Category: Linux

Tutorial: Automatic, Reproducible Preparation of Virtual Machine templates for your vSphere environment with HashiCorp’s Packer

Introduction

Let us say that you are using an vSphere (VMWare vCenter + one or more ESXi servers) environment in your company and you are creating virtual machines pretty often. Obviously you are familiar with the virtual machines template functionality but where could you get the templates? Well, one of the options is to look for them somewhere on the internet but that is not a very reliable thing to do since you have to trust a third-party to provide these to you. Most, if not all, operating systems distributors do not provide a VMWare template for their operating system. For example, Canonical only provides Ubuntu in a form of an ISO file. Thus, you have to change your approach and make the templates yourself. However, doing that manually that is tedious and very menial so you will look for solutions to this problem. Packer is something that solves this problem.

From here on, I will assume some level of familiarity with Packer here. It has a builder called vmware-iso that looks like it can do the trick. However, you will run into problems quickly. For example, the vsphere-template post-processor only works if you run the vmware-iso builder remotely. It does not work with your local virtual machine. Because of this and other issues you have to apply some tricks to make the post-processing section work. Also, after using that builder once, you might notice that there are some short-comings in how Packer detects what IP the virtual machine has. It takes the first DHCP lease from ethernet0 and uses that to communicate with your virtual machine even though it is, by default, the network that is used for network address translation (NAT). This post will show you how to make it all work. By the end of it, you should have a working pipe-line:

  • automatically a new virtual machine is created in VMware Workstation with needed parameters
  • specified ISO is mounted and the OS is installed using a “pre-seeded” configuration
  • some optional post-processing scripts will be run on the virtual machine
  • the resulting virtual machine template will be uploaded to the specified vCenter

As you can see, this process is perfect for making golden templates of various operating systems. Let us delve into by using Ubuntu 16.04 as an example.

Step 1: starting up the virtual machine

The vmware-iso builder is perfect for doing except we will have to apply some tricks to make Packer pick up the correct IP address. By default, the first and only network is used for network address translation which means that when the virtual machine will be deployed, Packer will pick up the given DHCP lease and it will try to connect to it via SSH for provisioning. Because network address translation is happening, there is no way to reach that deployed virtual machine without any tricks.

To solve this problem, I recommend you to specify custom VMX properties which will make ethernet0 be connected to vmnet8, and ethernet1 to vmnet1. vmnet8 is an internal, private network that will be used for communication between the host machine and the virtual machine. ethernet1 is connected to vmnet1, which is the network with an DHCP server and is used for network address translation which lets the virtual machine access the Internet. Such configuration of vmnet1 and vmnet8 is the default on VMWare Workstation so no changes are needed on that side.

This is achievable by specifying the following in the builders section of the JSON configuration file:

{
    "vmx_data": {
        "ethernet0.present": true,
        "ethernet0.startConnected": true,
        "ethernet0.connectionType": "custom",
        "ethernet0.vnet": "vmnet8",
        "ethernet1.present": true,
        "ethernet1.startConnected": true,
        "ethernet1.connectionType": "custom",
        "ethernet1.vnet": "vmnet1"
    }
}

You can find more information about VMX properties in this website: http://sanbarrow.com/vmx/vmx-network-advanced.html.

However, after provisioning the virtual machine, you might want to disable the second interface because it is a good default to only have one virtual NIC connected to a virtual machine template by default. If the user will want to add more networks and NICs – it is up to them. So to disable the second NIC after everything, add this to the configuration file that is passed to Packer:

{
    "vmx_data_post": {
        "floppy0.present": false,
        "ethernet1.present": false
    }
}

Note that this also disables the virtual floppy disk after provisioning. This is needed to unmount the floppy disk that has the pre-seed file. Obviously it is not needed in the final virtual machine template. This is all you need so far in this tutorial. You could run the file now and see the Ubuntu installer be automatically started. This is how my builders section looks like:

