Docker 17.06 introduces swarm service configs, which allow you to store non-sensitive information, such as configuration files, outside a service’s image or running containers. This allows you to keep your images as generic as possible, without the need to bind-mount configuration files into the containers or use environment variables.
Configs operate in a similar way to secrets, except that they are not encrypted at rest and are mounted directly into the container’s filesystem without the use of RAM disks. Configs can be added or removed from a service at any time, and services can share a config. You can even use configs in conjunction with environment variables or labels, for maximum flexibility. Config values can be generic strings or binary content (up to 500 kb in size).
Note: Docker configs are only available to swarm services, not to standalone containers. To use this feature, consider adapting your container to run as a service with a scale of 1.
Configs are supported on both Linux and Windows services.
Docker 17.06 and higher include support for configs on Windows containers. Where there are differences in the implementations, they are called out in the examples below. Keep the following notable differences in mind:
Config files with custom targets are not directly bind-mounted into Windows containers, since Windows does not support non-directory file bind-mounts. Instead, configs for a container are all mounted in C:\ProgramData\Docker\internal\configs (an implementation detail which should not be relied upon by applications) within the container. Symbolic links are used to point from there to the desired target of the config within the container. The default target is C:\ProgramData\Docker\configs.
When creating a service which uses Windows containers, the options to specify UID, GID, and mode are not supported for configs. Configs are currently only accessible by administrators and users with system access within the container.
On Windows, create or update a service using --credential-spec with the config://<config-name> format. This passes the gMSA credentials file directly to nodes before a container starts. No gMSA credentials are written to disk on worker nodes. For more information, refer to Deploy services to a swarm.
When you add a config to the swarm, Docker sends the config to the swarm manager over a mutual TLS connection. The config is stored in the Raft log, which is encrypted. The entire Raft log is replicated across the other managers, ensuring the same high availability guarantees for configs as for the rest of the swarm management data.
When you grant a newly-created or running service access to a config, the config is mounted as a file in the container. The location of the mount point within the container defaults to /<config-name> in Linux containers. In Windows containers, configs are all mounted into C:\ProgramData\Docker\configs and symbolic links are created to the desired location, which defaults to C:\<config-name>.
You can set the ownership (uid and gid) for the config, using either the numerical ID or the name of the user or group. You can also specify the file permissions (mode). These settings are ignored for Windows containers.
root) and that user’s default group (also often root).0444), unless a umask is set within the container, in which case the mode is impacted by that umask value.You can update a service to grant it access to additional configs or revoke its access to a given config at any time.
A node only has access to configs if the node is a swarm manager or if it is running service tasks which have been granted access to the config. When a container task stops running, the configs shared to it are unmounted from the in-memory filesystem for that container and flushed from the node’s memory.
If a node loses connectivity to the swarm while it is running a task container with access to a config, the task container still has access to its configs, but cannot receive updates until the node reconnects to the swarm.
You can add or inspect an individual config at any time, or list all configs. You cannot remove a config that a running service is using. See Rotate a config for a way to remove a config without disrupting running services.
To update or roll back configs more easily, consider adding a version number or date to the config name. This is made easier by the ability to control the mount point of the config within a given container.
To update a stack, make changes to your Compose file, then re-run docker stack deploy -c <new-compose-file> <stack-name>. If you use a new config in that file, your services start using them. Keep in mind that configurations are immutable, so you can’t change the file for an existing service. Instead, you create a new config to use a different file
You can run docker stack rm to stop the app and take down the stack. This removes any config that was created by docker stack deploy with the same stack name. This removes all configs, including those not referenced by services and those remaining after a docker service update --config-rm.
docker config commandsUse these links to read about specific commands, or continue to the example about using configs with a service.
This section includes graduated examples which illustrate how to use Docker configs.
Note: These examples use a single-Engine swarm and unscaled services for simplicity. The examples use Linux containers, but Windows containers also support configs.
The docker stack command supports defining configs in a Compose file. However, the configs key is not supported for docker compose. See the Compose file reference for details.
This simple example shows how configs work in just a few commands. For a real-world example, continue to Intermediate example: Use configs with a Nginx service.
Add a config to Docker. The docker config create command reads standard input because the last argument, which represents the file to read the config from, is set to -.
$ echo "This is a config" | docker config create my-config -
Create a redis service and grant it access to the config. By default, the container can access the config at /my-config, but you can customize the file name on the container using the target option.
$ docker service create --name redis --config my-config redis:alpine
Verify that the task is running without issues using docker service ps. If everything is working, the output looks similar to this:
$ docker service ps redis
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
bkna6bpn8r1a redis.1 redis:alpine ip-172-31-46-109 Running Running 8 seconds ago
Get the ID of the redis service task container using docker ps, so that you can use docker container exec to connect to the container and read the contents of the config data file, which defaults to being readable by all and has the same name as the name of the config. The first command below illustrates how to find the container ID, and the second and third commands use shell completion to do this automatically.
