Ansible: Making Things Happen

Ansible: Making Things Happen

Image

Shawn Powers
Tue, 01/30/2018 – 10:07

Finally, an automation framework that thinks like a sysadmin. Ansible,
you’re hired.

In my last article, I described how to configure your server and clients so
you could
connect to each client from the server. Ansible is a push-based automation
tool, so the connection is initiated from your “server”, which is usually
just a workstation or a server you ssh in to from
your workstation. In this article, I explain
how modules work and how you can use Ansible
in ad-hoc mode from the command line.

Ansible is supposed to make your job easier, so the first thing you
need to learn is how to do familiar tasks. For most sysadmins, that
means some simple command-line work. Ansible has a few quirks when it
comes to command-line utilities, but it’s worth learning the nuances,
because it makes for a powerful system.

Command Module

This is the safest module to execute remote commands on the client
machine. As with most Ansible modules, it requires Python to be installed
on the client, but that’s it. When Ansible executes commands using the
Command Module, it does not process those commands through the user’s
shell. This means some variables like $HOME are not available. It also
means stream functions (redirects, pipes) don’t work. If you don’t
need to redirect output or to reference the user’s home directory as
a shell variable, the Command Module is what you want to use. To invoke
the Command Module in ad-hoc mode, do something like this:


ansible host_or_groupname -m command -a "whoami"

Your output should show SUCCESS for each host referenced and then return
the user name that the user used to log in. You’ll notice that the user is not
root, unless that’s the user you used to connect to the client computer.

If
you want to see the elevated user, you’ll add another argument to the
ansible command. You can add
-b in order to “become” the elevated user
(or the sudo user). So, if you were to run the same command as above with a
“-b” flag:


ansible host_or_groupname -b -m command -a "whoami"

you should see a similar result, but the whoami
results should say root
instead of the user you used to connect. That flag is important to use,
especially if you try to run remote commands that require root access!

Shell Module

There’s nothing wrong with using the Shell Module to execute remote
commands. It’s just important to know that since it uses the remote
user’s environment, if there’s something goofy with the user’s account,
it might cause problems that the Command Module avoids. If you use the
Shell Module, however, you’re able to use redirects and pipes. You can
use the whoami example to see the difference. This command:


ansible host_or_groupname -m command -a "whoami > myname.txt"

should result in an error about > not being a valid argument. Since
the Command Module doesn’t run inside any shell, it interprets the
greater-than character as something you’re trying to pass to the
whoami
command. If you use the Shell Module, however, you have no problems:


ansible host_or_groupname -m shell -a "whom > myname.txt"

This should execute and give you a SUCCESS message for each host,
but there should be nothing returned as output. On the remote machine,
however, there should be a file called myname.txt in the user’s home
directory that contains the name of the user. My personal policy is to
use the Command Module whenever possible and to use the Shell Module
if needed.

The Raw Module

Functionally, the Raw Module works like the Shell Module. The key
difference is that Ansible doesn’t do any error checking, and
STDERR,
STDOUT and Return Code is returned. Other than that, Ansible has no idea
what happens, because it just executes the command over SSH directly. So
while the Shell Module will use /bin/sh by default, the Raw Module just
uses whatever the user’s personal default shell might be.

Why would a person decide to use the Raw
Module? It doesn’t require
Python on the remote computer—at all. Although it’s true that most servers
have Python installed by default, or easily could have it installed,
many embedded devices don’t and can’t have Python installed. For most
configuration management tools, not having an agent program installed
means the remote device can’t be managed. With Ansible, if all you have is
SSH, you still can execute remote commands using the Raw Module. I’ve used
the Raw Module to manage Bitcoin miners that have a very minimal embedded
environment. It’s a powerful tool, and when you need it, it’s invaluable!

Copy Module

Although it’s certainly possible to do file and folder manipulation with
the Command and Shell Modules, Ansible includes a module specifically
for copying files to the server. Even though it requires learning a new
syntax for copying files, I like to use it because Ansible will check
to see whether a file exists, and whether it’s the same file. That means it
copies the file only if it needs to, saving time and bandwidth. It even
will make backups of existing files! I can’t tell you how many times I’ve
used scp and sshpass in a
Bash FOR loop and dumped files on servers,
even if they didn’t need them. Ansible makes it easy and doesn’t
require FOR loops and IP iterations.

The syntax is a little more complicated than with Command, Shell or
Raw. Thankfully, as with most things in the Ansible world, it’s easy to
understand—for example:


ansible host_or_groupname -b -m copy 
    -a "src=./updated.conf dest=/etc/ntp.conf 
        owner=root group=root mode=0644 backup=yes"

This will look in the current directory (on the Ansible
server/workstation) for a file called updated.conf and then copy it to
each host. On the remote system, the file will be put in /etc/ntp.conf,
and if a file already exists, and it’s different, the original will
be backed up with a date extension. If the files are the same, Ansible
won’t make any changes.

