SSHbase

SSHbase is a purpose-built Bcfg2 plugin for managing ssh host keys. It is responsible for making ssh keys persist beyond a client rebuild and building a proper ssh_known_hosts file, including a correct localhost record for the current system.

It has two functions:

• Generating new ssh keys – When a client requests a dsa, rsa, or v1 key, and there is no existing key in the repository, one is generated.
• Maintaining the ssh_known_hosts file – all current known public keys (and extra public key stores) are integrated into a single ssh_known_hosts file, and a localhost record for the current client is added. The ssh_known_hosts file data is updated whenever any keys change, are added, or deleted.

Interacting with SSHbase

• Pre-seeding with existing keys – Currently existing keys will be overwritten by new, sshbase-managed ones by default. Pre-existing keys can be added to the repository by putting them in <repo>/SSHbase/<key filename>.H_<hostname>
• Pre-seeding can also be performed using bcfg2-admin pull ConfigFile /name/of/ssh/key
• Revoking existing keys – deleting <repo>/SSHbase/\*.H_<hostname> will remove keys for an existing client.

Aliases

SSHbase has support for Aliases listed in clients.xml. The address for the entries are specified either through DNS (e.g. a CNAME), or via the address attribute to the Alias.

Getting started

1. Add SSHbase to the plugins line in /etc/bcfg2.conf and restart the server. This enables the SSHbase plugin on the Bcfg2 server.
2. Add Path entries for /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_dsa_key.pub, etc., to a bundle.
3. Enjoy.

At this point, SSHbase will generate new keys for any client without a recorded key in the repository, and will generate an ssh_known_hosts file appropriately.

Supported key formats

SSHbase currently supports the following key formats:

• RSA1 (ssh_host_key, ssh_host_key.pub)
• RSA2 (ssh_host_rsa_key, ssh_host_rsa_key.pub)
• DSA (ssh_host_dsa_key, ssh_host_dsa_key.pub)
• ECDSA (ssh_host_ecdsa_key, ssh_host_ecdsa_key.pub)

Group-specific keys

New in version 1.2.0.

In addition to host-specific keys, SSHbase also supports group-specific keys, e.g., for a high-availability cluster or similar application. Group-specific keys must be pre-seeded; SSHbase cannot create group-specific keys itself.

To use group-specific keys, simply create SSHbase/<key filename>.Gxx_<group name>. For instance, ssh_host_dsa_key.pub.G65_foo-cluster.

Adding public keys for unmanaged hosts

If you have some hosts which are not managed by Bcfg2, but you would still like to have their public ssh keys available in ssh_known_hosts, you can add their public keys to the SSHbase directory with a .static ending.

Example:

a.static:

TEST1

b.static:

TEST2

The generated ssh_known_hosts file:

TEST1
TEST2

Static ssh_known_hosts file

New in version 1.2.0.

You can also distribute a fully static ssh_known_hosts file on a per-host or per-group basis by creating SSHbase/ssh_known_hosts.H_<hostname> or SSHbase/ssh_known_hosts.Gxx_<group name>. Those files will be entirely static; Bcfg2 will not add any host keys to them itself.

Permissions and Metadata

New in version 1.2.0.

SSHbase supports use of an info.xml file to control the permissions and other metadata for the keys and ssh_known_hosts file. You can use the <Path> directive in info.xml to change the metadata for different keys, e.g.:

<FileInfo>
  <Path name="/etc/ssh/ssh_host_dsa_key">
    <Info owner="root" group="wheel" perms="0660"/>
  </Path>
  <Path name="/etc/ssh/ssh_host_dsa_key.pub">
    <Info owner="root" group="wheel" perms="0664"/>
  </Path>
</FileInfo>

Default permissions are as follows:

File	owner	group	perms	sensitive	paranoid	encoding
ssh_known_hosts	root	root	0644	false	false	None
ssh_host_key	root	root	0600	false	false	base64
ssh_host_key.pub	root	root	0644	false	false	base64
ssh_host_[rsa|dsa|ecdsa]_key	root	root	0600	false	false	None
ssh_host_[rsa|dsa|ecdsa]_key.pub	root	root	0644	false	false	None

Note that the sensitive attribute is false, even for private keys, in order to permit pulling with bcfg2-admin. You should almost certainly set sensitive to “true” in info.xml.

Blog post

http://www.ducea.com/2008/08/24/using-the-bcfg2-sshbase-plugin/

Note

The linked post uses deprecated ConfigFile entries. Path entries have since replaced these. See Configuration Entries.