How to use docker hostname when running a container - docker

Question

I'm runnig RabbitMQ in docker
>docker run -d --hostname my-rabbit --name some-rabbit --rm --network my-bridge rabbitmq
My container name is some-rabbit and my hostname is my-rabbit
The container is connected to my-bridge network
My app is also connected to my-bridge network
docker network inspect my-bridge produces:
[
{
"Name": "my-bridge",
"Id": "c996f5318b2b3ff907b6680e41c4551f184f2c43b0df43fd6916fa8d1707e427",
"Created": "2018-12-26T19:40:46.6982856Z",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": {},
"Config": [
{
"Subnet": "172.21.0.0/16",
"Gateway": "172.21.0.1"
}
]
},
"Internal": false,
"Attachable": false,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"98d81ba991febbc9dc0775effc79a2b083f62fe563aad3ce146e56bab5b09f4c": {
"Name": "some-rabbit",
"EndpointID": "acb3da3e91d655a3ed245fa3938edd681318129e514b83db07e1b4efd4f2bfc2",
"MacAddress": "02:42:ac:15:00:02",
"IPv4Address": "172.21.0.2/16",
"IPv6Address": ""
},
"9fdf456e352e932b13530223f736028f357e63ead5ace042a13200f6d5744d62": {
"Name": "dockercompose8897046891659778892_publisher_1",
"EndpointID": "f45ba7f9c7832d3514d22ddf29bc19f1e74529cb371b335542c9eb54469fe7a4",
"MacAddress": "02:42:ac:15:00:04",
"IPv4Address": "172.21.0.4/16",
"IPv6Address": ""
}
},
"Options": {},
"Labels": {}
}
]
docker inspect some-rabbit produces:
...
"Config": {
"Hostname": "my-rabbit",
"Domainname": "",
"User": "",
"AttachStdin": false,
"AttachStdout": false,
"AttachStderr": false,
"ExposedPorts": {
"25672/tcp": {},
"4369/tcp": {},
"5671/tcp": {},
"5672/tcp": {}
},
"Tty": false,
"OpenStdin": false,
"StdinOnce": false,
"Env": [
"PATH=/usr/lib/rabbitmq/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"GOSU_VERSION=1.10",
"RABBITMQ_LOGS=-",
"RABBITMQ_SASL_LOGS=-",
"RABBITMQ_GPG_KEY=0A9AF2115F4687BD29803A206B73A36E6026DFCA",
"RABBITMQ_VERSION=3.7.8",
"RABBITMQ_GITHUB_TAG=v3.7.8",
"RABBITMQ_DEBIAN_VERSION=3.7.8-1",
"LANG=C.UTF-8",
"HOME=/var/lib/rabbitmq"
],
"Cmd": [
"rabbitmq-server"
],
"ArgsEscaped": true,
"Image": "rabbitmq",
"Volumes": {
"/var/lib/rabbitmq": {}
},
"WorkingDir": "",
"Entrypoint": [
"docker-entrypoint.sh"
],
"OnBuild": null,
"Labels": {}
},
"NetworkSettings": {
"Bridge": "",
"SandboxID": "bac2e155323c1ec94e145a37f45f8bc0d32e28a2b363cb1c9f3d4ed922a91a56",
"HairpinMode": false,
"LinkLocalIPv6Address": "",
"LinkLocalIPv6PrefixLen": 0,
"Ports": {
"25672/tcp": null,
"4369/tcp": null,
"5671/tcp": null,
"5672/tcp": null
},
"SandboxKey": "/var/run/docker/netns/bac2e155323c",
"SecondaryIPAddresses": null,
"SecondaryIPv6Addresses": null,
"EndpointID": "",
"Gateway": "",
"GlobalIPv6Address": "",
"GlobalIPv6PrefixLen": 0,
"IPAddress": "",
"IPPrefixLen": 0,
"IPv6Gateway": "",
"MacAddress": "",
"Networks": {
"my-bridge": {
"IPAMConfig": null,
"Links": null,
"Aliases": [
"98d81ba991fe"
],
"NetworkID": "c996f5318b2b3ff907b6680e41c4551f184f2c43b0df43fd6916fa8d1707e427",
"EndpointID": "acb3da3e91d655a3ed245fa3938edd681318129e514b83db07e1b4efd4f2bfc2",
"Gateway": "172.21.0.1",
"IPAddress": "172.21.0.2",
"IPPrefixLen": 16,
"IPv6Gateway": "",
"GlobalIPv6Address": "",
"GlobalIPv6PrefixLen": 0,
"MacAddress": "02:42:ac:15:00:02",
"DriverOpts": null
}
}
}
}
]
Concluding:
Both rabbitmq and my app are running in the same network
RabbitMQ has a custom hostname my-rabbit which can be observed when inspecting the container: "Hostname": "my-rabbit"
However when my app is trying to reach RabbitMQ at address my-rabbit, connection fails. Only if I use the container name as the address (some-rabbit) it works.
Is my understanding of --hostname wrong or am I using it wrong? When is it useful to use custom --hostname?

Answer 1

Hostname is only visible to the application inside that same container. Docker's embedded DNS will not resolve container hostnames. What you can resolve with docker's DNS are:
container name
container id
network alias for a container (including the service name in a compose file)