Listener Settings

Configuring Listener Settings

A listener is simply an IP address, port, and protocol combination that the server is accepting connections on. For example, you can add an FTP listener on port 21 and attach it to an IP address. It can be an IPv4 or IPv6 address. The “Default” listeners are listener templates that represent the settings that will be applied to newly detected listeners. There are several different parameters that each listener can have:

Types of Listeners

There are five types of listeners that you can add to an IP address:

The first two different forms of secure FTP as well as allow regular FTP while the SSH2 SFTP listener is for establishing connections over the SFTP protocol (a completely different protocol from FTP, despite the similar name). The HTTP and HTTPS listeners allow web client connections to the server using either the unsecure HTTP protocol or encrypted HTTPS protocol.

There are two types of secure FTP connections possible: FTPS and FTPES. 

FTPES, which is often referred to as explicit FTP with TLS/SSL security, is a modification of the FTP protocol that starts out over an insecure, normal, FTP connection and is then upgraded to a secure connection through FTP command extensions during login. This is the preferred method of secure FTP because it allows SPI firewalls to know that there is FTP traffic occurring on the connection. You establish FTPES sessions using a normal Cerberus FTP Server FTP listener, typically over port 21. Both unencrypted FTP and explicit TLS/SSL connections can be established to this type of listener. You cannot establish an implicit FTPS connection over this type of listener, only explicit.

FTPS is usually referred to as implicit FTP with TLS/SSL security. Its closest analog is HTTPS. It is basically the FTP protocol over a TLS/SSL-secured connection. This form of secure FTP is deprecated but widely supported and still in use. This is what a Cerberus FTP Server FTPS listener is for and this type of listener typically listens on port 990. Note, the settings “Require Secure Control” and “Require Secure Data” are meaningless for this type of listener as secure control and secure data are the only methods possible for this protocol. Connections established to an FTPS listener can only be established securely.

Adding a New Listener

Cerberus FTP Server supports adding multiple listeners for a given IP address. This allows you to have Cerberus accept connections from different protocols on multiple ports. The only requirement is that each listener is on a unique IP/port combination. You can add FTP, FTPS (for implicit secure FTP only), SSH2 SFTP, HTTP, or HTTPS listeners.

Click the 'New' button at the upper right of the listener page to add a new listener. A pop-up box will appear asking to specify the listener details (type, listener IP, and port combination).

Listener Settings

The “Default” Listeners

There is a Default listener for each type of listener (FTP, FTPS, SFTP, HTTP, HTTPS, HTTPS Admin). When a new IP address is detected, that IP address will receive an FTP, FTPS, and SFTP listener and each of those listeners will be assigned the values of the appropriate “Default” listener at the time of detection. For example, If the “Default FTP” listener was defined to be on port 21, then when a new listener is detected for the first time it will receive an FTP listener on port 21 with the values of the Default FTP listener. Those settings then become the settings for the newly detected listener. Note that the new listener’s settings are not linked to the “Default” listener in any way. The “Default” listener simply represents the values that newly detected listeners will be initialized with. Changing the values of the “Default” listener wouldn’t change any values on existing or previously detected listeners.

For example, when you first install Cerberus FTP Server, the “Default FTP” listener is set to port 21 (the default FTP listening port) and all listeners detected during that first start will receive FTP listeners with that port value. If you later change the “Default FTP” listener settings then that change will have no effect on existing listeners.

It is also worth noting that Cerberus remembers the settings for listeners that were previously detected but might have changed. For servers that have dynamic addresses that constantly change or cycle between a range of addresses, Cerberus will “remember” the old values and apply those instead of the “Default” settings if that listener address is later detected again.

Enabling/Disabling Listeners

Listeners can also be enabled or disabled from the main Cerberus FTP Server "Listeners" page in the Server Manager:

Select a listener and right-click.  Click the Enable/Disable menu item to toggle enabling or disabling a listener. Disabled listeners will no longer accept connections.