[ Tutorial ] Apache ZooKeeper – Setting ACL in ZooKeeper Client

Now let’s talk about setting the ACL of a znode in ZooKeeper. Before getting into the details, let’s talk more about the scheme and ID.

1. Scheme and ID

ID, as name suggests, is an identifier comprised of a username and password. By default, when the znode has an ACL set accessible by a specific group of users or an individual, the <username>:<password> is first hashed using an SHA-1 hashing algorithm, and then it (hex-string) is base-64 encoded.

As mentioned in earlier blog entries, scheme is like a group of users that are authorized to access a certain znode with a scheme-and-id-specific ACL set.

1.1. World Scheme

World scheme has one ID (anyone). This represents any user in the world.

For example, we type in the following command to set the znode accessible by anyone.

setAcl /newznode world:anyone:crdwa

By doing it correctly, You should get something like this in return:

1.2. Auth Scheme

Auth scheme represents a "manually" set group of authenticated users. According to the ZooKeeper documentation (http://zookeeper.apache.org/doc/r3.1.2/zookeeperProgrammers.html), auth does not utilize any ID. Unless I am mistaken, this seems not to be the case. Because if you try to set ACL on a znode using auth scheme and not provide any ID, it tells you that is not a valid ID, or some form of ID is needed. Below is a (bad) example:

setAcl /newznode auth:crdwa

as seen above, I did not provide any form of ID. This is what I get:

A correct way to use this scheme would be as follows:

setAcl /newznode auth:username:password:crdwa

Using auth scheme allows us to have multiple authorized users to access a single znode with the different username and password combination. Say we have 3 users:

username : password
user_123 : pwd_123
user_456 : pwd_456
user_789 : pwd_789

We can use the same syntax above by replacing username with user_123, user_456, or user_789 and password with pwd_123, pwd_456, or pwd_789 respectively.

1.2.1 addauth Command

One important thing to note is you must use the addauth command before proceeding to set the ACL of a znode using the auth scheme. If you try to set the ACL before executing the addauth command, you will get an error as below:

Correct way to do is to execute addauth command first, and then execute the setAcl command. Below is the syntax of command execution for addauth:

addauth /<node-name> digest <username>:<password>

By adding the authenticator and setting ACL accordingly, you can ensure that you set the ACL correctly.

Repeat the steps for additional username and password combo, and the ACL for that newznode looks like this:

1.3. Digest Scheme

Digest scheme represents an individual user with authentication. This uses username:password string that is hashed using the SHA-1 hashing algorithm, and that hashed string is in turn base64 encoded. According to the ZooKeeper website, it is stated that the MD5 hash of <username>:<password> is used as an ACL ID identity. Unless I am mistaken, that seems not to be the case. Instead, what I found was that <username>:<Base64 encoded SHA-1 hash of username:password> is used as an ACL ID (Please see above pictures under the Auth section).

What’s really funny is that if I authenticate an individual user on a znode using digest scheme on ZooKeeper client, instead of storing the username and encoded hash string of <username:password> like it should, it stores a clear, human-readable text of <username:password> as an ID. Executing the addauth command before setting the ACL with digest scheme does not work either. Below is the picture that illustrates my point:

Unless it is easy to work backwards – decoding the user_abc:pwd_abc, and then take that decoded string and undo the SHA-1 hashing part, it turns out setting ACL using digest scheme on a znode in ZooKeeper client is pointless.

Good thing is that if you setAcl a znode using digest scheme via client, you can delete it.

1.4. Host Scheme

Host scheme represents anyone within the same hosting server. I have not done enough with the host scheme yet, but I will come back to this with more details.

1.5. IP Scheme

IP scheme represents any user within the same IP address. Easiest example to use in this case would be 127.0.0.1, which represents the user of that any local machine, since any local machine will have 127.0.0.1 point to the localhost. Below is the syntax of setAcl using IP scheme:

setAcl /<node-name> ip:<IPv4-address>:<permission-set>

Using the syntax above, below is an example using the 127.0.0.1 IP address:

setAcl /newnode ip:127.0.0.1:crdwa

If done correctly, you should get the znode stat like the picture below:

That is it for now. On my next blog post, I will briefly talk about how to access them in Java; furthermore, I will talk more in detail about how username and password are stored. Thanks for reading as usual, and happy zookeeping!