LFD Clustering

The CLUSTSTER_* options in /etc/csf/csf.conf allow the configuration of an LFD cluster environment where a group of servers can share blocks and configuration option changes. In the configuration there are two comma separated lists of IP addresses:


If you want all members of the LFD cluster to send block notifications to each other then both settings should be them same. You also need to enable CLUSTER_BLOCK (enabled by default) for LFD to automatically send blocks to all members in CLUSTER_SENDTO.

However, you can also set up a cluster such that some members only provide notifications to others and do not accept blocks from others. For example, you may have a cluster of servers that includes one that hosts a support desk that you do not want to block clients from accessing. In such an example you might want to exclude the support desk server from the CLUSTER_SENDTO list, but include it in the CLUSTER_RECVFROM list.

The option CLUSTER_MASTER is the IP address of the master node in the cluster allowed to send CLUSTER_CONFIG changes to servers listed in the local CLUSTER_SENDTO list. Only cluster members that have CLUSTER_MASTER set to this IP address will accept CLUSTER_CONFIG changes.

There is another option, CLUSTER_NAT that should be used if the IP address of the server does not appear in ifconfig, for example if it is a NAT configuration. If this is the case, add the IP address of the server that this configuration is on and used in CLUSTER_SENDTO / CLUSTER_RECVFROM to CLUSTER_NAT.

CLUSTER_LOCALADDR can be set if you do not want to use the servers main IP, i.e. the first one listed via

The CLUSTER_PORT must be set to the same port on all servers. The port should NOT be opened in TCP_IN or TCP_OUT as CSF will automatically add appropriate in and out bound rules to allow communication between cluster members.

The CLUSTER_KEY is a secret key used to encrypt cluster communications using the Blowfish algorithm. It should be between 8 and 56 ASCII characters long, longer is better, and must be the same on all members of the cluster.

When blocks are sent around the cluster they will maintain their originals parameters, e.g. permanent/temporary, direction (in/out), ports, etc. All blocks are traded except for LT_POP3D and LT_IMAPD.

The cluster uses 10 second timeouts in its communications, if the timeout is reached then that cluster members notification will be lost.

You must restart CSF and then LFD after making any CLUSTER_* changes.

LFD Cluster CLI and Plesk Interface

See csf --help for the list of new CLI commands. Additional options will automatically become available in the UI once CLUSTER_SENDTO has been configured.

Only cluster members listed in CLUSTER_RECVFROM can send out requests to those members listed in CLUSTER_SENDTO.

Only the server listed in CLUSTER_MASTER will be accepted as the source of CLUSTER_CONFIG configuration option requests, such as: --cconfig, --cfile, --crestart

The CLI options --cfile and --cfiler allow you to synchronise csf configuration files throughout a cluster from the CLUSTER_MASTER server.

There is currently only provision for permanent simple IP denies and allows from the CLI (i.e. not Allow/Deny Filters).

The cluster PING sends a ping to each CLUSTER_SENDTO member which will report the request in their respective lfd.log files. This is intended as a test to confirm that cluster communications are functioning.

The options to change the configuration option in /etc/csf/csf.conf in cluster members should be used with caution to ensure that member specific options are not overwritten. The intention of the two options is that the --cconfig option be used if multiple changes are required and the final request is a --cconfigr to restart CSF and LFD to effect the requested changes immediately.

LFD Cluster Security

The clustering option is undoubtedly powerful in allowing servers to preemptively block access attempts as one server is hit before the attack can spread to other members of the cluster. This communication, however, does introduce a security risk. Since communications are made over the network, they are open to interception. Also, there is nothing to stop any local user from accessing the network port and sending data to it, though it will be discarded unless properly encrypted.

There are security measures implemented to help mitigate attacks:

  1. CSF constructs iptables rules such that only cluster members can communicate over the cluster port with each other.
  2. The clustered servers will only accept data from connections from IPs listed in CLUSTER_RECVFROM or CLUSTER_MASTER.
  3. All communications are encrypted using the Blowfish cipher through a Pure Perl cpan module using the Cipher Block Chaining module and the configured CLUSTER_KEY.
  4. CLUSTER_CONFIG set to 0 prevents the processing of configuration option requests.
  5. Only CLUSTER_MASTER will be accepted as the source of CLUSTER_CONFIG configuration option requests.

Should the configured secret key (passphrase) be compromised or guessed or a flaw found in the encryption modules or their implementation in CSF, a malicious connection could reconfigure the CSF firewall and then leverage a local or remote root escalation. This should be considered if you decide to use this option.


Related Files

File Description
/etc/csf/csf.conf CLUSTER_* configuration options