chef-client

A chef-client is an agent that runs locally on every node that is under management by Chef. When a chef-client is run, it will perform all of the steps that are required to bring the node into the expected state, including:

• Registering and authenticating the node with the Chef server
• Building the node object
• Synchronizing cookbooks
• Compiling the resource collection by loading each of the required cookbooks, including recipes, attributes, and all other dependencies
• Taking the appropriate and required actions to configure the node
• Looking for exceptions and notifications, handling each as required

Node Types

A node is any machine—physical, virtual, cloud, network device, etc.—that is under management by Chef.

The types of nodes that can be managed by Chef include, but are not limited to, the following:

Node Type	Description
	A physical node is typically a server or a virtual machine, but it can be any active device attached to a network that is capable of sending, receiving, and forwarding information over a communications channel. In other words, a physical node is any active device attached to a network that can run a chef-client and also allow that chef-client to communicate with a Chef server.
	A cloud-based node is hosted in an external cloud-based service, such as Amazon Web Services (AWS), OpenStack, Rackspace, Google Compute Engine, or Microsoft Azure. Plugins are available for knife that provide support for external cloud-based services. knife can use these plugins to create instances on cloud-based services. Once created, the chef-client can be used to deploy, configure, and maintain those instances.
	A virtual node is a machine that runs only as a software implementation, but otherwise behaves much like a physical machine.
	A network node is any networking device—a switch, a router—that is being managed by a chef-client, such as networking devices by Juniper Networks, Arista, Cisco, and F5. Use Chef to automate common network configurations, such as physical and logical Ethernet link properties and VLANs, on these devices.
	Containers are an approach to virtualization that allows a single operating system to host many working configurations, where each working configuration—a container—is assigned a single responsibility that is isolated from all other responsibilities. Containers are popular as a way to manage distributed and scalable applications and services.

The key components of nodes that are under management by Chef include:

Component	Description
	A chef-client is an agent that runs locally on every node that is under management by Chef. When a chef-client is run, it will perform all of the steps that are required to bring the node into the expected state, including: Registering and authenticating the node with the Chef server Building the node object Synchronizing cookbooks Compiling the resource collection by loading each of the required cookbooks, including recipes, attributes, and all other dependencies Taking the appropriate and required actions to configure the node Looking for exceptions and notifications, handling each as required RSA public key-pairs are used to authenticate the chef-client with the Chef server every time a chef-client needs access to data that is stored on the Chef server. This prevents any node from accessing data that it shouldn’t and it ensures that only nodes that are properly registered with the Chef server can be managed.
	Ohai is a tool that is used to detect attributes on a node, and then provide these attributes to the chef-client at the start of every chef-client run. Ohai is required by the chef-client and must be present on a node. (Ohai is installed on a node as part of the chef-client install process.) The types of attributes Ohai collects include (but are not limited to): Platform details Network usage Memory usage CPU data Kernel data Host names Fully qualified domain names Other configuration details Attributes that are collected by Ohai are automatic attributes, in that these attributes are used by the chef-client to ensure that these attributes remain unchanged after the chef-client is done configuring the node.

The chef-client Run

A “chef-client run” is the term used to describe a series of steps that are taken by the chef-client when it is configuring a node. The following diagram shows the various stages that occur during the chef-client run, and then the list below the diagram describes in greater detail each of those stages.

During every chef-client run, the following happens:

