Inspecting Docker container processes from the host

While I favour a containerize-all-the-things approach to new projects I still need to maintain systems that were designed several years ago around a combination of containers and host-based applications working together.

In these situations it is common enough to execute ps or iotop on the host and see all the host and container processes together with no obvious indication of which processes belong to which containers.

Here I will share some simple commands to help map the host-view of a containerised process to its container.

First, given a host PID, how do I know which container it belongs to?

$ cat "/proc/${host_pid}/cgroup"
...
4:cpu:/docker/769739f359ec192edf6c565f7756bb5ecabcfac3e691c2444794ab6a7d398e39
...

The procfs cgroup file will show the full Docker container ID which you can then use with docker inspect to get more container details.

Vice-versa if you have the container ID and want to locate the host process(es) you can use:

$ sudo ps -e -o pid,comm,cgroup | grep "/docker/${cid}"

Lastly, if you’re trying to debug a container process from the host and need the host-path to the process’ binary I have found a method to has been working reliably.

Unfortunately, because the procfs exe file is a symbolic link, not a hard link it won’t resolve to the file within the container’s layered file system so a few extra steps are required.

First, read the symlink to get the fully-qualified container-path to the binary:

$ exe=$(readlink "/proc/${host_pid}/exe")

Next, parse the process’ memory-mapped files to locate the first memory region referencing this file path:

$ map=$(grep -m1 -F "${exe}" "/proc/${host_pid}/maps" | cut -d' ' -f1)

Lastly read the symlink for this memory map from procfs’ map_files directory:

$ readlink "/proc/${host_pid}/map_files/${map}"

This final output should look something like this:

/var/lib/docker/aufs/diff/3cc533dae9a6cc96d6092844be3ce78c737db793cf1493b9f47e652e96bfd71e/bin/sleep

Note that the long identifier in that path is not the container ID, nor is it available via docker inspect, although I’m sure someone else has posted online how to locate this path via other means.

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