{
  "builders": [
    {
      "type": "vmware-iso",
      "iso_url": "http://releases.ubuntu.com/16.04/ubuntu-16.04.3-server-amd64.iso",
      "iso_checksum": "10fcd20619dce11fe094e960c85ba4a9",
      "iso_checksum_type": "md5",
      "ssh_username": "root",
      "ssh_password": "Giedrius",
      "vm_name": "Ubuntu_16.04_x64",
      "ssh_wait_timeout": "600s",
      "shutdown_command": "shutdown -P now",
      "output_directory": "ubuntu_1604",
      "boot_command": [
        "<enter><wait><f6><esc><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs>",
        "<bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs>",
        "<bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs>",
        "<bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs>",
        "/install/vmlinuz<wait>",
        " auto<wait>",
        " console-setup/ask_detect=false<wait>",
        " console-setup/layoutcode=us<wait>",
        " console-setup/modelcode=pc105<wait>",
        " debconf/frontend=noninteractive<wait>",
        " debian-installer=en_US<wait>",
        " fb=false<wait>",
        " hostname=ubuntu1604<wait>",
        " initrd=/install/initrd.gz<wait>",
        " kbd-chooser/method=us<wait>",
        " keyboard-configuration/layout=USA<wait>",
        " keyboard-configuration/variant=USA<wait>",
        " locale=en_US<wait>",
        " noapic<wait>",
        " preseed/file=/floppy/ubuntu_preseed.cfg",
        " -- <wait>",
        "<enter><wait>"
      ],
      "guest_os_type": "ubuntu-64",
      "floppy_files": [
        "./ubuntu_preseed.cfg"
      ],
      "vmx_data": {
        "ethernet0.present": true,
        "ethernet0.startConnected": true,
        "ethernet0.connectionType": "custom",
        "ethernet0.vnet": "vmnet8",
        "ethernet1.present": true,
        "ethernet1.startConnected": true,
        "ethernet1.connectionType": "custom",
        "ethernet1.vnet": "vmnet1"
      },
      "vmx_data_post": {
        "floppy0.present": false,
        "ethernet1.present": false
      }
    }
  ]}

Step 2: installing the operating system in the new virtual machine

There is almost no trick that has to be done here except that you have to make sure that DHCP is enabled on all network interfaces. Many operating systems’ installers only let you choose the default interface on which DHCP is enabled. This could be worked around with some custom amendments to the configuration.

For example, on Ubuntu 16.04 the easiest way to do this is to append some text to /etc/network/interfaces which will enable DHCP on the second interface as well. You can find more information on that file here: https://help.ubuntu.com/lts/serverguide/network-configuration.html. So, you should have something like this in your pre-seed file:

d-i preseed/late_command string in-target sh -c 'echo "auto ens33\niface ens33 inet dhcp" >> /etc/network/interfaces'

ens33 is the default name of the interface that it given to the second one on my set up so it is used here. As you can see, the network configuration is changed and DHCP client is enabled on the ens33 interface as well. If you are using some other operating system then refer to its manual to find out what exactly you have to change to enable the DHCP client on the second interface.

You are free to add anything you want to to the provisioners section of the Packer configuration. In my case, I am uploading some default configuration to /etc/profile.d and I am making those files executable so that they would be executed each time someone logs in.

Step 3: post processing a.k.a. uploading the actual template

This is an important trick that you have to do. The default post-processor only support this action if you use the vmware-iso builder in a remote configuration so some trickery is needed.

At first, you need to run the OVFTool to convert the *.vmx file to an *.ovf. Use the shell-local post-processor like this:

{
  "type": "shell-local",
  "inline": [
    "/c/Program\\ Files\\ \\(x86\\)/VMware/VMware\\\nWorkstation/OVFTool/ovftool.exe ubuntu_1604/Ubuntu_16.04_x64.vmx ubuntu_1604/Ubuntu_16.04_x64.ovf"
  ]
}

This will convert the resultant vmx file to an ovf one. Next, you have to use the artifice post-processor to change the list of artifacts so that the next post-processor that we are going to use would pick up the new ovf file as well.

