yosifkit / syslog-init

syslog-init

what is syslog-init

syslog-init is a simple init like tini or dumb-init to start a single child process, and wait for it to exit while reaping zombies and performing signal forwarding. As you might guess the main difference is the ability to request a syslog listener that will proxy the syslog messages to stdout.

why do I need syslog-init?

There are certain applications, like HAProxy, that can only log to syslog. To simplify our container deployment, we don't want to also run a full syslog server (either in another container or complicate our HAProxy container with a full supervisor). The standard for container setups is to log to stdout so that they can be processed though something like docker logs or redirected to a log driver.

See docker-library/haproxy#39 for why we don't just use busybox syslogd backgrounded from a shell script. The short version is that the container won't exit with a SIGTERM, but will stop haproxy from listening.

If you are already running a syslog server, then you don't need syslog-init and should just point the haproxy container config to log there.

using syslog-init

TLDR: get the syslog-init binary into your image and in the PATH. Set SYSLOG_SOCKET environment variable to /dev/log. Set ENTRYPOINT to ["syslog-init", "my-entrypoint"] and ensure CMD is using the same json syntax.

building syslog-init

Build the binary. Dockerfiles are provided for a stable and consistent build environment. Either Dockerfile.debian or Dockerfile.alpine will work and create a static binary. At some point, there will probably be signed releases.

$ docker build -t yosifkit/syslog-init -f Dockerfile.debian .

get syslog-init out of the image

Copy it to where it is needed. docker cp can be used once a container is running:

# start a container
$ docker run -d --name sysl yosifkit/syslog-init sleep 1000

# cp the syslog-init binary to the current directory
$ docker cp sysl:/go/bin/syslog-init ./

# clean up the container
$ docker rm -f sysl

Alternatively we could just use tar to pipe the file to the host:

$ docker run -i --rm syslog-init tar -c -C /go/bin/ syslog-init | tar -x -C ./

add syslog-init to my project

Assuming that you current Dockerfile ends something like this:

# other build steps....

ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["haproxy", "-f", "/usr/local/etc/haproxy/haproxy.cfg"]

Just change/add the following and you'll have a syslog listener available at /dev/log.

ENV SYSLOG_SOCKET /dev/log
COPY syslog-init /usr/local/bin/
ENTRYPOINT ["syslog-init", "/docker-entrypoint.sh"]
CMD ["haproxy", "-f", "/usr/local/etc/haproxy/haproxy.cfg"]