NOTE: If you intend on using techniques such as these and allowing such wide open functionality in Jenkins, I recommend that you run your entire Jenkins build system without outbound internet access by default. Allow access from the build system network segment only to approved endpoints while dropping the rest of the traffic. This will allow you to use potentially dangerous, but extremely powerful scripts while maintaining a high level of security in the system.

API Calls

Jenkinsfile calling Docker API

Making HTTP calls in a Jenkinsfile can prove tricky as everything has to remain serializable else Jenkins cannot keep track of the state when needing to restart a pipeline. If you’ve ever received Jenkins’ error in your console out, you know what I mean. There are not too many clear-cut examples of remote API calls from a Jenkinsfile so I’ve created an example Jenkinsfile that will talk to the Docker Hub API using GET and POST to perform authenticated API calls.

Explicit Versioning

The Docker image build process leaves a good amount to be desired when it comes to versioning. Most workflows depend on the :latest tag which is very ambiguous and can lead to problems being swallowed within your build system. In order to maintain a higher level of determinism and auditability, you may consider creating your own versioning scheme for Docker images.

For instance, a version of 2017.2.1929 #<year>-<week>-<build #> can express much more information than a simple latest. Having this information available for audits or tracking down when a failure was introduced can be invaluable, but there is no built-in way to do Docker versioning in Jenkins. One must rely on an external system (such as Docker Hub or their internal registry) to keep track of versions and use this system of record when promoting builds.

Versioning Scheme

This versioning scheme we are using is not based on Semver1, but it does encode within it the information we need to keep versions in lock and also will always increase in value. Even if the build number is reset, the date + week will keep the versions from ever being lower that the day previously. Version your artifacts however works for your release, but please make sure of these two things:

  • The version string never duplicates
  • The version number never decreases

Interacting with the Docker Hub API in a Jenkinsfile

For this example we are going to connect to the Docker Hub REST API in order to retrieve some tags and promote a build to RC. This type of workflow would be implemented in a release job in which a previously built Docker image is being promoted to a release candidate. The steps we take in the Jenkinsfile are:

Docker Registry

  • Provision a node
    • Stage 1
      • Make an HTTP POST request to the Docker Hub to get an auth token
      • Use the token to fetch the list of tags on an image
      • Filter through those tags to find a tag for the given build #
    • Stage 2
      • Promote (pull, tag, and push) the tag found previously as ${version}-rc
      • Push that tag to latest to make it generally available

This is a fairly complex looking Jenkinsfile as it stands, but these functions can be pulled out into a shared library2 to simplify the Jenkinsfile. We’ll talk about that in another post.


 NOTE: This Jenkinsfile has the following pre-requisites:
   - SECRET (id: docker-hub-user-pass): Username / Password secret containing your
     Docker Hub username and password.
   - ENVIRONMENT: Docker commands should work meaning DOCKER_HOST is set or there
     is access to the socket.
import groovy.json.JsonSlurperClassic; // Required for parseJSON()

// These vars would most likely be set as parameters
imageName = "technolog/serviceone"
build = "103"

// Begin our Scripted Pipeline definition by provisioning a node
node() {

    // First stage sets up version info
    stage('Get Docker Tag from Build Number') {

        // Expose our user/pass credential as vars
        withCredentials([usernamePassword(credentialsId: 'docker-hub-user-pass', passwordVariable: 'pass', usernameVariable: 'user')]) {
            // Generate our auth token
            token = getAuthTokenDockerHub(user, pass)

        // Use our auth token to get the tag
        tag = getTagFromDockerHub(imageName, build, token)

    // Example second stage tags version as -release and pushes to latest
    stage('Promote build to RC') {
        // Enclose in try/catch for cleanup
        try {
            // Define our versions
            def versionImg = "${imageName}:${tag}"
            def latestImg = "${imageName}:latest"

            // Login with our Docker credentials
            withCredentials([usernamePassword(credentialsId: 'docker-hub-user-pass', passwordVariable: 'pass', usernameVariable: 'user')]) {
                sh "docker login -u${user} -p${pass}"

            // Pull, tag, + push the RC
            sh "docker pull ${versionImg}"
            sh "docker tag ${versionImg} ${versionImg}-rc"
            sh "docker push ${versionImg}-rc"