$ docker ps --filter name=redis -q
5cb1c2348a59
$ docker container exec $(docker ps --filter name=redis -q) ls -l /my-config
-r--r--r-- 1 root root 12 Jun 5 20:49 my-config
$ docker container exec $(docker ps --filter name=redis -q) cat /my-config
This is a config
Try removing the config. The removal fails because the redis service is running and has access to the config.
$ docker config ls
ID NAME CREATED UPDATED
fzwcfuqjkvo5foqu7ts7ls578 hello 31 minutes ago 31 minutes ago
$ docker config rm my-config
Error response from daemon: rpc error: code = 3 desc = config 'my-config' is
in use by the following service: redis
Remove access to the config from the running redis service by updating the service.
$ docker service update --config-rm my-config redis
Repeat steps 3 and 4 again, verifying that the service no longer has access to the config. The container ID is different, because the service update command redeploys the service.
$ docker container exec -it $(docker ps --filter name=redis -q) cat /my-config cat: can't open '/my-config': No such file or directory
Stop and remove the service, and remove the config from Docker.
$ docker service rm redis
$ docker config rm my-config
This is a very simple example which shows how to use configs with a Microsoft IIS service running on Docker 17.06 EE on Microsoft Windows Server 2016 or Docker for Windows 17.06 CE on Microsoft Windows 10. It stores the webpage in a config.
This example assumes that you have PowerShell installed.
Save the following into a new file index.html.
<html>
<head><title>Hello Docker</title></head>
<body>
<p>Hello Docker! You have deployed a HTML page.</p>
</body>
</html>
If you have not already done so, initialize or join the swarm.
docker swarm init
Save the index.html file as a swarm config named homepage.
docker config create homepage index.html
Create an IIS service and grant it access to the homepage config.
docker service create
--name my-iis
--publish published=8000,target=8000
--config src=homepage,target="\inetpub\wwwroot\index.html"
microsoft/iis:nanoserver
Access the IIS service at http://localhost:8000/. It should serve the HTML content from the first step.
Remove the service and the config.
docker service rm my-iis
docker config rm homepage
This example is divided into two parts. The first part is all about generating the site certificate and does not directly involve Docker configs at all, but it sets up the second part, where you store and use the site certificate as a series of secrets and the Nginx configuration as a config. The example shows how to set options on the config, such as the target location within the container and the file permissions (mode).
Generate a root CA and TLS certificate and key for your site. For production sites, you may want to use a service such as Let’s Encrypt to generate the TLS certificate and key, but this example uses command-line tools. This step is a little complicated, but is only a set-up step so that you have something to store as a Docker secret. If you want to skip these sub-steps, you can use Let’s Encrypt to generate the site key and certificate, name the files site.key and site.crt, and skip to Configure the Nginx container.
Generate a root key.
$ openssl genrsa -out "root-ca.key" 4096
Generate a CSR using the root key.
$ openssl req \
-new -key "root-ca.key" \
-out "root-ca.csr" -sha256 \
-subj '/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA'
Configure the root CA. Edit a new file called root-ca.cnf and paste the following contents into it. This constrains the root CA to only sign leaf certificates and not intermediate CAs.
[root_ca] basicConstraints = critical,CA:TRUE,pathlen:1 keyUsage = critical, nonRepudiation, cRLSign, keyCertSign subjectKeyIdentifier=hash
Sign the certificate.
$ openssl x509 -req -days 3650 -in "root-ca.csr" \
-signkey "root-ca.key" -sha256 -out "root-ca.crt" \
-extfile "root-ca.cnf" -extensions \
root_ca
Generate the site key.
$ openssl genrsa -out "site.key" 4096
Generate the site certificate and sign it with the site key.
$ openssl req -new -key "site.key" -out "site.csr" -sha256 \
-subj '/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost'
Configure the site certificate. Edit a new file called site.cnf and paste the following contents into it. This constrains the site certificate so that it can only be used to authenticate a server and can’t be used to sign certificates.
[server] authorityKeyIdentifier=keyid,issuer basicConstraints = critical,CA:FALSE extendedKeyUsage=serverAuth keyUsage = critical, digitalSignature, keyEncipherment subjectAltName = DNS:localhost, IP:127.0.0.1 subjectKeyIdentifier=hash
Sign the site certificate.
$ openssl x509 -req -days 750 -in "site.csr" -sha256 \
-CA "root-ca.crt" -CAkey "root-ca.key" -CAcreateserial \
-out "site.crt" -extfile "site.cnf" -extensions server
The site.csr and site.cnf files are not needed by the Nginx service, but you need them if you want to generate a new site certificate. Protect the root-ca.key file.
Produce a very basic Nginx configuration that serves static files over HTTPS. The TLS certificate and key are stored as Docker secrets so that they can be rotated easily.
