{
  "_comment": "Build with `packer build -var-file=ubuntu1804.json ubuntu.json`",
  "vm_name": "ubuntu1804",
  "cpus": "1",
  "disk_size": "65536",
  "iso_checksum": "e2ecdace33c939527cbc9e8d23576381c493b071107207d2040af72595f8990b",
  "iso_checksum_type": "sha256",
  "iso_url": "http://cdimage.ubuntu.com/ubuntu/releases/18.04/release/ubuntu-18.04.4-server-amd64.iso",
  "memory": "512",
  "preseed" : "preseed.cfg",
  "boot_command_prefix": "<esc><esc><enter><wait>",
  "version" : "1"
}

{
  "_comment": "Build with `packer build -var-file=ubuntu1804.json ubuntu.json`",
  "vm_name": "ubuntu1804",
  "cpus": "1",
  "disk_size": "65536",
  "iso_checksum": "8c5fc24894394035402f66f3824beb7234b757dd2b5531379cb310cedfdf0996",
  "iso_checksum_type": "sha256",
  "iso_url": "http://cdimage.ubuntu.com/ubuntu/releases/18.04/release/ubuntu-18.04.5-server-amd64.iso",
  "memory": "512",
  "preseed" : "preseed.cfg",
  "boot_command_prefix": "<esc><esc><enter><wait>",
  "version" : "1"
}

Update Ubuntu 18.04 URL and Checksum
Update Ubuntu 18.04 URL and Checksum