Stages	Description
Get configuration data	The chef-client gets process configuration data from the client.rb file on the node, and then gets node configuration data from Ohai. One important piece of configuration data is the name of the node, which is found in the node_name attribute in the client.rb file or is provided by Ohai. If Ohai provides the name of a node, it is typically the FQDN for the node, which is always unique within an organization.
Authenticate to the Chef Server	The chef-client authenticates to the Chef server using an RSA private key and the Chef server API. The name of the node is required as part of the authentication process to the Chef server. If this is the first chef-client run for a node, the chef-validator will be used to generate the RSA private key.
Get, rebuild the node object	The chef-client pulls down the node object from the Chef server. If this is the first chef-client run for the node, there will not be a node object to pull down from the Chef server. After the node object is pulled down from the Chef server, the chef-client rebuilds the node object. If this is the first chef-client run for the node, the rebuilt node object will contain only the default run-list. For any subsequent chef-client run, the rebuilt node object will also contain the run-list from the previous chef-client run.
Expand the run-list	The chef-client expands the run-list from the rebuilt node object, compiling a full and complete list of roles and recipes that will be applied to the node, placing the roles and recipes in the same exact order they will be applied. (The run-list is stored in each node object’s JSON file, grouped under run_list.)
Synchronize cookbooks	The chef-client asks the Chef server for a list of all cookbook files (including recipes, templates, resources, providers, attributes, libraries, and definitions) that will be required to do every action identified in the run-list for the rebuilt node object. The Chef server provides to the chef-client a list of all of those files. The chef-client compares this list to the cookbook files cached on the node (from previous chef-client runs), and then downloads a copy of every file that has changed since the previous chef-client run, along with any new files.
Reset node attributes	All attributes in the rebuilt node object are reset. All attributes from attribute files, environments, roles, and Ohai are loaded. Attributes that are defined in attribute files are first loaded according to cookbook order. For each cookbook, attributes in the default.rb file are loaded first, and then additional attribute files (if present) are loaded in lexical sort order. All attributes in the rebuilt node object are updated with the attribute data according to attribute precedence. When all of the attributes are updated, the rebuilt node object is complete.
Compile the resource collection	The chef-client identifies each resource in the node object and builds the resource collection. Libraries are loaded first to ensure that all language extensions and Ruby classes are available to all resources. Next, attributes are loaded, followed by lightweight resources, and then all definitions (to ensure that any pseudo-resources within definitions are available). Finally, all recipes are loaded in the order specified by the expanded run-list. This is also referred to as the “compile phase”.
Converge the node	The chef-client configures the system based on the information that has been collected. Each resource is executed in the order identified by the run-list, and then by the order in which each resource is listed in each recipe. Each resource in the resource collection is mapped to a provider. The provider examines the node, and then does the steps necessary to complete the action. And then the next resource is processed. Each action configures a specific part of the system. This process is also referred to as convergence. This is also referred to as the “execution phase”.
Update the node object, process exception and report handlers	When all of the actions identified by resources in the resource collection have been done, and when the chef-client run finished successfully, the chef-client updates the node object on the Chef server with the node object that was built during this chef-client run. (This node object will be pulled down by the chef-client during the next chef-client run.) This makes the node object (and the data in the node object) available for search. The chef-client always checks the resource collection for the presence of exception and report handlers. If any are present, each one is processed appropriately.
Stop, wait for the next run	When everything is configured and the chef-client run is complete, the chef-client stops and waits until the next time it is asked to run.

Authentication

All communication with the Chef server must be authenticated using the Chef server API, which is a REST API that allows requests to be made to the Chef server. Only authenticated requests will be authorized. Most of the time, and especially when using knife, the chef-client, or the Chef server web interface, the use of the Chef server API is transparent. In some cases, the use of the Chef server API requires more detail, such as when making the request in Ruby code, with a knife plugin, or when using cURL.

The authentication process ensures the Chef server responds only to requests made by trusted users. Public key encryption is used by the Chef server. When a node and/or a workstation is configured to run the chef-client, both public and private keys are created. The public key is stored on the Chef server, while the private key is returned to the user for safe keeping. (The private key is a .pem file located in the .chef directory or in /etc/chef.)

Both the chef-client and knife use the Chef server API when communicating with the Chef server. The chef-validator uses the Chef server API, but only during the first chef-client run on a node.

Each request to the Chef server from those executables sign a special group of HTTP headers with the private key. The Chef server then uses the public key to verify the headers and verify the contents.

chef-validator

Every request made by the chef-client to the Chef server must be an authenticated request using the Chef server API and a private key. When the chef-client makes a request to the Chef server, the chef-client authenticates each request using a private key located in /etc/chef/client.pem.

However, during the first chef-client run, this private key does not exist. Instead, the chef-client will attempt to use the private key assigned to the chef-validator, located in /etc/chef/validation.pem. (If, for any reason, the chef-validator is unable to make an authenticated request to the Chef server, the initial chef-client run will fail.)

During the initial chef-client run, the chef-client will register with the Chef server using the private key assigned to the chef-validator, after which the chef-client will obtain a client.pem private key for all future authentication requests to the Chef server.

