can just use the service's name ("gor")
  # We need to intercept the port 8080, not 80 (that iptables redirect to 8080)
  /etc/daemon.conf:
    # Troubleshooting tips:
    # 1) Send stderr/out output of both daemon and the service to a file - see the commented-out line
    #    (it should be also possible to send it to syslog but that did not work for me)
    # 2) Add "foreground" to the options and start the service manually on the server
    content: |
      gor  respawn,command=/opt/gor --input-raw :8080 --output-http 'https://our-staging.example.com|1'
      #gor  output=/var/log/gor.log,respawn,command=/opt/gor --stats --output-http-stats --input-raw :8080 --output-http 'https://our-staging.example.com|1'
      #gor  errlog=/var/log/daemonapp.log,dbglog=/var/log/daemonapp.log,output=/var/log/gor.log,verbose,debug,respawn,command=/opt/gor --verbose --stats --output-http-stats --input-raw :8080 --output-http 'https://our-staging.example.com|1'


    # Use the commented-out line to enable stats logging to syslog (/var/log/messages) every 5s for troubleshooting
    content: |
      gor  respawn,command=/opt/gor --input-raw :8080 --output-http 'https://our-staging.example.com|1'
      #gor  respawn,output=gor.info,command=/opt/gor --stats --output-http-stats --input-raw :8080 --output-http 'https://our-staging.example.com|1'

  # HTTP traffic replicator; see https://github.com/buger/gor
  /opt/gor:
    source: "https://s3-eu-west-1.amazonaws.com/elasticbeanstalk-eu-west-1-<our id>/our_fileserver/gor"
    authentication: S3Access
    mode: "000755"
    owner: root
    group: root

  # System V service that supports chkconfig
  /etc/init.d/gor:
    mode: "000755"
    owner: root
    group: root
    content: |
      ## The chkconfig <levels> <startup order> <stop order> + descr. needed to
      ## support ensureRunning
      # chkconfig: 345 92 08
      # description: Gor copies traffic to staging
      ### BEGIN INIT INFO
      # Provides: gor
      # Short-Description: Start Gor to copy traffic to staging
      ### END INIT INFO

      # See how we were called.
      case "$1" in
        start)
              /usr/local/bin/daemon --name gor
              ;;
        stop)
              /usr/local/bin/daemon --name gor --stop
              ;;
        status)
              if ! /usr/local/bin/daemon --name gor --running; then
                  echo "gor is stopped"; exit 3
              fi
              ;;
        restart)
              /usr/local/bin/daemon --name gor --stop
              /usr/local/bin/daemon --name gor
              ;;
        *)
              echo $"Usage: $0 {start|stop|restart}"
              exit 2
      esac
      exit 0

commands:
  "Install daemon":
    command: rpm -i /opt/daemon.rpm
    test: /bin/sh -c "! rpm -q daemon" # It seems 'test: ! rpm -q daemon' ignores the !

services:
  sysvinit:
    gor:
      enabled: true
      ensureRunning: true

  # Allow files: to access our S3 bucket
  # See https://forums.aws.amazon.com/thread.jspa?messageID=557993
  # BEWARE: We have to explicitely allow access to any subdirectory by
  # editing the S3 bucket's policy and adding the subdir to the allowed Resources
  AWSEBAutoScalingGroup:
    Metadata:
      AWS::CloudFormation::Authentication:
        "S3Access": # reference this in the "authentication" property
          type: S3
          roleName: aws-elasticbeanstalk-ec2-role
          buckets: elasticbeanstalk-eu-west-1-<our id>

Highlights

1. Use the daemon utility to run Gor as a daemon (which it does not support out of the box). Daemon is small and works well. It will ignore gor's output and automatically restart it if it dies.
2. Create an init.d script for gor. To support ebextensions's ensureRunning, it has to support chkconfig
3. The test for whether daemon is installed cannot be just ! rpm -q daemon but needs to be /bin/sh -c "! rpm -q daemon"; the test property seems to require a single command to execute
4. The files are downloaded from a private S3 bucket (which needs to be accessible by the EC2 role used and have the policy to allow access to the files in question)