The post-processor that we will use is called vsphere-template. It sifts through the artifact list (that we will generate with artifice) and uploads the OVF file to the vCenter, and converts it into an template. You can find it here: https://github.com/andrewstucki/packer-post-processor-vsphere-template (props to andrewstucki!). You can get all of the details in there. To make it short:

  • download the executable file into some known location
  • edit %APPDATA%/packer.config and add something like this there (change the exact path depending on your set-up):
{
  "post-processors": {
    "vsphere-template": "C:\\packer-post-processor-vsphere-template_darwin_amd64.exe"
  }
}

For the final piece, you will have to create a post-processor chain with vsphere-template and artifice like it is described here: https://www.packer.io/docs/post-processors/artifice.html. You will need to enclose the last two post-processors in square brackets. The final version of the post-processor section looks like this:

{
  "post-processors": [
    {
      "type": "shell-local",
      "inline": [
        "/c/Program\\ Files\\ \\(x86\\)/VMware/VMware\\ Workstation/OVFTool/ovftool.exe ubuntu_1604/Ubuntu_16.04_x64.vmx ubuntu_1604/Ubuntu_16.04_x64.ovf"
      ]
    },
    [
      {
        "type": "artifice",
        "files": [
          "ubuntu_1604\\*"
        ]
      },
      {
        "type": "vsphere-template",
        "datacenter": "example_datacenter",
        "datastore": "example_datastore",
        "host": "example_host",
        "password": "example_password",
        "username": "example_username",
        "vm_name": "Ubuntu_16.04_x64",
        "resource_pool": "example_resourcepool",
        "folder": "example_folder"
      }
    ]
  ]
}

Links

The Packer JSON configuration file looks like this: https://gist.github.com/GiedriusS/04b9881595882fdee61a83d3c7dd3f3b

The pre-seed file that I used for Ubuntu 16.04: https://gist.github.com/GiedriusS/9053014e39ad17a7e4669e28bc7494bc

For other machines there is not much difference except you will have to figure out what boot options to use, the format of the installer configuration, and what to pass to the network management daemon that is used in the operating system to enable DHCP on all interfaces.

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Confusion With File Redirection And sudo

Introduction

I have noticed that newbies sometimes fail to understand why, for example, sudo echo “hi” > /tmp/test will create a file /tmp/test with whatever effective user ID and effective group ID the command is ran with. They fall into the trap thinking that root:root will be the owner of /tmp/test. However, that is certainly not true. This blog post will try to clarify why this happens and what the user can do to avoid this issue. This occurs due to peculiar parsing rules defined in POSIX. We will examine the standards which detail this behaviour, the source code of the popular shells bash/zsh, and some methods on how to avoid this problem with, for example, tee(1).

Why it happens

You, my dear reader, have to understand first of all that sudo is just a command, a binary just like any else. It is a special binary, though, because it runs whatever command you pass as arguments. However, the redirection part (> /tmp/test) is not part of the command. This special file redirection syntax is interpreted by the shell, not passed to sudo for it to be executed later. So the shell that you are running gets special instructions to run that command with file descriptor number 1 which will be opened to write to a newly created file /tmp/test.

Notice that at this point command has not been run yet. The shell, before starting the command, begins to prepare the first fd. The shell does this with whatever EUID/EGID that shell is running with so it creates that new file with not the root:root rights but with whatever EUID/EGID the shell is running with. Most shells probably prepare the file descriptor 1 with the dup*(2) family of functions or fcntl(2), and open(2) before doing a fork(2) and exec*(3) just after it.

This functionality is mandated by a section of POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07. All POSIX-compatible shells follow that section. Obviously, some shells are not compatible with POSIX. This article targets POSIX-compatible shells, however. In either case, to find out all of the gory details of how file redirection works, either refer to that section, read the fine manual, and/or read the source code of the shell that you are using. The latter option gives you the most detail and is the most trustworthy whereas the former one is more accessible. I will not go on a tangent here and I will redirect you to this article that I wrote before if you want to find more about this.

Next we will review the code of two popular shells: GNU/Bash and the Z shell, to see how they realize this functionality in code.