            // Push the RC to latest as well
            sh "docker tag ${versionImg} ${latestImg}"
            sh "docker push ${latestImg}"
        } catch(err) {
            // Display errors and set status to failure
            echo "FAILURE: Caught error: ${err}"
            currentBuild.result = "FAILURE"
        } finally {
            // Finally perform cleanup
            sh 'docker system prune -af'

// NOTE: Everything below here could be put into a shared library

// GET Example
// Get a tag from Docker Hub for a given build number
def getTagFromDockerHub(imgName, build, authToken) {

    // Generate our URL. Auth is required for private repos
    def url = new URL("${imgName}/tags")
    def parsedJSON = parseJSON(url.getText(requestProperties:["Authorization":"JWT ${authToken}"]))

    // We want to find the tag associated with a build
    // EX: 2017.2.103 or 2016.33.23945
    def regexp = "^\\d{4}.\\d{1,2}.${build}\$"

    // Iterate over the tags and return the one we want
    for (result in parsedJSON.results) {
        if ( {

// POST Example
// Get an Authentication token from Docker Hub
def getAuthTokenDockerHub(user, pass) {

    // Define our URL and make the connection
    def url = new URL("")
    def conn = url.openConnection()
    // Set the connection verb and headers
    conn.setRequestProperty("Content-Type", "application/json")
    // Required to send the request body of our POST
    conn.doOutput = true

    // Create our JSON Authentication string
    def authString = "{\"username\": \"${user}\", \"password\": \"${pass}\"}"

    // Send our request
    def writer = new OutputStreamWriter(conn.outputStream)

    // Parse and return the token
    def result = parseJSON(conn.content.text)
    return result.token


// Contain our JsonSlurper in a function to maintain CPS
def parseJSON(json) {
    return new groovy.json.JsonSlurperClassic().parseText(json)

Script Security

Due to the nature of this type of script, there is definitely a lot of trust assumed when allowing something like this to run. If you follow the process we are doing in Modern Jenkins nothing is getting into the build system without peer review and nobody but administrators have access to run scripts like this. With the environment locked down, it can be safe to use something of this nature.

Security Alert

Jenkins has two ways in which Jenkinsfiles (and Groovy in general) can be run: sandboxed or un-sandboxed. After reading Do not disable the Groovy Sandbox by rtyler (@agentdero on Twitter), I will never disable sandbox again. What we are going to do instead is whitelist all of the required signatures automatically with Groovy. The script we are going to use is adapted from my friend Brandon Fryslie and will basically pre-authorize all of the required methods that the pipeline will use to make the API calls.

Pre-authorizing Jenkins Signatures with Groovy

URL: http://localhost:8080/script

import org.jenkinsci.plugins.scriptsecurity.scripts.ScriptApproval

println("INFO: Whitelisting requirements for Jenkinsfile API Calls")

// Create a list of the required signatures
def requiredSigs = [
    'method groovy.json.JsonSlurperClassic parseText java.lang.String',
    'method flush',
    'method write java.lang.String',
    'method java.lang.AutoCloseable close',
    'method setRequestMethod java.lang.String',
    'method openConnection',
    'method connect',
    'method getContent',
    'method getOutputStream',
    'method setDoOutput boolean',
    'method setRequestProperty java.lang.String java.lang.String',
    'new groovy.json.JsonSlurperClassic',
    'staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods findAll java.lang.String java.lang.String',
    'staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods getText',
    'staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods getText java.util.Map',

    // Signatures already approved which may have introduced a security vulnerability (recommend clearing):
    'method openConnection',

// Get a handle on our approval object
approver = ScriptApproval.get()

// Aprove each of them
requiredSigs.each {

println("INFO: Jenkinsfile API calls signatures approved")

After running this script, you can browse to Manage Jenkins -> In-process Script Approval and see that there is a list of pre-whitelisted signatures that will allow our Jenkinsfile to make the necessary calls to interact with the Docker Hub API. You’ll notice there is one method in there that they mark in red as you can see the potential security issues with it. openConnection can be an extremely dangerous method to allow in an unrestricted environment so be careful and make sure you have things locked down in other ways.