In the current directory, create a new file called site.conf with the following contents:
server {
listen 443 ssl;
server_name localhost;
ssl_certificate /run/secrets/site.crt;
ssl_certificate_key /run/secrets/site.key;
location / {
root /usr/share/nginx/html;
index index.html index.htm;
}
}
Create two secrets, representing the key and the certificate. You can store any file as a secret as long as it is smaller than 500 KB. This allows you to decouple the key and certificate from the services that use them. In these examples, the secret name and the file name are the same.
$ docker secret create site.key site.key
$ docker secret create site.crt site.crt
Save the site.conf file in a Docker config. The first parameter is the name of the config, and the second parameter is the file to read it from.
$ docker config create site.conf site.conf
List the configs:
$ docker config ls
ID NAME CREATED UPDATED
4ory233120ccg7biwvy11gl5z site.conf 4 seconds ago 4 seconds ago
Create a service that runs Nginx and has access to the two secrets and the config. Set the mode to 0440 so that the file is only readable by its owner and that owner’s group, not the world.
$ docker service create \
--name nginx \
--secret site.key \
--secret site.crt \
--config source=site.conf,target=/etc/nginx/conf.d/site.conf,mode=0440 \
--publish published=3000,target=443 \
nginx:latest \
sh -c "exec nginx -g 'daemon off;'"
Within the running containers, the following three files now exist:
/run/secrets/site.key/run/secrets/site.crt/etc/nginx/conf.d/site.confVerify that the Nginx service is running.
$ docker service ls
ID NAME MODE REPLICAS IMAGE
zeskcec62q24 nginx replicated 1/1 nginx:latest
$ docker service ps nginx
NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
nginx.1.9ls3yo9ugcls nginx:latest moby Running Running 3 minutes ago
Verify that the service is operational: you can reach the Nginx server, and that the correct TLS certificate is being used.
$ curl --cacert root-ca.crt https://0.0.0.0:3000
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
body {
width: 35em;
margin: 0 auto;
font-family: Tahoma, Verdana, Arial, sans-serif;
}
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>
<p>For online documentation and support, refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>
<p><em>Thank you for using nginx.</em></p>
</body>
</html>
$ openssl s_client -connect 0.0.0.0:3000 -CAfile root-ca.crt
CONNECTED(00000003)
depth=1 /C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
verify return:1
depth=0 /C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
verify return:1
---
Certificate chain
0 s:/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
i:/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
---
Server certificate
-----BEGIN CERTIFICATE-----
…
-----END CERTIFICATE-----
subject=/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
issuer=/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
---
No client certificate CA names sent
---
SSL handshake has read 1663 bytes and written 712 bytes
---
New, TLSv1/SSLv3, Cipher is AES256-SHA
Server public key is 4096 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1
Cipher : AES256-SHA
Session-ID: A1A8BF35549C5715648A12FD7B7E3D861539316B03440187D9DA6C2E48822853
Session-ID-ctx:
Master-Key: F39D1B12274BA16D3A906F390A61438221E381952E9E1E05D3DD784F0135FB81353DA38C6D5C021CB926E844DFC49FC4
Key-Arg : None
Start Time: 1481685096
Timeout : 300 (sec)
Verify return code: 0 (ok)
Unless you are going to continue to the next example, clean up after running this example by removing the nginx service and the stored secrets and config.
$ docker service rm nginx
$ docker secret rm site.crt site.key
$ docker config rm site.conf
You have now configured a Nginx service with its configuration decoupled from its image. You could run multiple sites with exactly the same image but separate configurations, without the need to build a custom image at all.
To rotate a config, you first save a new config with a different name than the one that is currently in use. You then redeploy the service, removing the old config and adding the new config at the same mount point within the container. This example builds upon the previous one by rotating the site.conf configuration file.
Edit the site.conf file locally. Add index.php to the index line, and save the file.
server {
listen 443 ssl;
server_name localhost;
ssl_certificate /run/secrets/site.crt;
ssl_certificate_key /run/secrets/site.key;
location / {
root /usr/share/nginx/html;
index index.html index.htm index.php;
}
}
Create a new Docker config using the new site.conf, called site-v2.conf.
$ docker config create site-v2.conf site.conf
Update the nginx service to use the new config instead of the old one.
$ docker service update \
--config-rm site.conf \
--config-add source=site-v2.conf,target=/etc/nginx/conf.d/site.conf,mode=0440 \
nginx
Verify that the nginx service is fully re-deployed, using docker service ps nginx. When it is, you can remove the old site.conf config.
$ docker config rm site.conf
To clean up, you can remove the nginx service, as well as the secrets and configs.
$ docker service rm nginx
$ docker secret rm site.crt site.key
$ docker config rm site-v2.conf
You have now updated your nginx service’s configuration without the need to rebuild its image.
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https://docs.docker.com/engine/swarm/configs/