Simple build triggers with secured Jenkins CI

The jenkins continuous integration (CI) server provides several ways to trigger builds remotely, for example from a git hook. Things are easy on an open jenkins instance without security enabled. It gets a little more complicated if you like to protect your jenkins build environment.

Git plugin notify commit url

For git there is the “notifyCommitUrl” you can use in combination with the Poll SCM settings:


Note two things regarding this approach:

  1. The url of the source code repository given as a parameter must match the repository url of the jenkins job.
  2. You have to check the Poll SCM setting, but you do not need to provide a schedule

Another drawback is its restriction to git-hosted jobs.

Jenkins remote access api

Then there is the more general and more modern jenkins remote access api, where you may trigger builds regardless of the source code management system you use.
curl -X POST $JENKINS_URL/job/$JOB_NAME/build?token=$TOKEN

It allows even triggering parameterized builds with HTTP POST requests like:

curl -X POST $JENKINS_URL/job/$JOB_NAME/build \
--user USER:TOKEN \
--data-urlencode json='{"parameter": [{"name":"id", "value":"123"}, {"name":"verbosity", "value":"high"}]}'

Both approaches work great as long as your jenkins instance is not secured and everyone can do everything. Such a setting may be fine in your companies intranet but becomes a no-go in more heterogenious environments or with a public jenkins server.

So the way to go is securing jenkins with user accounts and restricted access. If you do not want to supply username/password as part of the url for doing HTTP BASIC auth and create users just for your repository triggers there is another easy option:

Using the Build Authorization Token Root Plugin!

Build authorization token root plugin

The plugin introduces a configuration setting in the Build triggers section to define an authentication token:

It also exposes a url you can access without being logged in to trigger builds just providing the token specified in the job:


Or for parameterized builds something like:



The token root plugin does not need HTTP POST requests but also works fine using HTTP GET. It does neither requires a user account nor the awkward Poll SCM setting. In my opinion it is the most simple and pragmatic choice for build triggering on a secured jenkins instance.

Self-contained projects in python

An important concept for us is the notion of self-containment. For a project in development this means you find everything you need to develop and run the software directly in the one repository you check out/clone. For practical reasons we most of the time omit the IDE and the basic runtime like Java JDK or the Python interpreter. If you have these installed you are good to go in seconds.

What does this mean in general?

Usually this means putting all your dependencies either in source or object form (dll, jar etc.) directly in a directory of your project repository. This mostly rules out dependency managers like maven. Another not as obvious point is to have hardware dependencies mocked out in some way so your software runs without potentially unavailable hardware attached. The same is true for software services somewhere on the net that may be unavailable, like a payment service for example.

How to do it for Python

For Python projects this means not simply installing you dependencies using the linux package manager, system-wide pip or other dependency management tools but using a virtual environment. Virtual environments are isolated Python environments using an available, but defined Python interpreter on the system. They can be created by the tool virtualenv or since Python 3.3 the included tool venv. You can install you dependencies into this environment e.g. using pip which itself is part of the virtualenv. Preparing a virtual env for your project can be done using a simple shell script like this:

python2.7 ~/my_project/vendor/virtualenv-15.1.0/ ~/my_project_env
source ~/my_project_env/bin/activate
pip install ~/my_project/vendor/setuptools_scm-1.15.0.tar.gz
pip install ~/my_project/vendor/six-1.10.0.tar.gz

Your dependencies including virtualenv (for Python installations < 3.3) are stored into the projects source code repository. We usually call the directory vendor or similar.

As a side note working with such a virtual env even remotely work like charm in the PyCharm IDE by selecting the Python interpreter of the virtual env. It correctly shows all installed dependencies and all the IDE support for code completion and imports works as expected:


What you get

With such a setup you gain some advantages missing in many other approaches:

  • No problems if the target machine has no internet access. This would be problematic to classical pip/maven/etc. approaches.
  • Mostly hassle free development and deployment. No more “downloading the internet” feeling or driver/hardware installation issues for the developer. A deployment is in the most simple cases as easy as a copy/rsync.
  • Only minimal requirements to the base installation of developer, build, deployment or other target machines.
  • Perfectly reproducable builds and tests in isolation. You continuous integration (CI) machine is just another target machine.

What it costs

There are costs of this approach of course but in our experience the benefits outweigh them by a great extent. Nevertheless I want to mention some downsides:

  • Less tool support for managing the dependencies, especially if your are used to maven and friends and happen to like them. Pip can work with local archives just fine but updating is a bit of manual work.
  • Storing (binary) dependencies in your repository increases the checkout size. Nowadays disk space and local network speeds make mostly irrelevant, especially in combination with git. Shallow-clones can further mitigate the problem.
  • You may need to put in some effort for implementing mocks for your hardware or third-party software services and a mechanism for switching between simulation and the real stuff.


We have been using self-containment to great success in varying environments. Usually, both developers and clients are impressed by the ease of development and/or installation using this approach regardless if the project is in Java, C++, Python or something else.

