ProFTPD: Filters

ProFTPD supports the ability to "filter" the FTP commands it receives from the client. These filters are written as regular expressions. This means that while the filters can be very powerful, they can also be complex and harder to construct.

First, there are the AllowFilter and DenyFilter configuration directives. These configuration directives are used to set filters on every FTP command. If an AllowFilter is used, the command parameters must match the given filter, otherwise the command will be denied. If a DenyFilter is used, the command parameters must not match the given filter, otherwise the command will be denied. If both AllowFilter and DenyFilter are used, then the AllowFilter will be checked first.

Second, there are some special filter configuration directives aimed at those FTP commands that cause changes to files and directories on the server system: PathAllowFilter and PathDenyFilter. Like AllowFilter and DenyFilter, the Allow filter, if present, is checked first, then a DenyFilter, if present. PathAllowFilter and PathDenyFilter are checked after the AllowFilter and DenyFilter directives. These Path filters are only applied to the following FTP commands: DELE, MKD/XMKD, RMD/XRMD, RNFR, RNTO, STOR, STOU, and to the SITE commands CHGRP and CHMOD. Note that using both PathAllowFilter and PathDenyFilter at the same time is not a good idea; only one filter is generally needed.

One property that often catches the unwary administrator is the fact that proftpd only operates on the first Filter directive defined in the configuration file; it does not cycle through multiple Filter directives. This is because multiple regular expressions can be combined into a single (albeit more complex) regular expression. The alternation metacharacter is helpful in creating such combined regular expressions. For example, if you had the following in your proftpd.conf:

  PathAllowFilter \.jpg$
  PathAllowFilter \.jpeg$
  PathAllowFilter \.mpeg$
  PathAllowFilter \.mpg$
  PathAllowFilter \.mp3$
only the first PathAllowFilter would be enforced. To enforce all Filter simultaneously, use:
  PathAllowFilter \.(jpg|jpgeg|mpeg|mpg|mp3)$
Matches are case-sensitive! Also note that if you surround your regular expression in quotation (") marks, any backslash will itself need to be escaped; ProFTPD's configuration parser interprets backslashes inside of quoted strings a little differently. Thus, the example above would look like this:
  PathAllowFilter "\\.(jpg|jpgeg|mpeg|mpg|mp3)$"
if using quotation marks.

Another characteristic to keep in mind is that Filters are only applied to FTP command parameters, not to the FTP command itself. Most of the time, this is not a problem. It would be useful sometimes, though, to be able to filter parameters of commands other than those filtered by the Path filter directives. It would be an easy change to the source code; however, there is no feature request for the ability to do this on with a good justification for such a feature. If you can think of one, please open such a request.

When developing your Filter directives, it often helps to keep an eye on the server debugging output, to see how well your filters are being applied.

To prevent clients from using paths which may contain non-printable characters (e.g. CR, LF, VB, etc), you can use the following PathDenyFilter pattern:

  PathDenyFilter [^[:print:]]
Alternatively, you could use a PathAllowFilter which only allows printable characters in paths:
  PathAllowFilter [[:print:]]
And if you want to prevent spaces and tabs from appearing in paths, you can use:
  PathDenyFilter [[:blank:]]

What if you want to prevent clients from uploading so-called "dot files", i.e. files whose names start with a period ('.'), so that they are "hidden"? To prevent uploading of all dot files, you can use the following PathDenyFilter configuration:

  PathDenyFilter \.[^/]*$

In ProFTPD 1.3.3rc1 and later, you can use the AllowFilter and DenyFilter configuration directives inside of <Limit> sections, so that those Filter directives only apply to the FTP commands listed in the <Limit> section. This means you can specify regular expression filters for the arguments for specific commands. For example, you may want to configure a directory that only allows uploads of files with specific extensions. You could use PathAllowFilter for this -- but PathAllowFilter also applies to the MKD command, and you might want to allow users to create subdirectories in your special directory. Thus you only want your regular expression to apply to the STOR command in your directory. Below is an example of how to do this using AllowFilter:

  <Directory /path/to/dir>
    <Limit STOR>
      Order deny, allow
      AllowFilter \.ext$
The key is the Order directive; without it, the configuration will not work as you expect.

Frequently Asked Questions
Question: How can I prevent SITE CHMOD commands which use the mode 777, but allow other SITE CHMOD commands?
Answer: For this, you would use the DenyFilter directive, like so:

  <Limit SITE_CHMOD>
    DenyFilter 777
However, the desired mode can also be expressed symbolically, using e.g. "SITE CHMOD a+rwx file". So our filter should take this into account, as well as some variants:
  <Limit SITE_CHMOD>
    DenyFilter (777|a+rwx|u=rwx,g=rwx,o=rwx)
Looking better, but still not perfect. A determined user could still work around our filter by using a series of SITE CHMOD commands like this:
  SITE CHMOD u+rwx file
  SITE CHMOD g+rwx file
  SITE CHMOD o+rwx file
Or the determined user could make the change even more granular:
  SITE CHMOD u+r file
  SITE CHMOD u+w file
  SITE CHMOD u+x file
  SITE CHMOD g+r file
  SITE CHMOD g+w file
  SITE CHMOD g+x file
  SITE CHMOD o+r file
  SITE CHMOD o+w file
  SITE CHMOD o+x file
Another trap is that the numbers "777" might appear in a legitimate file name, and thus the following allowed command would fail unnecessarily:
  SITE CHMOD 644 ServiceOrder777.doc

In short, it is possible to create filters for portions of commands, but you do need to be very careful to not inadvertently cause commands to fail that should succeed.

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