Modern Jenkins Unit 3 / Part 4: Configuring the GitHub Jenkins Plugin

GitHub + Jenkins

Setting up the Jenkins <–> Git Interactions

One task that just about every build system needs to complete before doing anything is getting the code that it will be testing and packaging. For most software development shops in the world, this code is kept in a source code repository in a version control system (VCS). The most common implementation that I have been exposed to is Git. Linus Torvalds with a few kernel developers wrote the initial version of git while working on the kernel after BitKeeper’s copyright holder revoked free use1. Git is what powers GitHub (surprise, surprise) and is just about the most popular VCS out there. We are even using it to develop our Jenkins CI system!

In order to interact with GitHub, we are going to need a couple of things setup:

Bender Rodriguez

  • A machine user that can be managed independently of your customers
  • A SSH key secret in the credentials store
  • Configuration that says to use the credentials in the store for cloning

Once we have these in place we should be able to create a job that clones a repo and does something.

Creating a machine user

Teams of developers change constantly with people being added and removed from the roster quite frequently. To use credentials that are tied to a person is not good practice as it does not separate the actions of the user and the actions of the system. For this reason we will create our own machine user in GitHub and configure her to do the builds.

Note: Do not stage or commit anything during this step. We will be encrypting the sensitive files in the next step.

  • Browse to in an Incognito window (we need to create a new account and you’re probably logged in).

  • Create an account for your machine user. It is not against GitHub’s T.O.S.2 to create a machine user, but it is to script this process. You can’t reuse your existing email, so you must have a secondary email setup for this user.

  • Verify the email and then browse to Settings in the upper right hand corner.

  • Fill out the info that makes sense and then head over to SSH and GPG Keys

  • We now need to create an ssh-key for the machine user. We will use a new type of key based on this article:

PWD: ~/code/modern-jenkins/

# cd into our secrets dir
cd secure

# Generate a ed25519 key in the new RFC4716 format with a random good passphrase.
# Something like: 't0$VQki3RWVim!K*rzA1' and then make sure to note the pw. We
# will be using it in a moment
ssh-keygen -o -a 100 -t ed25519 -f git-ssh-key.priv

# Rename the pubkey

# Record that passphrase we used to generate the key in secure/

# Record the GitHub username for the user in git-ssh-key.user
echo "CICDLifeBuildBot" > git-ssh-key.user

  • Now add the contents of to the SSH Keys in your GitHub Settings page with a good name. We now have a machine user and need to configure Jenkins to use it.

GitHub SSH Key

Add the secrets to the credential store

Add to Credential Store

Now that we have the Git credentials that should in theory work for cloning and pushing we need to get them added into the Jenkins credential store. How we will do this is via the init system Groovy and some Docker volume mounts.

Mount the secrets into the master container

Since we are still in development, let’s just add our secure dir to the compose file as a mount. When we get to prod we will change this up.


#  # Jenkins master's configuration
#  master:
#    image: mastering-jenkins/jenkins-master
#    ports:
#      - "8080:8080"
#    volumes:
#      - plugins:/usr/share/jenkins/ref/plugins
#      - warfile:/usr/share/jenkins/ref/warfile
       - ${PWD}/../../secure:/secure:ro
#  # Jenkins plugins configuration
#  plugins:

Restart the service using our script

PWD: ~/code/modern-jenkins/deploy/master

# Restart

# Confirm that the files have been mounted where we expect
docker exec -t master_master_1 ls /secure  git-ssh-key.priv  git-ssh-key.user

Restarting the master will mount the directory ~/code/modern-jenkins/secure at /secure inside the container. Since we specified ro at the end of the VOLUME definition, the secrets themselves will be read-only. With the secrets loaded into the environment, we can begin developing a script that configures GitHub.

Writing Groovy to configure GitHub

Configuring Jenkins plugins with Groovy can seem to be more like an art than a science. Each plugin operates a little bit differently and since Jenkins has been so popular for so long, plugins are in all different states of repair. The basic gist of how to do any plugin is:

  • Read the values in from the filesystem or ENV that you need to configure
  • Get a handle on the configuration object for the plugin
  • Create a new instance of the configuration objects with the values you want
  • Update the main configuration with your newly created one
  • Save the config object

Any plugin can be configured this way and it’s normally a matter of familiarizing yourself with the plugin’s data models + classes enough to create what you need. Let’s take a look at the (well documented) Groovy script that configures our GitHub plugin.

