authkeys

authkeys is a tool written in Go that you can use with OpenSSH as an AuthorizedKeysCommand. It'll reach out to LDAP and get keys and display them on stdout.

To learn more about our use of authkeys see our blog post.

• Authkeys: Key-Based SSH Authentication with Go

Pre-Requisites

You'll need an LDAP server that has a schema installed for storing SSH keys as part of an entry. Also, your LDAP server will need to use STARTTLS over port 389, as opposed to LDAPS.

Installation

To build a binary, you can use go get: go get -d github.com/threatstack/authkeys.

You'll need to put that binary somewhere (we use /usr/sbin because we make a package for it using fpm) and make sure the binary is chmod'ed to 0555.

Then, add to your sshd_config:

AuthorizedKeysCommandUser nobody
AuthorizedKeysCommand /usr/sbin/authkeys

Now when you log in, OpenSSH will run (in this example) /usr/sbin/authkeys with the username as the first argument. Authkeys will return the keys from LDAP, and the user should be logged in if there is a match. Of note: OpenSSH will prefer a local ~/.ssh/authorized_keys file over keys returned from AuthorizedKeysCommand, so make sure you test with a user that doesn't have that file.

At Threat Stack, we use Chef to deploy our authkeys package as part of our LDAP client setup -- just using a template and package resource. We leverage the OpenSSH cookbook using a node.override for the authorized_keys_command and authorized_keys_command_user variables.

Some LDAP installations require you to bind before searching. For example, Jumpcloud operates a user directory-as-a-service and allows users to self-service their SSH keys. You will need to provide a BindDN and BindPW in order to connect to the JumpCloud LDAP directory. See the documentation in this article for details: https://jumpcloud.com/engineering-blog/how-to-connect-your-application-to-ldap/

Configuration

Authkeys is configured using a JSON file. By default, it'll look in /etc/authkeys.json but you can override this with the AUTHKEYS_CONFIG environment variable for testing.

{
  "BaseDN": "",
  "DialTimeout": 5,
  "KeyAttribute": "",
  "LDAPServer": "",
  "LDAPPort": 389,
  "RootCAFile": "",
  "UserAttribute": "",
  "BindDN": "",
  "BindPW": ""
}

Variable	Type	Purpose	Possible Value
BaseDN	String	Base DN for your LDAP server	dc=spiffy,dc=io
DialTimeout	Int	A connection timeout if LDAP isnt reachable [Note 1]	5
KeyAttribute	String	LDAP Attribute for the SSH key	sshPublicKey
LDAPServer	String	Hostname of your LDAP server	ldap.spiffy.io
LDAPPort	Int	Port to talk to LDAP on	389
RootCAFile	String	A path to a file full of trusted root CAs [Note 2]	/etc/ssl/certs/ca-certificates.crt
UserAttribute	String	LDAP Attribute for a User	uid
BindDN	String	Bind DN for your LDAP server (LDAP service account)	uid=U,ou=Users,o=123,dc=jc,dc=com
BindPW	String	Password for the LDAP service account	password

Notes

1. Defaults to 5 seconds
2. If blank, Go will attempt to use system trust roots.

Usage

authkeys [username] will look up the user in LDAP and get their keys. Simple as that.

Changelog

If you're wondering why this started at version 2.0.0, it's because we've been using this tool internally for a while, and we cleaned it up for external consumption :)

Version 2.1.0 added a quicker TCP timeout. You can set this using the DialTimeout attribute.

Version 2.1.1 adds in error handling for when LDAP returns either no entries or too many (>1) entries.

Version 2.1.2 removes some superfluous os.Exit(1) calls, since log.Fatalf does that for you.

Version 2.1.3 added support for using a Bind DN for LDAP services such as Jumpcloud.com that require authentication.

Contribution

1. Fork
2. Create a feature branch
3. Commit your changes
4. Rebase your local changes against the master branch
5. Create a new Pull Request

Author

Patrick Cable (@patcable)