How can I get Docker Linux container information f

Question

I would like to make my docker containers aware of their configuration, the same way you can get information about EC2 instances through metadata.

I can use (provided docker is listening on port 4243)

curl http://172.17.42.1:4243/containers/$HOSTNAME/json

to get some of its data, but would like to know if there is a better way at least the get the full ID of the container, because HOSTNAME is actually shortened to 12 characters and docker seems to perform a "best match" on it.

Also, how can I get the external IP of the docker host (other than accessing the EC2 metadata, which is specific to AWS)

Answer 1

Docker sets the hostname to the container ID by default, but users can override this with --hostname. Instead, inspect /proc:

$ more /proc/self/cgroup
14:name=systemd:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
13:pids:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
12:hugetlb:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
11:net_prio:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
10:perf_event:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
9:net_cls:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
8:freezer:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
7:devices:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
6:memory:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
5:blkio:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
4:cpuacct:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
3:cpu:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
2:cpuset:/docker/7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605
1:name=openrc:/docker

Here's a handy one-liner to extract the container ID:

$ grep "memory:/" < /proc/self/cgroup | sed 's|.*/||'
7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605

Answer 2

A comment by madeddie looks most elegant to me:

CID=$(basename $(cat /proc/1/cpuset))

Answer 3

I've found out that the container id can be found in /proc/self/cgroup

So you can get the id with :

cat /proc/self/cgroup | grep -o  -e "docker-.*.scope" | head -n 1 | sed "s/docker-\(.*\).scope/\\1/"

Answer 4

WARNING: You should understand the security risks of this method before you consider it. John's summary of the risk:

By giving the container access to /var/run/docker.sock, it is [trivially easy] to break out of the containment provided by docker and gain access to the host machine. Obviously this is potentially dangerous.

---

Inside the container, the dockerId is your hostname. So, you could:

• install the docker-io package in your container with the same version as the host
• start it with --volume /var/run/docker.sock:/var/run/docker.sock --privileged
• finally, run: docker inspect $(hostname) inside the container

Avoid this. Only do it if you understand the risks and have a clear mitigation for the risks.

Answer 5

awk -F'[:/]' '(($4 == "docker") && (lastId != $NF)) { lastId = $NF; print $NF; }' /proc/self/cgroup

Answer 6

Short way:

basename `cat /proc/1/cpuset`
7be92808767a667f35c8505cbf40d14e931ef6db5b0210329cf193b15ba9d605