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Proxies, Docker and batect

tl;dr

batect will do its best to make things just work with proxies, but you'll need to configure proxies for pulling images yourself

Most applications expect to find proxy configuration in a number of environment variables. The most common are:

  • http_proxy: proxy to use for HTTP requests
  • https_proxy: proxy to use for HTTPS requests
  • ftp_proxy: proxy to use for FTP requests
  • no_proxy: comma-separated list of domains or addresses for which connections should bypass the proxy (ie. be direct to the destination)

There are three points where a proxy could be required during the lifecycle of a container:

  1. at image pull time
  2. at build time, after the image has been pulled
  3. at run time

Each of these are handled slightly differently by Docker, and so batect does its best to make your life easier.

At image pull time

When pulling an image, the Docker daemon uses any proxy-related environment variables in the Docker daemon's environment to determine whether or not to use a proxy. These settings cannot be set at image pull time, so batect can't configure these settings for you - you must configure them yourself.

On OS X, Docker defaults to using your system's proxy settings, and you can change these by going to the Docker icon > Preferences > Proxies.

On Linux, you may need to configure the Docker daemon's proxy settings yourself. This page in the Docker documentation gives an example of how to configure a Docker daemon running with systemd.

At build time, after the image has been pulled

After pulling the base image, all subsequent build steps use the environment variables of the build environment, which is a combination of:

  • any environment variables defined in the base image with ENV instructions
  • any build arguments defined in the Dockerfile with ARG instructions
  • any environment variables defined in the Dockerfile with ENV instructions
  • any of the pre-defined build arguments, if a value is provided for them

This last point is the most relevant to proxy settings - as http_proxy, https_proxy, no_proxy etc. are defined as pre-defined build arguments, we can pass the host's proxy environment variables into the build environment as build arguments.

batect automatically propagates any proxy environment variables configured on the host as build arguments unless the --no-proxy-vars flag is passed to batect.

Note that build arguments are not persisted in the image - they exist only as environment variables at build time. Furthermore, the pre-defined proxy-related build arguments (unlike normal build arguments) do not impact Docker's cache invalidation logic - so if an image build succeeded with http_proxy set to http://brokenproxy, changing http_proxy to http://workingproxy will not cause a rebuild. (The reasoning behind this is that if the build has succeeded with one proxy, then switching to another proxy should have no impact.)

At run time

The set of run time environment variables is defined by:

  • any environment variables defined in the image (including any base images) with ENV instructions
  • any container-specific environment variables specified in the container or task in batect.yml

batect automatically propagates any proxy environment variables configured on the host as environment variables unless the --no-proxy-vars flag is passed to batect.

Starting with v0.14, if propagating proxy environment variables is enabled, and any proxy environment variable recognised by batect is set, batect will also add the names of all containers started as part of the task to no_proxy and NO_PROXY (or create those environment variables if they're not set). This ensures that inter-container communication is not proxied.

Proxy environment variables recognised by batect

batect will propagate the following proxy-related environment variables:

  • http_proxy
  • HTTP_PROXY
  • https_proxy
  • HTTPS_PROXY
  • ftp_proxy
  • FTP_PROXY
  • no_proxy
  • NO_PROXY

Starting with v0.18, batect will add missing environment variables if only one in a pair is defined. (For example, if http_proxy is defined, but HTTP_PROXY isn't, then both http_proxy and HTTP_PROXY are propagated, with HTTP_PROXY set to the same value as http_proxy.)

Proxies running on the host machine

If you run a local proxy on your host machine such as Cntlm, referring to this proxy with localhost will not work from inside a Docker container, as localhost refers to the container, not the host machine.

Starting with v0.16, if you are running batect on OS X with Docker 17.06 or later, batect will automatically rewrite proxy-related environment variables that refer to localhost, 127.0.0.1 or ::1 so that they refer to the host machine.

If you are running batect on Linux, or using an older version of Docker, batect will not rewrite proxy-related environment variables. Support for Linux will be added in the future, check this issue on GitHub for updates.