After the initial chef-client run has completed successfully, the chef-validator is no longer required and may be deleted from the node. Use the delete_validation recipe found in the chef-client cookbook (https://github.com/chef-cookbooks/chef-client) to remove the chef-validator.

SSL Certificates

An SSL certificate is used between the chef-client and the Chef server to ensure that each node has access to the right data.

Signed Headers

Signed header authentication is used to validate communications between the Chef server and any node that is being managed by the Chef server. An API client manages each authentication request. A public and private key pair is used for the authentication itself. The public key is stored in the database on the Chef server. The private key is stored locally on each node and is kept separate from node data (typically in the /etc/chef/client.pem directory). Each request to the Chef server by a node must include a request signature in the HTTP headers. This signature is computed from a hash of request content and is encrypted using the private key.

During a chef-client Run

As part of every chef-client run, the chef-client authenticates to the Chef server using an RSA private key and the Chef server API.

SSL Verification

Chef server 12 enables SSL verification by default for all requests made to the server, such as those made by knife and the chef-client. The certificate that is generated during the installation of the Chef server is self-signed, which means the certificate is not signed by a trusted certificate authority (CA) that ships with the chef-client. The certificate generated by the Chef server must be downloaded to any machine from which knife and/or the chef-client will make requests to the Chef server.

For example, without downloading the SSL certificate, the following knife command:

$ knife client list

responds with an error similar to:

ERROR: SSL Validation failure connecting to host: chef-server.example.com ...
ERROR: OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3 ...

This is by design and will occur until a verifiable certificate is added to the machine from which the request is sent.

Changes Prior to Chef 12

The following changes were made during certain chef-client release prior to the chef-client 12 release:

• In the chef-client 11.8 release, the verify_api_cert setting was added to the client.rb file with a default value of false.

• In the chef-client 11.12 release, the local_key_generation setting was added to the client.rb file.

  The ssl_verify_mode continued to default to :verify_none, but now returned a warning: SSL validation of HTTPS requests is disabled..., followed by steps for how to configure SSL certificate validation for the chef-client.

  Two knife commands—knife ssl check and knife ssl fetch were added.

  A new directory in the chef-repo—/.chef/trusted_certs—was added.

  These new settings and tools enabled users who wanted to use stronger SSL settings to generate the private/public key pair from the chef-client, verify HTTPS requests, verify SSL certificates, and pull the SSL certificate from the Chef server down to the /.chef/trusted_certs directory.

• In the chef-client 12 release, the default value for local_key_generation was changed to true and the default value for ssl_verify_mode was changed to :verify_peer.

Starting with chef-client 12, SSL certificate validation is enabled by default and the knife ssl fetch is a necessary part of the setup process for every workstation.

/.chef/trusted_certs

The /.chef/trusted_certs directory stores trusted SSL certificates used to access the Chef server:

• On each workstation, this directory is the location into which SSL certificates are placed after they are downloaded from the Chef server using the knife ssl fetch subcommand
• On every node, this directory is the location into which SSL certificates are placed when a node has been bootstrapped with the chef-client from a workstation

SSL_CERT_FILE

Use the SSL_CERT_FILE environment variable to specify the location for the SSL certificate authority (CA) bundle that is used by the chef-client.

A value for SSL_CERT_FILE is not set by default. Unless updated, the locations in which Chef will look for SSL certificates are:

• chef-client: /opt/chef/embedded/ssl/certs/cacert.pem
• Chef development kit: /opt/chefdk/embedded/ssl/certs/cacert.pem

Keeping the default behavior is recommended. To use a custom CA bundle, update the environment variable to specify the path to the custom CA bundle. If (for some reason) SSL certificate verification stops working, ensure the correct value is specified for SSL_CERT_FILE.

client.rb Settings

Use following client.rb settings to manage SSL certificate preferences:

Setting	Description
local_key_generation	Whether the Chef server or chef-client generates the private/public key pair. When true, the chef-client generates the key pair, and then sends the public key to the Chef server. Default value: true.
ssl_ca_file	The file in which the OpenSSL key is saved. This setting is generated automatically by the chef-client and most users do not need to modify it.
ssl_ca_path	The path to where the OpenSSL key is located. This setting is generated automatically by the chef-client and most users do not need to modify it.
ssl_client_cert	The OpenSSL X.509 certificate used for mutual certificate validation. This setting is only necessary when mutual certificate validation is configured on the Chef server. Default value: nil.
ssl_client_key	The OpenSSL X.509 key used for mutual certificate validation. This setting is only necessary when mutual certificate validation is configured on the Chef server. Default value: nil.
ssl_verify_mode	Set the verify mode for HTTPS requests. Use :verify_none to do no validation of SSL certificates. Use :verify_peer to do validation of all SSL certificates, including the Chef server connections, S3 connections, and any HTTPS remote_file resource URLs used in the chef-client run. This is the recommended setting. Depending on how OpenSSL is configured, the ssl_ca_path may need to be specified. Default value: :verify_peer.
verify_api_cert	Verify the SSL certificate on the Chef server. When true, the chef-client always verifies the SSL certificate. When false, the chef-client uses the value of ssl_verify_mode to determine if the SSL certificate requires verification. Default value: false.

Bootstrap Operations

The knife bootstrap command is a common way to install the chef-client on a node. The default for this approach assumes that a node can access the Chef website so that it may download the chef-client package from that location.

The omnibus installer will detect the version of the operating system, and then install the appropriate version of the chef-client using a single command to install the chef-client and all of its dependencies, including an embedded version of Ruby, RubyGems, OpenSSL, key-value stores, parsers, libraries, and command line utilities.

The omnibus installer puts everything into a unique directory (/opt/chef/) so that the chef-client will not interfere with other applications that may be running on the target machine. Once installed, the chef-client requires a few more configuration steps before it can perform its first chef-client run on a node.

A node is any physical, virtual, or cloud machine that is configured to be maintained by a chef-client. A bootstrap is a process that installs the chef-client on a target system so that it can run as a chef-client and communicate with a Chef server. There are two ways to do this:

• Use the knife bootstrap subcommand to bootstrap a node using the omnibus installer
• Use an unattended install to bootstrap a node from itself, without using SSH or WinRM

The following diagram shows the stages of the bootstrap operation, and then the list below the diagram describes in greater detail each of those stages.

During a knife bootstrap bootstrap operation, the following happens:

Stages	Description
$ knife bootstrap	On UNIX- and Linux-based machines: The knife bootstrap subcommand is issued from a workstation. The hostname, IP address, or FQDN of the target node is issued as part of this command. An SSH connection is established with the target node using port 22. A shell script is assembled using the chef-full.erb (the default bootstrap template), and is then executed on the target node. On Microsoft Windows machines: The knife bootstrap windows winrm subcommand is issued from a workstation. (This command is part of the knife windows plugin .) The hostname, IP address, or FQDN of the target node is issued as part of this command. A connection is established with the target node using WinRM over port 5985. (WinRM must be enabled with the corresponding firewall rules in place.)
Get the install script from Chef	On UNIX- and Linux-based machines: The shell script that is derived from the chef-full.erb bootstrap template will make a request to the Chef website to get the most recent version of a second shell script (install.sh). On Microsoft Windows machines: The batch file that is derived from the windows-chef-client-msi.erb bootstrap template will make a request to the Chef website to get the .msi installer.
Get the chef-client package from Chef	The second shell script (or batch file) then gathers system-specific information and determines the correct package for the chef-client. The second shell script (or batch file) makes a request to the Chef website, and then downloads the appropriate package from s3.amazonaws.com.
Install the chef-client	The chef-client is installed on the target node.
Start the chef-client run	On UNIX- and Linux-based machines: The second shell script executes the chef-client binary with a set of initial settings stored within first-boot.json on the node. first-boot.json is generated from the workstation as part of the initial knife bootstrap subcommand. On Microsoft Windows machines: The batch file that is derived from the windows-chef-client-msi.erb bootstrap template executes the chef-client binary with a set of initial settings stored within first-boot.json on the node. first-boot.json is generated from the workstation as part of the initial knife bootstrap subcommand.
Complete the chef-client run	The chef-client run proceeds, using HTTPS (port 443), and registers the node with the Chef server. The first chef-client run, by default, contains an empty run-list. A run-list can be specified as part of the initial bootstrap operation using the --run-list option as part of the knife bootstrap subcommand.