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Swarm Mode Security

Lab Meta

Difficulty: Beginner

Time: Approximately 15 minutes

In this lab you'll build a new Swarm and view some of the built-in security features of Swarm mode. These include join tokens and client certificates.

You will complete the following steps as part of this lab.

Prerequisites

You will need all of the following to complete this lab:

  • Four Linux-based Docker hosts running Docker 1.13 or higher and not configured for Swarm Mode. You should use node1, node2, node3, and node4 from your lab.
  • This lab was built and tested using Ubuntu 16.04

NOTE: Things like IP addresses and Swarm join tokens will be different in your lab. Remember to substitute the values shown here in the lab guide for the real values in your lab.

Step 1: Create a new Swarm

In this step you'll initialize a new Swarm and verify that the operation worked.

For this lab to work you will need your Docker hosts running in single-engine mode and not in Swarm mode.

  1. Execute the following command on node1.

    node1$ docker swarm init
Swarm initialized: current node (kgwuvt1oqhqjsht0qcsq67rvu) is now a
manager.

To add a worker to this swarm, run the following command:

docker swarm join \
--token SWMTKN-1-4h5log5xpip966...y6gdy1-44v7nl9i0...k4fb8dlf21 \
172.31.45.44:2377

To add a manager to this swarm, run 'docker swarm join-token manager' and
follow the instructions.

The command above has created a brand new Swarm and made node1 the first manager of the Swarm. The first manager of any Swarm is automatically made the leader and the Certificate Authority (CA) for the Swarm. If you already have a CA and do not want Swarm to generate a new one, you can use the --external-ca flag to specify an external CA.

  1. Verify that the Swarm was created successfully and that node1 is the leader of the new Swarm with the following command.

    node1$ docker node ls
ID                       HOSTNAME   STATUS  AVAILABILITY  MANAGER STATUS
kgwuvt...0qcsq67rvu *    node1      Ready   Active        Leader

The command above will list all nodes in the Swarm. Notice that the output only lists one node and that the node is also the leader.

  1. Run a docker info command and view the Swarm related information.

    node1$ docker info
...
<Snip>
Swarm: active
  NodeID: kgwuvt1oqhqjsht0qcsq67rvu
  Is Manager: true
  ClusterID: ohgi9ctpbev24dl6daf7insou
  Managers: 1
  Nodes: 1
  Orchestration:
   Task History Retention Limit: 5
  Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
 ...

The important things to note from the output above are; nodeID, ClusterID, CA Configuration.

It is important to know that the docker swarm init command performs at least two important security related operations:

  • It creates a new CA (unless you specify --external-ca) and creates a key-pair to secure communications within the Swarm
  • It creates two join tokens - one to join new workers to the Swarm, and the other to join new managers to the Swarm.

We will look at these in the following steps.

Step 2: Add a new Manager

Now that you have a Swarm initialized, it's time to add another Manager.

In order to add a new Manager you must know the manager join token for the Swarm you wish to join it to. The process below will show you how to obtain the manager join token and use it to add node2 as a new manager in the Swarm.

  1. Use the docker swarm join-token command to get the manager join token.

    node1$ docker swarm join-token manager
To add a manager to this swarm, run the following command:

docker swarm join \
--token SWMTKN-1-4h5log5xpip966c6c...z2cy6gdy1-7y6lqwu6...goyf26yyg2 \
172.31.45.44:2377

The output of the command gives you the full command, including the join token, that you can run on any Docker node to join it as a manager.

NOTE: The join token includes a digest of the root CA's certificate, as well as a randomly generated secret. The format is as follows: SWMTKN-1-< digest-of-root-CA-cert>-< random-secret >.

  1. Copy and paste the command in to node2. Remember to use the command and join token for your lab, and not the value shown in this lab guide.

    node2$ docker swarm join \
--token SWMTKN-1-4h5log5xpip966c6c...z2cy6gdy1-7y6lqwu6...goyf26yyg2 \
172.31.45.44:2377

This node joined a swarm as a manager.

  2. Run the docker node ls command from either node1 or node2 to list the nodes in the Swarm.

    node1$ docker node ls
ID                      HOSTNAME   STATUS  AVAILABILITY  MANAGER STATUS
ax2cmh63...tvjp8trs4    node2      Ready   Active        Reachable
kgwuvt1o...qcsq67rvu *  node1      Ready   Active        Leader

The join token used in the commands above will join any node to your Swarm as a manager. This means it is vital that you keep the join tokens private - anyone in possession of it can join nodes to the Swarm as managers.

Step 3: Add a new Worker

Adding a worker is the same process as adding a manager. The only difference is the token used. Every Swarm maintains one manager join token and one worker join token.

  1. Run a docker swarm join-token command from any of the managers in your Swarm to obtain the command and token required to add a new worker node.

    node1$ docker swarm join-token worker
To add a worker to this swarm, run the following command:

docker swarm join \
--token SWMTKN-1-4h5log5xpip966c6c...z2cy6gdy1-44v7nl9...b8dlf21 \
172.31.45.44:2377

Notice that the join token for managers and workers share some of the same values. Both start with "SWMTKN-1", and both share the same Swarm root CA digest. It is only the last part of the token that determines if the token is for a manager or worker.

  1. Switch to node3 and paste in the command from the previous step.

    node3$ docker swarm join \
--token SWMTKN-1-4h5log5xpip966c6c...z2cy6gdy1-44v7nl9...b8dlf21 \
172.31.45.44:2377

This node joined a swarm as a worker.

