## ProFTPD module `mod_dnsbl`

A DNS blacklist is a way in which the DNS can be used to "blacklist" sites/addresses that have been deemd to be "bad" in some way. These blacklists are often used by email servers, for determining and rejecting email sent by addresses known to be sources of spam. More information on DNS blacklists can be found here:

      http://en.wikipedia.org/wiki/DNSBL

While DNS blacklists are well known for use by email servers, it is also possible to use them for other means, such as ways of checking whether an FTP client's address should be allowed or rejected by an FTP server. Thus the `mod_dnsbl` module was written for ProFTPD, for such a purpose.

The `mod_dnsbl` module is not compiled by default; build/installation instructions are discussed here.

The most current version of `mod_dnsbl` is distributed with the ProFTPD source code.

## Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

## Directives

- DNSBLDomain
- DNSBLEngine
- DNSBLLog
- DNSBLPolicy

### DNSBLDomain

Syntax: DNSBLDomain domain
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_dnsbl
Compatibility: 1.3.1rc1 and later

The `DNSBLDomain` directive is used to configure the DNS name of a DNS blacklist site, to be consulted when determining whether `mod_dnsbl` should allow or reject an FTP connection. This directive can be used multiple times, to configure multiple different DNS blacklist sites. When checking these sites, the `mod_dnsbl` module will check each `DNSBLDomain`, in the order they appear in the `proftpd.conf` file.

Example:

      DNSBLDomain sbl.spamhaus.org
      DNSBLDomain xbl.spamhaus.org

### DNSBLEngine

Syntax: DNSBLEngine on|off
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_dnsbl
Compatibility: 1.3.1rc1 and later

The `DNSBLEngine` directive toggles the use of DNS blacklists for access control for FTP client connections (e.g. `mod_dnsbl`). This is usually used inside a `<VirtualHost>` section to enable DNS blacklist use for a particular virtual host. By default `mod_dnsbl` is disabled for both the main server and all configured virtual hosts.

### DNSBLLog

Syntax: DNSBLLog file
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_dnsbl
Compatibility: 1.3.1rc1 and later

The `DNSBLLog` directive is used to specify a log file for `mod_dnsbl`'s reporting on a per-server basis. The file parameter given must be the full path to the file to use for logging.

Note that this path must not be to a world-writable directory and, unless `AllowLogSymlinks` is explicitly set to on (generally a bad idea), the path must not be a symbolic link.

### DNSBLPolicy

Syntax: DNSBLPolicy "allow,deny"|"deny,allow"
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_dnsbl
Compatibility: 1.3.1rc1 and later

The `DNSBLPolicy` directive determines whether the `mod_dnsbl` module (if enabled) will allow a connection by default or not.

If `DNSBLPolicy` is configured using "allow,deny", then the `mod_dnsbl` module will allow the connection, unless the connecting client is blacklisted by any of the configured `DNSBLDomain` sites.

If `DNSBLPolicy` is configured using "deny,allow", then the `mod_dnsbl` module will not allow the connection, unless the connecting client is listed by any of the configured `DNSBLDomain` sites.

## Installation

The `mod_dnsbl` module is distributed with ProFTPD. Simply follow the normal steps for using third-party modules in ProFTPD:

      $ ./configure --with-modules=mod_dnsbl
      $ make
      $ make install

Alternatively, `mod_dnsbl` can be built as a DSO module:

      $ ./configure --enable-dso --with-shared=mod_dnbsl ...

Then follow the usual steps:

      $ make
      $ make install

Logging
The `mod_dnbsl` module supports different forms of logging. The main module logging is done via the `DNSBLLog` directive. For debugging purposes, the module also uses trace logging, via the module-specific log channels:

- dnsbl

Thus for trace logging, to aid in debugging, you would use the following in your `proftpd.conf`:

      TraceLog /path/to/ftpd/trace.log
      Trace dnsbl:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

© Copyright 2007-2014 TJ Saunders
All Rights Reserved