URL: http://localhost:8080/script


// 02-configure-git-client.groovy
// Thanks to chrish:
import jenkins.model.*
import com.cloudbees.jenkins.plugins.sshcredentials.impl.*
import com.cloudbees.plugins.credentials.*;

// Read our values into strings from the volume mount
privKeyText = new File('/secure/git-ssh-key.priv').text.trim()
passPhraseText = new File('/secure/').text.trim()
sshUserText = new File('/secure/git-ssh-key.user').text.trim()

// Get a handle on our Jenkins instance
def jenkins = Jenkins.getInstance()

// Define the security domain. We're making these global but they can also
// be configured in a more restrictive manner. More on that later
def domain =

// Get our existing Credentials Store
def store = jenkins.getExtensionList(

// Create a new BasicSSHUserPrivateKey object with our values
gitHubSSHKey = new BasicSSHUserPrivateKey(
  new BasicSSHUserPrivateKey.DirectEntryPrivateKeySource(privKeyText),
  "GitHub Machine User SSH Creds"

// Add the new object to the credentials store
store.addCredentials(domain, gitHubSSHKey)

// Get the config descriptor for the overall Git config
def desc = jenkins.getDescriptor("hudson.plugins.git.GitSCM")

// Set the username and email for git interactions

// Save the descriptor

// Echo out (or log if you like) that we've done something
println("INFO: GitHub Credentials and Configuration complete!")

If things are setup and working right, you should see our INFO statement printed out below the console. To confirm it worked, browse to the credentials store confirm that we now have a SSH Key secret with the id of git-ssh-key. If you click on “Update” you should see the SSH key (don’t worry, it is armored and you can’t show the passphrase from here). While this is not 100% secure, it is still much better than baking the credentials into the image or a lot of other methods people use to expose secrets. Incremental improvements are what I always say!

Add the groovy to the Docker image

After confirming that the script works, add it to the plugins Docker image by dropping it in images/jenkins-plugins/files/init.groovy.d with the name of 02-configure-github-client.groovy. Then you can rebuild the image and restart the service to verify that everything is working on boot.

PWD: ~/code/modern-jenkins/

# Create images/jenkins-plugins/files/init.groovy.d/02-configure-github-client.groovy
# with the content from the script console.

# Build the image

# Restart the service
cd deploy/master
docker-compose logs -f

Watch the log output from the containers to confirm that the scripts are running (the name of the scripts is output in the console out) and that there are no stack traces or other unexpected errors. When the system is fully up and running, browse to the GUI and check to see if the credential has been installed. I think you’ll be pleasantly surprised!

Testing our changes

We should in theory be able to test our changes by creating a tiny little job that just clones a repo. Follow me and we’ll give it a run for it’s money:

  • Browse to http://localhost:8080
  • Click New Item in the top left
  • Name it what you like, select freestyle project, and click OK. This will drop you at the configuration screen.
  • Under Source Code Management, select Git
  • For the Repository URL enter:
  • For the Credentials choose GitHub Machine User SSH Creds Git Config
  • Scroll down to Build and add a shell step
  • Within the shell step type echo "INFO: dir contents are" && ls Shell Step
  • Save the job and run it
  • The job should be able to clone the repo and list the directory contents. If this is not the case, you must debug what is happening here and try to get it working again. It is critical that the changes we make in each part are working as expected. Console Out

Congratulations! You have just performed your first and certainly not last programmatic configuration of Jenkins! Give yourself a pat on the back because that was no easy feat.

Commit, Push, PR

Take extra care when pushing branches that are supposed to have encrypted secrets. Sometimes accidents happen and you want to know about them immediately. Since we have added a bunch of secrets in this branch, pay attention to make sure that all of them are encrypted and none are plaintext.

If you got wicked stuck (or even a little bit off track), take a look here to see the repo’s state at the end of this post.

Well, we can definitely run a job now so that’s good news. The bad news is that we created it by hand (yuck!). The next installment will introduce you to a way that eliminates use of the GUI for creating jobs. I personally think it has completely changed the way CI systems are built and I would never hand create a job EVER now.

Modern Jenkins Unit 3 / Part 3: Managing Secrets in GitHub

Encrypting files in the repo with Transcrypt


In production we will hopefully have a secrets management system, or at a very minimum private repos to store encrypted secrets in. For the purpose of this demo though I will be using a public repo and do not want to expose any sensitive data that we may need to add to this repo. WARNING: To be clear, storing any kind of secrets in a git repo, encrypted or not, may not be a good idea. Consult your local security team for advice. We however, don’t have any state secrets for this demo and very few options.


