Collecting docker logs and stats with Splunk

I’m working at Splunk, but this is my personal thoughts. I have some knowledge about Splunk obviously, but you should not consider this as an official Splunk manual. Everything I did here – I did only for my personal needs and my free time.

You cannot really feel safe for the services you run if you don’t monitor them. There are plenty of great tools which allow you to monitor your docker environments, like cadvisor and some other cloud solutions. I did not want to use cloud solutions, because they can also upload some sensitive information, like environment variables, where I could keep passwords for AWS backups. So I wanted to use something like cadvisor, but with historical information and also attached logs for the containers. I could not find anything which just works out of box, so I decided to start working on my own solution, which is built on top of Splunk Light.

Splunk Light is free if you need to index less than 500Mb, which will be more than enough for home use.

Settings up Splunk Light

First of all we need to setup Splunk Light. I have build my own Splunk docker image (on GitHub). You can use it to setup your Splunk Light container, this is my docker-compose.yml example

  image: busybox
    - /opt/splunk/etc
    - /opt/splunk/var

  hostname: splunk
  image: outcoldman/splunk:6.2-light
    - vsplunk
    - 8000:8000
    - 9997:9997
  restart: always

I have two containers here, I usually use Data Volume Containers to persist data. So first container is a Data Volume Container, second container is a Splunk Light. For Splunk Light I opened two ports:

  • 8000 for web access.
  • 9997 for data from forwarders.

To enable receiving data on this Splunk Light, just go to the Settings (it is on right top corner), Data, Receiving, after that click on New and add port 9997.


Settings up Splunk Forwarder to collect syslog data

The next set of containers which we want to setup is a Splunk Forwarder which will forward logs from syslog to Splunk. We can actually directly collect them from first Splunk container, but I prefer to separate them.

My docker-compose.yml file for Splunk Forwarder (again using my Splunk image)

  image: busybox
    - /opt/splunk/etc
    - /opt/splunk/var

  image: outcoldman/splunk:6.2-forwarder
    - vforwarder
    - 514:1514/udp
  restart: always

This image will use internal 1514 port for listening UDP traffic (syslog) instead of 514, because I run Splunk processes not under root and only root can get access to ports under 1024. But I still want to keep port 514 on host, because some applications does not allow you to change receiving port (like DD-WRT router).

Remember port 9997 which we just specified above? This image automatically will forward all logs to this port (just don’t forget to replace YOUR_DOCKER_HOSTNAME).

Now we need to enable listening on port 1514

docker exec -it splunk_forwarder_1 splunk add udp 1514 -sourcetype syslog

Forwarding logs from docker containers to Splunk

I usually keep default docker logging driver and only forward logs for specific containers, as I’m not interesting in all of them.

Example of my nginx proxy container is

  image: nginx
    - 80:80
    - 443:443
    - vdata
  restart: always
  log_driver: syslog
    syslog-tag: nginxproxy_nginx
    syslog-address: udp://MY_DOCKER_HOST

On previous chapter we mapped port 1514 to default syslog port on my docker host machine, so for this container we just need to forward logs to the syslog on current host and it will go to the Splunk Forwarder.

As you can see I also use syslog-tag which allows me to identify the right logs in syslog. The format of these logs will be something like

Aug 23 23:44:36 2015-08-23T16:44:36-07:00 docker_host_name docker/nginxproxy_nginx[2156]: LOG LINE

After that it will be easy to parse logs.

Forwarding logs from applications to Splunk

If application in your container does not write logs to the stdout and keep them in files you still can use Splunk Forwarder to monitor these files.

I will show you how to do that on GitLab example. It keeps all logs under /var/log/gitlab. So what we should do is to share these data in Data Volume Container between GitLab and Splunk forwarder images.

  image: busybox
    - /var/log/gitlab

After that we just need to add Splunk Forwarder

  image: busybox
    - /opt/splunk/etc
    - /opt/splunk/var

  hostname: gitlab
  image: outcoldman/splunk:6.2-forwarder
    - vlogs
    - vforwarder
  restart: always

And the last step, just tell Splunk Forwarder to monitor /var/log/gitlab/ by

docker exec -it YOUR_CONTAINER_NAME splunk add monitor "/var/log/gitlab/"

That is it. You should see your logs on the indexer after that.