How do the popular shells implement this

GNU/Bash uses the GNU/Bison project for specifying the input command syntax. The syntax file is written in the yacc format. You can find the special syntax described in the parse.y file, in the root directory of GNU/Bash source. There are two special types defined: redirection and redirection_list. Here is an excerpt from parse.y file which defines how redirecting stdout to a file works or how passing a file to stdin works:

redirection:	'>' WORD
			{
			  source.dest = 1;
			  redir.filename = $2;
			  $$ = make_redirection (source, r_output_direction, redir, 0);
			}
	|	'<' WORD 
			{
			  source.dest = 0;
			  redir.filename = $2;
			  $$ = make_redirection (source, r_input_direction, redir, 0);
			}
        | ...

And here is the excerpt which shows that the redirection list goes after a command:

command:	simple_command
			{ $$ = clean_simple_command ($1); }
	|	shell_command
			{ $$ = $1; }
	|	shell_command redirection_list
			{
			  ...
			}
	|	...

My point is that you can see that GNU/Bash really has a special syntax for file redirection, it is not passed to sudo. The shell command is separate from the redirection list. With the Z shell it is a bit different because there is no one single place where the grammar is defined. Instead, it has a big file dedicated for the hand-rolled parsing engine that is written in C. Thus, it will not be possible to present the whole file but here are the excerpts from the file which prove to you once again that the file redirection feature is indeed based on a special syntax, it is not passed to the actual binary as arguments. In the Z shell source code, Src/parse.c we can find this comment:

/*
 * cmd : { redir } ( for | case | if | while | repeat |
 * subsh | funcdef | time | dinbrack | dinpar | simple ) { redir }
 *
 * zsh_construct is passed through to par_subsh(), q.v.
 */

Indeed, redirection is parsed later in the function that is responsible for parsing the command:

static int
par_cmd(int *cmplx, int zsh_construct)
{
    int r, nr = 0;

    r = ecused;
    if (IS_REDIROP(tok)) {
        *cmplx = 1;
        while (IS_REDIROP(tok)) {
            nr += par_redir(&r, NULL);
        }
    }

    switch (tok) {
    case FOR:
          ...
    case FOREACH:
          ...
    case SELECT:
          ...
    case CASE:
          ...
    case IF:
          ...
    case WHILE:
          ...
    case UNTIL:
          ...
    case REPEAT:
          ...
    ...
    }

    if (IS_REDIROP(tok)) {
        *cmplx = 1;
        while (IS_REDIROP(tok))
            (void)par_redir(&r, NULL);
    }

    ...
    return 1;
}

As you can see, the redirections may be at either the end or the beginning of the command. Let me repeat again: file redirection is special syntax and it is not passed to the actual thing being run. The GNU/Bash shell supports the file redirection syntax at the beginning of the line as well but I have just decided to not include it for brevity.

So how to actually redirect output to a file as root?

I guess the most popular solution to this is to simply use a pipe to redirect output to a file. I am not sure but probably the program, tee(1), was made for this purpose. Or it was made as an extension of the tee system call but still it is the perfect tool to solve our issue. So, the solution to this problem would be:

echo "hi" | sudo tee /tmp/test >&-

This will at first (probably, depending on your set up) ask for your password. Then, “hi” will be written to a file descriptor which will get passed on to tee(1). Then, tee(1) will dump everything into /tmp/test. The >&- (or >/dev/null) is needed so that tee(1) would not output anything. tee(1), unfortunately, copies the content from standard input to standard output as well.

Another way to solve this is to pass more commands to run via root either by using the -s sudo option. For example, this works:

sudo -s <<EOF
exec >/tmp/test
echo "hi"
EOF

Or, run another shell using sudo which will redirect everything to /tmp/test:

sudo /bin/bash -c 'exec >/tmp/test; echo "hi"'

Or, run a script with sudo which will internally redirect stdout to /tmp/test:

cat >./script.sh <<EOF
exec >/tmp/test
echo "hi"
EOF
chmod +x ./script.sh
sudo ./script.sh

Obviously, there are more (complex) ways how you could achieve this but these are the main methods. You are free to adopt these examples to your own case. Please comment if you find any errors or if you want to add anything about this topic.

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