ProFTPD: Classes

What are Classes?
When configuring proftpd, it is sometimes nice, or even necessary, to tag or label a client as belonging to some group, based on that client's IP address or DNS hostname. A "class" is the name for such connection-based groupings in ProFTPD terms. A class is defined to have a name, and as having certain criteria such as IP addresses, IP subnets/masks, and DNS hostnames. A client that connects to the daemon that has matching characteristics is then labeled as belonging to that class. Note that a connecting client can belong to only one class; see the description below for how the winning class is selected for a session from among multiple possible matches.

How are Classes Defined?
To define a class, use a <Class> section in your proftpd.conf:

  <Class internal>
    From 192.168.0.0/16
  </Class>

This defines a class named "internal"; any client connecting from 192.168.0.0/16 will belong to this class. And if you wanted to define a class for all clients not connecting from 192.168.0.0/16 address space:

  <Class external>
    From !192.168.0.0/16
  </Class>

A more complicated class might include matching DNS names as well:

  <Class test>
    From 1.2.3.4
    From proxy.*.com
    From my.example.com
    From 5.6.7.8
  </Class>

This "test" class will then be used for a client with any of the defined characteristics.

Note that if your class rules use only DNS names, and proftpd is unable to resolve the IP address of a client to a DNS name, that class may not be matched as you might expect. This can be seen in the server debugging output, at level 10, as something like:

  comparing DNS name '1.2.3.4' to pattern 'proxy.*.com'

Here you see the 1.2.3.4 IP address, where a DNS name should be. In order for DNS name based class rules to function properly, both a) DNS resolution is needed (i.e. UseReverseDNS must be on, which is the default), and b) the IP address of a connecting client must be resolvable to a DNS name.

What if there are multiple classes defined, and the classes overlap, e.g. two classes both have:

  From *.example.com

Which one will be used for the connecting client? This will depend on the order in which classes are defined in the proftpd.conf file. When searching the list of classes for the one that matches the client, proftpd checks each class in the order in which they are defined. The first class definition (in order of appearance in proftpd.conf) that matches is used.

How do you define a class that includes all clients from a certain domain except one specific host in that domain? To define a class with these sorts of characteristics, use the Satisfy configuration directive:

  <Class foo>
    From *.example.com
    From !bad.example.com
    Satisfy all
  </Class>

Using Satisfy
The Satisfy directive, when used within a <Class> section, indicates whether any of the From rules in the section need to match, or whether all of the From rules in the section need to match. The default Satisfy setting for a <Class> section is "any".

To illustrate, the following class definition will never match:

  <Class impossible>
    From 127.0.0.1
    From !127.0.0.1
    Satisfy all
  </Class>

It is impossible to both an address and not match that same address, but that is what is demanded by the "Satisfy all" setting in the above class definition.

Now, where the use of "Satisfy all" comes in handy is when you have a general rule with exceptions:

  <Class customers>
    From .domain.com
    From !host1.domain.com !host2.domain.com
    Satisfy all
  </Class>

Specifically, the use of "Satisfy all" is necessary when you have multiple not matches (i.e. using the ! prefix), all of which need to be evaluated.

How are Classes Used?
By itself, a class does nothing. It is merely a way to define a set of clients and to give that set a name. Once that name is defined, though, it can be use as part of your configuration. There are a limited number of configuration directives that make use of classes directly:

AllowClass
DenyClass
DisplayGoAway
MaxClientsPerClass

The AllowClass and DenyClass directives are the main directives to use, for example in <Limit> sections:

  <Limit ALL>
    AllowClass internal
    DenyAll
  </Limit>

The mod_ifsession module also makes use of classes with its <IfClass> configuration section. Using classes and mod_ifsession, you can write a proftpd.conf that has specific configurations for specific classes of clients. Here's an example snippet demonstrating use of <IfClass>:

  <IfClass internal>
    MaxClients 100
  </IfClass>

  <IfClass !internal>
    MaxClients 25
  </IfClass>

This allows clients from class "internal" to see an effective MaxClients limit of 100 simultaneous clients, and clients not in class "internal" to see an effective limit of only 25.