NOTE: I’m thinking about automating step of automatically monitoring specified with environment variable folders/files. Probably will do that soon.

Forwarding docker stats and events

Another docker container I built is a Splunk Forwarder with preconfigured inputs, which monitors for the Docker stats, like top, inspect, stats and events. You can find this container on Docker Registry outcoldman/docker-stats-splunk-forwarder/ (or GitHub. It is very simple to set it up

  hostname: docker
  image: outcoldman/docker-stats-splunk-forwarder
    - /var/run/docker.sock:/var/run/docker.sock:ro
  restart: always

After that you will see all events in Splunk Indexer.

Some useful dashboards based on Docker stats

I’m still working on preconfigured Splunk Light image with all dashboards, or maybe Docker applications, but for now I can just share few Searches I use for my Dashboards.

These two screenshots of overall Docker information:

  • CPU% (800% because of 8 cores)
  • Memory Usage (one line is the maximum limit, another is how much is used right now)
  • CPU usage per container
  • Memory Usage per container (% of limit)
  • Network Input per container
  • Network Output per container
  • Last Events (excluding top as I query it regularly)
  • Top processes from all container



Also I worked on Dashboard for containers, select container and you will see

  • Top processes
  • Last events


To try them out you can use outcoldman/docker-stats-splunk which is Splunk Light image with predefined Dashboards.

Hi am new to splunk configuration apart from enabling the receiver is there anything you need to do for the splunk to accept forwareded inputs

my forwarder is fine I have checked the status by this command

mwass-MacBook-Pro:sms_platform francis$ docker exec -it smsplatform_forwarder_1 splunk list forward-server
Active forwards:

but somehow I can’t see my logs in splunk light

Francis Mwangi Muray
September 22, 2015

Francis, have you enabled Receive Data in Splunk Light? It is in the top of the blog post.

September 22, 2015

yep. I have enabled. Do you have like a sample app that is already running you give me the repo a try.

Francis Mwangi Muray
September 22, 2015

Hi there,
I’m having an interesting time whereby the stats splunk forwarder is not forwarding stats to splunk-light. After much head scratching, I found that the stats splunk forwarder is not able to read the docker statistics because it does not have rights to connect to /var/run/docker.sock.

If I log in (docker exec -ti sh) and then view the logs (cat /opt/splunk/var/log/splunk/splunkd.log) I do indeed see the error:
10-09-2015 14:37:02.075 +0000 ERROR ExecProcessor – message from “/opt/splunk/etc/apps/docker/bin/” Get http:///var/run/docker.sock/v1.20/containers/json: dial unix /var/run/docker.sock: permission denied.

If I run the command manually (as root) then it works a treat, but if I run as splunk, then it fails as above.

Therefore if I run (whilst logged in to the stats forwarding container)
chown root.root /opt/splunk/etc/apps/docker/bin/docker
chmod 4755 /opt/splunk/etc/apps/docker/bin/docker

then all the stats appear and it works perfectly.

Hope this helps!

Edward Hodd
October 9, 2015

I have coped the docker-compose exactly, but whenever I try the step “Settings -> Data -> Receiving -> Add New” the web UI throws the error “Your entry was not saved. The following error was reported: SyntaxError: Unexpected token <." I've even tried this with the new docker images 6.2.7-light and 6.3.1-light all with the same issue. Is there something I am missing?

Rob Cannon
December 4, 2015

Hi I am interested in the forward logs from app to splunk. But I couldn’t find the image outcoldman/splunk:6.2-forwarder in docker hub.
Could you please help provide the image url? Thank you so much!

February 1, 2016

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  1. […] Collecting docker logs and stats with Splunk (Den Gladkikh) […]

  2. […] it possible to cover more cases of collecting logs including from Docker. Previously I blogged on using the Splunk Universal Forwarder to collect logs from Docker […]