Wednesday, May 6, 2015

Chef Audit Mode Introduction [feedly]



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Chef Audit Mode Introduction
// Chef Blog

I have been working with the audit mode feature introduced in Chef version 12.1.0 – previously announced was the audit-cis cookbook. Audit mode allows users to write custom rules (controls) in Chef recipes using new DSL helpers. In his ChefConf 2015 talk, "Compliance At Velocity," James Casey goes into more of the background and reasoning for this. For now, I wanted to share a few tips with users who may be trying out this feature, too.

First, we need to update ChefDK to version 0.5.0, as that includes a version of test kitchen that allows us to configure audit mode for chef-client.

curl -L https://chef.io/chef/install.sh | sudo bash -s -- -P chefdk  
Next, create a new cookbook for the audit mode tests.
chef generate cookbook audit-test  cd audit-test  
Then, modify the audit-test cookbook's .kitchen.yml:
driver:    name: vagrant  provisioner:    name: chef_zero    client_rb:      audit_mode: :audit_only  platforms:    - name: ubuntu-12.04    - name: centos-6.5  suites:    - name: default      run_list:        - recipe[audit-test::default]      attributes:
This is the generated .kitchen.yml with client_rb added to the provisioner config to enable audit mode. Note that we must use the Ruby symbol syntax, :audit_only. The valid values for audit_mode are :enabled, :audit_only and :disabled. This will be translated to an actual Ruby symbol in the generated config file (/tmp/kitchen/client.rb):
audit_mode :audit_only
Next, let's write a control rule to test. Since we're using the default .kitchen.yml, which includes Ubuntu 12.04 and uses SSH to connect, we can assume that SSH is running, so port 22 is listening. The following control asserts this is true.
  control_group 'Blog Post Examples' do    control 'SSH' do      it 'should be listening on port 22' do        expect(port(22)).to be_listening      end    end  end  
Now run kitchen converge ubuntu to run Chef, but not tear down the VM afterward – we'll use it again for another example. Here's the audit phase output from the Chef run:
% kitchen converge ubuntu  Synchronizing Cookbooks:    - audit-test  Compiling Cookbooks...  Starting audit phase     Blog Post Examples    SSH      should be listening on port 22     Finished in 0.10453 seconds (files took 0.37536 seconds to load)  1 example, 0 failures  Auditing complete  
Cool! So we have asserted that the node complies with this control by default. But what does a failing control look like? Let's write one. Since we're working with SSH already, let's use the SSHd configuration. By default in the Vagrant base box we're using, root login is permitted, so this value is present:
PermitRootLogin yes
However, our security policy mandates that we set this to no, and we want to audit that.
control_group 'Blog Post Examples' do    control 'SSH' do      it 'should be listening on port 22' do        expect(port(22)).to be_listening      end         it 'disables root logins over ssh' do        expect(file('/etc/ssh/sshd_config').content).to contain('PermitRootLogin no')      end    end  end  
Rerun kitchen converge ubuntu and we see the validation fails.
Starting audit phase     Blog Post Examples    SSH      should be listening on port 22      disables root logins over ssh (FAILED - 1)     Failures:       1) Blog Post Examples SSH disables root logins over ssh       Failure/Error: expect(file('/etc/ssh/sshd_config').content).to contain('PermitRootLogin no')  expected File "/etc/ssh/sshd_config" to contain "PermitRootLogin no"       # /tmp/kitchen/cache/cookbooks/audit-test/recipes/default.rb:8:in `block (3 levels) in from_file'     Finished in 0.13067 seconds (files took 0.32089 seconds to load)  2 examples, 1 failure     Failed examples:     rspec  # Blog Post Examples SSH disables root logins over ssh  [2015-04-04T03:29:41+00:00] ERROR: Audit phase failed with error message: Audit phase found failures - 1/2 controls failed     Audit phase exception:  Audit phase found failures - 1/2 controls failed  
When we have a failure, we'll have contextual information about the failure, including the line number in the recipe where it was found, and a stack trace (cut from the output here), in case more information is required for debugging. To fix the test, we can simply edit the config file to have the desired setting, or we can manage the file with Chef to set the value accordingly. Either way, after updating the file, the validation will pass, and all will be well.

We can put as many control_group and control blocks with the it validation rules as required to audit our policy. If we have many validations, it can be difficult to follow with all the output if there are failures. Chef's audit mode is based on Serverspec, which is based on RSpec. We can use the filter_tag configuration feature of RSpec to only run the control blocks or it statements that we're interested in debugging. To do this, we need an RSpec.configuration block within the control_group – due to the way that audit mode is implemented, we can't do it outside of control_group.

For example, we could debug our root login configuration:

control_group 'Blog Post Examples' do    ::RSpec.configure do |c|      c.filter_run focus: true    end       control 'SSH' do      it 'should be listening on port 22' do        expect(port(22)).to be_listening      end         it 'disables root logins over ssh', focus: true do        expect(file('/etc/ssh/sshd_config').content).to contain('PermitRootLogin no')      end    end  end
The key here is to pass the argument focus: true (or if you like hash rockets, :focus => true) on the it block. This could also be used on a control block:
control 'SSH', focus: true do    it 'does stuff...'  end
Then, when running kitchen converge ubuntu, we see only that validation:
Starting audit phase     Blog Post Examples    SSH      disables root logins over ssh (FAILED - 1)     Failures:       1) Blog Post Examples SSH disables root logins over ssh       Failure/Error: expect(file('/etc/ssh/sshd_config')content).to contain('PermitRootLogin no')  
This example is simple enough that using focus isn't necessary, but if we were implementing audit mode checks for our entire security policy, that could be dozens or even hundreds of controls.

While audit mode is still in development as a feature, we think that it is highly valuable to organizations that need to ensure their systems are compliant with varying types of audit requirements. Also, because audit mode can be run on its own without converging resources in Chef recipes, it's great for assessing existing system's state before starting to implement Chef.


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