Starting cf-serverd sets up a line of communication between hosts. This daemon authenticates requests from the network and processes them according to rules specified in the [server control][cf-serverd#Control Promises] body and server bundles containing access promises.

The server can allow the network to access files or to execute CFEngine:

Unlike other approaches to automation, CFEngine does not rely on SSH key authentication and configuring trust, the communication between hosts is very structured and also able to react to availability issues. This means that you don’t have to arrange login credentials to get CFEngine to work. If large portions of your network stop working, then CFEngine on each individual host understands how to keep on running and delivering promises.

In particular, if the network is not working, CFEngine agents skip downloading new promises and continue with what they already have. CFEngine was specifically designed to be resilient against connectivity issues network failure may be in question. CFEngine is fault tolerant and opportunistic.

Connecting to server

In order to connect to the CFEngine server you need:

If all of the above criteria are met, connection will be established and data will be transferred between client and server. The client can only send short requests, following the CFEngine protocol. The server can return data in a variety of forms, usually files, but sometimes console output.

Bootstrapping

[Bootstrap][General Installation#Bootstrap] is the manual first run of cf-agent that establishes communication with the policy server. Bootstrapping executes the failsafe.cf policy that connects to the server, establishes trust to the server’s key, and that starts the CFEngine daemon processes cf-execd, cf-serverd and cf-monitord. The host that other hosts are bootstrapped to automatically assumes the role of policy server.

You should bootstrap the policy server first to itself:

$ /var/cfengine/bin/cf-agent --bootstrap [public IP of localhost]

Then execute the same step (using the exact same IP) on all hosts that should pull policy from that server. CFEngine will create keys if there are none present, and exchange those to establish trust.

CFEngine will output diagnostic information upon bootstrap. In case of error, investigate the access promises the server is making (run cf-serverd in verbose mode on the policy hub for more informative messages). Note that by default, CFEngine’s server daemon cf-serverd trusts incoming connections from hosts within the same /16 subnet.

After a host has been bootstrapped, the text file policy_server.dat in the CFEngine installation contains the IP address of the policy server.

Key exchange

The key exchange model used by CFEngine is based on that used by OpenSSH. It is a peer to peer exchange model, not a central certificate authority model. This means that there are no scalability bottlenecks (at least by design, though you might introduce your own if you go for an overly centralized architecture).

Key exchange is handled automatically by CFEngine and all you need to do is to decide which keys to trust. The server (cf-serverd) trusts new keys only from addresses in trustkeysfrom. Once a key has been accepted you should close down trustkeysfrom list. Then, even if a malicious peer is spoofing an allowed IP address, its unknown key will be denied.

Once you have arranged for the right to connect to the server, you must decide which hosts will have access to which files. This is done with access promises.

cf3 bundle server access_rules() { access: "/path/file" admit => { "127.0.0.1", "127.0.0.2", "127.0.0.3" }, deny => { "192.168.0.0/8" }; }

On the client side, i.e. cf-runagent and cf-agent, there are three issues:

  1. Choosing which server to connect to.
  2. Trusting the key of any previously unknown servers
  3. Choosing whether data transfers should be encrypted (with [encrypt][files#encrypt]) - not applicable if you are using new [protocol_version][Components and Common Control#protocol_version].

There are two ways of managing trust of server keys by a client. One is an automated option, setting the option [trustkey][files#trustkey] in a copy_from files promise, e.g.

cf3 body copy_from example { # .. other settings .. trustkey => "true"; }

Another way is to run cf-runagent in interactive mode. When you run cf-runagent, unknown server keys are offered to you interactively (as with ssh) for you to accept or deny manually:

 $ WARNING - You do not have a public key from host ubik.iu.hio.no = 128.39.74.25
 $ Do you want to accept one on trust? (yes/no)
 -->

Once public keys have been exchanged from client to server and from server to client, the issue of trust is solved according to public key authentication schemes. You only need to worry about trust when one side of a connection has never seen the other side before.

Time windows (races)

All security is based on a moment of trust that is granted by a user at some point in time – and is assumed thereafter (once given, hard to rescind). Cryptographic key methods only remove the need for a repeat of the trust decision. After the first exchange, trust is no longer needed, because the keys allow identity to be actually verified.

Even if you leave the trust options switched on, you are not blindly trusting the hosts you know about. The only potential insecurity lies in any new keys that you have not thought about. If you use wildcards or IP prefixes in the trust rules, then other hosts might be able to spoof their way in on trust because you have left open a hole for them to exploit. That is why it is recommended to set the system to the state of zero trust immediately after key transfer, by commenting or emptying out the trust options (trustkeysfrom on the server).

It is possible, though somewhat laborious, to transfer the keys out of band, by copying /var/cfengine/ppkeys/localhost.pub to /var/cfengine/ppkeys/user-aaa.bbb.ccc.mmm (assuming IPv4) on another host. e.g.

     localhost.pub -> root-128.39.74.71.pub

Other users than root

CFEngine normally runs as user “root” (except on Windows which does not normally have a root user), i.e. a privileged administrator. If other users are to be granted access to the system, they must also generate a key and go through the same process. In addition, the users must be added to the server configuration file.

Encryption

CFEngine provides encryption for keeping file contents private during transfer. If [protocol_version][files#protocol_version] is set to classic or 1, then file transfers must be explicitly encrypted by setting encrypt=>"true" in a copy_from body of a files promise. For newer protocol_version, all transfers are encrypted.

However, the main role of encryption in configuration management is for authentication. Secrets should not be transferred through policy, encrypted or not. Policy files should be considered public, and any leakage should not reveal secret information.

Troubleshooting

When setting up cf-serverd, you might see the error message

  Unspecified server refusal

This means that cf-serverd is unable or is unwilling to authenticate the connection from your client machine. The message is deliberately non-specific so that anyone attempting to attack or exploit the service will not be given information which might be useful to them.

There is a simple checklist for curing this problem:

  1. Make sure that you have granted access to the client’s address in the [server control][cf-serverd#Control Promises] body.
  2. Make sure the connecting client is granted access to the requested resources (files usually) in the access_rules promise bundle.
  3. See the verbose log of the server for the exact error message, since the client always gets the “Unspecified server refusal” reply from the server. To run the server in verbose, kill cf-serverd on the policy hub and run: $ cf-serverd -v and then manually run cf-agent on the client.
  4. In the unlikely case that you still get no indication of the denial, try increasing the agent run verbosity. cf-agent -I for info-level messages or even cf-agent -v for verbose.