Evolvability of Code: Uniform Access Principle

Most programmers like freedom. So there are many means of hiding implementations in modern programming languages, e.g. interfaces in Java, header files in C/C++ and visibility modifiers like private and protected in most object-oriented languages. Even your ordinary functions or public class interface gives you the freedom to change the implementation without needing to touch the clients. Evolvability in this sense means you can change and refine your implementations without requiring others, namely clients of your code, to change.

Changing the class interface or function signatures within a project is often possible and feasible, at least if you have access to all client code and use powerful refactoring tools. If you published your code as a library or do not want to break all client code or forcing them to adapt to your changes you have to consider your interface code to be fixed. This takes away some of your precious freedom. So you have to design your interfaces carefully with evolability in mind.

Some programming languages implement the uniform access principle (UAP) that eases evolvability in that it allows you to migrate from public attributes to properties/method calls without changing the clients: Read and write access to the attribute uses the same syntax as invoking corresponding methods. For clarification an example in Python where you may start with a class like:

class Person(object):
  def __init__(self, name, age): = name
    self.age = age

Using the above class is trivial as follows

>>> pete = Person("pete", 32)
>>> print pete.age
# a year has passed
>>> pete.age = 33
>>> print pete.age

Now if the age is not a plain value anymore but needs checking, like always being greater zero or is calculated based on some calendar you can turn it to a property like so:

class Person(object):
  def __init__(self, name, age): = name
    self._age = age

  def age(self):
    return self._age

  def age(self, new_age):
    if new_age < 0:
      raise ValueError("Age under 0 is not possible")
    self._age = new_age

Now the nice thing is: The above client code still works without changes!

Scala uses a similar and quite concise mechanism for implementing the UAP wheres .NET provides some special syntax for properties but still migration from public fields easily possible.

So in languages supporting the UAP you can start really simple with public attributes holding the plain value without worrying about some potential future. If you later need more sophisticated stuff like caching, computation of the value, validation or even remote retrieval you can add it using language features without touching or bothering clients.

Unfortunately some powerful and widespread languages like Java and C++ lack support for UAP. Changing a public field to a more complex property means the introduction of getter and setter methods and changing all clients. Therefore you see, especially in Java, many data classes littered with trivial getter and setter pairs doing nothing interesting and introducing unnecessary bloat to maintain the evolvability of the code.

Using passwords with Jenkins CI server

For many of our projects the Jenkins continuous integration (CI) server is one important cornerstone. The well known “works on my machine” means nothing in our company. Only code in repositories and built, tested and packaged by our CI servers counts. In addition to building, testing, analyzing and packaging our projects we use CI jobs for deployment and supervision, too. In such jobs you often need some sort of credentials like username/password or public/private keys.

If you are using username/password they do not only appear in the job configuration but also in the console build logs. In most cases this is undesirable but luckily there is an easy way around it: using the Environment Injector Plugin.

In the plugin you can “inject passwords to the build as environment variables” for use in your commands and scripts.inject-passwords-configuration

The nice thing about this is that the passwords are not only masked in the job configuration (like above) but also in the console logs of the builds!inject-passwords-console-log

Another alternative doing mostly the same is the Credentials Binding Plugin.

There is a lot more to explore when it comes to authentication and credential management in Jenkins as you can define credentials at the global level, use public/private key pairs and ssh agents, connect to a LDAP database and much more. Just do not sit back and provide security related stuff plaintext in job configurations or your deployments scripts!

Introduction to Debian packaging

In former posts I wrote about packaging your software as RPM packages for a variety of use cases. The other big binary packaging system on Linux systems is DEB for Debian, Ubuntu and friends. Both serve the purpose of convenient distribution, installation and update of binary software artifacts. They define their dependencies and describe what the package provides.

How do you provide your software as DEB packages?

The master guide to debian packaging can be found at It is a very comprehensive guide spanning many pages and providing loads of information. Building a usable package of your software for your clients can be a matter of minutes if you know what to do. So I want to show you the basic steps, for refinement you will need to consult the guide or other resources specific to the software you want to package. For example there are several guides specific to packaging of software written in Python. I find it hard to determine what the current and recommended way to debian packages for python is because there are differing guides over the last 10 years or so. You may of course say “Just use pip for python” 🙂

Basic tools

The basic tools for building debian packages are debhelper and dh-make. To check the resulting package you will need lintian in addition. You can install them and basic software build tools using:

  sudo apt-get install build-essential dh-make debhelper lintian

The python packaging system uses make and several scripts under the hood to help building the binary packages out of source tarballs.