Transcrypt is a shell script that uses OpenSSL to encrypt and decrypt files in your git repo that are noted in the .gitattributes file. Let’s initialize our repo and confirm that it is working.

PWD: ~/code/modern-jenkins/

# On MacOS you can use brew
brew install transcrypt

# On Linux you have to install OpenSSL then place the script in your path
yum install openssl
# Download the script to our PATH
wget -O /usr/local/sbin/transcrypt
# Make it executable
sudo chmod +x /usr/local/sbin/transcrypt

# Confirm it works
trancrypt --help

NOTE: More information on the software can be found on the README here:

Initialize the repo

We will initialize Transcrypt within the repo on a clean branch. This will hopefully ensure that we can configure Transcrypt prior to adding any secrets to the repo.

PWD: ~/code/modern-jenkins/

# Check out a branch and initialize transcrypt
git checkout -b chore-install_transcrypt

# Encrypt using which cipher? [aes-256-cbc]
# Generate a random password? [Y/n]
# Repository metadata:
#   GIT_WORK_TREE:  /Users/matt.bajor/code/modern-jenkins
#   GIT_DIR:        /Users/matt.bajor/code/modern-jenkins/.git
#   GIT_ATTRIBUTES: /Users/matt.bajor/code/modern-jenkins/.gitattributes
# The following configuration will be saved:
#   CIPHER:   aes-256-cbc
#   PASSWORD: <a really good password, trust me>
# Does this look correct? [Y/n]
# The repository has been successfully configured by transcrypt.

Add the generated PASSWORD to your LastPass, keychain, text document on your desktop, or wherever you store secure passwords. It is very important to not lose that passphrase.

We now want to get the .gitattributes file added up and merged in so we can test if secrets encryption is working as expected so let’s branch, commit, push and PR.

PWD: ~/code/modern-jenkins/

git add .
git commit -m "Initialize Transcrypt in the repo"
git push origin chore-install_transcrypt

# PR, review, merge and then catchup master
git checkout master
git pull
git checkout -b test-secrets

Let’s create a secrets dir and initialize it with a README to confirm that transcrypt is working as expected without potentially exposing secrets on the internet. The way transcrypt works is that it looks at the .gitattributes in the repo root for files to encrypt. If it sees that there are paths to encrypt, it will encrypt them before adding to the tree. It does still allow you to view the changes as plaintext locally which sometimes can lead to confusion as to whether or not its working.

PWD: ~/code/modern-jenkins/

# Look at the .gitattributes
cat .gitattributes
# #pattern  filter=crypt diff=crypt
# Echo our new pattern in there
echo 'secure/* filter=crypt diff=crypt' >> .gitattributes
# Confirm that the file looks good:
cat .gitattributes
# #pattern  filter=crypt diff=crypt
# secure/* filter=crypt diff=crypt 

# Add a README with a bit of info
mkdir -p secure/
echo "# Transcrypt Secrets\n\n This repo is encrypted with transcrypt" > secure/

# Commit and push
git add .
git commit -m "Add test secrets"
git push origin test-secrets

When opening the PR, you should notice that there are two files changed:

  • .gitattributes
  • secure/

Transcrypt Commit

You should also notice that the contents of is encrypted. If this is the case, give yourself a thumbs up, merge the PR, and update your local branch.


NOTE: If you see that is not encrypted, do not open the PR. Instead, delete your branch and start over. Follow the instructions on the Transcrypt site if you need more help or information. It is important to confirm that the encryption mechanism is working the way we expect before we put secrets into the repo.

Let’s take this time to add a line to the file reminding us to check for unencrypted secrets:


# #### Contribution checklist
# - [ ] The branch is named something meaningful
# - [ ] The branch is rebased off of current master}

Now that the repo has been initialized with Transcrypt, we can begin adding some configuration that depends on credentials. In the next segment, we will create a machine user for GitHub and configure the Git plugin to use those credentials when cloning.

The repo from this section can be found under the unit3-part3 tag here:

Next Post: Configuring the Jenkins GitHub plugin programmatically with Groovy