I tend to use the Copy Module when updating configuration files. It
would be perfect for updating configuration files on Bitcoin miners,
but unfortunately, the Copy Module does require that the remote machine has
Python installed. Nevertheless, it’s a great way to update common files
on many remote machines with one simple command. It’s also important
to note that the Copy Module supports copying remote files to other
locations on the remote filesystem using the
remote_src=true directive.

File Module

The File Module has a lot in common with the Copy Module, but if you try
to use the File Module to copy a file, it doesn’t work as expected. The
File Module does all its actions on the remote machine, so
src and dest
are all references to the remote filesystem. The File Module often
is used for creating directories, creating links or deleting remote files
and folders. The following will simply create a folder named /etc/newfolder
on the remote
servers and set the mode:


ansible host_or_groupname -b -m file 
       -a "path=/etc/newfolder state=directory mode=0755"

You can, of course, set the owner and group,
along with a bunch of other options, which you can learn about on the
Ansible doc site. I find I most often will either create a folder
or symbolically link a file using the File Module. To create a symlink:


sensible host_or_groupname -b -m file 
         -a "src=/etc/ntp.conf dest=/home/user/ntp.conf 
             owner=user group=user state=link"

Notice that the state directive is how you inform
Ansible what you actually want to
do. There are several state options:

  • link — create symlink.

  • directory — create directory.

  • hard — create hardlink.

  • touch — create empty file.

  • absent — delete file or directory recursively.

This might seem a bit complicated, especially when you easily could do
the same with a Command or Shell Module command, but the clarity of using
the appropriate module makes it more difficult to make mistakes. Plus,
learning these commands in ad-hoc mode will make playbooks, which consist
of many commands, easier to understand (I plan to cover this in my next
article).

File Management

Anyone who manages multiple distributions knows it can be tricky to
handle the various package managers. Ansible handles this in a couple
ways. There are specific modules for apt and yum, but there’s also a
generic module called “package” that will install on the remote computer
regardless of whether it’s Red Hat- or Debian/Ubuntu-based.

Unfortunately, while Ansible usually can detect the type of package
manager it needs to use, it doesn’t have a way to fix packages with
different names. One prime example is Apache. On Red Hat-based systems,
the package is “httpd”, but on Debian/Ubuntu systems, it’s
“apache2”. That
means some more complex things need to happen in order to
install the correct package automatically. The individual modules, however, are very
easy to use. I find myself just using apt or yum as appropriate, just
like when I manually manage servers. Here’s an apt example:


ansible host_or_groupname -b -m apt 
          -a "update_cache=yes name=apache2 state=latest"

With this one simple line, all the host machines will run
apt-get update
(that’s the update_cache directive at work), then install apache2’s latest
version including any dependencies required. Much like the File Module,
the state directive has a few options:

  • latest — get the latest version, upgrading
    existing if needed.

  • absent — remove package if installed.

  • present — make sure package is installed, but
    don’t upgrade existing.

The Yum Module works similarly to the Apt Module, but I generally
don’t bother with the update_cache directive, because yum updates
automatically. Although very similar, installing Apache on a Red Hat-based
system looks like this:


ansible host_or_groupname -b -m yum 
      -a "name=httpd state=present"

The difference with this example is that if Apache is already installed,
it won’t update, even if an update is available. Sometimes updating to
the latest version isn’t want you want, so this stops that from accidentally
happening.

Just the Facts, Ma’am

One frustrating thing about using Ansible in ad-hoc mode is that you don’t
have access to the “facts” about the remote systems. In my
next article, where I plan to explore
creating playbooks full of various tasks, you’ll see how you can reference the facts Ansible
learns about the systems. It makes Ansible far more powerful, but again,
it can be utilized only in playbook mode. Nevertheless, it’s possible to use
ad-hoc mode to peek at the sorts information Ansible gathers. If you run
the setup module, it will show you all the details from a remote system:


ansible host_or_groupname -b -m setup

That command will spew a ton of variables on your screen. You can
scroll through them all to see the vast amount of information Ansible pulls
from the host machines. In fact, it shows so much information, it can
be overwhelming. You can filter the results:


ansible host_or_groupname -b -m setup -a "filter=*family*"

That should just return a single variable,
ansible_os_family, which
likely will be Debian or Red Hat. When you start building more complex
Ansible setups with playbooks, it’s possible to insert some logic and
conditionals in order to use yum where appropriate and apt where the
system is Debian-based. Really, the facts variables are incredibly useful
and make building playbooks that much more exciting.

But, that’s for another article, because you’ve come to the end of the second
installment. Your assignment for now is to get comfortable using
Ansible in ad-hoc mode, doing one thing at a time. Most people think
ad-hoc mode is just a stepping stone to more complex Ansible setups,
but I disagree. The ability to configure hundreds of servers consistently
and reliably with a single command is nothing to scoff at. I love making
elaborate playbooks, but just as often, I’ll use an ad-hoc command in
a situation that used to require me to ssh in to a bunch of servers to
do simple tasks. Have fun with Ansible; it just gets more
interesting from here!

Read More

Leave a Reply