Your first package

First you need a tar-archive of the software you want to package. With python setuptools you could use python sdist to generate the tarball. Then you run dh_make on it to generate metadata and the package build environment for your package. Now you have to edit the metadata files, namely control, copyright, changelog. Finally you run dpkg-buildpackage to generate the package itself.  Here is an example of the necessary commands:

mkdir hello-deb-1.0
cd hello-deb-1.0
dh_make -f ../hello-deb-1.0.tar.gz
# edit deb metadata files
vi debian/control
vi debian/copyright
vi debian/changelog
dpkg-buildpackage -us -uc
lintian -i -I --show-overrides hello-deb_1.0-1_amd64.changes

The control file roughly resembles RPMs SPEC file. Package name, description, version and dependency information belong there. Note that debian is very strict when it comes to naming of packages, so make sure you use the pattern ${name}-${version}.tar.gz for the archive and that it extracts into a corresponding directory without the extension, e.g. ${name}-${version}.

If everything went ok several files were generated in your base directory:

  • The package itself as .deb file
  • A file containing the changelog and checksums between package versions and revisions ending with .changes
  • A file with the package description ending with .dsc
  • A tarball with the original sources renamed according to debian convention hello-deb_1.0.orig.tar.gz (note the underscore!)

Going from here

Of course there is a lot more to the tooling and workflow when maintaining debian packages. In future posts I will explore additional means for improving and updating your packages like the quilt patch management tool, signing the package, symlinking, scripts for pre- and post-installation and so forth.

4 questions you need to ask yourself constantly while programming

Most of today’s general purpose progamming languagues come with plethora of features. Often there are different levels of abstractions and intended use cases. Some features are primarily for library designers, others ease implementation of domain specific languages and application developers use mostly another feature set.

Some language communities are discussing “language profiles / levels” to ban certain potentionally harmful constructs. The typical audience like application programmers does not need them but removing them from the language would limit its usefulness in other cases. Examples are Scala levels (a bit dated), the Google C++ Style Guide or Profiles in the C++ Core Guidelines.

In the wild

When reading other peoples code I often see novice code dealing with low-level threading. Or they go over board with templates, reflection or meta programming.

I have even seen custom ClassLoaders in Java written by normal application programmers. People are using threads when workers, tasks, actors or other more high-level abstractions would fit much better.

Especially novices seem to be unable to recognize their limits and to stay off of inappropriate and potentially dangerous features.

How do you decide what is appropriate in your situation?

Well, that is a difficult question. If you find the task at hand seems hard you should probably take a step back because:

There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors.

-Jeff Atwood

Then ask yourself some simple questions:

  1. Someone must have done it before. Have I searched thoroughly for hints or solutions?
  2. Is there a (better) library, data structure or abstraction?
  3. Do I really have to do this? There must be a better/easier way!
  4. What do I gain using feature/library/tool X and what are its costs? What about the alternatives?


You need some experience to recognize that you are on the wrong path, solving problems you would not even have if doing the right thing in the first place.

Experience is what you got by not having it when you needed it.

-Author Unknown

Try to know and admit your limits – there is nothing wrong with struggling to get things working but it helps to frequently check your direction by taking a step back and reflecting.

Getting Shibboleth SSO attributes securely to your application

Accounts and user data are a matter of trust. Single sign-on (SSO) can improve the user experience (UX), convenience and security especially if you are offering several web applications often used by the same user. If you do not want to force your users to big vendors offering SSO like google or facebook or do not trust them you can implement SSO for your offerings with open-source software (OSS) like shibboleth. With shibboleth it may be even feasible to join an existing federation like SWITCH, DFN or InCommon thus enabling logins for thousands of users without creating new accounts and login data.

If you are implementing you SSO with shibboleth you usually have to enable your web applications to deal with shibboleth attributes. Shibboleth attributes are information about the authenticated user provided by the SSO infrastructure, e.g. the apache web server and mod_shib in conjunction with associated identity providers (IDP). In general there are two options for access of these attributes:

  1. HTTP request headers
  2. Request environment variables (not to confuse with system environment variables!)

Using request headers should be avoided as it is less secure and prone to spoofing. Access to the request environment depends on the framework your web application is using.

Shibboleth attributes in Java Servlet-based apps

In Java Servlet-based applications like Grails or Java EE access to the shibboleth attributes is really easy as they are provided as request attributes. So simply calling request.getAttribute("AJP_eppn") will provide you the value of the eppn (“EduPrincipalPersonName”) attribute set by shibboleth if a user is authenticated and the attribute is made available. There are 2 caveats though:

  1. Request attributes are prefixed by default with AJP_ if you are using mod_proxy_ajp to connect apache with your servlet container.
  2. Shibboleth attributes are not contained in request.getAttributeNames()! You have to directly access them knowing their name.

Shibboleth attributes in WSGI-based apps

If you are using a WSGI-compatible python web framework for your application you can get the shibboleth attributes from the wsgi.environ dictionary that is part of the request. In CherryPy for example you can use the following code to obtain the eppn:

eppn = cherrypy.request.wsgi_environ['eppn']

I did not find the name of the WSGI environment dictionary clearly documented in my efforts to make shibboleth work with my CherryPy application but after that everything was a bliss.


Accessing shibboleth attributes in a safe manner is straightforward in web environments like Java servlets and Python WSGI applications. Nevertheless, you have to know the above aspects regarding naming and visibility or you will be puzzled by the behaviour of the shibboleth service provider.