  2. Switch back to one of the manager nodes (node1 or node2) and run a docker node ls command to verify the node was added as a worker.

    node1$ docker node ls
ID                   HOSTNAME   STATUS  AVAILABILITY  MANAGER STATUS
ax2cm...vjp8trs4     node2      Ready   Active        Reachable
kgwuv...csq67rvu *   node1      Ready   Active        Leader
mfg9d...inwonsjh     node3      Ready   Active

    The output above shows that node3 was added to the Swarm and is operating as a worker - the lack of a value in the MANAGER STATUS column indicates that the node is a worker.

Step 4: Rotate Join Keys

In this step you will rotate the Swarms worker join-key. This will invalidate the worker join-key used in previous steps. It will not affect the status of workers already joined to the Swarm, this means all existing workers will continue to be valid workers in the Swarm.

You will test that the rotate operation succeeded by attempting to add a new worker with the old key. This operation will fail. You will then retry the operation with the new key. This time it will succeed.

  1. Rotate the existing worker key by execute the following command from either of the Swarm managers.

    node1$ docker swarm join-token --rotate worker
Successfully rotated worker join token.

To add a worker to this swarm, run the following command:

  docker swarm join \
  --token SWMTKN-1-4h5log5xpip...cy6gdy1-55k4ywd...z5xtns4eq \
  172.31.45.44:2377

    Notice that the new join token still starts with SWMTKN-1 and keeps the same digest of the Swarms root CA 4h5log5.... It is only the last part of the token that has changed. This is because the new token is still a Swarm join token for the same Swarm. The system has only rotated the secret used to add new workers (the last portion).

  2. Log on to node4 and attempt to join the Swarm using the old join token. You should be able to find the old join token in the terminal window of node3 from a previous step.

    node4$ docker swarm join \
--token SWMTKN-1-4h5log5xpi...duz2cy6gdy1-44v7nl9...4fb8dlf21 \
172.31.45.44:2377

Error response from daemon: rpc error: code = 3 desc = A valid join token
is necessary to join this cluster

The operation fails because the join token is no longer valid.

  1. Retry the previous operation using the new join token given as the output to the docker swarm join-token --rotate worker command in a previous step.

    node4$ docker swarm join \
--token SWMTKN-1-4h5log5...wzqlduz2cy6gdy1-55k4ywd...xtns4eq \
172.31.45.44:2377

This node joined a swarm as a worker.

Rotating join tokens is something that you will need to do if you suspect your existing join tokens have been compromised. It is important that you manage your join-tokens carefully. This is because unauthorized nodes joining the Swarm is a security risk.

Step 5: View certificates

Each time a new manager or worker joins the Swarm it is issued with a client certificate. This client certificate is used in conjunction with the existing Swarm public key infrastructure (PKI) to authenticate the node and encrypt communications.

There are three important things to note about the client certificate:

  1. It specifies which Swarm the node is an authorized member of
  2. It contains the node ID
  3. It specifies the role the node is authorized to perform in the Swarm (worker or manager)

Execute the following command from any node in your Swarm to view the nodes client certificate.

node1$ openssl x509 -in /var/lib/docker/swarm/certificates/swarm-node.crt -text

Certificate:
  Data:
      Version: 3 (0x2)
      Serial Number:
          59:53:84:47:3a:2d:15:5b:f0:39:46:93:dd:21:68:ad:70:62:40:d1
  Signature Algorithm: ecdsa-with-SHA256
      Issuer: CN=swarm-ca
      Validity
          Not Before: Mar 14 11:42:00 2017 GMT
          Not After : Jun 12 12:42:00 2017 GMT
      Subject: O=ohgi9...insou, OU=swarm-manager, CN=kgwuvt...csq67rvu
      ...

The important things to note about the output above are the three fields on the bottom line:

  • The Organization (O) field contains the Swarm ID
  • The Organization Unit (OU) field contains the nodes role
  • The Common Name (CN) field contains the nodes ID

These three fields make sure the node operates in the correct Swarm, operates in the correct role, and is the node it says it is.

You can use the docker swarm update --cert-expiry <TIME PERIOD> command to change frequency at which the client certificates in the Swarm are renewed. The default is 90 days (3 months).

Step 6: Rotate certificates

In this step you'll view the existing certificate rotation period for your Swarm, and then alter that period.

Perform the following commands from a manager node in your Swarm.

  1. Use the docker info command to view the existing certificate rotation period enforced in your Swarm.
node1$ docker info
Swarm: active
 NodeID: kgwuvt1oqhqjsht0qcsq67rvu
 Is Manager: true
 ClusterID: ohgi9ctpbev24dl6daf7insou
 Managers: 2
 Nodes: 4
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months

The last two lines of the output above show that the current rotation period (Expiry Duration) is 3 months.

  1. Use the docker swarm update command to change the rotation period.
node1$ docker swarm update --cert-expiry 168h
Swarm updated.

The --cert-expiry flag accepts time periods in the format 00h00m00s, where h is for hours, m is for minutes, and s is for seconds. The example above sets the rotation period to 168 hours (7 days).

  1. Run another docker info to check that the value has changed.
node1$ docker info
Swarm: active
 NodeID: kgwuvt1oqhqjsht0qcsq67rvu
 Is Manager: true
 ClusterID: ohgi9ctpbev24dl6daf7insou
 Managers: 2
 Nodes: 4
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 7 days

Congratulations, you have completed this lab on basic Swarm security.