[docker] How to get the hostname of the docker host from inside a docker container on that host without env vars

What are the ways get the docker host's hostname from inside a container running on that host besides using environment variables? I know I can pass the hostname as an environment variable to the container at container creation time. I'm wondering how I can look it up at run time.

foo.example.com (docker host)
  bar (docker container)

Is there a way for container bar running in docker host foo.example.com to get "foo.example.com"?

Edit to add use case:

The container will create an SRV record for service discovery of the form

_service._proto.name. TTL class SRV priority weight port target.
-----------------------------------------------------------------
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo.example.com.

where 20003 is a dynamically allocated port on the docker host for a service listening on some fixed port in bar (docker handles the mapping from host port to container port).

My container will run a health check to make sure it has successfully created that SRV record as there will be many other bar containers on other docker hosts that also create their own SRV records.

_service._proto.name. TTL class SRV priority weight port target.
-----------------------------------------------------------------
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo.example.com. <--
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo2.example.com.
_bar._http.example.com 60 IN SRV 5000 5000 20003 foo3.example.com.

The health check will loop through the SRV records looking for the first one above and thus needs to know its hostname.

aside

I'm using Helios and just found out it adds an env var for me from which I can get the hostname. But I was just curious in case I was using docker without Helios.

I know it's an old question, but I needed this solution too, and I acme with another solution.

I used an entrypoint.sh to execute the following line, and define a variable with the actual hostname for that instance:

HOST=`hostname --fqdn`

Then, I used it across my entrypoint script:

echo "Value: $HOST"

Hope this helps

I ran

docker info | grep Name: | xargs | cut -d' ' -f2

inside my container.

You can easily pass it as an environment variable

docker run .. -e HOST_HOSTNAME=`hostname` ..

using

-e HOST_HOSTNAME=`hostname`

will call the hostname and use it's return as an environment variable called HOST_HOSTNAME, of course you can customize the key as you like.

note that this works on bash shell, if you using a different shell you might need to see the alternative for "backtick", for example a fish shell alternative would be

docker run .. -e HOST_HOSTNAME=(hostname) ..

Another option that worked for me was to bind the network namespace of the host to the docker.

By adding:

docker run --net host

I think the reason that I have the same issue is a bug in the latest Docker for Mac beta, but buried in the comments there I was able to find a solution that worked for me & my team. We're using this for local development, where we need our containerized services to talk to a monolith as we work to replace it. This is probably not a production-viable solution.

On the host machine, alias a known available IP address to the loopback interface:

$ sudo ifconfig lo0 alias 10.200.10.1/24

Then add that IP with a hostname to your docker config. In my case, I'm using docker-compose, so I added this to my docker-compose.yml:

extra_hosts:
# configure your host to alias 10.200.10.1 to the loopback interface:
#       sudo ifconfig lo0 alias 10.200.10.1/24
- "relevant_hostname:10.200.10.1"

I then verified that the desired host service (a web server) was available from inside the container by attaching to a bash session, and using wget to request a page from the host's web server:

$ docker exec -it container_name /bin/bash
$ wget relevant_hostname/index.html
$ cat index.html

You can pass it as an environment variable like this. Generally Node is the host that it is running in. The hostname is defaulted to the host name of the node when it is created.

docker service create -e 'FOO={{.Node.Hostname}}' nginx  

Then you can do docker ps to get the process ID and look at the env

$ docker exec -it c81640b6d1f1 env                                                                                                                                    PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=c81640b6d1f1
TERM=xterm
FOO=docker-desktop
NGINX_VERSION=1.17.4
NJS_VERSION=0.3.5
PKG_RELEASE=1~buster
HOME=/root

An example of usage would be with metricbeats so you know which node is having system issues which I put in https://github.com/trajano/elk-swarm:

  metricbeat:
    image: docker.elastic.co/beats/metricbeat:7.4.0
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - /sys/fs/cgroup:/hostfs/sys/fs/cgroup:ro
      - /proc:/hostfs/proc:ro
      - /:/hostfs:ro
    user: root
    hostname: "{{.Node.Hostname}}"
    command:
      - -E
      - |
        metricbeat.modules=[
          {
            module:docker,
            hosts:[unix:///var/run/docker.sock],
            period:10s,
            enabled:true
          }
        ] 
      - -E
      - processors={1:{add_docker_metadata:{host:unix:///var/run/docker.sock}}} 
      - -E
      - output.elasticsearch.enabled=false
      - -E
      - output.logstash.enabled=true
      - -E
      - output.logstash.hosts=["logstash:5044"]
    deploy:
      mode: global

I'm adding this because it's not mentioned in any of the other answers. You can give a container a specific hostname at runtime with the -h directive.

docker run -h=my.docker.container.example.com ubuntu:latest

You can use backticks (or whatever equivalent your shell uses) to get the output of hosthame into the -h argument.

docker run -h=`hostname` ubuntu:latest

There is a caveat, the value of hostname will be taken from the host you run the command from, so if you want the hostname of a virtual machine that's running your docker container then using hostname as an argument may not be correct if you are using the host machine to execute docker commands on the virtual machine.